Advances in Catalytic Activation of Dioxygen by Metal Complexes (Catalysis by Metal Complexes)

ADVANCES IN CATALYTIC ACTIVATION OF DIOXYGEN BY METAL COMPLEXES

Catalysis by Metal Complexes Volume 26

Editors: Brian James, University of British Columbia, Vancouver, Canada Piet W. N. M. van Leeuwen, University of Amsterdam, The Netherlands Advisory Board: Albert S.C. Chan, The Hong Kong Polytechnic University, Hong Kong Robert Crabtee, Yale University, U.S.A. David Cole-Hamilton, University of St Andrews, Scotland István Horváth, Eotvos Lorand University, Hungary Kyoko Nozaki, University of Tokyo, Japan Robert Waymouth, Stanford University, U.S.A.

The titles published in this series are listed at the end of this volume.

ADVANCES IN CATALYTIC ACTIVATION OF DIOXYGEN BY METAL COMPLEXES edited by

László I. Simándi Chemical Research Center, Hungarian Academy of Sciences, Budapest

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

eBook ISBN: Print ISBN:

0-306-47816-1 1-4020-1074-5

©2002 Kluwer Academic Publishers New York, Boston, Dordrecht, London, Moscow Print ©2003 Kluwer Academic Publishers Dordrecht All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Kluwer Online at: and Kluwer's eBookstore at:

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v

Preface The subject of dioxygen activation and homogeneous catalytic oxidation by metal complexes has been in the focus of attention over the last 20 years. The widespread interest is illustrated by its recurring presence among the sessions and subject areas of important international conferences on various aspects of bioinorganic and coordination chemistry as well as catalysis. The most prominent examples are ICCC, ICBIC, EUROBIC, ISHC, and of course the ADHOC series of meetings focusing on the subject itself. Similarly, the number of original and review papers devoted to various aspects of dioxygen activation are on the rise. This trend is due obviously to the relevance of catalytic oxidation to biological processes such as dioxygen transport, and the action of oxygenase and oxidase enzymes related to metabolism. The structural and functional modeling of metalloenzymes, particularly of those containing iron and copper, by means of low-molecular complexes of iron, copper, ruthenium, cobalt, manganese, etc., have provided a wealth of indirect information helping to understand how the active centers of metalloenzymes may operate. The knowledge gained from the study of metalloenzyme models is also applicable in the design of transition metal complexes as catalytsts for specific reactions. This approach has come to be known as biomimetic or bioinspired catalysis and continues to be a fruitful and expanding area of research. This book is the sequel of the monograph Catalytic Activation of Dioxygen by Metal Complexes by the editor of the present volume, published by Kluwer Academic Publishers in 1992 as Volume 13 of the Series Catalysis by Metal Complexes. Ten years later it is appropriate to cover the developments in selected areas of the field, which has been the objective of this edited volume in the same series. It is my great pleasure to thank the authors of individual chapters for their excellent contributions. The following prominent scientists have accepted invitations to review major areas of dioxygen activation: Brian R.. James (Catalytic oxidations using ruthenium porphyrins), Kenneth D. Karlin (Copper-dioxygen complexes and their roles in biomimetic oxidation reactions), Roger A. Sheldon (Catalytic oxidations of alcohols), Takuzo Funabiki (Functional model oxygenations by nonheme iron complexes) and Craig L. Hill (Catalysis for selective aerobic oxidation under ambient conditions). A chapter on Catalytic oxidations using cobalt(II) complexes has been contributed by myself.

vi I am grateful to my wife Tatiana for her expert help at various stages of the work and critical reading of Chapter 6. I thank Mary Egresi for her assistance in the manuscript prepration. László I. Simándi Budapest August 2002

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Contents Preface Contributors About the Editor

Chapter 1 Catalytic oxidations using ruthenium porphyrins

v xiii xiv

1

Maria B. Ezhova and Brian R. James Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T1Z1, Canada

1. INTRODUCTION: OXYGENASE AND OXIDASE ACTIVITY 2. REACTIONS OF RUTHENIUM PORPHYRIN COMPLEXES WITH AND OTHER OXIDANTS 3. OXIDATION OF ORGANIC SUBSTRATES 3.1 Oxidation of phosphines, phosphites, arsines and stibines 3.2 Oxidation of thioethers 3.3 Epoxidation of olefins 3.4 Oxidation of saturated hydrocarbons 3.5 Oxidative-dehydrogenation of phenols and other arenes 3.6 Oxidative-dehydrogenation of alcohols 3.7 Oxidative dehydrogenation of amines 4. CONCLUSIONS 5. ABBREVIATIONS 6. REFERENCES

3 12 16 16 19 21 40 44 47 51 61 64 66

Chapter 2 Copper-dioxygen complexes and their roles in biomimetic oxidation reactions 79 Christiana Xin Zhang, Hong-Chang Liang, Kristi J. Humphreys and Kenneth D. Karlin Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA

1. INTRODUCTION 1.1 Practical Copper Oxidative Processes 1.2 Copper in Biology 1.2.1 Hemocyanin, Tyrosinase, and Catechol Oxidase 1.2.2 Amine Oxidases; Galactose Oxidase

79 80 80 81 84

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1.2.3 Cytochrome c Oxidases 2. COPPER-DIOXYGEN ADDUCTS 2.1 Copper-Dioxygen Complex Generation; 1984-1999 2.2 Recent Further Advances in Copper-Dioxygen Complex Generation 3. COPPER OXYGENASE CHEMISTRY 3.1 Aromatic Hydroxylation 3.2 Recent Tyrosinase Models 3.3 TPQ Biogenesis 3.4 Aliphatic Hydroxylation 4. COPPER OXIDASE MODELS; CATALYTIC ALCOHOL OXIDATION 5. COPPER-PHENANTHROLINE DNA OXIDATION 6. REFERENCES

Chapter 3 Catalytic oxidations of alcohols

86 86 87 93 97 97 99 101 103 107 111 116

123

R.A. Sheldon and I.W.C.E. Arends Biocatalysis and Organic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

1. 2. 3. 4. 5. 6. 7.

123 124 126 138 144 146

INTRODUCTION MECHANISMS RUTHENIUM-CATALYZED OXIDATIONS WITH PALLADIUM-CATALYZED OXIDATIONS WITH COPPER-CATALYZED OXIDATIONS WITH OTHER METALS AS CATALYSTS FOR OXIDATION WITH CATALYTIC OXIDATION OF ALCOHOLS WITH HYDROGEN PEROXIDE AND ALKYL HYDROPEROXIDES 8. CONCLUDING REMARKS 9. REFERENCES

148 151 152

Chapter 4 Functional model oxygenations by nonheme iron complexes

157

Takuzo Funabiki Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

1. INTRODUCTION

158

ix 2. HEME AND NONHEME OXYGENASES 3. FUNCTIONAL MODEL STUDIES ON NONHEME IRON DIOXYGENASES 3.1 Catechol Dioxygenases 3.1.1 Intradiol Cleavage Oxygenations 3.1.2 Extradiol Cleavage Oxygenations 3.1.3 Mechanisms of Oxygenations 3.2 Dioxygenases other than Catechol Dioxygenases 4. FUNCTIONAL MODEL SYSTEMS FOR NONHEME IRON MONOOXYGENASES 4.1 Functional Model Oxygenations by Diiron Complexes 4.1.1 Monooxygenation by Diiron Complexes with Peroxides 4.1.2 Monooxygenation by Diiron Complexes with Molecular Oxygen 4.2 Monooxygenation by Diiron Complexes 4.3 Functional Model Oxygenations by Iron Species in the Polyoxometalate and Heterogeneous Matrix 4.3.1 Oxygenation by Iron Species in the Homogeneous System 4.3.2 Oxygenation by Iron Species in the Heterogeneous System 4.4 Functional Model Oxygenations by Mono-Iron Species 4.4.1 Oxygenation by Mono-Iron Complex/Activated Oxygen System 4.4.2 Oxygenation by Mono-Iron System 4.4.3 Mono-Iron Oxygen Species 5. FROM FUNCTIONAL MODEL TO CATALYSIS 6. REFERENCES

Chapter 5 Catalysis for selective aerobic oxidation under ambient conditions. Thioether sulfoxidation catalyzed by gold complexes

159 161 161 162 170 173 179 181 183 183 187 187 189 189 190 191 191 198 202 204 207

227

Eric Boring, Yurii V. Geletii and Craig L. Hill Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA

1. INTRODUCTION CATALYTIC 2. DISCOVERY OF OXIDATION SYSTEM 3. STOICHIOMETRIC Au(III) REDUCTION BY THIOETHERS

228 230 231

x 4. IN SITU CATALYST PREPARATION 5. REACTION STOICHIOMETRY 6. EMPIRICAL REACTION RATE LAW 7. RATE LIMITING STEP 8. PROPOSED REACTION MECHANISM 9. MECHANISMS RULED OUT 10. ORIGIN OF OXYGEN IN SULFOXIDE PRODUCT; ROLE OF IN SULFOXIDATION 11. REOXIDATION OF Au(I) BY DIOXYGEN. CATALYST PREPARATION FROM Au(I) COMPLEX 12.EFFECT OF LIGANDS ON REACTIVITY 13.PRODUCT INHIBITION (DMSO EFFECT) 14. CO-CATALYSIS BY TRANSITION METAL IONS 15. SOLVENT EFFECTS 16.HETEROGENEOUS SYSTEMS 17.EFFECT OF AMINO ACIDS 18. OXIDATION OF THIOETHERS OTHER THAN CEES 19.EXPERIMENTAL DETAILS 20.CONCLUSIONS

Chapter 6 Catalytic oxidations using cobalt(II) complexes

232 233 235 237 238 242 244 245 247 249 251 252 255 256 257 259 261

265

László I. Simándi Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary

1. INTRODUCTION 2. COBALT DIOXYGEN COMPLEXES 3. OXIDATIONS CATALYZED BY Co(salen) COMPLEXES 3.1 Oxidation of substituted phenols 3.1.1 2,6-di-tert-butylphenol 3.2 Oxidation of 3,5-di-tert-butylcatechol 3.3 Oxidation of lignin phenolics 3.4 Nitrogen monoxide 3.5 Oxidative dehydrogenation 3.6 Oxidation of quercetin 3.7 Mercaptoethanol 3.8 Alkenes and alcohols 3.9 Alkene epoxidation 3.10 Primary amines 4. OXIDATIONS CATALYZED BY COBALOXMES

266 267 269 269 269 270 270 274 275 276 276 276 278 280 280

xi 4.1 4.2 4.3 4.4 4.5

Oxidation of o-phenylenediamine Oxidation of 2-aminophenol Oxidation of 3,5-di-tert-butylcatechol Oxidative cleavage of a stilbene derivative Oxygen insertions 4.5.1 Oxygen insertion to terminal P, C and N atoms 4.6.2 Insertion of into alkylcobaloximes 5. OXIDATIONS CATALYZED BY COBALT(II) PORPHYRINS 6. OXIDATION WITH COBALT(II) PHTHALOCYANINES 7. OXIDATIONS CATALYZED BY COBALT(II) AMINE COMPLEXES 7.1 Catalytic oxidation of substituted anilines 7.2 Oxidation of miscellaneous substrates 8. OXIDATIONS CATALYZED BY COBALT(II) PYRIDINE COMPLEXES epoxidation 8.1 9. COBALT-FENTON SYSTEMS 10. CATALYZED OXIDATIONS 10.1 10.2 Substituted phenols 10.3 Pinanediols 11. OXIDATIONS VIA ALKYLPEROXOCOBALT COMPLEXES 12.OXIDATIONS WITH Co-CYCLIDENE COMPLEXES 13. OXIDATIONS WITH COBALT PEPTIDE COMPLEXES 15.CARBOXYLATOCOBALT COMPLEXES AND SALTS 16.MISCELLANEOUS COBALT CATALYSTS 17.CONCLUSIONS 18. REFERENCES

281 282 283 287 289 289 290 292 298

Subject Index

329

301 301 306 307 307 308 309 309 309 310 311 313 316 317 320 322 323

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Contributors I.W.C.E. Arenda, Organic Chemisty and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Eric Boring, Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA Maria B. Ezhova, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T1Z1, Canada Takuzo Funabiki, Professor, Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 6068501, Japan Yurii V. Geletii, Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA Craig L. Hill, Professor, Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA Kristi J. Humphreys, Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA Brian R. James, Professor, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T1Z1, Canada Kenneth D. Karlin, Professor, Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA Hong-Chang Liang, Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA Roger A. Sheldon, Professor, Organic Chemisty and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands László I. Simándi, Professor, Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary Christiana Xin Zhang, Department University, Baltimore, MD 21218, USA

of Chemistry,

Johns

Hopkins

xiv

About the Editor László I. Simándi is Head of the Department of Coordination Chemistry at the Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences and Professor of Chemistry at L. Eötvös University (both in Budapest). His research interests include catalysis by cobalt, iron and manganese complexes (biomimetic dioxygen activation, catalytic oxidation and carbonylation), as well as kinetics and mechanisms of inorganic reactions in solution (fast redox and electron transfer). He was visiting professor at the University of Texas (Arlington) and lectured widely as invited speaker at conferences and seminars in the US, Europe and Japan. He was the organizer of the Fourth International Symposium on the Activation of Dioxygen and Homogeneous Catalytic Oxidation (Balatonfüred, Hungary, 1990), and co-organizer of the XXII International Conference on Coordination Chemistry (Budapest, 1982). He is member of the Hungarian Chemical Society and Fellow of the Royal Society of Chemistry. He is the author of the monograph Catalytic Activation of Dioxygen by Metal Complexes (Kluwer, 1992) and the editor of Dioxygen Activation and Homogeneous Catalytic Oxidation (Elsevier, 1991).

Chapter 1 Catalytic oxidations using ruthenium porphyrins

Maria B. Ezhova and Brian R. James Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1, Canada

Abstract: The major goal of the Chapter is to review developments in the use of Ru-porphyrin complexes as homogeneous (or matrix-supported) catalysts for oxygenation and oxidation processes. The subject was given impetus with the discovery of a remarkable reaction in which a Ru(II) porphyrin complex reacted with to give a trans-dioxo-Ru(VI) species. Such species, which can be formed from a wide range of O-atom donors, were shown subsequently to be capable of acting as a bis(monooxygenase) in transferring both the coordinated oxo ligands (as O-atoms) to olefinic substrates, saturated hydrocarbons, phosphines, and thioethers, and the processes become catalytic in the presence of excess of the O-atom donor. Further, the dioxo species can also exhibit oxidase-like activity, and effect stoichiometric or catalytic oxidative-dehydrogenation of phenols, alkoxyarenes, alcohols, and amines. Use of chiral porphyrins has led to catalytic, asymmetric epoxidation and hydroxylations, even though radical intermediates are invoked, as well as oxygenation of racemic substrates (phosphines and more interestingly tertiary alkanes) to yield chiral products by kinetic resolution processes. The reaction mechanisms invoked range from genuine O-atom transfer (from or species, where the disproportionation reaction is important), to free-radical induced processes, particularly when the porphyrin ligands are extensively halogenated, as Ru complexes generally of such porphyrins are extremely active in radical-type decomposition of hydroperoxides, often present as trace impurities in hydrocarbon substrates. 1 L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 1-77. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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The amine dehydrogenation chemistry has led to characterization of Ru-imine, -imido (i.e. nitrene), -oxo(imido), -amido, as well as -amine complexes, the types of species that are intermediates in some reductive enzyme processes, and in reactions such as oxyamination, amination, and aziridination; the last mentioned process involves transfer of the imido moiety (isoelectronic with the oxo group), and corresponding catalytic aziridination of alkenes and insertion of the imido group into alkanes to give amides can be catalyzed by Ru-porphyrin complexes; with chiral porphyrins, chiral recognition in binding of racemic aminoesters has been realized and enantioeselective amidation of hydrocarbon substrates has been demonstrated. The beginning of the Chapter gives a general introduction to catalyzed oxygenation/oxidation reactions involving and briefly describes enzymatic oxygenase and oxidase systems, particularly the monooxygenase, cytochrome P-450, as the attempted modelling of this system using Ruporphyrin complexes led to the discovery of the trans-dioxo-Ru(VI) species and their chemistry. Key words:

amidation reactions, asymmetric hydroxylations, asymmetric epoxidation of alkenes, aziridination,

oxidation, chiral porphyrins, C=C

cleavage, Cu-containing proteins, cytochrome P-450, dioxoruthenium(VI) species, dioxygenases, epoxidation, galactose oxidase, Haber-Weiss reactions, halogenated porphyrins, hemoglobin, metalloporphyrins, monooxygenases, mustard gas, myoglobin, nitrene complexes, nitrene transfer, nitrous oxide, Oatom donors, oxidases, oxidation of olefins, oxidation of phosphines, oxidation of saturated hydrocarbons, oxidation of thioethers, oxidative dehydrogenation, oxidative dehydrogenation of alcohols, oxidative dehydrogenation of amines, oxidative dehydrogenation of phenols, porphyrins, pyridine-oxides, pyrocatechase, rebound mechanism, reductive activation of

ruthenium

amido complexes, ruthenium(IV) disproportionation, ruthenium imido complexes, ruthenium imine/amine complexes, ruthenium porphyrins, steroid epoxidation, supported catalysts, tryptophan dioxygenase.

1. Catalytic oxidations using ruthenium porphyrins

1.

3

INTRODUCTION: OXYGENASE AND OXIDASE ACTIVITY

Molecular oxygen is the most abundant and inexpensive oxygenating/oxidizing agent, which can effect in the presence of an appropriate catalyst a variety of useful oxidation reactions. These range from so-called di- or monooxygenase systems where two or one O-atoms of the are incorporated into the organic substrate (eqs. 1 and 2, respectively) to oxidative-dehydrogenation systems, where H-atoms are removed from organics as or (eqs. 3 and 4, respectively). In terms of strict, modern nomenclature, “oxygenase activity” (or O-atom incorporation) is effected by enzymatic di- or monooxygenases, and is represented by eqs. 1 and 2, while “oxidase activity” (H-atom abstraction) is effected by oxidases, and is represented by eqs. 3 and 4. Many studies with models (i.e. nonprotein systems) aim to mimic the enzyme systems, particularly their high selectivity (formation of a single product) and operation under ambient conditions.

Dioxygen is clearly an attractive oxidant (whether as an oxygenation or oxidizing agent), and is highly desirable when environmental requirements are considered as any inorganic co-product is typically water (eqs. 2 and 4). This may be contrasted with a classical stoichiometric oxidant such as dichromate (e.g. for alcohol oxidations), where undesirable Cr(III) is the coproduct. However, because of its biradical nature, non-coordinated reacts with organic substrates preferably according to a radical-chain mechanism, which generally operates with low selectivity. A typical example is of cyclohexene to give ene-one, ene-ol and epoxide via the intermediate cyclohexyl hydroperoxide, formed by attack of on the allylic radical, itself produced by hydrogen-atom abstraction (eq. 5): such reactions are catalyzed by transition metal salts and complexes which initiate

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Maria B. Ezhova and Brian R. James

decomposition of hydroperoxides via so-called Haber-Weiss pathways (eqs. 6 and 7)1.

Such radical reaction pathways do not involve directly at the metal. Much more selective oxidation processes can be realized by activation of the through coordination to a metal center; the coordination is generally followed by transfer of electrons from the metal to the oxygen moiety (see below). The reactions shown in eqs. (1) - (4) can be exemplified by specific enzyme systems that incorporate such and these will be discussed briefly, but with greater emphasis on the mono-oxygenase type (eq. 2) because: (a) this type of activity is better understood, and (b) the majority of reported studies on model oxygenation systems has involved attempts at mimicking mono-oxygenase activity. A volume entitled “Oxygenases and Model Systems” has appeared within this series2, and discusses in detail reactions of the types shown in eqs. 1 and 2. As noted below, Fe-porphyrin complexes embedded in a protein environment are responsible in human systems for binding and transport of dioxygen, as well as for incorporation of one or two O-oxygen atoms into organic substrates via mono- or dioxygenase activity. Interest in the second-row analogues, Ruporphyrins, stems from this general “oxygen chemistry” but, before discussing the Ru systems in subsequent sections, a brief introduction to the enzyme chemistry of eqs. 1-4 is appropriate. Dioxygenase activity (eq. 1). Two examples of dioxygenase systems are provided by a pyrocatechase, which effects oxygenation of catechol to cis-muconic acid (eq. 8), and tryptophan dioxygenase, which catalyzes ring cleavage of tryptophan derivatives to the N-formyl products, called formylkynurenines (eq. 9). The active site within protocatechuate 3,4-dioxygenase (the enzyme that effects reaction 8, with as determined crystallographically, is high spin, 5-coordinate Fe(III) containing 2 tyrosine and 2 histidine ligands, and a coordinated water or hydroxide ((1), see Fig. 1) , and a postulated mechanism, based largely on EXAFS and EPR data, for the

1. Catalytic oxidations using ruthenium porphyrins

5

dioxygenase activity is shown in Figure 12,3. The catechol coordinates to

give (2) with the substrate phenolic protons used to remove the coordinated as water and one coordinated tyrosine; with (2) being written as an Fe(II)/o-semiquinone, subsequent binding of generates an intermediate

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Maria B. Ezhova and Brian R. James

(3) set up to activate the substrate for attack by a dioxygen moiety, although details of the oxygen insertion and cleavage reactions remain obscure. No crystallographic data are available for L-tryptophan 2,3-dioxygenase (TPO), but the active site is now an Fe-heme centre2,4; that is, there is coordination to a porphyrin ligand, specifically protoporphyrin IX (PpIX), the same porphyrin present in myoglobin (Mb) and hemoglobin (Hb), the Fe(II)-containing proteins used by humans for storage and transportation of respectively. TPO is isolated as the Fe(III) form and spectroscopic data suggest that there is an axial histidine with water as a possible 6th ligand; at least in acid pH, the system is high spin. The system has to be reduced to the Fe(II) form before activity is realized and, although there is no direct evidence for formation of an (or intermediate, mechanisms involving prior binding of have been suggested, as well as mechanisms showing prior activation of tryptophan before reaction with particularly as L-tryptophan does combine with the Fe(III) and Fe(II) forms of the enzyme in the absence of Figure 2 shows suggested pathways involving reaction of a preformed species interacting with a deprotonated tryptophan to form eventually a dioxetane (4) that leads to the formyl products; as written here, the function of the heme is to localize and activate the Of note, ignoring the protein environment, deoxy-Mb and deoxy-Hb have active sites essentially identical to that of reduced TPO, and there are no tryptophan residues near the pocket5, and so the ‘simple’ reversible occurs efficiently unaccompanied by potentially deleterious ‘oxidation’ processes.

Monooxygenase activity (eq. 2). Monooxygenase activity is exemplified best by the well understood enzyme cytochrome P-450; this contains

1. Catalytic oxidations using ruthenium porphyrins

7

Fe(PpIX) as the prosthetic group with an axial thiolate ligand provided by a cysteinyl amino acid residue of the protein. Investigations on the structure and oxidizing ability of cytochrome P-450, as well as on the related model (biomimetic) systems, are extensive and the topic is well developed6-16. The protein and model studies have brought about detailed understanding of the activation of by P-450 and related systems; the scheme depicted in Figure 3 outlines the essential features. The scheme incorporates the so-called “reductive activation of dioxygen”, and demonstrates the counter-intuitive concept of requiring the presence of a reducing agent in order to execute transfer of a single O-atom in to an organic. The process is accomplished by coordination of dioxygen, a net transfer of two electrons (via the Fe center) and two protons, concomitant with the heterolytic cleavage of the oxygen-oxygen bond; one O-atom is

incorporated into the substrate via a high-valent metal-oxo species, while the second O-atom is reduced to water. The following steps of the process have been distinguished within the protein or model systems: (a) Addition of the substrate (S) to the resting state of the enzyme, a low spin Fe(III) state (5), gives a more easily reducible, high spin, Fe(III) enzyme-substrate complex

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(6); crystallographic data for a enzyme that hydroxylates camphor have revealed that the camphor is held by the protein in close proximity to the Fe(III) center16. (b) A 1-electron reduction of (6) gives a high spin, 5coordinate Fe(II) complex (7). (c) (7) binds to give a low spin complex (8). (d) A second 1-electron reduction of the dioxygenated adduct (8) gives what is probably a high spin Fe(III)-peroxide (9)8,17. (e). Heterolytic cleavage of the oxygen-oxygen bond within (9), with consumption of protons, generates water and a high-valent oxo-iron species (10), written here as Fe(V)=O, although model studies favor a O=Fe(IV)(porp+.), porphyrin cation-radical species7-11. (f) Incorporation of the O-atom into the substrate gives the oxygenated product with concomitant regeneration of the initial Fe(III) state (5). The net reaction is that shown in eq. (2); the 2electrons are supplied by NADPH, and involve coupling to flavin reductase and putidaredoxin systems. The addition of the O-atom to S is usually written as a “rebound” mechanism, involving H-atom abstraction from the substrate by the Fe=O moiety and subsequent rebound of the OH group7. The protein pocket, in which cytochrome P-450 is embedded together with the amino-acid cysteine axial ligand, provides favorable and presumably optimal conditions for monooxygenase activity. In vitro (i.e. outside of a protein) metalloporphyrins, especially those containing naturally occurring porphyrins such as PpIX, can undergo undesirable side-reactions such as dimerization and aggregation and, under can be irreversibly oxidized (e.g. Fe and Ru-porphyrins give species - see below) with resulting loss of their oxygen-activating abilities. Efforts have thus been made to minimize these undesirable effects by introducing substituent alkyl or halogen substituents into, for example, the phenyl rings of the well known, easily synthesized meso-tetraphenylporphyrin ligand such modifications create steric hindrance against formation of species (loosely called “dimerization”), but also, of course, change the electrophilicity of the metal (or metal oxo) centre. The majority of biomimetic studies, including the use of a plethora of metal complexes, and not just metalloporphyrins, can be categorized and rationalized in terms of the pathways illustrated in Figure 3, especially when the so-called “shunt pathways” A and B are included. Basically, and of key importance, the P-450 studies show that generation of a high-valent metaloxo species such as (10) is needed to mimic monooxygenase activity. Clearly one way to achieve formation of model species akin to (10) via (9) is to follow pathways (b) - (d), i.e. utilize a metalloporphyrin (or other metal complex), and a reducing agent to supply the electrons11-15,18-31; a metal (M) in oxidation state (III) could generate a species. This type of system represents genuine monooxygenase-like character, i. e. is employed; however, a problem arises in that both the substrate (S) and the

1. Catalytic oxidations using ruthenium porphyrins

9

reducing agent will compete with the active form of the catalyst, the highvalent oxo species, and this renders such systems catalytically less efficient the competing reaction, catalytic of the reductant instead of (S) takes place! Many reducing agents, including borohydride, ascorbic acid, Zn dust (the GIF-type systems), aldehydes, CO and as well as electrons, have been tested with varying degrees of success. Such competition is avoided, of course, in the enzyme systems by membrane separation of the key components. Another general way to form species like (10) is to treat a precursor such as (6) with a net O-atom donor (XO, see Figure 3, pathway B); examples include iodosylbenzene, hypochlorite, chlorate, periodate, amine oxides, and Closely related are systems providing the dioxygen already reduced by 2-electrons, i.e. at the peroxide level using reagents such as organic hydroperoxides, peroxy acids, and magnesium monoperoxyphthalate, which again are net O-atom donors (see Figure 3, pathways A and B). A huge number of papers have been devoted to studies using the general “shunt pathways” exemplified by A and B (for reviews see refs. 10, 11, 14, and 32). Extensive investigations on metalloporphyrin-catalyzed oxidations of organic substrates using O-atom donors have shown that high-valent oxo-metal species generated in situ are the active species in the oxygenation of alkenes and alkanes6-15. However, in some cases, it has also been suggested that a key step of the catalytic reaction is the formation of an intermediate ternary complex composed of the metalloporphyrin, oxidizing agent and substrate33. Reactivity of metalloporphyrins with As both certain mono- and dioxygenase systems mentioned above utilize metalloporphyrins for activation (by binding) of it is worth reflecting on such systems that are known to react with Of the 1st-row transition metalloporphyrins, only those of Cr(II), Fe(II), Mn(II) and Co(II) bind dioxygen34,35. The products are type complexes with end-on geometry for the coordinated superoxide, except in the case of manganese which yields a Mn(IV) side-on peroxide type species. The Fe and Mn dioxygenated adducts of “non hindered” porphyrins are stable only at low temperatures, and at room temperature the species are too transient to be of catalytic use31. However, in the presence of a reducing agent and protons, bound and stabilized (as by the Fe(II)-picket fence porphyrin can be cleaved heterolytically and the system under showed some catalytic activity in the 24 oxidation of olefins . In contrast to the Fe and Mn systems, well defined adducts can be detectable in solutions at room temperature, although “dimerization” to bridged peroxo species is facile for non-sterically hindered porphyrins36; in any case, the bound is not a strong enough electrophile or nucleophile to effect epoxidation of olefins or hydroxylation of alkanes31.

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The 2nd-row transition metal porphyrin complexes of Rh(II) and Ru(II) also react with but the resulting species are usually quite different in character. Rhodium octaethyl- and tetraphenylporphyrin oxygen adducts are like the Co analogues with an end-on superoxide; however, the octaethylporphyrin (OEP) species is only stable below -80°C and on warming to 20° C converts to a Rh(III) complex, analogous to the Co systems37,38. The interaction of oxidants, including with Ru porphyrins, and subsequent oxygenation/oxidation catalysis, constitute the topics of the remaining sections of this chapter; this review updates earlier 1992 and 1994 reviews from this laboratory on this topic15, while related reviews from the group of Che39 and Groves40 appeared in 1999 and 2000, respectively. Under certain conditions, “non-hindered” Ru(II) porphyrins can bind reversibly to give species that contain either coordinated superoxide or peroxide, although irreversible oxidation to a dinuclear species is more common (Section 2). Ru(II) porphyrins, initially containing bulky substituents at ortho-positions of meso-phenyl rings present in the porphyrin ligand, were shown to have a remarkable and unique reactivity with yielding trans-dioxo species which are stable in solutions at room temperature; an example is shown in eq. 10, where TMP = the dianion of 5,10,15,20-tetramesitylporphyrin, see Figure 4 in Section 2). Some nonhindered Ru(II)-porphyrins were later shown to form, in alcohol solutions, less stable trans-dioxo species (Section 2).

Over the last 17 years or so, studies based on these trans-Ru(VI)-dioxo complexes have led to the development of new, catalytic and selective oxygenation and oxidation systems that generally operate via non-radical pathways and that involve direct reaction between a metal complex and as illustrated in eq. 10. The Ru(VI)-dioxo species, depending on experimental conditions and substrates, can exhibit: (i) monooxygenase activity in terms of effecting addition of one O-atom of to a molecule of substrate, although both O-atoms of the are utilized for two molecules of substrate (cf. eq. 1), and so the systems are in effect illustrating dioxygenase activity based on the stoichiometric use of the molecule (Sections 3.13.5). Perhaps the term bis(monooxygenase) activity is appropriate; (ii) oxidase activity of the type illustrated in eq. 4 for certain dehydrogenation reactions (Section 4); and (iii) free-radical type activation of (eqs. 5-7). A very wide range of organic oxidation reactions is catalyzed by nonporphyrin complexes of Ru, including a vast literature on the use of Ru-oxo

1. Catalytic oxidations using ruthenium porphyrins

11

species; these systems are more commonly based on the use of O-atom donors. There are systems; however, these are typically radicalinduced processes and do not involve formation of oxo species. This extensive literature on Ru/non-porphyrin systems can be traced through refs. 41-43. Oxidase activity (eqs. 3 and 4). As the complexes (porp = porphyrin dianion) introduced above do exhibit oxidase activity (Sections 3.5-3.7), some brief elaboration of the enzymatic oxidase systems seems appropriate. To our knowledge, there are no oxidases that utilize metalloporphyrin centres as the active catalytic site. The majority of oxidases are Cu-containing protein systems44,45, and those exemplified in eq. 3 typically convert a primary alcohol moiety in a sugar residue to an aldehyde group. The “simplest” is probably galactose oxidase, which is one of a growing class of “free radical” enzymes, and it is only recently that the nature of the active site has been elucidated46,47. There is only a single redoxactive metal centre, which, in the Cu(II) state, is a so-called “normal, non blue, type-2 Cu”; the second required redox site needed to mediate the overall 2e-redox reaction is provided by a modified tyrosine radical, present as an equatorial ligand within an overall square pyramidal Cu site that contains (at pH 7) 2 histidines and water as the other equatorial ligands and a tyrosine as the axial ligand44,47. The axial tyrosine is thought to play a mechanistic role by facilitating abstraction of a proton from the substrate alcohol47. An oxidase system as outlined in eq. 4 typically transforms polyphenols/diols to quinone/diketones, as exemplified by lactase and ascorbate oxidase (eq. 11). Both these enzymes in the oxidized state contain, as well as the normal type-2 Cu, the “blue, type-1 Cu” (which exhibits an

intense blue colour, and an EPR signal with small Cu hyperfine coupling due to delocalization of the spin density towards a cysteine S-atom), and “type-3 Cu” (a centre that is EPR silent because of anti-ferrromagnetic exchange coupling between the Cu-atoms via a bridging ligand)48. The structure of ascorbate oxidase shows the 4 Cu centres quite close together: the type-1 Cu is coordinated by 2 histidines, 1 methionine, and 1 cysteine residue, the type-2 Cu has 3 histidines and a water and the type-3,

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Maria B. Ezhova and Brian R. James

dimetallic centre contains a single bridging-OH and 3 terminal histidines on each Cu48. Oxidase mechanisms are not well understood in detail, but those presented here generally function via an electron transfer chain from the substrate to the various Cu centres through to which may or may not be coordinated to the metal. Studies on models for ascorbate oxidase have invoked reactivity via a ternary species, as well as oxidation by “free” essentially, the oxidizes the reduced metal and the ascorbate via its anionic, semiquinone radical1c.

2.

REACTIONS OF RUTHENIUM PORPHYRIN COMPLEXES WITH

AND OTHER

OXIDANTS Before detailing the catalytic oxygenations/oxidations that have been effected by Ru-porphyrin species, it is instructive to consider the reactions of these complexes with various oxidants in the absence of the substrates to be oxidized; clearly any such Ru products must be considered as catalysts or, at least, catalyst precursors. Studies on Ru-porphyrins in the laboratories here were initiated in the mid-1970s, with the aim of mimicking biological oxygenation/oxidation processes, especially those effected by their 1st-row, often naturally occurring Fe-based analogues8,15,49-53. In polar, aprotic solvents such as DMF, DMA (N,N’dimethylacetamide) or pyrrole, complexes of the type (L = solvent; porp = an anion of OEP or TPP) bind reversibly at ambient conditions to yield complexes50, but such species are readily converted in the presence of trace water to dinuclear Ru(IV) species of the type In toluene, under 50 undergoes a slow irreversible oxidation to a species , which is the usual oxidation product of Ru(II) “non hindered” porphyrin complexes using or an O-atom donor as oxidant8,53. The reaction of TPP complexes with in DMF at ~0°C is 1st-order in both metalloporphyrin and and the findings, particularly a Hammett plot, were rationalized in terms of formation of a Ru(IV), peroxo species54. Of note, myoglobin, which has been reconstituted with Ru(II)mesoporphyrin IX, is 6-coordinate low spin and undergoes, like

1. Catalytic oxidations using ruthenium porphyrins

13

(Im = imidazole), to the so-called met form via the outer-sphere process exemplified in eq. 12 (one L = histidine, the other L being unknown); the oxidation rate is faster than that of an axial ligand dissociation, which, for example, is the initial step in formation of Ru(porp)L(CO) via reaction with CO51.

A complex derived from has been stabilized within the protected cap on one side of a “picnic basket” porphyrin55, and at low temperature within a species56. An unusual demetallation reaction of a Ru(II) porphyrin complex with to give the insoluble, black and the free-base porphyrin is illustrated in eq. 13, trace again being the key ingredient57. The phosphine ligands were considered to be “burned off as phosphine oxides,

with a Ru(IV)=O species acting as oxidant the remaining ‘Ru(OEP)’, known in the form of the dimer then giving the known that was shown to react with to give (eq. 15). In the presence of excess the phosphine dissociation required

to generate the vacant site for coordination of (eq. 14) is prevented, and superoxide is formed via an outer-sphere electron transfer, according to eq. 12. Introduction of substituents at the ortho positions of the phenyl rings of creates steric hindrance against ‘dimerization’of the Ru derivatives (cf. eq. 15) and allows for an initial binding of that finally yields readily isolable, stable trans-Ru(VI)-dioxo species; e.g. in benzene, (L = MeCN15,59 or THF60,61) reacts with (or air) to yield (eq. 10, Figure 4). Its formation is thought to occur via steps of the type shown in eq. 14, followed by disproportionation of the Ru(IV)-monooxo

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Maria B. Ezhova and Brian R. James

species, eq. 168,15,40,60,61. This equilibrium is evidenced by formation of on mixing samples of and 40,61 ; see also Section 3.3. in benzene under Ar at ambient conditions

The species has been detected by as a monomeric, intermediate in the reaction of with air in benzene at ambient conditions en route to The monooxo species has 2 unpaired electrons, typical of paramagnetic, high spin, porphyrin species that obey a Curie plot49,62. A corresponding species was also evident from the reaction of with 1 mole equivalent of in benzene that generates which was initially thought to be noncoordinated40,61, although later kinetic data imply that the product is see Section 3.1; the v(RuO) value of for the dioxo64 was shifted to for the monooxo species, and correct isotope shifts were observed on using species. The oxidation state was also supported by an oxidation state marker at a narrow range in this IR region has been found to be very sensitive to the oxidation state of the metal in Ru-porphyrin complexes61,65-68.

The trans-Ru(VI)-dioxo species can effect stoichiometric and/or catalytic oxygenations/oxidations of organic substrates such as phosphines, thioethers,

1. Catalytic oxidations using ruthenium porphyrins

15

olefins, alkanes, alcohols, phenols and amines (see Section 3), via reaction pathways that illustrate the whole gamut of reactivity patterns”: oxygenase, oxidase, and free-radical activity, see Section 1. More details illustrating the importance of the disproportionation step (eq. 16) in the catalytic oxygenations are given later (especially in Section 3.3). Single oxygen atom donors, such as iodosylbenzene, mchloroperbenzoic acid (m-CPBA), and amine oxides, have also been reacted with Ru-porphyrins. In terms of mimicking the Fe(III) resting state of cyto P-450 (Fig. 3), a Ru(III) precursor is an obvious candidate; for example, reacts with PhIO according to the stoichiometry of eq. 17 to yield a green complex, isolable at room temperature and tentatively formulated as the oxo-Ru(IV) cation radical species, (11)8,69; an EPR signal at g = 2.00, the visible spectrum, and the

demonstrated stoichiometry of eq. 18, supported the formulation of (11)70, which showed some catalytic oxidation activity8,69 (Section 3.3). The Ru(III) precursors Ru(OEP)X (X = Cl, Br), synthesized from the corresponding dihalo-Ru(IV) complexes49,62 according to the chemistry outlined in eq. 1971, react with PhIO or m-CPBA again to generate a green solution, but work-up

procedures yield only species such as (12)71. Of note, corresponding Fe(IV)-oxoporphyrin cation-radical species are also green72,73. Addition of a 2-fold excess of m-CPBA, PhIO or to or benzene solutions of the sterically hindered Ru(porp)CO complexes (porp = TMP or TDCPP, Figure 4, cf. eq. 10) gives the species, which are well characterized15,40,61,64,74, including a crystal structure of the TDCPP complex40. The trans-dioxo complexes are sensitive to decomposition by traces of acids64 (see Section 3.4). Corresponding oxidation of the non-hindered Ru(TTP)CO with m-CPBA gave the species where L = OH or mchlorobenzoate64; similarly, Ru(OEP)CO with gives a product75. The derivatives with the non-hindered TPP and OEP can be made by reaction of the appropriate Ru(porp)CO with m-CPBA in alcohol, the formation of species being inhibited by coordination of

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Maria B. Ezhova and Brian R. James

alcohol at the vacant site of the initially formed Ru(IV)(porp)O; the nonhindered dioxo complexes are reasonably stable in the solid state and in organic solvents, although they react readily with to form Useful precursors for a myriad of derivatives are the ‘bare” species Ru(TMP)59,76, and the dimeric and species58. For example, Ru(TMP), a 14-electron species, reacts readily with donor ligands ranging from (which gives CO, olefins, acetylenes, MeCN and ethers59,76,77. Figure 5 summarizes synthetic routes to the trans-dioxo species using or an O-atom donor59-61,67 .

3.

OXIDATION OF ORGANIC SUBSTRATES

3.1

Oxidation of phosphines, phosphites, arsines and stibines

of can be catalyzed by Ru(II) non-hindered porphyrin species57. The mechanism in benzene solution involves an initial outer-sphere oxidation of a 6-coordinate bis(phosphine) species which generates superoxide according to eq. 12 (porp = OEP; ). Although readily reduces species, the presence of protons (needed as a cocatalyst) forces the equilibrium of eq. 12 to the right via stabilization and subsequent disproportionation of to and (eq. 20). The oxidizes free while is regenerated using

1. Catalytic oxidations using ruthenium porphyrins

17

the 2-equiv. reducing power of phosphines in the presence of (eqs. 21, 22); the overall oxidation (reactions 12, 20-22) is catalytic in Ru, and That the oxidation proceeds faster than substitution of (e.g. by CO) supports the outer-sphere process, while EPR data on the system (eq. 12) showed formation of a mixture of the hydrated superoxide (g = 2.00, 2.10) and a low spin Ru(III) species (g = 1.98, 2.30)57.

In principle, any substrate that is oxidizable by can be oxidized stoichiometrically by a metal complex that allows for outer-sphere generation of and a catalytic process results if an appropriate reducing agent, preferably the substrate itself, is present to generate the lower-valent metal complex - in this case a Ru(II)-porphyrin. A related system (see Section 3.2) using with leads to catalytic oxidation of thioethers to sulfoxides, where the reaction corresponding to eq. 12 is photo-assisted78. The system gave non-reproducible kinetic data57, and it is possible that a photochemical pathway contributes to the reaction. The species (cf. eq. 18) can be reduced by excess to generate and thus can be used as a catalyst precursor for catalytic of at least under the outer-sphere conditions. can oxidize 2 moles of (or stoichiometrically, and a 3rd mole of can subsequently coordinate, according to eq. 23 in the presence of catalytic oxidation of

the phosphine can be readily effected at ambient conditions61,63,64,80. Initially, the detectable (13) was written without the axial ligand61,64, but kinetic data for the stoichiometric reaction point to the oxide being coordinated63. and were determined for the first O-atom transfer to (step a) by stopped-flow kinetics, as well as corresponding data for other phosphines; X = OMe, Me, F, Cl, values increase with increasing electron-withdrawing power of the p- substituent position, consistent with electrophilic attack of a Ru=O moiety on the lone-pair of the phosphine. The negative values

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Maria B. Ezhova and Brian R. James

show an interesting trend becoming more favorable with increasing mass of the phosphines (and of and that were similarly oxidized), and this was rationalized in terms of O-atom transfer occurring via strong Ru=O vibrational coupling63,81. The kinetics for the slower, subsequent stages of the stoichiometric reaction, which monitored directly the conversion of to (again in the stopped-flow regime), were 1 st-order in the Ru complex, 1 st- to zero-order in and inverse on consistent with the steps shown in eqs. 24 -26. There was no direct evidence for the disproportionation step (eq. 25), but the kinetic data were most consistent with its involvement; some exhange data40 using imply that Ru(porp)O undergoes rapid exchange with water, while exchanges slowly, and the findings suggest that the disproportionation reaction is generally faster than, for example, Ru(porp)O reacting directly with a 2nd mole of oxidizable substrate, in this case (see also Section 3.3). Under 1 atm (or, less efficiently, air) at 24°C in benzene, in the presence of excess PPh3, does effect catalytic oxidation of but at a rate much lower than the stoichiometric reaction; the catalysis is governed by the rate of conversion by of the product of eq. 26, back to the dioxo species63.

Use of

containing a chiral picket-fence type porphyrin see Fig. 10, Section 3.3) in or under at 25°C can stoichiometrically oxidize the racemic phosphine to 82,83 chiral phosphine oxide with 41% ee . This reaction effects a kinetic resolution of the racemate, with the O-atom transfer occurring with retention of the configuration at the P-atom. The other reaction product is Ru(porpcontaining the phosphines as a mixture of SS : RS : RR isomers in the ratios 38 : 54 : 9. The stereoselectivity was explained by differences in reactivity of and Ru(porp)(O) toward the R- and S-isomers of the phosphine; this implies that Ru(porp)(O) is also an effective O-atom donor to phosphines, which conflicts with the kinetic interpretation discussed above. The relative capabilities of the dioxo versus the mono species for O-atom transfer is a question that will arise again in later sections. Equilibrium titration data for the addition of to were interpreted in

1. Catalytic oxidations using ruthenium porphyrins

19

terms of the dioxo species being a more powerful O-atom donor61; a key factor will be the nature of the ligand coordinated trans to the oxo ligand, and to draw a general conclusion is likely untenable. The mechanistic details of net O-atom transfer processes from metal-oxo species generally are of intense current interest14,15,32,80,84,85.

3.2

Oxidation of thioethers

Selective of thioethers to sulfoxides is of industrial importance74,86-88 and has been accomplished for dialkyl sulfides using complexes (porp = TMP, TDCPP)74,89. Kinetic and data in benzene solution near ambient conditions were first interpreted in terms of the chemistry shown in eq. 27. Under Ar, a stoichiometric reaction gives the bis(O-bonded sulfoxide) product (14), while under or air the labile sulfoxides are displaced to regenerate the trans-dioxo species and the

process becomes catalytic. The k values for the TMP system at 20°C are 0.0075, 0.012 and 0.11 respectively, for and the differences resulting more from differences in than values74; as with the phosphine systems, the determined negative values are consistent with O-atom transfer occurring via electronic coupling induced by strong Ru=O vibrational motion81, the data implying an easier Oatom transfer process with bulkier substrates. Alkylaryl and diaryl sulfides do not react with the trans-dioxo species74,89,90 presumably because of their decreased nucleophilicity. Of note, an Fe(TPP)Cl/PhIO system effects oxidation of dialkyl, alkylaryl and diaryl sulfides, via a proposed ClFe(TPP)O intermediate91; the axial ligand trans to the oxo ligand is likely critical. The chemistry of eq. 27 would imply that is a more efficient O-atom donor than while data for phosphine oxidation (see Section 3.1) imply that the Rumonooxo species disproportionates more quickly than reacting with phosphine. The kinetic data for thioether systems could be consistent with the type of mechanism described for the phosphine systems, and a careful reevaluation of the thioether systems is needed. With Fe systems, of course, disproportionation to a dioxo species is unknown. Relevant to this discussion are the findings that LRu(porp)(O) species are better catalyst precursors than the corresponding for oxidation of saturated hydrocarbons

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Maria B. Ezhova and Brian R. James

(Section 3.4.) and some observations for the Ru(III)(OEP) systems (Section 3.3.), which again demonstrate the likely critical role of the axial ligand trans to the oxo ligand. That this is a key factor in biologically important7-9,11 and biomimetic Fe-porphyrin systems92 is well documented. The catalytic thioether oxidation (eq. 27) is limited by isomerization of the bis(O-bonded sulfoxide) to the more substitution inert bis(S-bonded) species, and by degradation of the TMP ligand74,89. The catalyst is more effective, the k value for oxidation being 10 times greater; complete conversion of 0.035 M to the sulfoxide and sulfone (4:1 mixture) can be accomplished at 100°C using mM catalyst with no degradation of the catalyst74. Substitution of halogens in to a porphyrin favours O-atom transfer by increasing the electrophilicity of the oxo ligand, and the decreased electron density in the ring also makes any metal-oxo catalyst less susceptible to attack by itself (self-destruction); this is related to an increase of reduction potentials within the systems92-94 (see also Section 3.3). In the thioether systems, the chlorine substituents perhaps reduce the extent of and slow the 15 isomerization process . The presence of chlorine in the thioether decreases the nucleophilicity of the S-atom to the extent that no O-atom transfer occurs from trans-Ru-dioxo species; such studies on bis(2-chloroethyl)sulfide (Mustard Gas) have led to isolation of the first (and possibly last!) structurally characterized Mustard Gas complex, Phenyl methyl and benzyl sulfides have been catalytically oxidized to corresponding sulfoxides (>80%) and sulfones (<10%) using lutidine Noxide as oxidant in the presence of the reaction takes days at ambient conditions, and depends especially on the coordination ability of the sulfide and sulfoxide96. Considering the non-reactivity of aryl sulfides toward the dioxo species, the amine oxide may well play a role as an axial ligand in these systems (see Sections 3.3 and 3.4). The selective, stoichiometric of the “non-hindered” (R = alkyl) to the bis(S-bonded sulfoxide) complex, in containing acids as cocatalyst, is initiated by an initial reaction to form superoxide97,98, as seen for (Sections 2 and 3.1, eqs. 12, 20-21). The formed oxidizes one mole of (cf. eq. 21). The regeneration of Ru(II) (cf. eq. 22) now involves a disproportionation: 2 followed by aquation of the Ru(IV) to generate a Ru(IV)=O species, which then reacts with thioether to give (after isomerization) the species, as summarized in eq. 28, where Ru = Ru(OEP)98. Such pathways lead eventually to the overall stoichiometry: TheRu(OEP)(Scomplex was characterized structurally. Catalytic conversion of

1. Catalytic oxidations using ruthenium porphyrins

21

to occurs in benzene solutions containing and benzoic acid at mM concentrations under 1 atm at 35°C, in the presence

of visible radiation; initial turnovers of 350 are seen, and activity decreases after a total turnover of ~ 10,000 because of build-up of relatively inactive species78; the radiation is required to provide energy for the otherwise thermodynamically unfavourable outersphere electron transfer from the metal to dioxygen (cf. eq. 12).

3.3

Epoxidation of olefins

Approximately half of the papers on Ru porphyrins appearing during the last 5 years or so deal with olefin epoxidation. Epoxides are found in naturally occurring compounds99 and are important intermediates in organic synthesis; in the case of asymmetric epoxidation, stereospecific ring-opening of the epoxide generates the chiral alcohols100. The first report on oxidation of olefins with Ru-porphyrins appeared in 1983 and involved the use of or in or MeCN with PhIO at ~ 20°C8,69. The sterically hindered TMP system gave the highest activity with a turnover of 130 in 6 h being attained for epoxide formation from norbornene; styrene was also selectively epoxidized, but cyclohexene gave a mixture of epoxide, ene-one, ene-ol, cyclohexanone and cyclohexylbromide with a turnover of 100 in 6 h. Marginal, non-selective activity for cyclohexane oxidation to give the alcohol, ketone and bromide was also reported. The product distributions implied a contribution from a free-radical pathway, and the O=Ru(IV)(porp+.) cation radical, isolated as a green bromo derivative (see Section 2, eq. 17), was proposed to be the catalyst acting via a free-radical pathway involving the so-called oxygen-rebound mechanism7,101. The later discovered could well have been playing a role in the TMP systems as this species does give selective oxidation of olefins to epoxides (see below). Of particular interest, use of Ru(OEP)Br (eq. 19) with PhIO under the conditions used with is not effective for oxidation of cyclohexene71, but addition of one equiv. of (which is formed during reaction 17) does regenerate the catalytic activity102. The implication is that the plays a role in the catalysis, perhaps binding as an axial ligand trans to the oxo group.

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Maria B. Ezhova and Brian R. James

Both O-atoms of can be transferred to an olefin in a stoichiometric reaction to generate 2 moles of epoxide, and the species also catalyzes of olefins at ambient conditions in benzene with high selectivity and yields with norbornene; the order of olefin reactivity is: norbornene > > cyclooctene > while epoxidation of cis- and proceeds with retention of configuration60. The suggested catalytic cycle shown in Figure 6 implies the key disproportionation of a Ru(IV)=O intermediate to the and Ru(II) species (see also Section 2, eq. 16, and Section 3.1, eq. 25)60,

while a plausible route for reoxidation of Ru(II) is shown in eq. 1440,74. In catalyzed epoxidations of cis-but-2-ene in benzene, using in the presence of excess a 10:1 ratio of to was produced, and the 2 different pathways outlined in Figure 7 were considered in order to rationalize the data40: the dioxo species could be formed by disproportionation, or by cleavage of a dimer. Separate experiments had shown that the dioxo species undergoes oxygen exchange slowly with (see Fig. 7), and that epoxides do not undergo exchange; thus the pathway would be expected to generate a 1:1 mixture of the and epoxides, while the disproportionation pathway would yield in principle 100% The conclusion was that the observed 10:1 ratio thus favored the disproportionation pathway40, which is reasonable, with the presumption that undergoes rapid exchange, and that the does not. Although these data suggest that the fastest step in the epoxidations (Fig. 6) is the disproportionation reaction (ref. 40, see Fig. 7), incorporation from added to a reaction mixture does not necessarily imply the intermediacy of a metal-oxo species; other intermediates such as M-OOH or M(O-atom donor) could also have exchangeable O-atoms103. These selective of olefins, a commercially important process104, are based on the somewhat exotic hindered porphyrins, but are highly significant. Such O-atom transfer via an or air-generated metal-

1. Catalytic oxidations using ruthenium porphyrins

23

dioxo species represents unique chemistry, even though turnovers are only up to at 10 mM Ru and 0.5 M olefin. Stoichiometric epoxidation of para-substituted styrenes is 1st-order in both Ru-dioxo and styrene, at least for [styrene] up to 30 mM, and a Hammett plot shows increasing rate constants with electron-donating substituents, the data being consistent with a concerted process involving an electrophilic metal-oxo species40. Two important factors for applications of these Ru(VI)-dioxo/olefin systems using the TMP, TDCPP or TBCPP systems, and that could contribute to the low epoxidation rates, include: (a) a competitive binding of the olefin and the epoxide product at the Ru40,77,84,105 and (b) a build-up of catalytically inactive Ru(porp)CO species40,106. Ru(II) complexes generally have a strong affinity for a carbonyl moiety and, indeed, Ru-porphyrins under appropriate conditions can catalyze decarbonylation of aldehydes107, which are detected in trace amounts during epoxidation of styrenes40. Even a coordinated methyl group at a Ru-porphyrin centre has been transformed to a coordinated carbonyl108. After a catalyzed of oct-1-ene and propene to give 1,2-epoxyoctane and propene oxide, respectively, Ru(TMP)CO was isolated106; aldehyde was formed during the propene oxidation, while there was no reaction between aldehydes (or ketones) with Use of -oct-1-ene led to formation of and in a 3:1 ratio, showing that most of the CO was derived from the terminal C-atom of the olefin106. With cyclooctene at concentrations of ~0.5 M), the epoxidation rate becomes independent of [cyclooctene], implying a rate-determining step other than O-atom transfer60; this could result if either of factors (a) or (b) play a role, when regeneration

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Maria B. Ezhova and Brian R. James

of the Ru-dioxo could become the rate-determining step and a dependence on would be expected; alternatively, genuine saturation kinetics might be operative, implying the build-up of a kinetically important ‘Ru-dioxoolefin’ ternary intermediate (cf. the benzhydrol system in Section 3.6.). Irradiation of the cyclooctene system with visible light from a tungsten lamp gives a five-fold increase in the epoxidation rate40, and this could result from photolytic decarbonylation of Ru(TMP)CO109. The isolable ethylene complex is stable to torr at 20°C, but the cyclohexene analogue readily dissociates the coordinated alkene77. The competition of alkene binding versus reactivity to give dioxo species is reminiscent of mechanisms in catalytic homogeneous hydrogenation, where hydride and unsaturated routes have been identified110; the former operates by olefin attack on a metal-hydride, and the latter by attack of on a metal-alkene species. Epoxides coordinate to Ru(II)-porphyrins, and Ru(TDCPP)(CO)(styrene oxide) has been characterized by X-ray analysis84. The bent geometry of the coordinated epoxide ring (Fig. 8) may be similar to the transition state geometry for olefin epoxidation, with a side-on approach of the alkene allowing for favourable interactions between its filled and the metal-oxygen orbitals; solution data show that the 84,105 . The corresponding coordinated epoxide rotates about the Ru-O axis thioepoxide and aziridine complexes also have bent geometries analogous to that of the epoxide40. Several other mechanistic possibilities, invoking a metallaoxetane, carbon radical, carbocation, ion-pair, or charge transfer species as an intermediate or transition state, have been proposed for epoxidations catalyzed by 1st-row metalloporphyrin monooxo species13,32,111.

A further complication in the catalyzed olefin epoxidation is a catalyzed cis-trans isomerization of the coordinated epoxide84,105. For example, in a reaction that is 1st-order in metal complex, catalyzes isomerization of cis- or oxide in benzene to give a 1:5 equilibrium mixture of the cis and trans forms. The non-hindered tetra-p-

1. Catalytic oxidations using ruthenium porphyrins

25

tolylporphyrin analogue exhibited similar reactivity, while Ru(TMP)CO was catalytically inactive. Coordinating olefins such as styrene, norbornene, cyclooctene and inhibited the isomerization of methylstyrene oxide, while the more weakly coordinating methylstyrene was a poorer inhibitor. The relative binding constants for olefin and epoxide are critical in evaluating any catalytic epoxidation processes. The binding constant of for the replacement of MeOH within Ru(TMP)(CO)(MeOH) is > at -50°C105, while that for styrene oxide coordination to Ru(TDCPP)CO is at -40°C84. Studies on chiral oxide suggested that the isomerization involved coordination of the epoxide, homolytic cleavage of the bond to give a benzylic radical, and then rotation about the single bond and subsequent reclosure to give coordinated epoxide105. The (air) catalyst has found an application at ambient conditions for epoxidation of steroids containing C=C bonds (Fig. 9)112-117. The catalyst (~5 mM), generated in situ in benzene from Ru(TMP)CO and

m-CPBA, was first applied to (at ~0.1 M) bearing a C5-C6 double bond, the reactivity depending very much on the nature of the C3 substituent 112-114 . Thus, there is no epoxidation if C3 has an OH group, perhaps because of reactivity of the OH with the Ru-dioxo species (Section 3.6). Protection of the OH as an ester or with a silyl group allows for up to 90% epoxidation with up to 99% initially surprising as the is more crowded and as epoxidation with peroxy acids takes place on the The best results were obtained with an acetate ester at C3; longer chain aliphatic esters and the benzoate ester required longer reaction

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times of up to 3-6 days113,114. The cholesteryl acetate epoxidation revealed a 2 h induction period, with subsequent maximum rates roughly proportional to the initial [Ru]; complete epoxidations after 5 h were realized for a substrate : catalyst ratio of ~25. Reuse of the catalyst gave much lower rates but the product selectivities were retained; catalyst deactivation was attributed to the formation of hydroxylic species via protonation of the oxo ligands113. Although slow, the epoxidations are synthetically useful. With steroids having additional C=C bonds, either elsewhere in the nucleus or in the C17 side-chain, epoxidation still predominates at the 5,6position; for conjugated 5,7-dienes, the epoxidation is regioselective at C5C6 but with loss of stereoselectivity115. There is no effective epoxidation of cholest-4-ene-3-one, which has a carbonyl conjugated with the olefin bond, but ketalization of the conjugated carbonyl shifts the double bond to the 5,6-position and epoxidation occurs as described above114. The non-conjugated cholest-5-ene-3-one yields a mixture of epimeric 6-hydroxy-4-ene-3-ones, where the C=C bond has been shifted, and a 4-ene-3,5-dione116; this reaction was insensitive to the addition of a radical inhibitor, indicating a non-radical process. Ru(TMP)CO also catalyzes equally well this same reaction, but the true catalyst was again the trans-dioxo species formed from the carbonyl via reaction with a 6hydroperoxy-4-ene-3-one (cf. Fig. 5), formed by radical-initiated, incipient autoxidation of the cholest-5-ene-3-one. The reactivity and high of the epoxidations have been rationalized generally in terms of steric interactions between the catalyst and substrate117. Steroids containing a Me group on C6, or a double bond in ring C or D, are not epoxidized because of non-bonded interactions between the steroid and the porphyrin ring for a side-on approach of the alkene moiety, with the mean plane of the steroid orthogonal and not planar to the Ru=O bond (Fig. 9; cf. Fig. 8). A rationale for the has emerged from the structures of cholesteryl ethyl carbonate and its epoxide, and is based on conformational differences between the two structures along the C5-C10 bond117. Molecular modeling indicates that epoxidation on the ‘folds’ the A-B junction and allows for an easier approach of the substrate by releasing steric strain that results from interactions between the C3-ester and porphyrin mesityl; epoxidation on the has no effect on the A-B junction and is relatively disfavoured. NMR data confirm that only the interacts with the metal in 117 Ru(TMP)CO . The activity and stereoselectivity of other related metalloporphyrin systems depend on the metal and on the oxidant used; Fe(III)- or Mn(III)TMP complexes are weakly active or inactive for the aerobic oxidations9,14,40,118-120.

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in effects the stoichiometric epoxidation of olefins such as norbornene, styrene and cis- and trans-stilbene according to eq. 29; cis-stilbene yields cis- and trans-epoxides in a 1:2.7 ratio, and transstilbene gives mainly trans-epoxide67. The supposed Ru(IV)=O co-product reacts with present to form inactive and no catalysis occurs even up to 10 atm The 2nd-order rate constant for reaction 29 with norbornene was at 25°C67. In pyridine solution, a reaction similar to 29 generates and two equivalents of epoxide.

In EtOH, the monooxo species is thought to be stabilized as Ru(OEP)O(EtOH) and a very slow catalytic epoxidation, observed at 1 atm was attributed to slow of Ru(IV)=O to Ru(VI)-dioxo species67; whether disproportionation of the Ru(IV)=O occurs under these conditions (eq. 16) is unclear. As noted for the thioether oxidations, introduction of halogen substituents into the porphyrin, especially in positions to prevent sterically formation of Ru species, can increase and/or prolong catalytic activity. More generally, extensive halogenation of TPP-type porphyrins at the mesophenyl rings and positions gives rise to the so-called third generation metalloporphyrins9,92,94, and excitement became intense when such Fe-porphyrins were found to effect catalytic of light alkanes with high activity, and a mechanism involving high-valent Fe=O intermediates was proposed92; the mechanism was shown subsequently to be free-radical in nature, involving hydroperoxide decomposition pathways (eqs. 6, 7)121. Such substituents on the porphyrin ring can also modify the conformation of the porphyrin, and out-of-plane distortion can enhance catalytic activity, as first suggested for some Fe(III)-porphyrin systems.121 One rationale for increased activity via out-of-plane distortion is an imposed, so-called “unidirectionality of electron transfer”122. The highly distorted (X-Ray data revealed that its precursor, Ru(DPP)(CO)(py), also exhibits both saddle and ruffle distortion)123 catalyzes in benzene-MeCN (9:1) some epoxidation of norbornene, styrene, cyclohexene, cyclooctene and cis-stilbene under 1 atm with turnovers of 8 - 40 in 4 h before deactivation of catalyst123; the numbers are comparable with those for and greater than for and There were also co-oxidation products: benzaldehyde (7 turnovers) and trace phenylacetylaldehyde were formed from styrene, cyclohex-2-en-1-ol (20 turnovers) and cyclohex-2-en-1-one (11 turnovers) from cyclohexene, and trace benzaldehyde and trans-stilbene

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oxide (3 turnovers) from cis-stilbene. did not oxidize trans-stilbene under these conditions. Stoichiometric alkene oxidation by in in the presence of pyrazole was also studied; allylic C-H oxidation was seen for cyclohexene, while styrene and cis-stilbene gave solely styrene oxide and cis-stilbene oxide, respectively. the final Ru product, was characterized by X-Ray123. The Stoichiometric alkene oxidations obeyed a rate law of the form = k[Ru(porp)][alkene], with k values for norbornene and styrene (in at 25.9°C) of 3.8 and respectively, but whether this referred to a radical process or an Oatom transfer process was uncertain. Use of perhalogenated Ru porphyrins certainly leads to radical autoxidation processes65,124. This is reflected in the fact that is significantly less active catalytically than the carbonyl for of cyclohexene124; a total turnover of 20 to radical products (epoxide, 2-cyclohexen-1-ol, 2cyclohexen-1-one) was achieved using M in at ambient conditions. In contrast, can catalyze similar radical of cyclohexene (and styrene) with turnovers up to 300; cyclooctene gave only epoxide, but such high product selectivity is not unusual for this substrate even in radical, autoxidation processes125. Of note, these systems were photo-initiated with visible light. Closely related are the autoxidations catalyzed by or the carbonyl precursor65. Use of M of these complexes in neat hydrocarbon substrate at ~90°C gives extremely efficient catalyzed autoxidations; turnovers of up to are found for non-selective cyclohexene oxidation (trace cyclohexene hydroperoxide was also detected), while cyclooctene gives >80% selectivity to epoxide65. Corresponding autoxidations using TDCPP systems were completely inhibited by addition of a radical inhibitor such as BHT65. The TDCPP-Cl8 systems similarly catalyze autoxidations of saturated hydrocarbons; see Section 3.4 for further details on radical processes, where the presence of the porphyrin ligand is not always essential. Reports have appeared on the rates of decomposition of cyclohexyl hydroperoxide (an intermediate in the industrial oxidation of cyclohexane126,127) to cyclohexanol and cyclohexanone catalyzed by Ru(porp)CO and systems (porp = tCPP , mCtPP , TDCPP, TMCPP, TMP, TPP) either in solution or anchored to polystyrene or silica128-130. The systems were studied in 20 : 1 at 25°C, when decompositions in the 28-66% range were observed after 2 h, and close to 100% after 48 h129,130. Several, plausible reaction pathways were presented for decomposition of the alkyl hydroperoxides130. Introduction of bulky and chiral substituents at the 5,10,15,20 (meso)positions of the porphyrin ring allows for aerobic, enantioselective epoxidation of olefins 131 . Use of chiral Fe(III)- and especially Mn(III)-

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porphyrin catalysts (using PhIO and LiOCl as oxidants), respectively, for regio- and enantioselective olefin epoxidation had been demonstrated earlier132, but use of chiral Ru-porphyrins for enantioselective epoxidation has been explored only recently131,133-139. Thus, (see Fig. 10) can catalyze aerobic oxidation of several olefins. Under the conditions given in eq. 30, only small turnovers were achieved, e.g. 10 for styrene oxide with 70% ee; in toluene, 21 turnovers were seen for conversion of

methylstyrene to a 7:1 cis/trans ratio of oxide (73% ee), 11 turnovers for the epoxidation of p-chlorostyrene (52% ee), and 14 turnovers for oxidation of 2-vinylnaphthalene The low turnovers were attributed to deactivation of the catalyst after ~24 h through formation of diamagnetic Ru(II) complexes containing epoxide (cf. Fig. 8); in the case of oxidation, the epoxide complex was isolated and characterized by FAB MS131. Application of the chiral dioxoruthenium(VI) porphyrin (Fig. 10) for aerobic oxidation of in benzene (with 9 atm 40 h) gave trans-epoxide 139 with 59% ee after 7 turnovers . The same types of chiral porphyrins have been used in conjunction with O-atom donors for asymmetric epoxidation (see below)133-139. The systems were developed largely from a non-chiral one that utilized in conjunction with pyridine N-oxides and other heteroatomic N-oxides that had been used for selective epoxidation of olefins96,140,141; these had proceeded efficiently under mild conditions under Ar and, in some cases (e.g. using 2,3,5,6-tetramethylpyrazine oxide), styrene was epoxidized over one day in 100% yield based on both styrene and N-oxide. Amine oxides with large substituents, such as phenyls, ortho to the nitrogen-oxo group were inert, presumably because of no interaction between the amine O-atom and the Ru. Strong coordination of the reduced amine oxide at the Ru (e.g. 4,6-dimethyltriazine formed from the N-oxide) also destroys the catalysis96. High selectivities, comparable to those obtained utilizing were observed using 2,6-lutidine N-oxide with cis- and methylstyrene, and with cis- and trans-stilbene; the cis moiety of trans,cis,trans-1,5,9-cyclododecatriene was the most reactive, while the 6,7-

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Maria B. Ezhova and Brian R. James

double bond in some terpene acetates was also selectively epoxidized141. The catalyzed amine-oxide oxidation of 2-vinylnaphthalene was faster than the stoichiometric oxidation using and thus Ru(TMP)(O)(amine oxide) was considered a viable active intermediate; some data using labeled water were interpreted in terms of a reaction pathway other than via Some representative data showing the use of chiral Ru-porphyrins for asymmetric epoxidation using amine oxides or PhIO as O-donors are given in Table 1. The system134 effects epoxidation with 5-88% yields and 28-77% ee values. Either or

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31

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in combination with PhIO are also active; however, yields do not exceed 71% with ee values from 7 to 63%136. Comparison of data for stoichiometric and catalytic epoxidations using and in the presence and absence of PhIO led to the conclusion that the dioxo-species was the key oxidizing intermediate using this oxidant136. Formation of was suggested as an intermediate, based on stoichiometric styrene oxidation seen in the presence of pyrazole (Hpz), eq. 31, where was detected by UV-vis spectroscopy (by comparison with data for an authentic sample). The pyrazolate complex

was similarly isolated from the reaction of with an alkene in the presence of pyrazole123. Kinetic studies on the stoichiometric oxidation of alkenes by in the presence of pyrazole revealed the rate law: with k values of and at 25°C for styrene, cisstilbene and trans-stilbene, respectively136. The rate-determining step for the O-atom transfer for epoxidation of was given as formation of the radical intermediate that explained generation of some oxide (Fig. 11). Such long-lived radical intermediates have been invoked in earlier catalytic epoxidation work using homochiral Fe- and Mn-porphyrins135,142-144. The system (see Fig. 10) was also thought to involve the Ru(VI)-dioxo species as the most active catalyst133, but with an N-oxide as O-donor an alternative mechanism has been suggested (see Fig. 12) 135 . With such oxidants, there is a strong dependence of the reactivity and selectivity of the epoxidations on the solvent and nature of the oxidant135,137, 145-147 . A general increase in enantioselectivity for epoxidations in benzene vs. those in (e.g. with for epoxidation of styrene 42 vs. 5%135, and with 68 vs. 58%137 (see Table 1)) has been considered to result from a strong interaction of Ru with the aromatic solvent137, and there is, for example, NMR evidence for formation of species59. Association of benzene with species (or perhaps some other active Ru(IV) or Ru(V)

1. Catalytic oxidations using ruthenium porphyrins

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intermediates) has been suggested to affect the solution structure of the chiral cavity associated with chiral induction135. Further, the dependence of enantioselectivity on the nature of the oxidant has been considered to indicate that the structure of the active oxidant includes an axially coordinated molecule of the O-atom donor; for example, a Ru(porp)(O)(pyNO) species has been proposed as an active intermediate in 2,3-dimethyl2-butene epoxidation catalyzed by (Fig. 12)137. Such possible intermediates have been discussed prior to reports on the asymmetric epoxidation (see Section 3.1.). The initial cycle starts with slow oxidation of the olefin by to give and epoxide (step a), the monooxo complex then reacting rapidly with N-oxide to form complex (15) (step b). (15) is then considered to effect very rapid epoxidation with liberation of the pyridine (step c). Regeneration of from (15) (step d) was ruled out, as this step was slower than epoxidation via (15). The role of coordinated pyNO was rationalized in terms of its effect on the chiral environment on the trans oxo-coordination site137. Much improved chiral induction resulted from the introduction of Cl-atoms into the meta-positions of the bridged phenyl rings in (Fig. 10): thus, under the catalysis conditions listed in Table 1, styrene was epoxidised with 79% ee at 551 turnover numbers, while cis- and yielded epoxides with 57 and 69% ee at 244 and 487 turnovers, respectively. Trans-stilbene was also oxidized to a mixture of epoxides with 38% ee at 242 turnovers. The authors concluded that overall, the observed chiral induction was higher for terminal and trans-olefins, versus cis-olefins138.

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Attractive from an environmental point of view, nitrous oxide has been employed as an oxidising agent. acyclic and steroidal substrates have been epoxidized using Ru(TMP)-based systems148,149. Stoichiometric epoxidation of is effected by generated via reaction of with but catalytic oxidation of cholesteryl esters has been reported149. Although the conditions are relatively severe: 0.2 mmol of olefins, 5.0 mol % at 10 atm in chlorobenzene at 140°C, the yields and selectivities to the steroidal epoxidation are good; only were formed with yields from 48 to 99%. Use of polar solvents (THF, EtOH, or use of other complexes (porp = TPP, OEP, 4MeO-TPP) resulted in no epoxidation149. Application of Ru-porphyrin complexes for catalytic, homogeneous alkene epoxidation is limited because of small turnover numbers, only moderate enantioselectivities in the case of chiral systems, the cost of porphyrin and the O-atom donor and often limited stability of the porphyrin ring under the oxidizing conditions. In attempts to improve possible applicability, Ru-porphyrins have been “heterogenized” on various supports150-152, following methodology developed previously for Fe- and Mn-

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porphyrin systems14,153. The Ru(porp)(CO)(EtOH) complexes (porp = TCPP150 and TDCPP151) were successfully encapsulated into the channels of mesoporous material MCM-41154, modified by treatment with 3(aminopropyl)triethoxysilane M-41(m); the ligand exchange reaction utilized is shown in Figure 13 150,151 . Ru(TCPP)(CO)/M-41(m) containing 8.3 wt.% Ru showed good catalytic activity for alkene epoxidation using as

oxidant in under at r.t.150. A higher catalytic activity at lower Ru content was attributed to efficient site isolation, coupled with diffusion reaction pathways, conditions that precluded formation of stable, catalytically inactive species. Total turnovers of 9000 were achieved with 0.1 wt.% of Ru for oxidation of norbornene to give 53% exo-epoxide (yield based on consumption of TBHP), in comparison with only 216 turnovers using the 8.3 wt.% Ru catalyst; 230 turnovers were achieved with Ru(TCPP)(CO)(EtOH) as a homogeneous catalyst in In general, the turnovers were 20-40 times higher with the supported catalyst, which was stable under the oxidation conditions, with activities not changing after 2 days of reaction. Although the turnover numbers are reasonable, the chemoselectivity for the supported catalyst is poor. Thus, styrene gave only 11.0% of styrene oxide with benzaldehyde being the co-product (26.5%); cyclohexene gave 2.2% epoxide, cyclohex-2-en-l-ol (7%) and cyclohex-2en-l-one (12.3%); cis- and trans-stilbene gave trans-stilbene oxide as the major product for both isomers (8.6 and 9.35%, respectively), with cis-oxide (1.4 and 0.18%) and benzaldehyde (0.94 and 0.27%) being the other coproducts150. Ru(TDCPP)(CO)/M-41(m) also catalyzed selective alkene epoxidation using as an O-donor in the presence of HC1151. Aromatic and aliphatic alkenes gave epoxides in good yields (up to 98%, based on the amount of substrate consumed) with complete selectivities, styrene, cisstilbene, norbornene and octene1 giving only cis-products; trans-stilbene was not oxidized. Of note, the formation of cis-oxides from cis-alkenes here is quite different from the preferred formation of trans-oxides from cis- or trans-stilbene catalyzed by

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the Ru(TDCPP)CO/TBHP system150. The catalyst retains 67% activity after 11,700 turnovers under the selected reaction conditions: 1 mmol of alkene, 1.1 mmol of ~ 0.3 mmol of HC1, 50 mg of 0.4 wt.% Ru/M-41(m) in 5 ml of The loss of activity was attributed to decomposition of the catalyst. The possibility of being an active intermediate was ruled out based on product distribution studies for and cyclohexene oxidation, and on Hammett correlation studies for the stoichiometric, epoxidation of p- and msubstituted styrenes; a species was proposed as a key intermediate, particularly as a paramagnetic species, assumed to be Ru(III) (presumably formed after O-atom transfer from a moiety) was detected in the reaction mixture after the catalysis (see also Fig. 20, Section 3.4)151. The longevity of the catalyst 150,151 is not the only advantage of using Ru-porphyrins supported on mesoporous materials. Differences in selectivity for the heterogeneous and homogeneous catalysis may also be an asset. Thus oxidation of (+)-limonene catalyzed by gives the 8,9epoxide as major product (61%) and 27 and 10% of the cis- and trans-1,2epoxides, respectively (Fig. 14). Under homogeneous conditions, in gives preferential formation of

1,2-epoxides (55 and 19% of cis- and trans-l,2-epoxides) and 22% 8,9epoxide151. Restricted space in the M-41(m) channels likely makes the more sterically hindered, trisubstituted C=C bond less accessible to the active metal center, compared with the exocyclic methylene. Another example of the influence of steric constraint of M-41(m) on selectivity is oxidation of the bulky 3,4,6-tri-O-acetyl-D-glucal. Under homogeneous conditions, the catalyst system affords stereoselectively the correspondent glycal epoxide (this undergoes methanolysis with an inversion of configuration, leading to methyl (77%))151. In contrast, under heterogeneous conditions, a 3 : 1 mixture of was formed

1. Catalytic oxidations using ruthenium porphyrins

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(Figure 15) 151 . The more bulky benzyl-D-glycal was not oxidized under the heterogeneous conditions. Further developments in supported Ru-porphyrin catalysts include attachment to a polymer through a covalent linkage152. Ruthenium complexes with the unsymmetrical 5,10,15-tris(4-R-phenyl)-20-(4-hydroxy-

phenyl)porphyrins (R = Cl, Me) (abbreviated, for example, have been attached through the hydroxy group to Merrifield’s peptide resin (MPR) bearing benzyl chloride groups according to Figure 16. The resulting catalysts and

(4-MPR)TPP)CO epoxidized a variety of alkenes with as oxidant, the chloro system giving higher yields (56-98%) with Ru (8.6 wt.%.) : oxidant: substrate = 1 : 1400 : 1000, in benzene, at r. t. for 24 h. Cis-stilbene and norbornene yield exclusively cis-epoxides. In contrast, with the M-41(m) system, the polymer oxidized trans-stilbene and to the corresponding trans epoxides in 90 and 86 % yields, respectively. Several alkenes were oxidized catalytically for the first time with high selectivity: 3,4-dihydronaphthalene yielded 62% epoxide,

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1,5-cyclooctadiene gave only bis-epoxide (76%), and cis-1-phenyl-3-penten1-yne only gave cis-epoxide (91%). The 3,4,6-tri-O-acetyl-D-glucal was epoxidized to only (56%), while the system gave a 3 : 1 mixture of and (see Fig. 15). Of note, were selectively oxidized to threo-(amino)epoxides in 87-89% yields. The system was consecutively reused four times without detectable leaching of activity, giving for example epoxide yields of 96, 93, 90, 92 and 91% from styrene. Lowering the Ru concentration to 0.17 wt.% decreased the yields (e.g. 68% for styrene oxidation), but the turnover numbers remained high ( for styrene). The remarkable activity of was attributed to the ability of MPR to solvate and swell in some organic solvents152. That Ru(II)-porphyrins do not effect deoxygenation of epoxides has been ascribed to the relatively low oxophilicity of the incipiently formed Ru(IV)-monooxo species105; thus such species are able to transfer the oxo ligand to olefins, and data on the amine-oxides donor systems described above and for related Ru non-porphyrin systems41,42,155 support such an inference. Ruthenium porphyrins in the presence of O-donors complexes are also capable of cleaving C=C double bonds. Thus a Ru(TDFPP)/TBHP system can cleave the C=C bond in to give acetophenone156, while cleavage of trans,trans-1,4-diphenyl-1,3-butadiene by Ru(TDCPP)/TBHP yields benzaldehyde and cinnamaldehyde157. Epoxidation is proposed as the first step, followed by a TBHP/Ru-porphyrin-mediated fragmentation of the epoxide. Another example is reaction of a Ru supramolecular system ((16), essentially Ru(TPP) with two units linked to two trans-phenyl rings through metheneoxo linkages, Fig. 17) with In a

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Maria B. Ezhova and Brian R. James

biphasic system ((hexane and (9:1)) : water = 10 : 1), preference for the cleavage of E-double bonds yields a mixture of aldehydes (15’- , 12’and l0’-apocarotenals (1:0.95:0.5)) in 30% yield. The “simpler” system also can mediate oxidative cleavage of to give a mixture of and Isomerization of trans- to cis-isomers of first takes place, followed by fast epoxidation of the trans-isomers. Simultaneously oxidation of the initially present trans-C=C double bonds yields the corresponding 5,6and 5,8-epoxides, with subsequent cleavage leading to the mixture of products (Fig. 18)158.

3.4

Oxidation of saturated hydrocarbons A

marginally

catalytic oxidation of cyclohexane using a system was mentioned above (Section 3.3.). The species is thermally unreactive toward cyclooctane at 1 atm at 70°C, but some O-atom insertion into saturated C-H bonds has been achieved under photolytic conditions and with the electrochemically generated species40; few details are available, but the latter system showed for adamantane oxidation the usual radical selectivity (tertiary > secondary carbon). Work in this laboratory has shown also that the complexes (porp = TMP, TDCPP, and are “practically” inactive for thermal of saturated hydrocarbons65. Some activity data for 0.2 mM Ru solutions in benzene under air at ~25°C for “optimum” substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only ~0.2 per day; the maximum turnover realized was ~15 after 40 days for the TDCPP system! Nevertheless, this was a non-radical catalytic processes; there was < 10% decomposition of the and a genuine O-atom transfer process was envisaged65. Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm an essentially linear oxidation rate over 55 h gave about ~70% conversion of the olefin with ~ 80% selectivity to the epoxide; however, the system was completely bleached after ~ 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,00065! As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 6, 7) 65,124,128-130 . The radical nature was more obvious in a corresponding

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oxidation of cyclohexene using 0.03 mM Ru, when the usual mixture of enol, enone, epoxide, and other products were observed, with again a turnover in the region of ~105 being realized65. At much higher 1.0 mM Ru concentration with cyclohexene in benzene at 50°C and 1 atm a selective O-atom transfer oxygenation to epoxide is evident, but at only ~ 1 turnover per day! Similarly, at 0.01 - 0.03 mM Ru in neat methylcyclohexane, using either or Ru(PCP)CO, at 90°C and 1 atm turnovers up to 20,000 were attained in non-selective, radical oxidations giving mainly tertiary alcohols, secondary alcohols, and several ketones65. Of note, several other Ru non-porphyrin complexes initiate (as effectively as the “exotic” porphyrin complexes) the reasonably selective but radical oxidation of neat cyclooctene at 90°C and 1 atm even the essentially insoluble hydrate, and show moderate activity (~16% conversion in 16 h, with ~85% selectivity to epoxide), while the soluble at (!) gives 90% conversion with 69% selectivity (a turnover of at 1 mM, the phosphine complex gives only 50% conversion (at 79% selectivity) implying a contribution from a less effective pathway, possibly O-atom transfer, which has been invoked for Ruphosphine/O-atom donor systems159. Three important points are evident: (i) The introduction of halogen substituents into the porphyrin ring does stabilize Ru(VI)-dioxo species for executing O-atom transfer catalysis, but the halogenated species are more effective than the non-halogenated species for catalyzing decomposition of hydroperoxides; and these, if present in trace amounts, can lead to efficient radical oxidation pathways, (ii) At lower concentrations of a Ru(porp) complex (or apparently many other Ru non-porphyrin complexes) in neat hydrocarbon (typically ~10 M) under where there will be trace amounts of hydroperoxides, the favored reaction appears to catalyzed decomposition of the ROOH to generate radicals with accompanying bleaching (i.e. destruction) of the porphyrin. (iii) At higher Ru concentrations (~1 mM) in, for example, benzene solutions of the substrate (typically ~ 0.1 M) under where there will be less hydroperoxide, the radical chemistry is less evident and conditions for O-atom transfer from a Ru(VI)-dioxo species are improved. The dioxo species can, of course, be generated by reaction of a Ru-precursor and a hydroperoxide, the latter acting an O-atom donor (Section 2, Fig. 5), and so which pathway dominates will depend critically on concentrations of Ru and hydroperoxide (particularly within a second-order process) and the relative activation energies of the reaction pathways. The Ru(porp)-based oxidizing systems using as oxidant (see Section 3.3 for use in olefin epoxidation) have been reported to be highly efficient for oxidation of alkanes138,160-163. or

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was initially used as a catalyst precursor in benzene containing “2-3 drops of concentrated aqueous HCl or HBr” under Ar at 20-40°C, with molecular sieves added to maintain anhydrous conditions160. Essentially complete conversion of adamantane yielded adamantan-1-ol (up to 68%), adamantan1,3-diol (25%) and adamantan-2-one (1%), while methylcyclohexane was oxidized to 1-methylcyclohexanol (77%) and methylcyclohexanones (6%); ethylbenzene and cyclohexanol were converted in 88% yield to acetophenone and cyclohexanone, respectively. The acids were essential, and NMR data suggest that these convert the dioxo species into the known, paramagnetic species (X = halide)49,71. The dihalo complexes, as well as Ru(porp)CO complexes (porp = TMP, TDCPP, TPFPP, and even the non-sterically hindered TPP) were also effective catalyst precursors161,162. For example, the (or HBr) system effected hydroxylation of adamantane to 1-adamantanol (76.2%) and adamantan-1,3diol (13%) with turnovers of depending on the while cis-decalin was converted to (Z)-9-decalol (79.6%) and (Z)-decal-9,10diol (4.2%) at up to 64 turnovers . The TPP, TMP, and TDCPP systems also effected oxidation of steroids, with retention of configuration for those containing chiral centers; for example, in the presence of Ru(TPP)CO gave the 25-hydroxy product (11%) (Fig. 19), while Ru(TMP)CO and Ru(TDCPP)CO generated other alcohols as well161.

Analysis of product evolution during adamantane and cis-decalin hydroxylation by the Ru(TPFPP)CO system, coupled with observation of intermediates (18 – 20) by UV-vis, IR and ESR spectroscopies, led to the mechanism outlined in Figure 20162. An observed induction time was attributed to formation of (20), and this induction period could be reduced by initial treatment of the precursor (17) with O-atom transfer occurs from (21), a Ru(V)-oxo (or a Ru(IV)-oxo-porphyrin cation radical160, which had been suggested much earlier8,69 - see above), and its formation was

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43

considered rate-determining under conditions when there was a zero-order dependence on substrate. At lower [substrate], the reaction of (21) with the hydrocarbon became rate-determining, when kinetic deuterium isotopic effects for hydroxylation of adamantane and cisdecalin were measured162. It is clear that the dioxo species, does not participate in these systems using pyridine oxides as an O-atom donor. Some rate data had suggested that the HCl/HBr acids may also accelerate deoxygenation of the N-oxide by the Ruporphyrin Total turnovers up to 18,800 were reached for adamantane oxidation and, for the TDCPP system, a high turnover of was attained160. An example of enantioselective hydroxylation of a benzylic C-H bond

using or complexes (see Fig. 10) has been reported by Che’s group163. The stoichiometric oxidation of substituted ethylbenzenes, 2-ethylnaphthalene, indane and dehydronaphthalene by in containing pyrazole at r.t., led to the corresponding alcohols (27-48% yield) and ketones (24-34% yield), with ee values of 9-58% (S) for the alcohols. The second-order rate constant determined for oxidation, for example, of ethylbenzene was at 25°C with a kinetic isotope effect for of 8.9 at 40°C and 11.2 at 25°C. A dual-parameter Hammett correlation with data for 4-substituted ethylbenzenes was considered consistent with a ratelimiting step involving C-H bond cleavage (Fig. 21). Preferential formation of the S-isomer was explained by preferential collapse of the benzylic radical

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Maria B. Ezhova and Brian R. James

on the pro-S face versus pro-R face during an O-atom rebound step, resulting from the fit of the substrate in the chiral cavity (see Fig. 10). Chiral induction via radical intermediates is a remarkable result. Under catalytic conditions (5 ml benzene, 25°C, alkane : : catalyst = 0.5 mmol : 0.55 mmol : ), effectively oxidizes the substrates to alcohols in 28 to 72% yields, with ee values of 12-76% (S). Use of the system138 (see Fig. 10) for hydroxylation of the tertiary alkanes (rac-2-phenylbutane and -2-phenylhexane) under catalytic conditions ( of Ru, oxidant and 1 mmol alkane, 48% aq. HBr, 50 mg of 4A molecular sieves in 1 ml benzene under Ar at 25°C) results in tertiary alcohols with modest enantioselectivity and yields, again remarkable considering the possible radical nature of the process (see above). Hydroxylation of the alkanes gave

PhC(Me)(R)OH (16% ee, 41% yield, 103 TON for R = Bu; 27% ee, 54% yield, 135 TON for R = Et). The highest ee (38%) was obtained for hydroxylation of the 2-phenylbutane at 10°C; some kinetic resolution of the unreacted alkane (up to 8%) was evident138. This is the first report of catalytic enantioselective hydroxylation of tertiary alkanes.

3.5

Oxidative-dehydrogenation of phenols and other arenes

Preliminary studies had suggested that benzene solutions of react stoichiometrically with phenol under 1 atm to give the paramagnetic species (23), via a detected species, according to eq. 32, and data were given for the second-order rate constant Evidence for (23) included data,

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elemental analysis and a magnetic moment showing 2 unpaired electrons.

More detailed studies have shown subsequently that the product is, in fact, (24 in Fig. 22 below)65. (24) was synthesized independently from the metathesis reaction between with excess phenol under Ar, and mass spectral data distinguished the product from the supposed (23), which was probably the dihydrate of (24). A revised mechanism for the stoichiometric phenol oxidation is presented in Figure 22. The reaction initially produces p-hydroquinone, which is further oxidized to p-benzoquinone at a rate faster than the rate of phenol oxidation, and this was confirmed by direct oxidation of p-hydroquinone with The generated species is then thought to react reversibly with water to give the dihydroxy species (the X-ray structure for has been reported164) that then undergoes metathesis with

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phenol to give (24). The disproportionation of to and (see Sections 2 and 3.3) is thought to be a competing pathway for the reaction with water, and this would give rise to the observed which was also formed during the anaerobic reaction of with excess phenol. Under anaerobic conditions, the kinetic dependence on phenol for loss of the reactant becomes > 1 because of the faster reaction of the p-hydroquinone than phenol. The selective attack of at the para-position of the phenol is presumably imposed by the steric restraints of the TMP ligand. The mechanism of the net O-atom insertion into C-H bond could involve either sequential electron and H-atom transfer processes as suggested for the reaction between phenol and transor something akin to the oxygen-rebound 76,101 mechanism with initial H-atom abstraction from the phenol at a Ru=O site. Effective catalytic hydroxylation/oxidation of phenol to give phydroquinone and or p-quinone has yet to realized using these Ru porphyrin systems. N,N-Dimethylaniline is marginally catalytically oxidized by 3.3 x 10-3 mM benzene solutions of under aerobic conditions at 50°C perhaps to the p-hydroxy-derivative; the turnover of 2.1 after 30 h, was limited by decomposition of the catalyst to Ru(TMP)CO via a decarbonylation process, presumably of the phenolic product65. The preliminary findings could be consistent with initial attack of an electrophilic oxo moiety at the para-position of the aniline, as suggested for the phenol oxidation (see above), or attack at the moiety (see Section 3.7). Of note, methoxybenzene, toluene, chlorobenzene, bromobenzene and nitrobenzene were not oxidized by The Ru(porp)CO/HX/2,6-Cl2pyNO systems (porp = TMP, TPP; X = Cl, Br; see Sections 3.3 and 3.4) can, however, convert alkoxyarenes eventually to the p-benzoquinone derivatives selectively, as exemplified in Fig. 23 l66 . The yields of 11-97% varied with the structure of the substrates, but the most electron-rich C-atom was preferentially attacked by an

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electrophilic oxygen; thus 1,3,5-trimethoxybenzene was oxidized to 2,6dimethoxy-p-quinone with 97% yield, the total turnover reaching 33,000. Oxidation of m-dimethoxybenzene gave 2-methoxy-p-quinone (74%), while o-dimethoxybenzene yielded the corresponding p-quinone in only 11% yield, and p-dimethoxybenzene was not oxidized at all; the chemoselectivity of these reactions is clear. Similarly, oxidation of 2,3’,4,4’- or 2,2’,4,5’tetramethoxy-1,1’-biphenyl resulted in 2-methoxy-5-(3,4-dimethoxyphenyl)and 2-methoxy-5-(2,5-dimethoxyphenyl)-p-quinones in 77 and 46% yields, respectively (Fig. 24). Oxidation of naphthalene and phenanthrene yielded 1,4-naphthoquinone (29%) and 9,10-phenanthroquinone (40%), respectively; benzene was not oxidized. The nature of the catalytic Ru-oxo species involved was considered uncertain (see Section 3.3.). The intermediate phenol formed in the first step was isolated in the m-dimethoxy system, and the second stage involved loss of MeOH. An experiment showed that at least one ketonic O-atom of the p-quinone originated from the pyridine oxide.

3.6

Oxidative-dehydrogenation of alcohols

at 1 mM concentrations in benzene catalyzes aerobic oxidative-dehydrogenation of 2-propanol to acetone65,74,167,168, benzylalcohols to the corresponding benzaldehydes65,167,168, and various benzhydrols (solid substrates, where X = F, H, MeO) to the corresponding benzophenones169, and in each system water is the coproduct, but the turnovers are small. After 24 h under 1 atm air at 50°C, for substrates typically at ~0.2 M, 6 turnovers were observed for 2-propanol, and 50 for a maximum 23 was seen (after 45 h) for (p-

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Addition of increases the turnover numbers by factors of 2-6, depending on the substrate, while addition of aq. KOH or NaOH increases turnovers by a further factor of up to 6. Thus addition of KOH increases turnovers for isopropanol and benzylalcohol by factors of 56. Maximum turnovers over 24 h of ~200 (100% yield) were achieved for 2-, 3-, or 4-MeO substituted benzyl alcohols, and the catalyst still remains active. Oxidation of primary alcohols (such as 1-butanol) to the aldehyde was less efficient, with turnovers of up to 40 being noted over 24 h, with no formation of carboxylic acids. The use of rather than air did not increase the oxidation rates 65,167,168 . In general, use of instead of for the aerobic oxidations made little difference, i.e. the activities were similar over 24 h, but the TDCPP analogue was more susceptible to deactivation via a decarbonylation process that generated the Ru(CO) derivative168. For the oxidation of benzhydrols, the perchloro derivative exhibited activity similar to that of The stoichiometric reaction between and the alcohols (2propanol, and under Ar conforms to that shown in eq. 33 or Ph(H)), as evidenced by NMR. The

paramagnetic bis(alkoxy)ruthenium(IV) complexes were isolated and fully characterized by NMR, UV-vis and IR spectroscopies, magnetic moment data (2 unpaired electrons), and X-ray structures for and The NMR spectra show upfield, paramagnetic shifts for the hydrogens to the to -34 region (vs. 8 - 9 for diamagnetic or complexes), consistent with Ru(IV) species possessing symmetry49,170. Detailed kinetic studies on the stoichiometric oxidation of 2-PrOH, benzyl alcohol and several benzhydrols by have led to the mechanism outlined in Figure 25; the rapid pre-equilibrium (K) to give a Ru-dioxo-alcohol intermediate (25), followed by a rate-determining (k) hydride transfer from the conforms to the measured saturation kinetic behaviour and the rate-law given in eq. 34169. The reactions were

monitored by NMR and UV-Vis spectroscopies but even at higher [ROH], when the rate is zero-order in ROH and (25) should be fully formed,

1. Catalytic oxidations using ruthenium porphyrins

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there was no spectroscopic evidence for this intermediate. Such species are commonly proposed for alcohol oxidation using other Ru complexes171-175. For the porphyrin dioxo system, the “more sensitive” v(Ru=O) IR region was investigated, but the data were inconclusive169. A primary kinetic isotope effect at 20°C of ~15 (e.g., for and for with no isotope effect found for oxidation of is consistent with the rate-determining hydride transfer. Large kinetic isotope effects of between 8176 and 50177 have been reported for cleavage of bonds in alcohol oxidation by non-porphyrin transand complexes, respectively. A Hammett plot of log against for oxidation the p-substituted MeO-, H- and F-benzhydrols gave a linear relationship with a slope (the F-system being least active), implying that transfer of electron density from the to the Ru oxidant occurs in the formation of the transition state; the Ru=O...H bonding within (25) is akin to H-bonding but involving a hydride H-atom and an electrophilic O-atom169. The presence of a small and reasonably constant quantity of (see Section 3.3.) was detected by NMR during the stoichiometric oxidation of and its formation was suggested to take place concomitantly with that of the ketone via a net loss of from the intermediate and protonated ketone (Fig. 25). Production of the bis(alkoxide) was written as involving a ligand exchange reaction

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Maria B. Ezhova and Brian R. James

between the alcohol and (formed reversibly from the oxo species). There is no direct evidence for the existence of the isolation of this compound has been claimed (from crystal structure data on an alcohol solvated species)66, but other work has suggested that the structure reported is that of a species167. Nevertheless, crystals of have been isolated during a synthesis of and analyzed by X-ray crystallography (see Section 3.5) 164 . Isomerization of to species via proton migration has been invoked within some non-porphyrin systems176. Studies on the catalytic alcohol oxidation process involved investigation of the above systems under an atmosphere of air or The initial rate of production of the ketone product is governed by the rate of formation of the bis(alkoxy) species, while subsequently the catalysis rate is likely determined by regeneration of the dioxo species via disproportionation of the species (see Sections 2 and 3.3.); however, this will depend on the concentration of which increases as the catalysis proceeds, and indeed water has been considered to accelerate the disproportionation reaction by increasing the rate of dioxo formation (eq. 35)65. Of note, the Ru-

bis(alkoxide) species are stable in dry benzene under an atmosphere of dry while in air in wet benzene the species are slowly regenerated over several hours, with eventual formation of 2 mole equivalents of ketone (eq. 36); such a reaction suggests the likely involvement of the bis(alkoxy)species as intermediates in the catalysis65. No radical

intermediates were detected during the above alcohol oxidations, and addition of radical inhibitors such as BHT in small amounts did not significantly effect the rate of ketone production, although there was some NMR evidence for reaction of BHT with possibly to form a bis(phenoxy) species169. The O-atom donor, lutidine-N-oxide, has been used with to catalyze the room temperature oxidation of alcohols to the corresponding aldehydes or ketones in ~80%96,145. Thus, under Ar, allyl alcohols were oxidized selectively to aldehydes selectively, and to was not oxidized. Cyclohexanol and adamantanol gave the corresponding ketones. The related system, mentioned in the previous section, catalytically oxidizes cyclohexanol to cyclohexanone160.

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Oxidations of racemic secondary alcohols ArCH(R)OH by system in the presence of HC1 proceed with some kinetic resolution138. The smaller molecules were oxidized with larger selectivity. Thus oxidation of when carried out to ~50% of conversion, yielded ~ 50% methylphenylketone, and the remaining alcohol was enantiomerically enriched (24% ee). Similarly, oxidation of alcohol gave 32% of ketone, with the ee of remaining alcohol being 10%; yielded 25% of ketone, and 25% ee for the remaining alcohol; and yielded 41% of ketone, and 2% ee for residual alcohol138.

3.7

Oxidative dehydrogenation of amines

The oxidation of amines is important biologically7,178,179. In particular, oxidative N-dealkylation (eq. 37) is one specific reaction catalyzed by cytochrome P-450 monooxygenase (Section 1)7,180, and several models

mimicking this reactivity have been reported using iron porphyrins181 and Ru non-porphyrin182 systems. More generally, oxidation of amines catalyzed by transition metal complexes can lead to this dealkylation, formation of amine or dehydrogenation183-185. Ruthenium(II) precursors have been used with a variety of oxidizing agents, including oxygen 186 , t BuOOH 182, 187 , PhIO188, 189, and bromamine-T190 . Reports utilizing high-valent Ru-oxo complexes include the use of (L = N- and P- donors)191 , species (see below); it is this last mentioned type of the system that is the focus of this Section. As first shown in the laboratory here, can oxidatively dehydrogenate primary and secondary amines under stoichiometric and catalytic conditions in benzene at 50°C (with as 185 oxidant) . For the former, the reaction stoichiometry depends on the number of H-atoms in the of the amine, and for primary amines can be presented primarily by eqs. 38 and 39 (Ru = Ru(TMP)): imines or nitriles are formed with generation of isolable Ru(II)-bis(amine) complexes, and an X-ray structure of was determined185.

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Pyridine was not oxidized under these conditions185. Reactions of and with can lead to formation of complexes, and corresponding reactions with can lead to complexes of the type which was structurally characterized195; however, the fate of the oxo ligand and reaction stoichiometries in these systems were not determined (see below also)195,196. In the catalytic work185, subsequent to formation of the imine from secondary amines, secondary products such as aldehydes are seen due to imine hydrolysis by the co-product, The maximum turnover after 24 h was 20, observed for the conversion of to and to 18 and 15 turnovers were found for conversion of to Ph(Me)C=NH, and to respectively; after this time, most of the dioxo complex has been syphoned off as the Ru(II)-bis(amine) species, and the catalysis depends on the subsequent conversion of this back to the dioxo species. Figure 26 outlines the essential paths suggested for the catalysis185. An initial 2-electron oxidation of amine to imine by takes place, with formation of Ru(TMP)(O), which was detected; this again presumably disproportionates to and and the latter can then react with to regenerate the dioxo species (Sections 2 and 3.3), or with amine to give species. Imines with an atom can undergo a second dehydrogenation by or possibly Ru(TMP)(O).

Application of the halogenated porphyrin complexes and as catalysts for dehydrogenation of and gave higher rates than the TMP system, but the chloro-systems were visibly less stable (i.e. some bleaching occurred)185.

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More detailed studies on reactions between and amines have revealed, at least by NMR studies (see below), the presence of several Ru-intermediates en route to the final and coproducts (imine or nitrile, and water, eqs. 38, 39)197. UV-Vis studies at ~ M Ru(VI) for the reaction with (R = H, Me, Ph.) in benzene at 20°C show rapid spectral changes with clean isosbestic points for implying the absence of substantial amounts of any intermediates; attempted kinetic studies were thwarted by irreproducibility, although the substituted benzylamines reacted more slowly185,197. The kinetic inconsistencies possibly result from photosensitivity of the or other species (see below) in the light beam197. At the higher concentrations used for NMR, and in the darkness of the probe, kinetic data became reproducible within stoichiometric conditions (20°C under Ar). The stoichiometric reactions (cf. eq. 38) followed the simple rate-law k[Ru] [amine] for loss of Ru(VI), over an amine concentration range of ~ 0.01 - 0.3 M, with k values of 9.74, 1.72 and at 20°C for the R = H, Ph, and Me systems, respectively197, and there was no sign of saturation kinetics as found for the related alcohol systems (see Section 3.6). A kinetic isotope effect measured for the systems is perhaps consistent with formation of an activated complex of the type Ru=O....H---N, via an H-bonding interaction with H-atoms on the N-atom; the suggestion corresponds to that shown in Figure 25 for the alcohol systems where a C-H bond is initially stretched. Cleavage of the N-H bond of the amine versus the C-H bond of the alcohol would account for the generally faster stoichiometric reactions of the species with amines versus alcohols under analogous conditions. In contrast to studies on the alcohol systems, two Ru-intermediates were detected in the NMR studies en route to the bis(amine) product for the slower R = Ph and Me systems (eq.40)197. Intermediate (26) has symmetry and thus two different axial ligands (this being readily diagnosed

by the presence of two signals for the o-Me groups of the mesityl substituents49,60,196,198,199), while intermediate (27) has symmetry and two identical axial ligands. Potential intermediates such as and were ruled out, at least for the system, where the is readily available, as the bis(imine) and (amine)(imine) species with readily identifiable NMR spectra were synthesized independently from The findings are

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Maria B. Ezhova and Brian R. James

consistent with (27) being the bis(imido) species and indeed a corresponding complex has been isolated and characterized by NMR in the reaction of and (27) was also synthesized from oxidation of by Formation of the bis-imido species implies a net loss of both H-atoms attached to the N-atom of the amine (to give rather than one H-atom from each of the N- and Most relevant to this is the interesting, observed isomerization of the bis(imine) species to the bis(imido) species in benzene at ambient conditions during the oxidation of by which is stoichiometric and forms stepwise the Ru(TMP)(amine)(imine) and then Ru(TMP)(imine)2 species (eq. 41); water and/or Ru species may play a role in such isomerization. Attempts to make species directly

from have not yet been successful. Other evidence for the bis(imido) formulation for (27) includes its reaction with which displaces the imido (or nitrene) moieties to generate the bis(amine) complex and a polymeric nitrene product197. Related nitrene transfer reactions from and (X = H, Cl, I, Me) to pyridine and tertiary phosphines, respectively, have been demonstrated (see below). Substitution of the imido ligand in (porp = TTP, 4-C1-TPP) by to generate has also been reported (see below)200. Identification of (26) is less definitive: most likely it is the oxo-imido species as corresponding species have been characterized (see below). For the system, data for the (26) and (27) intermediates were quite different, and were more consistent with them being the mixed imine/amine species (Me)Ph] and the bis(imine) respectively197. The chemistry of systems differentiated by a Ph vs. Me substituent thus appear to be remarkably different. In some systems with water added initially (prior to its formation as a co-product), significant amounts of were 197 seen . Mixed Ru(II)-amine/imine complexes of the type have been isolated from the reaction of = the dianion of the chiral 10,15,20-tetrakis[o-(2methoxy-2-phenyl-3,3,3-trifluoropropanolamino)phenyl]porphyrin) with excess esters198,202,203. For example, reaction of the methyl ester of

1. Catalytic oxidations using ruthenium porphyrins

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L-alanine leads to complexes such as (28) in up to 80% yield Analogous products are formed using the methyl esters of Lvaline (55% yield), L-leucine (44% yield), L-phenylalanine (50% yield), and L-glycine (30% yield)). The complexes were also formed as co-products usually in < 5% yield, although the bis(glycine ester) complex was formed in 30% yield202; a complex was structurally characterized203. The coordinated imine is the expected product from oxidative-dehydrogenation of the reactant amine, and only the E-configuration was seen for the coordinated imines. It seems plausible that a bis(imine) species is formed initially and then one coordinated imine is displaced by the ester amine; the authors did not

mention production of free imine or its potential decomposition products. The amino ester/imino ester species could also be formed from electrochemical oxidation of the Ru(II)-bis (amino ester) complexes202. The reaction of a species with excess L-alanine methyl ester gave a 1:1 mixture of the amino ester/imino ester and bis amino ester complexes202, consistent with disproportionation of the precursor to and (Sections 2 and 3.3). The complexation of the amino esters to Ru(II) did not give any chiral recognition. For example, reaction of with 10 equiv. of rac-valine or -leucine esters yielded a racemic mixture of the corresponding bis (amino ester) complexes (DD : DL : LL = 1 : 2 : 1)203; however, chiral recognition to 52% (for leucine methyl ester) was observed for the complexation of the rac-leucine esters to and the oxidation of the amino esters by the chiral (or perhaps, more precisely, coordination of amino ester to the Ru imino ester complex)

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Maria B. Ezhova and Brian R. James

was usually selective. Thus, oxidation of rac-valine ester (10 equiv.) yielded the bis(amino ester) complex with 66% ee; ee values for esters of alanine, leucine and phenylalanine were 10, 26, and 0%, respectively203. There are clearly several complications to the simplified scheme proposed for the oxidation of amines (see Fig. 26), although the essential feature of a disproportionation of to and likely remains. The can react with imine as well as amine, and oxo-imido or bis(imido) species have also been identified within certain amine systems, and the concentrations of the dehydrogenated amine species and co-product water, which increase as the stoichiometric or catalytic reactions proceed, will certainly effect the behavior of the system (see Section 3.6.). Reactions of other complexes with excess of primary or secondary amines have been reported195,196,205; with the porp = TPP, TTP, 4Cl-TPP, 3,5-Cl-TPP and 3,4,5-MeO-TPP, and with R = H, cyclohexyl, n-octyl, and n-dodecyl, and (R = Me, Et), the complexes are formed, and X-ray structures of and were solved; however, no information was given on the amine dehydrogenation products that must result from consumption of the oxo ligands. Of interest, Ru(IV)-bis(amido) complexes have been isolated when the secondary amine was used as reactant, and the stability of these complexes was strongly dependent on the substituent on the phenyl groups of TPP: and were isolated presumably via the stoichiometry of eq. 42, although this was not established196; the bis(amido) species can be reoxidized back to the dioxo complex using meta-chloroperbenzoic acid170. Other porphyrinato ligand systems generated a mixture of products196, in contrast with corresponding

chemistry of the complexes that cleanly gave the 206 bis(amido) products . The only X-ray structures of Ru(porp) mixed amido complexes were obtained for Ru(porp)(NHTs)(pz) (porp = TPP207, OEP208), which were isolated during some reactions of with alkene in the presence of pyrazine (see below). Somewhat analogous to eq. 42, reaction of (porp =TTP, 3,4,5-MeO-TPP) with the imine leads to formation of the corresponding methyleneamido complexes of which the trimethoxy derivative was structurally characterized.

1. Catalytic oxidations using ruthenium porphyrins

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More generally, Ru(porp)-bis(imido) and -oxo(imido) species, products of oxidative-dehydrogenation of bis(amine) complexes, are of considerable interest as metalloporphyrin imido complexes have been postulated in the nitrite reductase cycle209, and can be used as models for metabolism of natural amines201. Further, the imido ligand is isoelectronic with the oxo ligand, and transition metal imido complexes, including metalloporphyrin derivatives, are the key reagents or intermediates in reactions such as oxyamination210, amination211-215 and aziridination201b,207,208,213,214,216 of olefins. Outside of Ru species, metalloporphyrin complexes with imido ligands are well demonstrated and include those of Ti217, Cr218, Mn216, Fe201, and Os68,200,206,219. Well characterized complexes are the ptolylsulfonyl derivatives (porp = TPP, TTP, 4-C1-TPP, 4MeO-TPP, OEP, TMP, prepared by Che’s group via reaction of Ru(porp)(CO)(MeOH) with PhINTs207,208,213; these tosylimido complexes are stable in the solid state for a few days at –15°C, but in at r. t. they decompose after a few hours, and are not very reactive toward water. They readily react with to yield the bis(phosphine) complexes208. Formation of species with other substituents on the N atom and has been proposed in oxidation of and oxidation of primary amine by (see above). has been isolated200, but it quickly hydrolyzes in air to give oxo(imido) and species68,196,200. In comparison, complexes are more stable and have been characterized by X-ray analysis200. A mono(imido) complex where X = Me, H, Cl, I) (analogous to has been prepared via reaction with This mono(imido) species can react with tertiary phosphines to yield phosphinimines and complexes (Fig. 28); the imido transfer reaction for the system obeys the rate law = showing standard saturation kinetics behavior199. The electron-deficient arylimide systems were more reactive than the electron-rich analogues [k = 39.5 (X = I), 16.0 (X = H), 13.6 (X = Cl), 25.0°C, in toluene]; and the more reactive the imido complex, the lower its affinity for the phosphine [K = 1.15 (X = I), 13.6 (X = H), 2.39 (X = Cl), . The proposed mechanism involves reversible binding of phosphine to and subsequent rate-limiting irreversible intramolecular imido-group transfer (Figure 28). Unfortunately, no spectroscopic evidence was given for species (29) (or (30)) under conditions where they should be present.

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Analogous to O-atom transfer from Ru-oxo species, imido transfer can occur from and species. Stoichiometric imido transfer from (31) to alkenes to give aziridines has been demonstrated when porp = TPP, OEP, TTP, 4-C1-TPP, 4MeO-TPP, the reaction being formally analogous to epoxidation using species; for example, when was used in in the presence of pyrazole, aziridines are formed in 22 and 10% yields from cis- and trans-stilbenes, respectively, 75 and 72% yields from cis- and respectively, and in 67-82% yields for p207,208 substituted styrenes . The Ru is recovered as the complex Ru(porp)(NHTs)(pz) (32), the overall stoichiometry being shown in eq. 43207,208. Insertion of the imido group into a C-H bond of alkanes correspondingly gives the tosylamide; e.g. toluene gives the tosyl derivative of benzylamine (eq. 44), in 9% yield208,213,214, while benzyl alcohol gives benzaldehyde in 95% yield (eq. 45)207. The rate-determining step for the aziridination of alkenes by was considered to be formation

of a carboradical intermediate (Fig 29, cf. Fig. 8 for epoxide formation); clean isosbestic points were observed during kinetic studies at 25°C by UVvis spectroscopy, the rate-law was simply in each of and

1. Catalytic oxidations using ruthenium porphyrins

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styrene, and the small isotopic effects determined at the and at the were considered to be consistent 208 with the nonstereospecificity of such reactions . The net oxidation of benzyl alcohol also obeyed the rate law The insertion of the imido group into alkane C-H bonds occurs selective at the tertiary centers. For example, with adamantane, N-(1adamantyl)tosylamide is formed in 52 and 60% yields from and respectively, under stoichiometric conditions in the 208 presence of pyrazole . Cyclohexane gave the cyclohexylamide in only 10% yield; ethylbenzene gave (~80%), isopropylbenzene yielded (~75%), while cyclohexene produced 3-(N-tosylamido) cyclohexene (~85%), the insertion occurring at a C-H bond to an carbon. Formation of a carboradical was again considered rate-determining for the alkane reactions, with the relatively large isotope effect being found for tosylamidation of cyclohexane by

(Fig. 30)208.

Application of the more electron deficient complex Ru(TPFPP)CO as a catalyst for such imido transfer reactions with PhINTs as donor, in the absence of pyrazole, led to reasonable yields of the corresponding aziridines and amides214. For example, aziridination of allylbenzene, cyclooctene and

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Maria B. Ezhova and Brian R. James

1-octene in at 40°C for 2 h (Ru : substrate : PhINTs = 1 : 75 : 150) generated the aziridines with conversions of 32, 43 and 42%, respectively214. The amidation reactions under such conditions (for example, with indane, adamantane and 1,2-dihydronaphthalene) resulted in corresponding amides in high conversions (92-99%) of substrate. Use of a lower temperature, or use of benzene or MeCN, gave poorer results214. Of interest, use a mixture of and (R = Ts, Ns, instead of PhINR itself, for aziridination and amidation of alkenes and alkanes with Mn(TPFPP)Cl as a catalyst, gave excellent substrate conversions, but the corresponding Ru system was not tested214. Enantioselective amidation of saturated C-H bonds has also been reported by Che’s group213. Application of or a mixture of as a catalyst 40°C, Ru : PhINTs : substrate = 1 : 100 : 500) for a range of substrates such as ethylbenzene afforded corresponding amides in yields of 16 to 47% and ee values of 6-48%, all with excess S-configuration213. Attempts to synthesis bis(imido) compounds via oxidative bromination of led to formation of the oxo(imido) species (eq. 46, porp = TPP, TTP, 4-C1-TPP, 3,4,5-MeO-TPP, 3,5-Cl-TPP), that were isolated and characterized68,196,200, where the source of the oxo-ligand was most likely trace amount of water present in the solvent68,196. The oxo(imido) complexes were more stable than the corresponding dioxo species, being air-stable in the solid state, but gradually decomposing in to form the nitrosyl complexes Ru(porp)(NO)(OH). The species react with amines or phosphines to yield the respective bis(amine)- and complexes200, but

the fates of the oxo and imido ligands were not determined. The corresponding and species do not react with and reactivity with is very slow. The nitrido complex is said to be formed via reaction of with aq. in although an attempt to make the nitride complex via oxidation of with mCPBA, or air was not successful220. In contrast, oxidation of complexes (porp = TPP, TTP, 4-C1-TPP) with m-CPBA in a mixture under reflux gave the corresponding species200.

1. Catalytic oxidations using ruthenium porphyrins

4.

61

CONCLUSIONS

Studies on Ru-porphyrin chemistry first appeared in the early 1970s and, as in many other areas of chemical research, the number of publications dealing with the topic has grown exponentially, with much of the work developed from a theme and rationale to mimic the extensive biological chemistry of naturally occurring, enzymatic Fe-porphyrin systems; some of these are involved in oxygenase systems that carry out highly selective oxidation processes of the types which are very attractive from an industrial point of view. As an alternative to using enzymes industrially, efforts remain intense to develop protein-free, “model complexes” that will activate for oxygenation/oxidation processes. Currently the emphasis probably lies in the use of non-porphyrin complexes of Fe and Cu to mimic, for example, methane-monooxygenase221 and oxidase systems44,222, respectively, while the use of Ru, the “slowed down” 2nd-row analogue of Fe, is an obvious choice that has found emphasis in porphyrin-based systems. Certainly, the work from the laboratories here has developed over the last 30 years from the hypothesis that Ru systems would allow for easier detection of intermediates in oxidation pathways and a better understanding of such reactions, this hopefully eventually leading to design of improved catalysts (based on Ru or Fe). The contributions, plus those from many other groups, have led to identification of species with coordinated superoxide and peroxide, and bridging and terminal oxo ligands. Correspondingly, increased understanding of enzyme systems (particularly cytochrome P-450) has contributed to development of the chemistries of Fe- and Ru-porphyrins. The published coordination chemistry of Ru-porphyrins with the metal in oxidation states II to VI is now very extensive, and is discussed in several sections of this Chapter. The ability of Ru(II)-porphyrins to split to generate complexes, and the subsequent transfer of both these coordinated “O-atoms” to organic substrates, both represent truly remarkable and unique chemical processes: to our knowledge, no other metallic center exhibits the reactivity pattern, let alone such “bis(monooxygenase)” activity. There is no doubt that the catalyzed oxygenations of P-, As-, Sb-, and S-based substrates operate by genuine O-atom transfer processes, and involve a key reversible disproportionation of to and Such pathways also pertain in epoxidation of olefins using porphyrin systems (based on sterically hindered porphyrins such as TMP and TDCPP (or OCP)), but activity is slow (a maximum of ~100 turnovers per day) and limited by decomposition chemistry; efforts to heterogenize the systems have been initiated. Use of corresponding with chiral porphyrins has demonstrated their

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Maria B. Ezhova and Brian R. James

feasibility for asymmetric epoxidation, but ee values of only ~70% have been achieved with small total turnovers (<50) because of catalyst decomposition. Use of O-atom donors instead of generally gives somewhat better turnovers (up to ~1000) and, in chiral porphyrin systems, ee values up to ~80% have been realized. Within O-atom donor systems generally, alternative mechanisms appear to be operating, involving catalytic species such as where L is the O-atom donor. The asymmetric olefin epoxidation systems are not economically competitive on a laboratory practical scale with the chiral Schiff-base systems based usually on Mn223 or the recently reported Fe-based, chiral, chelating amine systems224. More commercially attractive oxidation catalysts are likely to be based on cheaper, non-porphyrin ligand sets, Schiff-base ligands or purely O-based ligand sets as in zeolites or heteropolyacids; the latter have been considered as the oxidatively resistant, inorganic analogues of metalloporphyrins225. Oxygenation of saturated hydrocarbons via the systems under typical catalytic conditions (e.g. ~ 1 mM in Ru) is marginally detectable, and even this activity may be in part photo-initiated. Remarkably active for thermal oxygenation of saturated hydrocarbons are O-atom transfer systems based on an O-atom donor such as a pyridine oxide, and these probably operate via a catalysis cycle and again the Oatom donor may be a trans-ligand; in these systems, a precursor is not essential and Ru(porphyrin)-carbonyls and -dihalo complexes (even with the non-sterically hindered TPP) may be used. Total turnovers close to 20,000 have been seen for oxidation of adamantane. Commercial application of these systems is limited by formation of the stoichiometric co-product (e.g. the pyridine) with the oxygenated hydrocarbon, and their development rests on efficient regeneration of the O-atom donor via an oxidation process using preferably or In related systems using a chiral porphyrin, enantioselective hydroxylation of benzylic C-H bonds leads to secondary alcohols in up to 76% ee, while generation of chiral tertiary alcohols (with up to 54% ee) from racemic tertiary alkanes is noted for the first time. Attempts to improve stability (by reducing self-destruction) of the porphyrin catalysts for oxygenation of hydrocarbons, by incorporation of substituents such as halogens into the meso-phenyl substituents (the so-called 2nd generation catalysts) and then also into the pyrrole rings (3rd generation catalysts), has led to the findings that such halogenated species are very active for decomposition of trace hydroperoxides present in the systems, with the result that extremely active, and generally non-selective, free-radical oxidations are initiated within olefinic and saturated hydrocarbons. Again, the dioxo species is not a mandatory precursor - the nature of the axial ligands seems incidental. In

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63

retrospect, the systems become akin to some that have been known for decades, involving the use of simple, commercially available Ru compounds such as hydrated and Nevertheless, the studies have led to advances in the basic coordination chemistry of Ru-porphyrins, albeit it fairly exotic and utilizing expensive ligands. The catalytic oxidative-dehydrogenations of phenols, alcohols and amines are chemically and mechanistically complicated by the formation of water, the co-product; this can certainly react with the species, and can play a role as an axial ligand when conversions to coordinated hydroxo and oxo are possible. In the case of certain primary amine systems, the water can hydrolyze the imine product, in a process that might also be metal-catalyzed. Studies on the alcohol systems have led to novel alkoxide complexes, which are of interest in their own right, while the amine systems are of more interest (versus the net dehydrogenation catalysis) regarding better understanding of some reductase enzymes and the metabolism of natural amines; and again the discoveries of the oxo-imido, bis(amido), bis(imido), bis(imine) and mixed imine/amine complexes are significant contributions to coordination chemistry in general. The transfer of the imido (nitrene) group to alkenes and alkanes is analogous to O-atom transfer, and systems based on Ru-porphyrins are discussed. An emphasis throughout this Chapter (and indeed in much of the literature on homogeneous catalytic oxidations) is on selectivity in product formation using O-atom transfer mechanisms, and indeed this is a worthy aim. However, it should be noted that the major industrial homogeneous catalytic process (from an economic standpoint) currently in operation is the of p-xylene to terephthalic acid (the Mid-Century Amoco process), which is catalyzed by an inexpensive mix of Co and Mn acetates and bromides in acetic acid, the system operates at 225°C under 15 atm This is a radical-initiated process that has been in operation for over 30 years, and the details of the mechanistic pathways continue to be studied226. In essence, the system has been fine-tuned from an empirical point of view, and is a classic case of the practical industrial approach versus the basic academic approach: a mechanistically messy process but one that is commercially extremely important The investigations of Ru-porphyrins for catalytic oxidations, especially for hydrocarbon substrates, have necessarily impinged on aspects of the organometallic chemistry of these compounds, and this is mentioned occasionally in the Chapter. In a more general sense, the studies have contributed substantially to such organometallic chemistry which is now quite extensively developed49,l08,227,228. More generally, the development of catalyzed by metal complexes is perhaps at the stage that homogeneously catalyzed

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64

hydrogenations was 35 years ago; the mono- and dioxo- species, at least in a figurative sense, correspond to the transition metal mono- and dihydrides that began to proliferate in the 1960s. The extensive synthetic work, with concurrent kinetic and mechanistic studies on hydrogenation of unsaturated organics, has led in certain cases to a sophisticated understanding of the pathways in the catalysis, such that systems have been fine-tuned for chemo-, regio-, and enantioselective processes110,229,230. Corresponding advances in catalyzed oxidation reactions are emerging and findings gleaned from systems utilizing Ru-porphyrins will form a significant fraction of the required data-base.

5.

ABBREVIATIONS BHT: bromamine-T:

DMA: DMF: DPP: Hb: hist: Hpz (pz): Im: (3,4,5-MeO-TPP): Mb: MCM-41: m-CPBA: mCtPP:

MpIX: MPR:

2,6-ditertbutyl-4-methylphenol. N-bromo-p-toluenesulfonamide. 5,10,15-tris(4-Cl-phenyl)-20-(4-hydroxypheny l)porphyrinato. 5,10,15-tris(4-Cl-phenyl)-20-(4-Merrifield peptide resin phenyl)porphyrinato. 2,6-dichloropyridine-N-oxide. N,N-dimethylacetamide. N,N-dimethylformamide. 2,3,5,7,8,10,12,13,25,27,28,20-dodecaphenyl(porphyrinato). hemoglobin. histidine. general porphyrin. pyrazine (pyrazolato). imidazole. meso-5,10,15,20-tetrakis(3,4,5-(trimethoxyphenyl)porphyrinato (and see TPP). myoglobin. mesoporous silicous molecular sieves. meta-chloroperbenzoic acid. 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrinato. 5,10,15-tris(4-methylphenyl)-20-(4-Merrifield peptide resin phenyl)porphyrinato. mesoporphyrin IX. Merrifield’s peptide resin.

1. Catalytic oxidations using ruthenium porphyrins Ns: NTs: OEP: PpIX: porp:

r.t: Ru:

TBCPP: TBHP: TBPP: TCPP or (4-Cl-TPP): tCPP:

TDFPP:

TMP: TMCPP:

TPFPP: TPO: TPP: 3,5-Cl-TPP: TTP: tyr:

norbornyl sulfonate. imidotoluene sulfonate. 2,3,7,8,12,13,17,18-octaethyl(porphyrinato). protoporphyrin IX. dianion of general porphyrin. 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6, 7,8-octahydro-1,4,5,8-dimethanoanthracen-9yl]porphyrinato (see Fig. 10). see Fig. 10 room temperature. Ru(porp). trans-dioxo ruthenium porphyrin species. meso-5,10,15,20-tetra-(2,6-dichloro-4tertbutyl)phenyl(porphyrinato) (and see TPP). tert-butylhydroperoxide. meso-5,10,15,20-tetrakis(p-tert-butylphenyl)porphyrinato. meso-5,10,15,20-tetrakis(4-chlorophenyl)porphyrinato. 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato. meso-5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinato.

(porphyrinato) (and see OEP and TPP). meso-5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrinato. octachlorotetrakis(pentafluorophenyl)porphyrinato (and see OEP and TPP). meso-5,10,15,20-tetramesityl(porphyrinato). meso-5,10,15,20-tetrakis(2-chlorophenyl)porphyrinato. meso-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato. tryptophan 2,3-dioxygenase. meso-5,10,15,20-tetraphenyl(porphyrinato). meso-5,10,15,20-tetrakis(3,5-dichlorophenyl)porphyrinato. meso-5,10,15,20-tetra-p-tolyl(porphyrinato). tyrosine.

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66

6. 1.

2.

3. 4. 5.

6. 7.

8.

9.

10. 11.

12. 13. 14.

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Paulson, D.R.; Addison, A.W.; Dolphin, D.; James, B.R. J. Biol. Chem., 1979, 254, 7002. James, B.R. Stud. Surface Sci. Catal., 1991, 66, 195. Collman, J.P.; Barnes, C.E.; Brothers, P.J.; Collins, T.J.; Ozawa, T.; Gallucci, J.C.; Ibers, J.A. J. Am. Chem. Soc., 1984, 106, 5151. Barringer Jr., L.F.; Rillema, D.P.; Ham, J.H. J. Inorg. Biochem., 1984, 21, 195. Collman, J.P.; Brauman, J.I.; Fitzgerald, J.P.; Sparapany, J.W.; Ibers, J.A. J. Am. Chem. Soc., 1988, 110, 3486. Paeng, I.R.; Nakamoto, K. J. Am. Chem. Soc., 1990, 112, 3289. James, B.R.; Mikkelsen, S.R.; Leung, T.W.; Williams, G.H.; Wong, R. Inorg. Chim. Acta, 1984, 85, 209. Collman, J.P.; Barnes, C.E.; Sweptson, P.N.; Ibers, J.A. J. Am. Chem. Soc., 1994, 106, 3500. Camenzind, M.J.; James, B.R.; Dolphin, D. J. Chem. Soc., Chem. Commun., 1986, 1137. Groves, J.T.; Quinn, R. J. Am. Chem. Soc., 1985, 107, 5790. Groves, J.T.; Ann, K.-H. Inorg. Chem., 1987, 26, 3831. Sishta, C.; Ke, M.; James, B.R.; Dolphin, D. J. Chem. Soc., Chem. Commun., 1986, 787. Cheng, S.Y.S.; James, B.R. J. Mol. Catal., 1997, 117, 91. Groves, J.T.; Quinn, R. Inorg. Chem., 1984, 23, 3844. Cheng, S.Y.S. Ph.D. Dissertation, University of British Columbia, Vancouver, 1996. Leung, W.-H.; Che, C.-M.; Yeung, C.-H.; Poon, C.-K. Polyhedron, 1993, 12, 2331. Leung, W.-H.; Che, C.-M. J. Am. Chem. Soc., 1989, 111, 8812. Huang, J.-S.; Che, C.-M.; Poon, C.-K. J. Chem. Soc., Chem. Commun., 1992, 161. (a) Dolphin, D.; James, B.R.; Leung, T. Inorg. Chim. Acta, 1983, 79, 25. (b) Leung, T.W.; James, V.; Dolphin, D. ibid. 1983, 79, 180. Barley, M.; Becker, J.Y.; Domazetis, G.; Dolphin, D.; James, B.R. Can. J. Chem., 1983, 61, 2389. Sishta, C. Ph. D. Dissertation, University of British Columbia, Vancouver, 1990. Balch, A.L.; Latos-Grazynski, L.; Renner, M.W. J. Am. Chem. Soc., 1985, 107, 2983. Yamaguchi, K.; Watanabe, Y.; Morishima, I. J. Chem. Soc., Chem. Commun., 1992, 1721. Rajapakse, N.; James, B.R.; Dolphin, D. Stud. Surface Sci. Catal., 1990, 55, 109.

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76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86.

87. 88. 89. 90. 91. 92. 93. 94.

95. 96. 97. 98.

1. Catalytic oxidations using ruthenium porphyrins 99. 100.

101. 102. 103.

104. 105. 106. 107. 108. 109. 110.

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Chapter 2 Copper-dioxygen complexes and their roles in biomimetic oxidation reactions Christiana Xin Zhang, Hong-Chang Liang, Kristi J. Humphreys and Kenneth D. Karlin Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA

Abstract Copper-dioxygen interactions are essential from both industrial and biological perspectives. This review will focus on recent advances in copper-dioxygen complexes with regard to their relevance to bioinorganic chemistry. New, room-temperature stable copperperoxo compounds serve as models for the dioxygen transporting protein hemocyanin. Other copper-dioxygen complexes with formally-cleaved O-O bonds are also of interest as models for dioxygen activation. Interest in copper-dioxygen complexes which can oxidize organic compounds stems from their roles in biological substrate oxidation chemistry as well from their industrial utility. Copper-dioxygen complexes capable of hydroxylating arenes are reported which model tyrosinase, an enzyme which catalyzes the ortho-oxygenation of phenols. Synthetic copper compounds that catalyze the hydroxylation of aliphatic C-H bonds serve not only as models for industrial processes involving copper catalysts but also as mimics for copper monooxygenases such as dopamine peptidylglycine monooxygenase, and particulate methane monooxygenase. Copper-dioxygen compounds which oxidize benzylic C-H bonds as well as non-activated C-H bonds are reported, including recent studies where stereoselective hydroxylation occurs. Catechol oxidase models that catalyze the oxidation of o-diphenols to their corresponding o-quinones are also reported. Other copper oxidase models include mononuclear copper complexes which catalyze the two-electron oxidation of alcohols to aldehydes, coupled with the reduction of to through a Cu(II) phenoxyl-radical species, closely mimicking the mechanism of galactose oxidase. A dinuclear copper complex is reported which also catalyzes the aerobic oxidation of alcohols to the corresponding aldehydes and ketones. DNA as a substrate for oxidation by copper-phenanthroline compounds is overviewed. Key Words: bioinorganic chemistry, model compounds, dioxygen activation, binuclear copper, catalytic oxidation, alcohol and DNA oxidation

1. INTRODUCTION In this review, we provide a bioinorganic perspective in overviewing recent advances in the chemistry of copper-dioxygen [dioxygen; molecular 79 L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 79-121. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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oxygen, complexes. Copper ions are among the most common and useful industrial and synthetic oxidation catalysts as well as one of the most ubiquitous redox active metals utilized by biological systems.

1.1

Practical Copper Oxidative Processes

Industrially,1,2 copper has long been used as a catalyst in the Glaser process, which couples terminal acetylenes to give diacetylenes using cuprous chloride and molecular oxygen. This is a historically important (but now obsolete) catalytic process for producing the precursor to chloroprene, which is then used to produce neoprene rubber. Another important industrial process which utilizes copper/dioxygen chemistry is the oxidative coupling of 2,6-xylenol to form a para-phenylene oxide polymer by coupling an oxygen of one phenol molecule to the para carbon of another to form an aromatic polyether by the trade name PPO. This polyether is a high melting plastic that is extremely resistant to heat and to water, and it is useful as an engineering themoplastic. Another less common process also uses copper(I) chloride to catalyze the analogous oxidation of 2,6-diphenylphenol to form an even more rigid and higher melting material than PPO. An important industrial process that indirectly utilizes copper/dioxygen chemistry is the Wacker process which produces acetaldehyde from ethylene.3 In this process, both palladium chloride and cupric chloride are used. Although the copper catalyst is not directly involved in the oxidation of the ethylene substrate, it is crucial for catalyzing the re-oxidation of to Pd(II) to sustain the catalytic cycle. Synthetically, copper/dioxygen reactivity has been utilized in a similar fashion in the osmium-catalyzed dihydroxylation of olefins. Cupric is used to catalyze the re-oxidation of osmium from a formal oxidation state of +6 to +8. Other synthetic uses of copper/dioxygen catalysts include the oxidation of various substrates such as aniline, aromatic diamines, alcohols, and thiols.4

1.2

Copper in Biology

Copper ion is an essential trace element found in living systems, and its importance resides in its role as a protein or enzyme active-site constituent.5-9 Recently described chaperone 1 0 , 1 1 proteins and 7 metallothioneins aid the cell in the copper ion trafficking, and thus copper ion homeostasis. But the major role of copper proteins involves oxidationreduction (i.e., ‘redox’) activity. Donors for copper ion complexes which are typically available in protein matrices include the side-chain imidazole group of histidine (His), the phenol oxygen donor of tyrosine, or the sulfur atom of the thiol group from cysteine (Cys). With these ligands, the Cu(I) and Cu(II) oxidation states are readily accessible and interconvertible under

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physiological conditions, using available oxidants (e.g., dioxygen) or reducing agents such as glutathione or ascorbic acid (vitamin C). Protein active-site copper ions perform the functions of electron transfer (e.g. as electron carriers in photosynthetic organisms or in respiratory pathways in certain bacteria),6 reversible and transport,12 mono- or dioxygenation of organic substrates [oxygenases, incorporating one or both atoms of or oxidation-dehydrogenation of substrates accompanied by to either hydrogen peroxide or water [oxidases]. 12 Comparable functions are carried out by heme and non-heme iron enzymes. Copper mediated oxidative processes ‘gone wrong’ have been discussed,13-15 including in possible connection with aging, “Lou Gehrig’s disease” (Cu/Zn superoxide dismutase mutation), Alzheimer’s disease, and others. Separate copper proteins participate in the biological nitrogen cycle, catalyzing the reduction of nitrite and nitrous oxide.16,17 Copper nitrite reductases (i.e., possess a active site which binds substrate; this center receives electrons which are transferred from a nearby ‘type 1’ ‘blue’ copper center with ligation (Met, methionine).16 A very recent protein x-ray structural study reveals a tetranuclear cluster with His ligation (so called ) as the active site which effects the conversion, after receiving reducing equivalents passed on by the binuclear electron-transfer center (with ligation).18-20 The major classes of copper proteins involved in are given in Table 1. Some details of the active sites and chemistry of proteins most relevant to the chemistry described in this article is provided below. 1.2.1

Hemocyanin, Tyrosinase, and Catechol Oxidase. Dioxygen transporting hemocyanins occur in the hemolymph of mollusks and arthropods. Dioxygen binding occurs with a reaction stoichiometry, formally an oxidative addition reaction, with ligation in the ‘oxy’ form occurring in a side-on peroxo fashion, as indicated in Scheme 1.12,21,22

An x-ray structure is not available for the monooxygenase tyrosinase, but spectroscopic and biochemical insights reveal its active site to be very similar to the binuclear site in hemocyanins.12 The dioxygen-adduct

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intermediate responsible for ortho-oxygenation of phenols is thus nearly identical to oxy-hemocyanin. A major difference in the proteins is that the

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protected buried active site in oxy-hemocyanin is instead accessible to phenolic substrates in tyrosinase. Detailed insight into how the peroxo moiety transfers to phenol substrate still needs to be resolved; recent experiments on model systems and the protein suggest that tyrosinase may effect a direct phenol-to-quinone conversion rather than phenol to o-catechol transformation. This is discussed further below. Catechol oxidases, ubiquitous plant enzymes containing binuclear copper centers, catalyze the oxidation of o-diphenols to the corresponding oquinones. The resulting highly reactive o-quinones can undergo autopolymerization to form brown polyphenolic catechol melanins, which protect the damaged plant from pathogens or insects. A recent x-ray structure23 of an oxidized catechol oxidase from sweet potato shows that in the active site each copper ion is coordinated by three histidine residues and the two copper ions are 2.9 Å apart with a bridging solvent molecule, most likely a hydroxide ion. Upon reduction the two copper ions move further apart with a metal-metal distance of 4.4 Å, while no significant conformational change around the copper centers is observed. In the reduced state, has a distorted trigonal pyramidal geometry with three coordinated histidine residues and a water molecule, while engages in a square planar geometry with one missing coordination site. Based on biochemical, spectroscopic and structural data, the proposed catalytic mechanism (Scheme 2),23 similar to that delineated for the catecholase activity observed for tyrosinase12,24 (and even certain

hemocyanins)25,26 involves simultaneous binding of dioxygen and catechol substrate (CAT) to the reduced enzyme to form catechol

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complex, in which the dioxygen binds in a side-on mode with a distance of 3.8 Å. Electron transfer from the coordinating catecholate to the peroxide followed by protonation of the peroxide group and O-O bond cleavage yields o-quinone and the hydroxide-bridged dicupric state of the enzyme. The oxidized enzyme then regenerates itself to the dicuprous form by oxidizing another molecule of catechol to o-quinone. 1.2.2

Amine Oxidases; Galactose Oxidase Amine oxidases catalyze oxidative deamination reactions involving functions such as the crosslinking of collagen and elastin and the regulation of blood plasma biogenic amines.27,28 A carbonyl-containing cofactor has long been known to be present, identified by Klinman and co-workers as topaquinone (TPQ) (B).29 Recent protein x-ray crystal structures30-33 confirm this conclusion, while also verifying the spectroscopically deduced coordination environment (A) with 3 histidine ligands. The TPQ lies close to the copper ion but is not coordinated, and it is shown that this group is conformationally flexible. The enzyme reaction can be divided into two halfreactions: a reductive half which involves initial Schiff-base formation of amine substrate with quinone, and an oxidative half which requires copper ion and dioxygen to recycle the reduced enzyme back to its oxidized resting state, as well as release of ammonia and hydrogen peroxide. The role of copper is to facilitate reoxidation of the reduced cofactor by (after reactions with substrate); a Cu(I) semiquinone intermediate (in equilibrium with Cu(II)quinone) has been detected and is presumed to interact directly with (also see below).28

Another aspect of considerable interest is the self-processing of TPQ, known to be derived from a protein tyrosine precursor. Copper ion is implicated in this cofactor biogenesis and the copper-dioxygen chemistry involved is discussed below. Recently, Knowles and co-workers34 using flash-freezing techniques obtained x-ray crystal structures of several species related to the oxidative half reaction of the amine oxidase from Escherichia coli. The structure of the anaerobically β-phenylethylamine-reduced amine oxidase shows that the reduced cofactor TPQ exists in the aminoquinol form possessing a hydrogen bond between the O-2 of the aminoquinol and axial water that coordinates to the copper ion, while the product phenylacetaldehyde remains bound at the

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back of the substrate-binding pocket. Upon aerobic exposure to excess phenylethyleneamine, the structure of the intermediate reveals dioxygen replacing the axial water ligand to the copper ion while the product phenylactaldehyde remains bound at the back of the active site. The kinetic studies of the oxidative half-reaction for bovine serum amine oxidase suggest that the rate-limiting electron transfer to is possibly from aminoquinol. A single electron transfer results in the formation of superoxide and semiquinol, which is absent based on the visible spectrum, suggesting formation of the hydrogen peroxide species through the second electron transfer from the cofactor. The crystal structure shows that the two oxygen atoms of are 2.8 and 3.0 Å from the copper ion and the Cu-O-O angle is 88 °, which the authors state is consistent with a peroxide, or perhaps hydroperoxide species. Galactose oxidase is an extracellular fungal enzyme, catalyzing the stereospecific two-electron oxidation of D-galactose and other primary alcohols. Prior to any knowledge from a protein x-ray structure, Whittaker

and co-workers35,36 deduced that a stabilized ligand-protein radical-cation was involved, which explained a long-standing puzzle of how a single active site copper ion could effect a two-electron process. The x-ray structure of the protein37 revealed a novel protein co-factor consisting of a thioether bond linking a cysteine to the ortho-position of a copper-coordinated tyrosine ligand, with the latter also undergoing a interaction with a tryptophan residue. A recent report indicates that the formation of the

Cys-Tyr linked active-site cofactor is a self-processing reaction, i.e., mediated by copper-dioxygen reactivity occuring at the active site of the pro-enzyme following the binding of copper(II) ion.38

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The reaction to oxidize alcohols (eqs. 1 & 2) can be thought of as proceeding by oxidation of a reduced protein and copper(I) center by dioxygen, in a two-electron process, to give oxidized copper(II) and proteinligand radical cation The “stored” oxidizing equivalents can then oxidize a coordinated alcohol substrate, producing aldehyde and restoring the reduced enzyme center. Proton transfers from substrate to dioxygen (as product peroxide) are suggested to be mediated via protonation-deprotonation of the coordinated tyrosinate/tyrosine ligands, which are more likely to be protonated and not coordinated when the copper ion is reduced.

Cytochrome c Oxidases These are ubiquitous proteins that occur in anaerobic organisms and are part of a superfamily of heme-copper oxidases which function as proton pumps; the proton and charge gradient thus produced is utilized for ATP synthesis by ATPase enzymes.39,40 A heme-copper binuclear active site functions to bind and receive electrons and protons in the four-electron, four-proton reduction of to water. A copper-dioxygen 1:1 initial adduct is implicated39,41 in the earliest stage of the protein reaction of the active site. An subsequent intermediate has been considered in the past, but more recent studies abate the importance of such a species, suggesting that an iron-bound receives electrons from the heme, and a modified active site tyrosine (Y) phenol, directly cleaving the O-O bond and forming a ferryl and (plus radical) ‘P’ intermediate.42,43 X-ray structures are now available for various states of three cytochrome oxidases.44-47 In one case, a protein form with peroxo species bridging the heme and copper ion is claimed.44 Further details can be found in the references cited. 1.2.3

2.

COPPER-DIOXYGEN ADDUCTS

As mentioned above, there are several cases of protein dioxygencopper adducts which have been characterized only by spectroscopy (i.e., tyrosinase)12 and/or protein x-ray crystallography. The latter cases include oxyhemocyanins, 21,22 and lower resolution examples are reported for amine oxidase34 and cytochrome c oxidase.44 Far more detailed structures are now available for small molecule synthetic complexes, as described below. There are four principal binding modes previously established for the interaction of transition metal complexes with dioxygen (Chart 1), and all are suggested or proven for copper. These include end-on binding, as found in

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hemoglobin, and side-on binding, as in hemocyanin. Both may be combined with stoichiometry, leading to a superoxoand copper(II) and peroxo-dicopper(II) products, respectively. In addition, in copper-dioxygen chemistry, a stoichiometry can or lead to complexes with a dicopper or tricopper core containing copper(III), respectively, as described below. Peroxo-tetracopper species are now also known (see below), although it is unclear if they are directly formed in reactions.

2.1

Copper-Dioxygen Complex Generation; 1984-1999

The first case of reversible dioxygen (and CO) binding to copper, where the reaction product was also proven to possess an intact O-O bond, involves a dicopper complex with bridging phenoxide ligand. The x-ray structurally characterized dicopper(I) complex reversibly reacts with one mole of at –80 ºC to form the adduct, 48 This species is a peroxo-dicopper(II) complex as determined from resonance Raman and X-ray absorption 49-51 spectroscopic studies. Structural insights from Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy (Cu...Cu = 3.31 Å) and a mixed isotope resonance Raman experiment50 suggest the peroxo ligand is terminally bound (possessing inequivalent O-atoms), consistent with the presence of two peroxo-to-copper(II) charge transfer bands, assigned as (610 nm) transitions. serves as a (505 nm) and prototype for terminally bound peroxo-Cu complexes.12,52 The kinetics and

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thermodynamics of dioxygen binding to were also studied by stopped-flow techniques; rapid dioxygen binding occurs (183 K), with (183 K), and 53,54

The first x-ray crystallographic description for a copper-dioxygen adduct, synthetic or protein, came with our characterization of the 2:1 adduct (1), with counter-anions, formed by or reversible oxygenation of copper(I) precursor complex (1a, R = Me, Et, TMPA = tris[2-pyridylmethyl]amine) (Chart 2).55,56 The dication 1 is best described as a peroxo-dicopper(II) species. It is essentially diamagnetic, possessing a moiety with and O-O = 1.432 Å. Other relevant physical properties are (resonance Raman), and UV-vis (i.e., peroxo-to-Cu(II) LMCT) bands at 440 nm (11,500), ~ 600 nm (sh, ~ 7600) and a d-d band at 1035 nm (180).57 Kinetics studies54,58 reveal that (1a) reacts with to give a transient (at -80°C, 410 nm) 1:1 superoxo-copper(II) adduct, (1b), with

suggested end-on coordination (Chart 1). The full kinetics and thermodynamics of these reactions, and those with complexes with ligands TMPAE, BPQA, BQPA, plus binucleating analogues D1 and DO (Chart 3), have been elucidated and described.54,58-60 We have shown that ligand DO confers room temperature stability to the resulting dioxygen adduct 60 Reedijk and co-workers61 have also been able to obtain similar room-temperature stability for a adduct with a macrocyclic binucleating ligand, which possesses pyrazolyl and alkylamine donors. The

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resulting complex has physical and spectroscopic properties very similar to those of 1.

Kitajima and co-workers62,63 used a sterically hindered tridentate ligand in characterizing (2, Chart 2) (HB(3,5= hydrotris(3,5-diisopropyl-pyrazolyl)borate anion), which has a sideon ligated peroxodicopper(II) structure, with physical properties (i.e., Cu…Cu = 3.56 Å, O-O = 1.412 Å, (resonance Raman), 551 nm (790)) closely matching those of Limulus polyphemus oxy-hemocyanin. This important contribution to synthetic modeling of metalloproteins in fact preceded confirmation of the

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protein side-on structure. Subsequently, 6 4 a side-on superoxocopper(II) complex has been generated and structurally characterized by the same group; (3) has (resonance Raman). (2,330), O-O = 1.22 (3) Å and Certain other tridentate ligands with nitrogen donors were also found to give copper(I) complexes which bind to give side-on peroxo dicopper(II) products having intense UV (345-365 nm) charge-transfer bands like those found in oxy-hemocyanin and oxy-tyrosinase. 9,62,65-77 Detailed electronic structural studies indicate this is characteristic of this binding mode and is accompanied by the low v(O-O) values due to Cu(II)-to-peroxo electron donation to the peroxide O-O orbital.78

Based on a low-temperature EXAFS studies, we first76 proposed this structure for binuclear complexes containing linked PY2 chelates (PY2 = bis[2-(2-pyridyl)ethyl]amine), i.e., ligands Nn (see diagram).

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Dioxygen (and CO) react reversibly with dicopper(I) complexes producing (PY = 2-pyridyl) (or which are stable at –80 °C in solution.77 A recent resonance Raman study confirms that possess side-on ligation, and the variation in UV-Vis (i.e., shift in a 400-500 nm CT absorption) and values (765, 751, for N3, N4 and N5 complexes, respectively) are ascribed to distortions of the moiety from planarity, probably due to ligand constraints.79 Mononucleating ligands RPY2 (R = Me, Ph, ) can also be utilized to generate similar complexes, where for example 65-67 reacts with giving In fact, this exists as a mixture of both side-on peroxo and species, both in solution and in the solid state.67 Further discussion of this interconversion or equilibrium is given below. It is notable that the value for the peroxodicopper(II) solution species, , is lower than observed for The kinetics of formation of these dioxygen adducts have recently been compared;80 all Nn species show relatively similar tendencies, but the bridged peroxo adducts form expectedly faster for the intramolecular process in ligands, compared to the intermolecular reaction for A special case of these ligands and their chemistry occurs when the linker is a m-xylyl moiety (in XYL–H; diagram), leading to a monooxygenase model system, discussed below. Others have shown that species also form with tris(1-R-4-R’-imidazolyl)phosphines 69-71 or trisubstituted triazacyclononane (TACN) ligands. For the latter case, Tolman and coworkers72-75 discovered important new chemistry, Scheme 3. Oxygenation of 4a can give both adducts 5b or 5 (Scheme 3 & Chart 2). For R = iPr in solvent at low temperatures, 5b

is formed reversibly. When the oxygenation is carried out in tetrahydrofuran solvent, 5 is obtained. Its properties include UV-Vis features at 324 and 448 nm, and The (580 using

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equilibrium between side-on peroxo 5b and dicopper compound 5 is sensitive to the nature of the counter-anion and solvent mixture. Species 5a and 5 have been shown to be interconvertible, an important finding, since the reaction represents a unique O-O bond forming reaction.81 The structure of 5 (Chart 2, R = benzyl) reveals O...O = 2.287 Å, with a short value of Cu...Cu = 2.794 Å. Complex 5 is best described as a dicopper(III) species.82 The relationship of this structure to side-on species has been studied.83 84,85 Stack and co-workers have also demonstrated O-O bond breaking reactions in copper(I)-dioxygen reactivity studies utilizing ligands possessing the simple ethylenediamine core. Three-coordinate copper(I) complexes, (6a, L = (1R,2R)- cyclohexanediamine), react with dioxygen at -80 °C in to afford diamagnetic species (6). An x-ray structure of one derivative (Chart 2; reveals a squareplanar core surrounding copper, with very short Cu-O bonds (Ave. = 1.81 Å), Cu...Cu = 2.743 (1) Å, and O...O = 2.344(1) Å. Cu absorption Kedge data corroborate the +3 copper oxidation state in these complexes.86 The same research group also discovered a significant new reaction stoichiometry.87 reacts with to give a trinuclear cluster compound (7) with two ligands (O...O = 2.37 Å) and three copper ions. An x-ray structure (Chart 2) reveals one unique Cu atom, with short Cu-O bond distances (~ 1.83 Å, compared to the distances of ~ 2.00 ave. Å). X-ray absorption measurements and comparisons86 indicate this is a copper(III) ion, affirming the overall reaction to be a four-electron reduction and O-O cleavage of dioxygen. A novel peroxocopper complex with is formed when copper(II) perchlorate is reacted with tridentate ligand 4-methyl-2,6bis(pyrrolidinomethyl)phenol (HL) in methanol solution, in the presence of air, triethylamine base and 3,5-di-tert-butylcatechol.88 The four copper(II) ions in (8, Chart 2) form a near-planar rectangle, with alternate methoxo (Cu...Cu = 3.03 Å) and phenolato (Cu...Cu = 2.99 Å) bridging. Weaker axial ligation is completed by bridging perchlorato oxygen atoms. The peroxide ligand has a typical O-O bond length of 1.453(4) Å. The nature of any chemistry in the generation of 8 is not clear. A mononuclear copper(II)-hydroperoxo complex was characterized by Masuda and co-workers,89 using a ligand similar to TMPA, bis(6pivalamine-2-pyridylmethyl)-2-pyridylmethyl)amine (complex 4; Chart 2). The compound forms from reaction of with a copper(II) complex of this ligands and N-H hydrogen bonding provides stabilization of the bound ligand.

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2.2

93

Recent Further Advances in Copper-Dioxygen Complex Generation

Biomimetic copper-dioxygen chemistry has advanced considerably since the first structurally-characterized copper-dioxgygen adduct. However, it has been difficult to simulate the room-temperature stability of hemocyanin in these model complexes due to the fact that unlike the enzyme active sites, these models usually do not possess protective environments which can help stabilize potentially reactive copper-dioxygen species. Recently, two roomtemperature stable copper-dioxygen complexes have been synthesized which come closer to the goal of mimicking the dioxygen carrier hemocyanin. Using 1,2-bis[2-(bis(6-methyl-2-pyridyl)methyl)-6-pyridyl]ethane (Scheme 4), a ligand with two sterically hindered tripyridylmethane units tethered by an ethylene spacer, Kodera and co-workers90 were able to reversibly generate a peroxo complex with a half-life of 25.5 h at 25 °C in dichloromethane. The peroxo compound (Scheme 4) which has been characterized by x-ray crystal analysis, can be synthesized either by the reaction of the copper(I) precursor with or by treating the complex with As the crystal structure shows, the core of sits in a protective environment surrounded by the methyl moieties of the pyridine rings and by the ethylene linker (Scheme 4). This may explain in part the stability of the peroxo compound

Gorun and co-workers91 also reported on the synthesis of a roomtemperature stable copper-dioxygen adduct, by replacing C-H bonds in the vicinity of the core with C-F bonds. The ligand, 3trifluoromethyl-5-methyl-1-pyrazolyl borate is analogous to the

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non-fluorinated substituted pyrazolyl borate systems originally studied by Kitajima and co-workers. The UV-vis spectrum of the is similar to that of other crystallographically-characterized peroxo species, with absorptions at 334 and 510 nm, and resonance Raman studies have confirmed its peroxo-binding mode. Compound is stable indefinitely at 25 °C, probably due to the fact that the dicopper(II)peroxo center is protected by robust trifluoromethyl groups. Unfortunately, has not yet been characterized crystallographically.

Besides the two room-temperature stable dicopper(II) peroxo compounds mentioned above, a new dicopper(III) complex has recently been structurally-characterized by Suzuki and co-workers.92 Although we have previously reported on the partial formation of a dicopper(III) compound with two pyridyl side arms,66 no dicopper(III) complex containing aromatic nitrogen donors had ever been structurally-characterized until the work of Suzuki and co-workers. They showed that by introducing two 6-methylpyridyl groups into the TMPA ligand to form bis-(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine, a dicopper(III) complex is formed instead of a trans peroxo)dicopper(II) complex generated from TMPA (see Sect. 2.1). This complex can be reversibly converted between the copper-dioxygen adduct and the copper(I) precursor, and it shows an intense absorption band at 378 nm ~19,000 ) in dichloromethane, which is higher in energy than other known complexes. This formulation has been confirmed by both resonance Raman spectroscopy (with an isotope-sensitive band at 590 ) and by x-ray crystallography. The distance of 2.758 Å is comparable to other structurally characterized dicopper(III) complexes. The distances between the copper centers and the nitrogens of the 6-methylpyridyl groups are 2.55 and 2.48 Å, much longer than the other two copper nitrogen distances of 1.97 and 1.91 Å. In this way, the formally copper(III) centers can adopt a square-planar geometry favored by d8 metal centers and converts the traditionally tetradentate, tripodal TMPA ligand to a quasi-bidentate ligand capable of supporting a dicopper(III) core. It should be noted that in the copper(I) oxidation state

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the ligand is thought to be a tetradentate ligand with a trigonal pyramidal structure (i.e., with the two 6-methylpyridyl groups binding strongly to the copper center).

Hofmann and co-workers recently reported on a new dicopper complex formed by treating a neutral copper(I) ethylene complex containing the anionic iminophosphanamide ligand with dioxygen at –25 °C.148 This dicopper complex is the first reported neutral dicopper complex, and it has a UV-vis absorption maximum at 444 nm which is characteristic of other reported dicopper complexes (~385–455 nm). X–ray characterization of shows that its Cu–Cu distance of 2.906(1) Å is significantly longer than that of other structurally–characterized dicopper compounds, which have been in the range of 2.743–2.794 Å.84

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In addition to forming dicopper(III) species with the bidentate ethylene diamine ligands, Stack and co-workers have also shown that certain of these systems can form dicopper side-on peroxo complexes which are interconvertible and in equilibrium with their dicopper(III) isomer.149 Generally, the rates of formation for dicopper side-on peroxo complexes and for their isomeric counterparts are equal, as are their thermal decomposition rates, presumably due to the fact that the rate of isomer interconversion is faster than their formation or decomposition rates. This has made it difficult to study the differential reactivity between side-on peroxo and complexes. Recently, however, they established a system which exhibits slower isomer interconversion rates in the solvent 2-methyl tetrahydrofuran (2-MeTHF), such that the decay of these species occur at measurably different rates (see diagram). This has made it possible to study differences in the reactivities of the side-on peroxo and complexes towards substrates such as PPh3 or 2,4-di-tertbutylphenol. Preliminary results indicate that the side-on peroxo isomer decays more quickly with addition of triphenylphospine, indicating that it is a better oxygen-atom transfer agent than the isomer. Because of the differences in core structures between the isomers (CuCu distance in dicopper side-on peroxo complexes ~ 3.6 Å vs. ~2.7 Å in compounds), accessibility of the substrate is thought to be a key factor in the observed differential reactivities of the two isomers towards PPh3. When the mixture of isomers are treated with 2,4-di-tertbutylphenol, the side–on peroxo complex also decays more quickly than the form. However, the formation of the expected oxidative coupling product 3,3’,5,5’-tetra-tert-butyl 2,2’-biphenol does not track with the decay of the side-on peroxo. Therefore, the authors conclude that the side–on peroxo isomer decays via the displacement of the peroxide by the phenol to form a phenoxide–bound Cu(II) complex instead of via oxidation of the phenol substrate. Conversely, although slower than the decay of the peroxo isomer, the degradation of the form does correlate with the oxidative coupling of the phenol. Therefore, the authors conclude

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that experimentally, the dicopper side-on peroxo isomer is a better oxygen–atom transfer reagent while the isomer is a better hydrogen-atom acceptor. Recently, Meyer and Pritzkow9 3 reported on the X-ray crystallographic characterization of a novel complex using a multidentate pyrazolate ligand by treating the pyrazolate ligand with 2 equivalents of and excess and allowing the product to crystallize. The tetranuclear complex with a central peroxo ligand has four copper(II) centers in Jahn-Teller-distorted square-pyramidal environments, and the O-O bond distance of 1.497(5) Å is within the range for known peroxo structures. However, it is the longest O-O bond distance reported to date on structurally characterized copper-peroxo complexes. No resonance Raman data was reported for this peroxo compound.

3.

COPPER OXYGENASE CHEMISTRY

3.1

Aromatic Hydroxylation

The enzyme tyrosinase functions by utilizing a complex to hydroxylate an arene. In a model system for copper hydroxylases from our own laboratories, dicopper(I) complex reacts with (reversibly), giving an intermediate possessing a bridged side-on bound ligand which transforms to the xylyl-ligand hydroxylated dicopper(II) complex (Scheme 5). Thus, an unactivated arene (XYL-H) is converted to the phenol XYL-OH. Detailed mechanistic studies indicate that the hydroxylatton reaction occurs by electrophilic attack of the xylyl substrate by a side-on bound peroxo group (in As in an enzyme active site, the XYL-H substrate is held in ideal proximity to the reacting species. Although stoichiometric, this monooxygenase model remains as one of very few chemical systems where an unactivated C-H bond substrate is rapidly hydroxylated under very mild conditions using dioxygen (-80 °C; 9,94-98

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Tolman and co-workers99 have also synthesized a model system which undergoes endogenous aromatic hydroxylation using an amine and a pyridine ligand tethered to an arene. (Scheme 6) Unlike our XYL-H system, oxygenation of copper(I)-complex forms a complex instead of a peroxodicopper(II) complex. Upon decomposition, the complex forms an unidentified copper(II) product which yields both the hydroxylated and the original ligand in a ~2:1 ratio, which corresponds to ~60% yield in monoxygenase activity. Previous to this study, the Tolman group had also reported100 on complexes similar to the XYL-H model system which employ dinucleating triazacyclononane moieties linked by m- or p-xylyl groups. These systems form either complexes or complexes, depending on conditions such as solvent, temperature, and concentration. Based on this report, the complex leads to arene hydroxylation, similar to the XYL-H system, whereas the complex favors monooxygenation leading to Ndealkylation.

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While these studies show that both the and the cores are capable of hydroxylating arenes in model systems, the question still remains of whether the enzyme tyrosinase hydroxylates its substrates directly with its observed core or if O-O bond breaking occurs first before C-H bond activation.

3.2

Recent Tyrosinase Models

Casella and co-workers 101,102 have recently reported on catechol oxidase and phenol hydroxylase activities of a series of binuclear copper complexes with benzimidazole-containing ligands (Chart 4). These model compounds catalyze the oxidation of 3,5-di-tert-butylcatecholn (DTBC) to 3,5-di-tert-butylquinone as well as the ortho hydroxylation of methyl 4hydroxybenzoate to give methyl 3,4-dihydroxybenzoate. The catalytic oxidations of DTBC are biphasic; the first phase is a fast stoichiometric step involving electron transfer from the bridging catechol anion to the dicopper(II) centers, which is affected by the reduction potential of the Cu(II)/Cu(I) couple. Among the four dicopper(II) complexes, has the highest activity due to its high redox potential. The second phase involves the oxygenation of the dicopper(I) species, binding of the catechol to the copper-dioxygen intermediate and the electron transfer between the catechol anion and the dioxygen moiety. Various factors can affect the efficiency of this second phase reaction. For the case, the slow oxygenation reaction is the rate-determining step, therefore limiting the efficiency of this complex. Contrarily, the reactions of dioxygen with dicopper(I) complexes of L-55 and EBA are very fast, thus the binding of the catechol and the subsequent electron transfer reaction become the rate-

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determining steps. The authors proposed that the higher efficiencies of the and complexes are due to the small size of the chelate rings, which make the more reactive and facilitate binding of the substrate by displacing the weak axial ligand. Phenol hydroxlase activity of these complexes have also been studied. With methyl-4hydroxybenzoate as a substrate, appreciable quantities of catecholate product

are obtained from the ortho-hydroxylation reactions of phenol mediated by the dicopper(I) complexes of L-55 and L-66, while warming or longer

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reaction times lead to Michael adducts as the major product, claimed to arise from a slower oxidative coupling of phenol and catechol. Recently, the authors report that under the same reaction conditions, the oxygenation of methyl-4-hydroxybenzoate mediated by the dicopper(I) complex gives only catechol, with no trace of Michael adduct, even at room temperature. Only when the reaction times are on the order of 20 h is the formation of the ADDUCT observed with the depletion of catechol, which remains as the major product. A controversy exists, however, pertaining to the identity of the primary reaction product and course of reaction. Sayre and Nadkarni103 also carried out experiments using but they determined that the ADDUCT forms as a Michael addition product of phenol and 4carbomethoxy-1,2-benzoquinone, nothing that 4-carbomethoxy-1,2-catechol is inert to oxidation under the conditions employed. Thus, Sayre concludes that the benzoquinone is the direct product of oxygenation in the tyrosinase like reaction, Such a reaction thus proceeds like a dioxygenase, regenerating catalyst and not requiring external reductant. It is notable that Kitajima62 previously suggested a similar pathway and mechanism for tyrosinase reaction, based on observed reaction tendencies of phenol substrates with his own complex. In fact, very recent investigations104, 105 suggest that in the enzyme itself, catechols are produced in an indirect route not involving enzymatic reactions. These workers uphold the view that tyrosinase does not act as a tyrosinase hydroxylase, but only produces o-quinones either from mono- or diphenol substrates. If correct, this may require rethinking of the detailed mode of dioxygen activiation in binuclear complexes, at least with respect to phenolic substrates. Other reviews cover earlier tyrosinase models and proposed mechanisms.9,106

3.3

TPQ Biogenesis

A carbonyl-containing cofactor has been long known to exist in amine oxidases, identified by Klinman and co-workers as topaquinone (TPQ).27 This cofactor derives from a protein tyrosine precursor and it is thought that a copper(II) ion is required for its generation to TPQ. A mechanism for the TPQ biogenesis has been proposed28,107 involving the initial formation of the [Cu(I)-Tyr] radical species and binding to copper ion yielding a copper(II)-superoxide species which subsequently oxidizes the phenol of the cofactor to o-quinone. Conjugate addition of water to the o-quinone mediated by copper(II) ion forms the reduced triol form of the TPQ followed by aerobic oxidation to TPQ (Scheme 7). A recent study108 on catechol autoxidation reported by Sayre and coworkers demonstrated, however, that conjugate addition of water to o-quinone

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in basic aqueous media as a model for the biogenesis of the cofactor TPQ is chemically unfavorable. Upon treatment of 4-alkylcatechol to aqueous base, hydroxyquinone is generated but with variable yield, while starting with oquinone the yield of hydroxyquinone is lower than that starting with catechol.

This indicates that the reaction sequence proposed for biogenesis of TPQ cofactor of copper amine oxidase is not applicable in this simple chemical system. The catalase effect is shown to imply the involvement of in the transformation of catechol to hydroxyquinone under model conditions. In addition, no corresponding triol is observed when o-quinone is treated with aqueous base anaerobically with or without added Cu(II), which indicates that conjugate addition of water to o-quinone does not occur under this solution chemistry condition. The authors rationalize that the enzyme performs the conjugate addition of water to o-quinone yielding hydroxyquinone TPQ by taking advantage of tri-histidine-bound copper(II) ion in the active site with

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special proximity or stereoelectronic features that are absent in the solution model chemistry.

Overview. The chemistry described in Scheme 5 (XYL system) may best model the hydroxylation function of tyrosinase with its dicopper active site. Other very different copper monooxygenases include dopamine and peptidylglycine monooxygenase) (PAM) (Table 1), which are important pharmacological targets due to their roles in regulating neurotransmitters and polypeptide hormones. Models for benzylic hydroxylation chemistry in (see below) and oxidative Ndealkylation chemistry in PAM9 have appeared. From the interest in C-H activation chemistry, an important target for future chemical modeling is the copper-dependent particulate methane monooxygenase

3.4

Aliphatic Hydroxylation

Synthetic copper complexes which can activate dioxygen and hydroxylate aliphatic C-H bonds are of great importance, both for shedding light on common industrial processes which employ copper catalysts and also as models for the copper monooxygenases mentioned above, such as PAM, and pMMO. One such system is the one reported by Itoh and co-workers,109 which employs a tridentate ligand, N,N -bis[2-(2-pyridyl)ethyl]-2-phenylethylamine (PY2Phe), similar to N,N -bis[2-(2-pyridyl)ethyl]-benzylamine (PY2Bz), a ligand that was previously studied in our laboratories.65 When the mononuclear Cu(II) complex was treated with one equivalent of a two electron reductant (a 1,2-enediolate derived from benzoin and triethylamine) under Ar, then exposed to for several hours, the dinuclear copper(II) compound with bridging alkoxide ligands forms, demonstrating that benzylic hydroxylation of the ligand occurs. Removal of the copper from the ligand confirms that the yield of the ligand hydroxylation was 100%. When the well-characterized copper(I) complex was treated with the isolated yield of the hydroxylated ligand was the maximum 50% (Scheme 8). Although the only copper-dioxygen adduct detected in these reactions is a species, the authors

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propose that the ligand hydroxylation involves intramolecular C-H bond activation by an unobserved intermediate, based on the results of mechanistic studies.

More recently, Itoh and co-workers110 reported on similar benzylic hydroxylation chemistry using an analogous ligand, N-ethyl-N-[2-(2pyridyl)ethyl]-2-phenylethylamine. This system employs a bidentate ligand instead of the tridentate PY2Phe, which forms the instead of a intermediate, presumably because bidentate ligands help stabilize the formally Cu(III) centers of the core due to the fact that transition metal compounds prefer

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square planar geometry. When the compound formed from the reaction of the Cu(I) complex with at –78 °C, is warmed to 25 °C and kept under an atmosphere of for 20 h, benzylic hydroxylation of the phenylethyl group occurs in 46% yield, close to that of the maximum 50% yield observed in the PY2Phe system (Scheme 9). The authors suggest that the ligand hydroxylation may occur via the abstraction of a benzylic hydrogen atom by the core followed by the rebinding of a hydroxyl group (rebound mechanism) or through a concerted mechanism.

Using RPY2 ligands similar to our PY2Bz, Reglier and coworkers111,112 have reported on model systems that can hydroxylate aliphatic C-H bonds in a stereospecific manner. While one of these systems, IndPY2, contains an indane moiety with a benzylic C-H bond, two other systems, nPrPY2 and cPtPY2, have non-activated C-H bonds (with n-propyl and cyclopentyl moieties, respectively) which are hydroxylated stereospecifically. Similar to Itoh’s PY2Phe system, hydroxylation of the benzylic position in IndPY2 can either take place via reaction of the copper(I) complex with for ~50% hydroxylation yield or by reducing the copper(II) complex with benzoin/triethylamine under Ar and then exposing the unidentified reduced species to for ~100% hydroxylation. The stereoselective hydroxylation of the cyclopentyl and the n-propyl moieties in nPrPY2 and cPtPY2 are more complicated, however. For instance, when the Cu(I) complexes and react with andthe resulting copper(II) complexes were demetallated, the starting ligands nPrPY2 and cPtPY2 were recovered with no products resulting from oxygenation of the ligand. Only treatment of Cu(II) compounds and

106

with products.

C. X. Zhang, H.-C. Liang, K. J. Humphreys and K. D. Karlin

prior to exposure to dioxygen resulted in oxygenation

Kodera and co-workers113 have also reported on copper-mediated C-H bond oxygenation in two tris-pyridyl methane ligands at the methine position when the corresponding Cu(II) complexes are exposed to Labeling experiments with confirm that the oxygen atom comes from dioxygen.

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However, it is highly unlikely that the active oxygenation species is a discrete copper-dioxygen adduct in this system. In fact, the authors suggest that it is probably a Cu(II)-catalyzed autoxidation reaction that leads to the oxygenation of a methine position in the ligand.

Yoshizawa and co-workers recently reported on the characterization of a transient copper(II) hydroperoxo species = bis{2-N,N-bis(2-pyridylethyl)-amino]-1,1-dimethylethyl} disulfide) by treating the disulfide-bridged dicopper(I) complex with (see diagram).150 This hydroperoxo species has absorption maxima at 295 nm, 325 nm, and 670 nm and exhibits resonance Raman features with and close to that of observed for a structurally characterized hydroperoxo-copper(II) complex.89 The of is red-shifted to 781 and when is used, indicating that two kinds of hydroperoxo species are formed, possibly differing between having trans or cis Cu-OOH moieties. Because is thought to be the intermediate responsible for the catalyzed oxidation of cyclohexane to cyclohexanol with it may serve as a useful functional model for understanding PHM or DβM activities, both of which have S-donors to their copper centers.

4.

COPPER OXIDASE MODELS; CATALYTIC ALCOHOL OXIDATION

Galactose oxidase, a mononuclear copper enzyme, catalyzes the twoelectron oxidation of alcohols to aldehydes, coupled with the reduction of to through a Cu(II) phenoxyl-radical active species. Modeling this biological reaction is of great interest from the perspective of its importance as an organic transformation. The crystal structure of the enzyme shows that the copper ion is square pyramidal, with two tyrosine phenolate, two histidine imidazole ligands and one exogenous ligand. The equatorial tyrosinate has the unusual thioether linkage to a nearby cysteine via an ortho C-S bond. Many research groups have carried out efforts to model the structure and ligation of the protein copper center and develop systems mimicking

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functional aspects of the alcohol to aldehyde conversion. There also have been successful efforts to generate and physically characterize Cu(II)phenoxyl radical complexes or their zinc(II) analogues.106,114-117

Here, we focus on some recent developments in the catalytic oxidation of alcohols to aldehydes or ketones using copper catalysts. Stack and co-workers118 reported on one of the first functional models for galactose oxidase, in which the catalytic oxidation of benzylic and allylic alcohols to their corresponding aldehydes or ketones occurs using under mild conditions; moreover, the mechanism observed appears to faithfully replicate that observed for galactose oxidase. The copper Schiff-base ligand complexes possess two phenolate donors, each with bulky ortho and para substituents. A representative complex, [Cu(II)(BSP)], which contains o-thiophenyl and ptert-butyl groups in its ligand, is shown in Scheme 11. The key feature of the [Cu(II)(L)] (L = dianionic tetradentate ligand) complexes is their distorted non-square-planar structure, deduced from an xray structure and solution EPR spectra. One-electron oxidation (i.e., using leads to EPR silent Cu(II)-phenoxyl radical complexes, as corroborated by Cu K-edge x-ray absorption spectra and EPR spectra on analogous one-electron oxidized zinc(II) phenoxyl radical complexes. The radical cation complexes bind alcoholates in a 1:1 stoichiometry to form pentacoordinate alkoxide species, and for anaerobic conversion to benzaldehyde and a copper(I) complex occurs.

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Under an atmosphere, neat alcohol substrates (with a catalytic amount of base) are converted to aldehydes or ketones, catalytically, some with more than 1,000 turnovers. The mechanistic scheme, (Scheme 11), has been deduced from studies with [Cu(BSP)] and benzyl alcohol using kinetic measurements and kinetic-isotope effects (KIE = 5.3) plus Hammet relationships with para-substituted benzylic alcohols. These data are consistent with bond cleavage in the rate-determining step. Hydrogen atom transfer is suggested to occur from substrate to the coordinated phenoxyl-radical, leading to release of the -OH moiety from the reduced Cu(I)-product complex. Two-electron oxidation of [Cu(I)(BSP)]- to form is suggested. While no direct spectroscopic evidence is available for the hydroperoxo complex product of dioxygen reaction, its formation is inferred from an observed and the fact that it is EPR silent. uptake (aldehyde product : Recently, Wieghardt and co-workers reported a dinuclear phenoxyl radical species that catalyzes the aerobic oxidation of primary and secondary alcohols to the corresponding aldehyde, ketone and /or 1,2-glycol derivatives as well as generating This work has been reviewed elsewhere.115-117 More recently, Wieghardt and co-workers119 reported on a mononuclear catalyst that selectively oxidizes primary alcohols with to aldehydes, forming The tridentate ligand can be easily oxidized to the radical dianion and then to the diamagnetic monoanion (Scheme 12). When was mixed with in under anaerobic conditions in the presence of excess then exposed to air at 20 °C, the neutral complex forms. The crystal structure of complex shows that the

ion in exists in a slightly distorted square-planar environment. The compound has a diamagnetic ground state due to the antiferromagnetic coupling between the ion and the ligand radical. When a solution of in THF is treated with the benzyl and ethyl alcohol under anaerobic condition, it yields the corresponding aldehydes and probably On the other hand, catalytic oxidations of primary alcohols are observed when a THF solution of was mixed with large excess of benzyl alcohol or ethanol as substrates and stirred

110

C. X. Zhang, H.-C. Liang, K. J. Humphreys and K. D. Karlin

under air at 20 °C for 20 hours, yielding the corresponding aldehydes with approximate 55 % yield. The proposed mechanism for this catalytic oxidation

reaction is shown in Scheme 12. The catalytic cycle starts with complex binding an alcohol to form an alkoxide compound and then in the rate-determining step ligand is reduced to through hydrogen-atom transfer from the atom of the alkoxide. The resulting coordinated ketyl radical anion transfers one electron to ion rapidly to form the and the aldehyde which dissociates. The regeneration of the catalyst occurs by oxidizing the complex with through a superoxide species ] and release of The superoxide species can be generated on the benchtop and characterized spectroscopically ( and 650 nm). A copper-based aerobic catalytic system that transforms a wide range of alcohols to the corresponding aldehydes or ketones under mild condition was reported by Marko et al.120 Upon treatment with a mixture of 5 % CuCl, 5 % phenanthroline, 5% (di-t ert-butyl hydrazodiformate) o r (1,2-dicarbethoxyhydrazine) and 2 equiv in toluene under or air at 70 ° to 90 °C, a wide range of primary, secondary, allylic and benzylic alcohols can be oxidized to the corresponding aldehydes and ketones with very good yields. The proposed reaction mechanism (Scheme 13)

2. Copper-Dioxygen Complex Biomimetic Reactions

111

involves an initial hydrogen-atom transfer within the copper-alkoxide/azo complex generating the carbonyl-bound hydrazino-copper species. Then the binding of dissociates the product aldehyde or ketone and forms a binuclear copper(II) peroxide species which undergoes homolytic cleavage followed by hydrogen-atom abstraction from the coordinated hydrazine giving a Cu(I)-hydroxy complex. The Cu(I)-hydroxy complex binds to another molecule of the alcohol and releases water to regenerate the copperalkoxide/azo complex. However, Cu(I) hydroxy complexes are unknown in the literature. Therefore, the exact nature of the catalytic cycle is unclear.

5.

COPPER-PHENANTHROLINE DNA OXIDATION

Due to the toxicity and carcinogenicity of metal ions and metal complexes there is considerable interest in understanding metal-mediated oxidation of biopolymers. i.e., nucleic acids and proteins. Copper, in particular is highly regulated in the cell to prevent the deleterious reactions that can result from its interaction with dioxygen or reactive oxygen species such as superoxide or hydrogen peroxide.121 The reactive nature of copper complexes, make them potential drug candidates and powerful tools for studying the solution structure of biological substrates. The chemistry of copper with DNA results in both reaction at the deoxyribose sugar and some base oxidation. Although, some compounds

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C. X. Zhang, H.-C. Liang, K. J. Humphreys and K. D. Karlin

carry out predominantly direct strand scission to the near exclusion of nucleobase chemistry, there are no examples of copper complexes that only react with the bases.122 Both types of reaction utilize a reactive oxygen species, usually hydroxyl radical and are hydrogen peroxide dependent. For clarification the numbering scheme for the sugar is presented herein. Because of the limited and unspecific nature of reaction with the bases, their structures and numbering are not presented. Distinctions will be made between experiments that utilized restriction fragments or oligonucleotides to assess cleavage and those which employed plasmid, a considerably more sensitive assay, the results of which do not always correlate with work done on shorter DNA fragments.

In the presence of thiols, ascorbate, or NADH, free copper ions have been observed to facilitate direct strand scission of DNA. In the thioldependent reaction, the observed reactivity remains unchanged despite the use of different thiols suggesting that the reaction is independent of the nature of the reductant, and proceeds through a copper(I) dioxygen intermediate which generates hydroxyl radical as opposed to a thiyl or alkyl radical.123-125 This is confirmed by inhibition of the cleavage reaction with plasmids in the presence of hydroxyl radical scavengers.126 Contrarily, in the ascorbate case there appears to be no inhibition by hydroxyl radical scavengers, while catalase almost completely arrests the reaction, implicating hydrogen peroxide as an essential component.127,128 The previous statement also holds true for reaction of Cu(II) and NADH with restriction fragments.129 In all of the above cases where a restriction fragment or oligonucleotide was treated with piperidine after reaction with copper and an initiator, there was an observed enhancement in the reaction. Piperidine treatment results in strand scission at alkaline labile sites where the base has been removed or modified. The enhancement is presumed to be due to base oxidation, although specific products consistent with this assertion were not isolated or characterized. There is also considerable reactivity with copper(II), hydrogen peroxide and DNA. In the absence of another reagent such as thiol or ascorbate, the reaction in the presence of can be quenched by the addition of radical scavengers, suggesting that there are at least two different mechanisms possible. In one instance, the researchers reported cleavage selectivity for polyguanosines within a restriction fragment upon piperidine treatment.130 In a conflicting study, selectivity for thymine and guanine, especially when located 5’ to a guanine was shown.131

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Due to the role of the ligand in tuning the redox potential of metal ions in a complex, there are examples of compounds that are both more and less reactive than uncomplexed copper. The best studied and to-date most reactive DNA-cleavage systems are based on the bis(1,10phenanthroline)cuprous complex, The reduction of the (phenanthroline)cupric complex by mercaptopropionic acid usually initiates the reaction to give the cuprous form and a disulfide. (Eq. 1) The cuprous complex can then react with dioxygen yielding superoxide and the copper(II) starting material. (Eq. 2) Spontaneous dismutation of superoxide in the presence of protons produces hydrogen peroxide and dioxygen. (Eq. 3) Strand scission results from interaction of hydrogen peroxide with the cuprous compound bound in the minor groove of the DNA duplex. The identity of the reactive intermediate has yet to be determined; although, inhibition studies like those done with copper ions suggest that a diffusible radical is not involved. Other possibilities include

The major reaction pathway was thought to proceed via hydrogen atom abstraction at the C-1’ position of the deoxyribose giving a 2’deoxyribonolactone intermediate and resulting in 5’-phosphate, 3’-phosphate, 5-methylene furanone, and free base as products (Scheme 14). Recent work showed that the carbonyl oxygen of the 5-methylene furanone is derived from water and a 1’,2’dehydronucleotide was proposed as the intermediate.134 However, a mononucleotide analogue of the 1’,2’ dehydronucleotide did not decompose to the observed cleavage products resulting from reaction of DNA with Decomposition of a 2’-deoxyribonolactone model to 5methylene furanone and free phosphate suggested a catalyzed elimination of the lactone intermediate.135 A contradictory report discounted both intermediates and proposed that direct strand scission is a result of Hatom abstraction at the C-4’ and C-5’ positions with C-1’ abstraction producing alkaline labile sites, but not strand breaks.136 Under certain conditions, a secondary reaction at the C-4’ position is also observed yielding 5’-phosphate, 3’-phosphoglycolate, and an unidentified 3-carbon fragment.137

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Using B-DNA as a substrate, (phenanthroline)-copper complexes will react with every base, although there appears to be a slight preference for A T stretches. There is no reaction with single-stranded DNA indicating that the complex requires some secondary structure in the substrate for the reaction to occur. This is supported by a decrease in reactivity with A-DNA and a lack of reactivity with Z-DNA.138 Covalent linkage of phenanthroline to an RNA primer or a DNA binding protein allows for specific cleavage at the site of interaction between the recognition unit and the DNA substrate.139,140 In the absence of a tethered DNA recognition unit there is limited specificity in the observed cleavage.

·

The 1,10-phenanthroline-copper complex is usually generated in situ where it exhibits maximal cleavage efficiency at a copper to phenanthroline ratio of 1:4. This results from the smaller binding constant of the second phenanthroline in the complex. At high thiol concentrations where there is competitive binding between phenanthroline and the thiol this can be problematic. By covalently attaching two phenanthrolines using a short

flexible arm containing an exogenous primary amine, a 2:1 phenanthrolinecopper complex can be maintained.141 The new compound, “clip-phen” has been shown to be more reactive than with plasmid DNA. Vectorization of “clip-phen” at the amine using acridine, a known intercalator, resulted in compounds possessing even greater reactivity. 142 Although, neither "clip-phen" nor phenanthroline:copper are capable of mediating strand scission in the absence of a reductant, the ortho-quinacridine compounds, and facilitate the conversion of supercoiled plasmid

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DNA to nicked under these conditions when complexed with copper.143 Although the exact mechanism is unknown, the ligand is suspected to act as an internal reducing agent, generating the copper(I) species. Addition of hydrogen peroxide to the reaction results in an enhancement of the nuclease activity. The cleavage reaction with peroxide was inhibited by radical, and scavengers suggesting an oxidative cleavage mechanism similar to those already discussed. However, addition of the same scavengers to the reaction without peroxide resulted in no inhibition. Less work has been done with other types of ligand systems. DNA 144 cleavage has been observed for alkylresorcinols with copper and As in the case of the (phenanthroline)-copper system, the active complex is generated in situ. Addition of a copper(I) specific chelator, which suppressed the reaction, demonstrated that Cu(I) is an obligatory intermediate in the reaction and quenching by a radical scavenger and catalase showed that hydroxyl radical and play important roles.145 Two other natural products have been isolated that give rise to oxidative cleavage of DNA in the presence of copper.146,147 In both cases, the reactive species is believed to be formed by oxidation of the ligand to give a cation and Cu(I), which can then react with dioxygen. This observation is supported by the complete inhibition of the reaction by catalase. A freely diffusible hydroxyl radical was ruled out by the failure of radical scavengers to inhibit the reaction.

Despite the high DNA cleavage activity of copper and copper complexes in conjunction with dioxygen, very few compounds have been as thoroughly studied as the phenanthroline-copper system. All of the above chemistry shares in common a dependence on copper and hydrogen peroxide with the reactive species responsible for DNA cleavage believed to be either hydroxyl radical or a coordinated isoform. Some of the results regarding specificity are conflicting, particularly for reactions with copper ion. In general, for a compound not possessing a recognition unit, any degree of specificity derives from the local structure of the duplex determined by the

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base sequence. High reactivity and the possibility of conveying sequence specificity suggest that this is a field that has not yet been exhausted, especially in light of the large number of copper complexes which have been studied for their reactivity with dioxygen. This knowledge combined with what is already understood about the oxidative cleavage and modification of nucleic acids could contribute to a new class of therapeutic agents.

6.

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C. X. Zhang, H.-C. Liang, K. J. Humphreys and K. D. Karlin Blackburn, N. J.; Strange, R. W.; Cruse, R. W.; Karlin, K. D. J. Am. Chem. Soc. 1987, 109, 1235-1237. Solomon, E. I.; Tuczek, F.; Root, D. E.; Brown, C. A. Chem. Rev. 1994, 94, 827-856. Cruse, R. W.; Kaderli, S.; Karlin, K. D.; Zuberbühler, A. D. J. Am. Chem. Soc. 1988, 110, 6882-6883. Karlin, K. D.; Kaderli, S.; Zuberbühler, A. D. Acc. Chem. Res. 1997, 30, 139147. Jacobson, R. R.; Tyeklár, Z.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am. Chem. Soc. 1988, 110, 3690-3692. Tyeklár, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.; Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677-2689. Baldwin, M. J.; Ross, P. K.; Pate, J. E.; Tyeklár, Z.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 8671-8679. Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Niklaus, P.; Zuberbuhler, A. D. J. Am. Chem. Soc. 1993, 115, 9506-9514. Lee, D.-H.; Wei, N.; Murthy, N. N.; Tyeklar, Z.; Karlin, K. D.; Kaderli, S.; Jung, B.; Zuberbühler, A. D. J. Am. Chem. Soc. 1995, 117, 12498-12513. Karlin, K. D.; Lee, D.-H.; Kaderli, S.; Zuberbühler, A. D. Chem. Commun. 1997, 475-476. Bol, J. E.; Driessen, W.; Ho, R. Y. N.; Maase, B.; Que, J., L.; Reedijk, J. Angew. Chem. Int. Ed Engl. 1997, 36, 998-1000. Kitajima, N.; Moro-oka, Y. Chem. Rev. 1994, 94, 737-757. Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto, S.; Kitagawa, T.; Toriumi, K.; Tasumi, K.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 1277-1291. Fujusawa, K.; Tanaka, M.; Moro-oka, Y.; Kitajima, N. J. Am. Chem. Soc. 1994, 116, 12079-12080. Sanyal, I.; Mahroof-Tahir, M.; Nasir, S.; Ghosh, P.; Cohen, B. I.; Gultneh, Y.; Cruse, R.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. Inorg. Chem. 1992, 31, 4322-4332. Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.; Ralle, M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberbühler, A. D.; Karlin, K. D. J. Am. Chem. Soc. 1998, 120, 12960-12961. Pidcock, E.; DeBeer, S.; Obias, H. V.; Hedman, B.; Hodgson, K. O.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 1870-1878. Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Que Jr., L.; Tolman, W. B. J. Am. Chem. Soc. 1994, 116, 9785-9786. Lynch, W. E.; Kurtz, D. M., Jr.; Wang, S.; Scott, R. A. J. Am. Chem. Soc. 1994, 116, 11030-11038. Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg. Chem. 1995, 34, 952-960. Allen, W. E.; Sorrell, T. N. Inorg. Chem. 1997, 36, 1732-1734. Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young Jr., V. G.; Que Jr., L.; Zuberbühler, A. D.; Tolman, W. B. Science 1996, 271, 1397-1400. Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.; Young, J., V. G.; Cramer, C. J.; Que, J., L.; Tolman, W. B. J. Am. Chem. Soc. 1996, 118, 11555-11574. Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161-2168. Holland, P. L.; Tolman, W. B. Coord. Chem. Rev. 1999, 190-192, 855-869.

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Monzani, E.; Quinti, L.; Perotti, A.; Casella, L.; Gullotti, M.; Randaccio, L.; Geremia, S.; Nardin, G.; Faleschini, P.; Tabbi, G. Inorg. Chem. 1998, 37, 553562. Monzani, E.; Battaini, G.; Perotti, A.; Casella, L.; Gullotti, M.; Santagostini, L.; Nardin, G.; Randaccio, L.; Geremia, S.; Zanello, P.; Opromolla, G. Inorg. Chem. 1999, 38, 5359-5369. Sayre, L. M.; Nadkarni, D. J. Am. Chem. Soc. 1994, 116, 3157-3158. Cooksey, C. J.; Garratt, P. J.; Land, E. J.; Pavel, S.; Ramsden, C. A.; Riley, P. A.; Smit, N. P. M. J. Biol. Chem. 1997, 272, 26226-26235. Clews, J.; Cooksey, C. J.; Garratt, P. J.; Land, E. J.; Ramsden, C. A.; Riley, P. A. Chem. Commun. 1998, 77-78. Kopf, M.-A.; Karlin, K. D. In Biomimetic Oxidations; Meunier, B., Ed.; Imperial College Press: London, 2000; pp Ch. 7, pp 309-362. Ruggiero, C. E.; Smith, J. A.; Tanizawa, K.; Dooley, D. M. Biochemistry 1997, 36, 1953-1959. Mandal, S.; Lee, Y.; Purdy, M. M.; Layre, L. M. J. Am. Chem. Soc. 2000, 122, 3574-3584. Itoh, S.; Nakao, H.; Berreau, L. M.; Kondo, T.; Komatsu, M.; Fukuzumi, S. J. Am. Chem. Soc. 1998, 120, 2890-2899. Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.; Que Jr., L.; Fukuzumi, S. Angew. Chem. Int. Ed. 2000, 39, 398-400. Blain, I.; Bruno, P.; Giorgi, M.; Lojou, E.; Lexa, D.; Réglier, M. Eur. J. Inorg. Chem. 1998, 1297-1304. Blain, I.; Giorgi, M.; De Riggi, I.; Réglier, M. Eur. J. Inorg. Chem. 2000, 393398. Kodera, M.; Tachi, Y.; Kita, T.; Kobushi, H.; Sumi, Y.; Kano, K.; Shiro, M.; Koikawa, M.; Tokii, T.; Ohba, M.; Okawa, H. Inorg. Chem. 2000, 39, 226-234. Mahadevan, V.; Klein Gebbink, R. J. M.; Stack, T. D. P. Curr. Opin. Chem. Biol. 2000, 4, 228-234. Itoh, S.; Taki, M.; Fukuzumi, S. Coord. Chem. Rev. 2000, 198, 3-20. Jazdzewski, B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200-202, 633-685. Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50, in press. Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T. D. P. Science 1998, 279, 537-540.

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Chapter 3 Catalytic oxidations of alcohols

R.A. Sheldon and I.W.C.E. Arends Biocatalysis and Organic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Abstract: The scope and mechanisms of catalytic methodologies for the selective oxidation of primary and secondary alcohols, using dioxygen, hydrogen peroxide or alkyl hydroperoxides as the stoichiometric oxidant, are critically reviewed Emphasis is placed on homogeneous transition metal catalysts. Catalytic oxidations with dioxygen generally involve late transition elements, e.g. Ru, Pd and Cu that operate via elimination from a lowvalent alkoxymetal intermediate (hydridometal mechanism) as the key step. In contrast, catalytic oxidations with hydrogen peroxide and alkyl hydroperoxides generally involve early transition metals, e.g. Ti, Mo, W and Re, and high-valent peroxometal complexes or first-row elements (V, Mn and Cr) and high-valent oxometal species as the active oxidant, respectively. Ruthenium forms an exception in that it is able to catalyze the aerobic oxidation of alcohols via a hydridometal or an oxometal mechanism.

Keywords: Alcohol oxidations, peroxometal pathway, oxometal pathway, hydridometal pathway, ruthenium catalyzed oxidations, palladium catalyzed oxidations, copper catalyzed oxidations, hydrogen peroxide, tert-butyl hydroperoxide, dioxygen

1.

INTRODUCTION

The catalytic aerobic oxidation of alcohols has a long history dating back to Döbereiner's observation, in 1820, that ethanol is oxidized to acetic acid over platinum blacki. Indeed, this preceded the coining of the term catalysis, by Berzelius in 18351. The catalytic effect of platinum on the aerobic oxidation of cinnamyl alcohol was described by Strecker in 1855ii and in the period 1912-1921 Wieland showed that finely divided palladium catalyzes the aerobic oxidation of primary alcohols to aldehydes in aqueous 123

L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 123-155. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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was concluded that these reactions involve a dehydrogenation mechanism, followed by oxidation of hydride by dioxygen. More recently, noble metalcatalyzed oxidative dehydrogenations have been widely applied to the selective oxidations of alcohols and vicinal diols4. Similarly, following the pioneering work of Heyns and Paulsen5, the liquid phase aerobic oxidation of carbohydrates over supported noble metal catalysts has been extensively studied by groups in Delft6, Eindhoven7, Lyon8 and Zürich9. Noble metal salts, e.g. of Pd(II) or Pt(II) undergo reduction by primary and secondary alcohols in homogeneous solution. Indeed, the ability of alcohols to reduce Pd(II) was already described in 1828 by Berzelius who showed that was reduced to palladium metal in an aqueous elimination from an ethanolic solution10. The reaction involves a alkoxymetal intermediate and is a commonly used method for the preparation of noble metal hydrides (Reaction 1). In the presence of dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol, e.g. with palladium salts11-15.

The oxidation of primary and secondary alcohols to the corresponding carbonyl compounds plays a central role in organic synthesis16. Traditionally, such transformations have been performed with stoichiometric quantities of inorganic oxidants, notably chromium VI reagents17. However, from both an economic and an environmental viewpoint, there is a growing demand for atom efficient, catalytic methods that employ clean oxidants such as and In this review we will focus on the use of homogeneous metal catalysts to mediate the selective oxidation of alcohols using or as the primary oxidant. Heterogeneous catalysts have been extensively reviewed elsewhere4 and will be covered only where they are relevant to the discussion.

2.

MECHANISMS

As noted above, the aerobic oxidation of alcohols catalyzed by lowvalent late transition metal ions, particularly those of Group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (see Figure 1) a hydridometal species, formed by elimination from an alkoxymetal intermediate, is reoxidized by dioxygen, presumably via Alternatively, an insertion of into the M-H bond with formation of alkoxymetal species can decompose to a proton and the reduced form of the catalyst (see Figure 1), either directly or via the intermediacy of a

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hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or elimination. Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III).

Metal-catalyzed oxidations of alcohols with peroxide reagents can be conveniently divided into two categories, involving peroxometal and oxometal species, respectively, as the active oxidant (Figure 2). In the peroxometal pathway the metal ion remains in the same oxidation state throughout the catalytic cycle and no stoichiometric oxidation is observed in

the absence of the peroxide. In contrast, oxometal pathways involve a twoelectron change in oxidation state of the metal ion and a stoichiometric oxidation is observed, with the oxidized form of the catalyst, in the absence Indeed, this is a test for distinguishing between the two of e.g. pathways.

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Peroxometal pathways are typically observed with early transition metal ions with a configuration, e.g. Mo(VI), W(VI), Ti(IV) and Re(VII), that are relatively weak oxidants. Oxometal pathways are characteristic of late transition elements and first row transition elements, e.g. Cr(VI), Mn(V), Os(VIII), Ru(VI) and Ru(VIII), that are strong oxidants in high oxidation states. Some metals can operate via both pathways depending, inter alia, on the substrate, e.g. vanadium(V) operates via a peroxometal pathway in olefin epoxidations but an oxometal pathway is involved in alcohol oxidations18. In both of these mechanisms the product-forming step involves a elimination from an alkoxymetal species (see Figure 2) analogous to the above described aerobic oxidations catalyzed by low-valent metal ions. In some cases, notably ruthenium, the aerobic oxidation of alcohols is catalyzed by both low- and high-valent forms of the metal (see later). In the former case the reaction involves (see Figure 1) the formation of a hydridometal species (or its equivalent) while the latter involves an oxometal intermediate (see Figure 2) which is regenerated by reaction of the reduced form of the catalyst with dioxygen instead of a peroxide. Since both mechanisms involve a elimination it is difficult to distinguish between the two and one should bear in mind, therefore, that aerobic oxidations with high-valent oxometal catalysts could involve the formation of low-valent species, even the (colloidal) metal, as the actual catalyst.

3.

RUTHENIUM-CATALYZED OXIDATIONS WITH

Ruthenium compounds are widely used as catalysts in organic synthesis19,20 and have been extensively studied as catalysts for the aerobic oxidation of alcohols21. In 1978, Mares and coworkers22 reported that catalyzes the aerobic oxidation of secondary alcohols into the corresponding ketones, albeit in modest yields. Subsequently, and were shown to catalyze the aerobic oxidation of activated allylic and benzylic alcohols under mild conditions23, e.g. the oxidation of retinol to retinal (Reaction 2).

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Aliphatic primary and secondary alcohols were more efficiently oxidized using trinuclear ruthenium carboxylates, as the catalysts24. With lower aliphatic alcohols, e.g. 1-propanol, 2-propanol and 1-butanol, activities were ca. 10 times higher than with and Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Bäckvall and coworkers25’26 used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiff s base complex. The proposed mechanism is shown in Figure 3. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant regeneration of the ruthenium catalyst. The cobalt-Schiffs base complex catalyzes the subsequent aerobic oxidation of the hydroquinone to benzoquinone to complete the catalytic cycle.

More recently, the same group described a further improvement in which a zeolite-encapsulated cobalt-Schiff's base complex was used as the cocatalyst27. The regeneration of the benzoquinone can also be achieved with dioxygen in the absence of the cobalt cocatalyst. Thus, Ishii and coworkers28 showed that a combination of hydroquinone and dioxygen, in as solvent, oxidized primary aliphatic, allylic and benzylic alcohols to the corresponding aldehydes in quantitative yields (Reaction 3).

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A combination of and the stable nitroxyl radical, 2,2',6,6'tetramethylpiperidine-N-oxyl (TEMPO) is a remarkably effective catalyst for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, respectively, in >99% and selectivity29. The best results were obtained using 1m% of 3m% of TEMPO (Reaction 4).

The results obtained in the oxidation of representative primary and secondary aliphatic alcohols and allylic and benzylic alcohols using this system are shown in Tables 1 and 2.

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Primary alcohols give the corresponding aldehydes in high selectivity, e.g. 1-octanol affords 1-octanal in >99% selectivity. Over-oxidation to the corresponding carboxylic acid, normally a rather facile process, is completely suppressed in the presence of a catalytic amount of TEMPO. For example, attempted oxidation of octanal under the reaction conditions, in the presence of 3m% TEMPO, gave no reaction in one week. In contrast, in the absence of TEMPO octanal was completely converted to octanoic acid within 1 h under the same conditions. These results are consistent with overoxidation of aldehydes occurring via a free radical autoxidation mechanism. TEMPO suppresses this reaction by efficiently scavenging free radical intermediates resulting in the termination of free radical chains, i.e. it acts as an antioxidant. Allylic alcohols were selectively converted to the corresponding unsaturated aldehydes in high yields. No formation of the isomeric saturated ketones via intramolecular hydrogen transfer, which is known to be promoted by ruthenium phosphine complexes30, was observed. Although, in separate experiments, secondary alcohols are oxidized faster than primary ones, in competition experiments the Ru/TEMPO system displayed a preference for primary over secondary alcohols. This can be explained by assuming that initial complex formation between the alcohol and the ruthenium precedes rate-limiting hydrogen transfer and determines substrate specificity, i.e. complex formation with a primary alcohol is favoured over a secondary one. An oxidative hydrogenation mechanism, analogous to that proposed by Bäckvall for the Ru/quinone system (see earlier), can be envisaged for the Ru/TEMPO system (see Figure 4). The intermediate hydridoruthenium species is most probably as was observed in hydrogen transfer 31 exhibits the same activity as reactions . The observation that in the Ru/TEMPO catalyzed aerobic oxidation of 2-octanol is consistent with this notion.

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The TEMPO acts as a hydrogen transfer mediator by promoting the regeneration of the ruthenium catalyst, via oxidation of the ruthenium hydride, resulting in the concomitant formation of the corresponding hydroxylamine, TEMPOH. The latter then undergoes rapid re-oxidation to TEMPO, by molecular oxygen, to complete the catalytic cycle (see Figure 4). A linear increase in the rate of 2-octanol oxidation was observed with increasing TEMPO concentration in the range 0-4mol% but above 4mol% further addition of TEMPO had a negligible effect on the rate. Analogous results were observed by Bäckvall and coworkers32 in the Ru/benzoquinone system and were attributed to a change in the rate-limiting step. Hence, by analogy, we propose that at relatively low TEMPO/Ru ratios (up to 4:1) reoxidation of the ruthenium hydride species is the slowest step while at high ratios dehydrogenation of the alcohol becomes rate-limiting. Under an inert atmosphere catalyzes the stoichiometric oxidation of 2-octanol by TEMPO, to give 2-octanone and the corresponding piperidine, TEMPH, in a stoichiometry of 3:2 (Reaction 5).

This result can be explained by assuming that the initially formed TEMPOH (see above) undergoes disproportionation to TEMPH and the oxoammonium cation (Reaction 6). Reduction of the latter by the alcohol affords another molecule of TEMPOH and this leads, ultimately, to the formation of the ketone and TEMPH in the observed stoichiometry of 3:2. The observation that attempts to prepare TEMPOH33 under an inert

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atmosphere always resulted in the formation of TEMPH is consistent with this hypothesis.

Based on the results discussed above the detailed catalytic cycle depicted in Figure 5 is proposed for the Ru/TEMPO catalyzed aerobic oxidation of alcohols.

According to a recent report by Ishii and coworkers34 supported on active charcoal is able to catalyze the oxidative cleavage of vic-diols to aldehydes, using dioxygen as the oxidant, in at 60°C (Reaction 7).

The alcohol oxidations discussed above involve as a key step the oxidative dehydrogenation of the alcohol to form low-valent hydridoruthenium intermediates. On the other hand, high-valent oxoruthenium species are also able to dehydrogenate alcohols via an

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oxometal mechanism (see Figure 2). It has long been known that ruthenium tetroxide, generated by reaction of ruthenium dioxide with periodate, smoothly oxidizes a variety of alcohols to the corresponding carbonyl compounds35. Griffith and coworkers36 reported the synthesis of the organic soluble tetra-n-butylammoniumperruthenate (TBAP), in 1985. They later found that tetra-n-propylammoniumperruthenate (TPAP), is even easier to prepare, from and in 37,38 water . TBAB and TPAP are air-stable, non-volatile and soluble in a wide range of organic solvents. Griffith and Ley39,40 subsequently showed that TPAP is an excellent catalyst for the selective oxidation of a wide variety of alcohols using N-methylmorpholine-N-oxide (NMO) as the stoichiometric oxidant (Reaction 8).

More recently, the groups of Ley41 and Marko42 independently showed that TPAP is able to catalyze the oxidation of alcohols using dioxygen as the stoichiometric oxidant. In particular, polymer supported perruthenate (PSP), prepared by anion exchange of with a basic anion exchange resin (Amberlyst A-26), has emerged as a versatile and recyclable catalyst for the aerobic oxidation (Reaction 9) of alcohols41, albeit with an activity ca. 4 times lower than homogeneous TPAP. Analogous to the above described Ru/TEMPO system PSP displays a marked preference for primary versus secondary alcohol functionalities41.

Examples illustrating the scope of PSP-catalyzed aerobic oxidation of primary alcohols to the corresponding aldehydes are shown in Table 3.

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In a more recent variation on this theme a tetraalkylammonium perruthenate tethered to the internal surface of mesoporous silica (MCM-41) was shown43 to catalyze the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols (Figure 6). Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM41. No mechanistic interpretation for this surprising observation was offered by the authors.

Indeed, only sparse attention has been paid to the mechanism of perruthenate-catalyzed alcohol oxidations. Although TPAP can act as a three-electron oxidant the fact that it selectively oxidizes cyclobutanol to cyclobutanone militates against free radical intermediates

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and is consistent with a heterolytic, two-electron oxidation44. Presumably, the key step involved elimination from a high-valent, e.g. alkoxyruthenium(VII) intermediate followed by reoxidation of the lower valent ruthenium by dioxygen. However, as shown in Figure 7, if this involved the Ru(VII)/Ru(V) couple the reoxidation would require the close proximity of two ruthenium centres, which would seem unlikely in a polymer-supported catalyst. A plausible alternative, which can occur at an isolated ruthenium centre involves the oxidation of a second molecule of alcohol, resulting in the reduction of ruthenium(V) to ruthenium(III), followed by reoxidation of the latter to ruthenium(VII) by dioxygen (see Figure 7).

More detailed mechanistic studies are obviously necessary in order to elucidate the details of this fascinating reaction. It is worth noting, in this context, that the reaction of TPAP with 2-propanol was found to be autocatalytic, possibly due to the formation of colloidal A possible alternative too is one involving the initial formation of oxoruthenium(VI), followed by cycling between ruthenium(VI), ruthenium(IV) and possibly ruthenium(II). We note, in this context, that James and coworkers46 showed that a trans-dioxoruthenium(VI) complex of meso-tetrakismesitylporphyrin dianion (tmp) oxidizes isopropanol, in a stoichiometric reaction, with concomitant formation of a dialkoxyruthenium(IV) tmp complex (Reaction 10).

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The oxoruthenium(VI) complex was prepared by exposing a benzene solution of to air at 20°C. Addition of isopropanol to the resulting solution, in the absence of air, afforded the dialkoxyruthenium(IV) complex, in quantitative yield, within 24 hours. In the presence of air, benzene solutions of the dioxoruthenium(VI) or the dialkoxyruthenium(IV) complex effected catalytic oxidation of isopropanol at room temperature, albeit with a modest rate (1.5 catalytic turnovers per day). Interestingly, with the dialkoxyruthenium(IV) complex, catalytic oxidation was observed with air but not with dry oxygen, suggesting that hydrolysis to an oxoruthenium(IV) complex is necessary for a catalytic cycle. Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. For example, both and 5% Ru-on-charcoal catalyze the aerobic oxidation of activated alcohols such e.g. Reaction 11. as allylic alcohols47 and

More recently, Kagan and coworkers49 have described the use of ruthenium supported on ceria, as a catalyst for the aerobic oxidation of alcohols. Primary and secondary alcohols are oxidized to the corresponding aldehydes (carboxylic acids) and ketones, respectively, at elevated temperatures (>140°C). Surprisingly, allylic alcohols, such as geraniol, and some cyclic alcohols, e.g. menthol, are unreactive. The former result suggests that low-valent ruthenium species are possibly involved and that coordination of ruthenium to the double bond inhibits alcohol oxidation. Ruthenium-exchanged hydrotalcites were shown, by Kaneda and coworkers50, to be heterogeneous catalysts for the aerobic oxidation of reactive allylic and benzylic alcohols. Hydrotalcites are layered anionic clays consisting of a cationic Brucite layer with anions (hydroxide or carbonate) situated in the interlayer region. Various cations can be introduced in the Brucite layer by ion exchange. For example, ruthenium-exchanged hydrotalcite with the formula, was prepared by

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treating an aqueous solution of and with a solution of NaOH and followed by heating at 60°C for 18 h50. The resulting slurry was cooled to room temperature, filtered, washed with water and dried at 110°C for 12 h. The resulting ruthenium-hydrotalcite showed the highest activity amongst a series of hydrotalcites exchanged with e.g. Fe, Ni, Mn, V and Cr. Subsequently, the same group showed that the activity of the ruthenium-hydrotalcite was significantly enhanced by the introduction of cobalt(II), in addition to ruthenium(III), in the Brucite layer51. For example, cinnamyl alcohol underwent complete conversion in 40 min. in toluene at 60°C, in the presence of Ru/Co-HT, compared with 31% conversion under the same conditions with Ru-HT. A secondary aliphatic alcohol, 2-octanol, was smoothly converted into the corresponding ketone but primary aliphatic alcohols, e.g. 1-octanol, exhibited extremely low activity. The authors suggested that the introduction of cobalt induced the formation of higher oxidation states of ruthenium, e.g. Ru(IV) to Ru(VI), leading to a more active oxidation catalyst. However, on the basis of the reported results it is not possible to rule out low-valent ruthenium species as the active catalyst in a hydridometal pathway. The results obtained in the oxidation of representative alcohols with Ru-HT and Ru-Co-HT are compared in Table 4.

Ruthenium pyrochlore oxides, mixed oxides of ruthenium and bismuth or lead with the general formula (A = Pb or Bi; 0 < x < 1 ; 0 < y < 0.5) catalyze the oxidative cleavage of vic-diols to the corresponding (di)carboxylic acids with NaOCl52 or dioxygen53,54 as the stoichiometric oxidant. In the latter case two equivalents of sodium hydroxide are required to neutralize the carboxylic acid product, otherwise the catalyst is deactivated. For example, cyclohexane-1,2-diol afforded sodium adipate in 70% yield (Reaction 12).

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The use of these ruthenium pyrochlore oxides as catalysts for the aerobic oxidation of alcohols would seem to be worthy of further investigation. Presumably the function of the bismuth (or lead) is to facilitate the reoxidation of the ruthenium, by dioxygen, in these catalysts. A ruthenium(III) salen complex anchored to a chloromethylated styrenedivinylbenzene copolymer (see Figure 8) was reported54,55 to be 75 times as active as the homogeneous analogue in the aerobic oxidation of benzyl alcohol. However, recycling of the catalyst led to a 35% loss in activity due to facile leaching of the ruthenium. Moreover, we note that it is highly unlikely that the oxidation sensitive salen ligand survives the reaction conditions.

The aerobic oxidation of alcohols proceeds smoothly at room temperature in the presence of one equivalent of an aldehyde, e.g. acetaldehyde, and a catalyst comprising a 1:1 mixture of and in ethyl acetate (Reaction 13)56.

Representative examples are shown in Table 5. The results were rationalized by assuming that the corresponding percarboxylic acid is formed by cobalt-mediated free radical autoxidation of the aldehyde. Subsequent reaction of ruthenium(m) with the peracid affords oxoruthenium(V) carboxylate which is the active oxidant. Compared to the aerobic oxidations

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discussed above the method suffers from the drawback that one equivalent of a carboxylic acid is formed as a coproduct.

4.

PALLADIUM-CATALYZED OXIDATIONS WITH

Palladium(II) is also capable of mediating the oxidation of alcohols via the hydridometal pathway shown in Figure 1. Blackburn and Schwarz first reported57 the aerobic oxidation of alcohols in 1977. However, activities were very low, with turnover frequencies of the order of 1 Subsequently, much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For example, the giant palladium cluster, was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding aldehydes (Reaction 14)59.

in combination with as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in combination with sodium in fairly good yields60. Similarly, carbonate and a tetraalkylammonium salt, Adogen 464, as a phase transfer catalyst, catalyzed the aerobic oxidation of alcohols, e.g. 1,4- and 1,5-diols afforded the corresponding lactones (Reaction 15)61,62.

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However, these methods suffer from low activities and/or narrow scope. Uemura and coworkers63,64 reported an improved procedure involving the use of (5m%) in combination with pyridine (20m%) and 3A molecular sieves (500 mg per mmol of substrate) in toluene at 80°C. This system catalyzed the smooth aerobic oxidation of primary and secondary aliphatic alcohols to the corresponding aldehydes and ketones, respectively, in addition to benzylic and allylic alcohols. Representative examples are summarized in Table 6. 1,4- and 1,5-Diols afforded the corresponding lactones.

A catalytic cycle (Figure 9) was proposed which involved elimination from a pyridine-palladium(II) alkoxide to give the carbonyl compound and a palladium(II) hydride. Insertion of dioxygen in the Pd-H bond affords a Pd(II)-hydroperoxide which undergoes ligand exchange with alcohol to regenerate the palladium(II) alkoxide and

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Separate experiments showed that the molecular sieves accelerated the decomposition of under the reaction conditions. Presumably, another function of the molecular sieves is to remove the water produced in the reaction, which may have a deleterious effect on the catalyst activity (but see later). Although this methodology constitutes an improvement on those previously reported, turnover frequencies were still generally and, hence, there is considerable room for further improvement. Recently, we described the use of a water-soluble palladium(II) complex of sulfonated bathophenanthroline as a stable, recyclable catalyst for the aerobic oxidation of alcohols in a two-phase aqueous-organic medium, e.g. in Reaction 1665.

Reactions were generally complete in 5 h at 100°C/30 bar air with as little as 0.25m% catalyst. No organic solvent is required (unless the substrate

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is a solid) and the product ketone is easily recovered by phase separation. The catalyst is stable and remains in the aqueous phase which can be recycled to the next batch. A wide range of alcohols were oxidized with TOFs ranging from 10 to 100, depending on the solubility of the alcohol in water (since the reaction occurs in the aqueous phase the alcohol must be at least sparingly soluble in water). Thus, in a series of straight-chain secondary alcohols the TOFs decreased from 100 to 13 on increasing the chain length from 1-pentanol to 1-nonanol. Representative examples of secondary alcohols that were smoothly oxidized using this system are collected in Table 7. The corresponding ketones were obtained in >99% selectivity in virtually all cases. Primary alcohols afforded the corresponding carboxylic acids via further oxidation of the aldehyde intermediate, e.g. 1-hexanol afforded 1hexanoic acid in 95% yield. It is important to note, however, that this was achieved without the requirement of one equivalent of base to neutralize the carboxylic acid product (which is the case with supported noble metal catalysts4). In contrast, when 1m% TEMPO (4 equivalents per Pd) was added the aldehyde was obtained in high yield, e.g. 1-hexanol afforded 1hexanal in 97% yield. Some representative examples of primary alcohol oxidations using this system are shown in Table 8.

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Compared to existing systems for the aerobic oxidation of alcohols the Pd-bathophenanthroline system is at least an order of magnitude more reactive, requires no solvent and product/catalyst isolation involves simple phase separation. The system has broad scope but is not successful with all alcohols. Some examples of unreactive alcohols are shown in Figure 10. Low reactivity was generally observed with alcohols containing functional groups which could strongly coordinate to the palladium.

The reaction is half-order in palladium and first order in the alcohol substrate, when measured with a water soluble alcohol to eliminate the complication of mass transfer. A possible mechanism is illustrated in Figure 11. The resting catalyst is a dimeric complex containing bridging hydroxyl

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groups. Reaction with the alcohol in the presence of a base, added as a cocatalyst (NaOAc) or free ligand, affords a monomeric alkoxy palladium(II) intermediate which undergoes elimination to give the carbonyl compound, water and a palladium(0) complex. Oxidative addition of dioxygen to the latter affords a palladium(II) complex which can react with the alcohol substrate to regenerate the catalytic intermediate, presumably with concomitant formation of hydrogen peroxide as was observed in analogous systems66.

It is worth noting, in this context, that palladium complexes of substituted phenanthrolines were recently shown67 to catalyze the formation of hydrogen peroxide, by reaction of a primary or a secondary alcohol with dioxygen, in the presence of an acid cocatalyst, e.g. in a biphasic chlorobenzene/water medium at 70°C and 5 bar. Turnover frequencies up to were observed. The hydrogen peroxide is formed in the organic phase, via palladium catalyzed oxidation of the alcohol, but is subsequently extracted into the water phase where it is protected from decomposition by the palladium complex. The same catalyst system was also used for the production of hydrogen peroxide from a mixture of carbon monoxide, water and dioxygen, with turnover frequencies up to according to Reaction 1768.

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COPPER-CATALYZED OXIDATIONS WITH

Copper would seem to be an appropriate choice of metal for the catalytic oxidation of alcohols with dioxygen since it comprises the catalytic centre in a variety of enzymes, e.g. galactose oxidase, which catalyze this conversion in vivo 69,70. However, despite extensive efforts71 synthetically useful copperbased systems have generally not been forthcoming. Semmelhack reported72 that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzylic and allylic alcohols and activities were low (10mol% of catalyst was needed for smooth reaction). More recently, Marko and coworkers73,74 reported that a combination of CuCl (5m%), phenanthroline (5m%) and di-tert-butylazodicarboxylate, DBAD (5m%), in the presence of 2 equivalents of catalyzes the aerobic oxidation of allylic and benzylic alcohols (Reaction 18). Primary aliphatic alcohols, e.g. 1-decanol, could be oxidized but required 10m% catalyst for smooth conversion.

The nature of the copper counterion was critical, with chloride, acetate and triflate proving to be the most effective. Polar solvents such as acetonitrile inhibit the reaction whereas smooth oxidation takes place in apolar solvents such as toluene. An advantage of the system is that it tolerates a variety of functional groups (see Table 9 for examples). Serious drawbacks of the system are the low activity, the need for two equivalents of (relative to substrate) and the expensive DBAD as a cocatalyst. can be reduced to 0.25 According to a later report75 the amount of equivalents by changing the solvent to fluorobenzene.

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The active catalyst is heterogeneous, being adsorbed on the insoluble (filtration gave a filtrate devoid of activity). Besides fulfilling a role as a catalyst support the acts as a base and as a water scavenger. The mechanism illustrated in Figure 12 was postulated to explain the observed results.

Osborn and coworkers76,77 reported that CuCl in combination with (TPAP) catalyzes the aerobic oxidation of alcohols. The or scope is rather limited, however, and the system would not appear to have

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any advantages over the earlier described ruthenium- and palladium-based systems.

6. OTHER METALS AS CATALYSTS FOR OXIDATION WITH In addition to ruthenium, other late and first-row transition elements are capable of dehydrogenating alcohols via an oxometal pathway. Some are used as catalysts, in combination with or for the oxidative dehydrogenation of alcohols (see later). By analogy with ruthenium, one might expect that regeneration of the active oxidant with dioxygen would be possible. For example, one could easily envisage alcohol oxidation by oxovanadium(V) followed by reoxidation of the resulting vanadium(III) by dioxygen. However, scant attention appears to have been paid to such possibilities. The aerobic oxidation of 1-propanol to 1-propanal (94-99% selectivity), in the gas phase at 210°C over a catalyst modified with an alkaline earth metal oxide (10m%), was described78 in 1979. However, to our knowledge vanadium-catalyzed aerobic oxidation of alcohols have not been further investigated, in the liquid or gas phase79. Chromium(VI) catalyzes the oxidation of alcohols with alkyl hydroperoxides80. Chromium-incorporated molecular sieves, in particular chromium-substituted aluminophosphate-5 (Cr-APO-5) were shown81 to be effective for the aerobic oxidation of secondary alcohols to the corresponding ketones (Reaction 19). This, and related catalysts, were first believed to be heterogeneous but more detailed investigations82 revealed that the observed catalysis is due to small amounts of soluble chromium that are leached from the framework by reaction with hydroperoxides. Reaction 19 may involve initial chromium-catalyzed free radical autoxidation of the alcohol to the hydroperoxide followed by chromiumcatalyzed oxygen transfer with the latter and/or (formed by its dissociation) via an oxochromium(VI)-chromium(IV) cycle.

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in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Reaction 2083.

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers84, Reaction 20 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of alkylperoxy radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the bond of the alcohol to propagate the autoxidation chain (Reactions 21-23).

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7. CATALYTIC OXIDATION OF ALCOHOLS WITH HYDROGEN PEROXIDE AND ALKYL HYDROPEROXIDES In the aerobic oxidations discussed in the preceding sections the most effective catalysts tend to be late transition elements, e.g. Ru and Pd, that operate via oxometal or hydridometal mechanisms. In contrast, the most effective catalysts with or as the oxidant tend to be early transition metal ions with a d° configuration, e.g. Mo(VI), W(VI) and Re(VII), that operate via peroxometal pathways. Ruthenium and palladium are generally not effective with because they display high catalase activity, i.e. they catalyze rapid decomposition of Early transition elements, on the other hand, are generally poor catalysts for decomposition. One of the few examples of ruthenium-based systems is the bromide combination reported by Sasson and coworkers85. This system catalyzes the selective oxidation of a variety of alcohols, at high (625:1) substrate:catalyst ratios, in an aqueous/organic biphasic system. However, 3-6 equivalents of were required, reflecting the propensity of ruthenium for catalyzing nonproductive decomposition of Jacobsen and coworkers86 showed, in 1979, that anionic molybdenum(VI) and tungsten(VI) peroxo complexes are effective oxidants for the stoichiometric oxidation of secondary alcohols to the corresponding ketones. Subsequently, Trost and Masuyama87 showed that ammonium molybdate, (10m%), is able to catalyze the selective oxidation of secondary alcohols, to the corresponding ketones, using hydrogen peroxide in the presence of tetrabutylammonium chloride and a stoichiometric amount of a base It is noteworthy that a more hindered alcohol moiety was oxidized more rapidly than a less hindered one, e.g. Reaction 24.

The above mentioned reactions were performed in a single phase using tetrahydrofuran as solvent. Subsequently, the group of Di Furia and Modena

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reported88 the selective oxidation of alcohols with 70% aq. using or as the catalyst and methyltrioctylammonium chloride (Aliquat 336) as a phase transfer agent in a biphasic (dichloroethane-water) system. More recently, Noyori and coworkers89,90 have achieved substantial improvements in the sodium tungstate-based, biphasic system by employing a phase transfer agent containing a lipophilic cation and bisulfate as the anion, e.g. This afforded a highly active catalytic system for the oxidation of alcohols using 1.1 equivalents of 30% aq. in a solvent-free system. For example, 1-phenylethanol was converted to acetophenone with turnover numbers up to 180,000. As with all Mo- and Wbased systems, the Noyori system shows a marked preference for secondary alcohols, e.g. Reaction 25.

Unsaturated alcohols generally undergo selective oxidation of the alcohol moiety (Reactions 26 and 27) but when an allylic alcohol contained a reactive trisubstituted double bond selective epoxidation of the double bond was observed (Reaction 28).

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Molybdenum- and tungsten-containing heteropolyanions are also 91-96 effective catalysts for alcohol oxidations with . For example, or in combination with cetylpyridinium chloride as a phase transfer agent, were shown by Ishii and coworkers90-94 to be effective catalysts for alcohol oxidations with in a biphasic, chloroform/water system. Methyltrioxorhenium (MTO) also catalyzes the oxidation of alcohols with via a peroxometal pathway 97,98. Primary benzylic and secondary aliphatic alcohols afforded the corresponding aldehydes and ketones, respectively, albeit using two equivalents of In the presence of bromide ion the rate was increased by a factor 1,00098. In this case the active oxidant may be hypobromite (HOBr), formed by MTO-catalyzed oxidation of bromide ion by Molybdenum99-102 and vanadium compounds103 have also been widely investigated as catalysts for the oxidation of alcohols with tert-butyl hydroperoxide (TBHP) as the oxidant. With the former a peroxometal pathway is involved while with the latter an oxovanadium(V) intermediate is the active oxidant. As with the systems described above, these systems exhibit a preference for the oxidation of secondary hydroxyl functionalities over primary ones. In contrast, zirconyl acetate, catalyzes the selective oxidation of primary alcohol moieties with TBHP (Reaction 29)104.

Polymer-supported tetrabromooxomolybdate(V) was claimed to be a heterogeneous catalyst for alcohol oxidations with TBHP102. However, it seems likely that molybdenum is leached from the surface and the observed catalysis may be, at least partially, homogeneous in nature. The same applies to Cr(III) and Ce(IV) catalysts supported on a perfluorinated sulfonic acid resin (Nafion®K) which catalyze the oxidation of alcohols with TBHP105. Similarly, vanadium-pillared montmorillonite clay (V-PILC)106 and a zeolite-encapsulated vanadium picolinate complex107 were shown to catalyze

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alcohol oxidations with 30% aq. and a adduct, respectively. However, it seems highly likely that the observed catalysis is due to leached vanadium. Indeed, as we have noted elsewhere82, heterogeneous catalysts based on Mo, W, Cr, V, etc. are highly susceptible towards leaching by or alkyl hydroperoxides. Hence, in the absence of rigorous experimental proof, it is questionable whether the observed catalysis is heterogeneous in nature. In contrast, titanium silicalite (TS-1), an isomorphously substituted molecular sieve108 is a truly heterogeneous catalyst for oxidations with 30% aq. including the oxidation of alcohols109. Late and first row transition elements can catalyze the oxidation of via an oxometal pathway. Chromium and alcohols with or vanadium have already been mentioned (see above). Ruthenium compounds, e.g. also catalyze the oxidation of alcohols with TBHP110, presumably involving a high-valent oxoruthenium species as the active oxidant. A dinuclear manganese(IV) complex of trimethyl triazacyclononane (tmtacn) catalyzed the selective oxidation of reactive benzylic alcohols with hydrogen peroxide in acetone111. However, a large excess (up to 8 equivalents) of was required, suggesting that there is substantial nonproductive decomposition of the oxidant. Moreover, we note that the use of acetone as a solvent for oxidations with is not recommended owing to the formation of explosion-sensitive peroxides. The exact nature of the catalytically active species in this system is rather obscure; for optimum activity it was necessary to pretreat the complex with in acetone. Presumably the active oxidant is a high-valent oxomanganese species but further studies are necessary to elucidate the mechanism.

8.

CONCLUDING REMARKS

The economic importance of alcohol oxidations in the fine chemical industry will, in the future, continue to stimulate the quest for effective catalysts that utilize dioxygen or hydrogen peroxide as the primary oxidant. Although much progress has been made in recent years there is still room for further improvement with regard to catalyst activity and scope in organic synthesis. A better understanding of mechanistic details regarding the nature of the active intermediate and the rate-determining step would certainly facilitate this since many of these systems are poorly understood. It may even lead to the development of efficient methods for the enantioselective oxidation of chiral alcohols, e.g. the ruthenium-based system recently described by Katsuki and coworkers112.

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R.A. J. Am. Chem. Soc. 2001, 123, 6826; for a related study see Inokuchi, T.; Nakagawa, K.; Torii, S. Tetrahedron Lett., 1995, 36, 3223. Bäckvall, J.-E.; Andreasson, U. Tetrahedron Lett., 1993, 34, 5459; Trost, B.M.; Kulawiec, R.J. Tetrahedron Lett., 1991, 32, 3039. Aranyos, A.; Csjernyik, G.; Szabo, K.J.; Bäckvall, J.-E. Chem. Commun., 1999, 351. Karlson, U.; Wang, G.-Z.; Bäckvall, J.-E. J. Org. Chem., 1994, 59, 1196. Paleos, C.M.; Dais, P. J. Chem. Soc. Chem. Commun., 1977, 345. Takezawa, E.; Sakaguchi, S.; Ishii, Y. Org. Lett., 1999, 1, 713. Beynon, P.J.; Collins, P.M.; Gardiner, D.; Overend, W.G. Carbohydr. Res., 1968, 6, 431; see also Friedrich, H.B. Plat. Met. Rev., 1999, 43, 94. Dengel, A.C.; Hudson, R.A.; Griffith, W.P. Trans. Met. Chem., 1985, 10, 98. Griffith, W.P.; Ley, S.V.; Whitcombe, G.P.; White, A.D. Chem. Commun., 1987, 1625. Dengel, A.C.; El-Hendawy, A.M.; Griffith, W.P. Trans. Met. Chem., 1989, 40, 230. Griffith, W.P.; Ley, S.V. Aldrichim. Acta, 1990, 23, 13. Ley, S.V.; Norman, J.; Griffith, W.P.; Marsen, S.P. Synthesis, 1994, 639. Hinzen, B.; Lenz, R.: Ley, S.V. Synthesis, 1998, 977. Marko, I.E.; Giles, P.R.; Tsukazaki, M.; Chelle-Regnaut, I.; Urch, C.J.; Brown, S.M. J. Am. Chem. Soc., 1997, 119, 12661. Bleloch, A.; Johnson, B.F.G.; Ley, S.V.; Price, A.J.; Shepard, D.S.; Thomas, A.N. Chem. Commun., 1999, 1907. Rocek, J.; Ng, C.-S. J. Am. Chem. Soc., 1974, 96,1522. Lee, D.G.; Wang, Z.; Chandler, W.D. J. Org. Chem., 1992, 57, 3276. Cheng, S.Y.S.; Rajapakse, N.; Rettig, S.J.; James, B.R. J. Chem. Soc. Chem. Commun.,

1994, 2669; see also Rajapakse, N.; James, B.R.; Dolphin, D. Stud. Surf. Sci. Catal., 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58.

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1990, 55, 109. Matsumoto, M.; Watanabe, N. J. Org. Chem., 1984, 49, 3435. Matsumoto, M.; Ito, S. Synth. Commun., 1984, 14, 697. Vocanson, F.; Guo, Y.P.; Namy, J.L.; Kagan, H.B. Synth. Commun., 1998, 28, 2577. Kaneda, K.; Yamashita, T.; Matsushita, T.; Ebitani, K. J. Org. Chem., 1998, 63, 1750. Matsushita, T.; Ebitani, K.; Kaneda, K. Chem. Commun., 1999, 265. Emons, C.H.H.; Kuster, B.F.M.; Vekemans, J.A.J.M.; Sheldon, R.A. Tetrahedron Asymm., 1991, 2, 359. Felthouse, T.R. J. Am. Chem. Soc., 1987, 109, 7566. Arts, S.J.H.F.; Rantwijk, F. van; Sheldon, R.A. J. Carbohydr. Chem., 1996, 15, 317. Dulal, M.K.; Upadhyay, M.J.; Ram, R.N. J. Mol. Catal. A: Chemical, 1999, 142, 325. Murahashi, S.-I.; Naota, T.; Hirai, N. J. Org. Chem., 1993, 58, 7318. Blackburn, T.F.; Schwartz, J. J. Chem. Soc. Chem. Commun., 1977, 157. Vargaftik, M.N.; Zagorodnikov, V.P.; Storarov, I.P.; Moiseev, I.I. J. Mol. Catal., 1989, 53, 315; see also Moiseev, I.I.; Vargaftik, M.N. In Catalysis by Di- and Polynuclear Metal Cluster Complexes, Adams, R.D.; Cotton, F.A., Eds., Wiley-VCH: Weinheim, 1998, p. 395. Kaneda, K.; Fujii, M.; Morioka, K. J. Org. Chem., 1996, 61, 4502; Kaneda, K.; Fujie, Y.; Ebitani, K. Tetrahedron Lett., 1997, 38, 9023.

Peterson, K.P.; Larock, R.C. J. Org. Chem., 1998, 63, 3185. Ait-Mohand, S.; Hénin, F.;. Muzart, J. Tetrahedron Lett., 1995, 36, 2473. Ait-Mohand, S.; Muzart, J. J. Mol. Catal. A: Chemical, 1998, 129, 135. Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahedron Lett., 1998, 39, 6011. Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem., 1999, 64, 6750; Nishimura, T.; Ohe. K.; Uemura, S. J. Am. Chem. Soc., 1999, 121, 2645.

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Brink, G.-J. ten; Arends, I.W.C.E.; Sheldon, R.A. Science, 2000, 287, 1636. Bianchi, D.; Bortolo, R.; D'Aloisio, R.; Ricci, M. Angew. Chem. Int. Ed, 1999, 38, 706; J. Mol. Catal A: Chemical, 1999, 150, 87. 67. Bortolo, R.; Bianchi, D.; D'Aloisio, R.; Querici, C.; Ricci, M. J. Mol. Catal. A: Chemical, 2000, 153, 25. 68. Bianchi, D.; Bortolo, R.; D'Aloisio, R.; Ricci, M. Angew. Chem. Int. Ed., 1999, 38, 706; J. Mol. Catal. A: Chemical, 1999, 150, 87. 69. Ito, N.; Phillips, S.E.V.; Stevens, C.; Ogel, Z.B.; McPherson, M.J.; Keen, J.N.; Yadav, K.D.S.; Knowles, P.P. Nature, 1991, 350, 87. 70. Drauz, K.; Waldmann, H. Enzyme Catalysis in Organic Synthesis, VCH: Weinheim, 1995, Chapter 6. 71. For example see: Skibida, I.P.; Sakharov, A.M. Catal. Today, 1996, 27, 187; Sakharov, A.M.; Skibida, I.P. J. Mol. Catal., 1988, 48, 157; Feldberg, L.; Sasson, Y. J. Chem. Soc. Chem. Commun., 1994, 1807; Capdevielle, P.; Sparfel, D.; Baranne-Lafont, J.; Cuong, N.K.; Maumy, D. J. Chem. Res. (S), 1993, 10; Munakata, M.; Nishibayashi, S.; Sakamoto, S. J. Chem. Soc. Chem. Commun., 1980, 219; Bhaduri, S.; Sapre, N.Y. J. Chem. Soc. Dalton Trans., 1981, 2585; Jallabert, C.; Rivière, H. Tetrahedron Lett., 1977, 1215; Jallabert, C.; Lapinte, C.; Rivière, H. J. Mol. Catal., 1980, 7, 127 and J. Mol. Catal, 1986, 14, 75; Jallabert, C.; Rivière, H. Tetrahedron, 1980, 36, 1191. 72. Semmelhack, M.F.; Schmid, C.R.; Cortes, D.A.; Chou, C.S. J. Am. Chem. Soc., 1984, 106, 3374. 73. Marko, I.E.; Giles, P.R.; Tsukazaki, M.; Brown, S.M.; Urch, C.J. Science, 1996, 274, 2044; Marko, I.E.; Tsukazaki, M.; Giles, P.R.; Brown, S.M.; Urch, C.J. Angew. Chem. Int. Ed. Engl., 1997, 36, 2208. 74. Marko, I.E.; Giles, P.R.; Tsukazaki, M.; Chellé-Regnaut, I.; Gautier, A.; Brown, S.M.; Urch, C.J. J. Org. Chem., 1999, 64, 2433. 75. Marko, I.E.; Gautier, A.; Chellé-Regnaut, I.; Giles, P.R.; Tsukazaki, M.; Urch, C.J.; Brown, S.M. J. Org. Chem., 1998, 63, 7576. 76. Coleman, K.S.; Lorber, C.Y.; Osborn, J.A. Eur. J. Inorg. Chem., 1998, 1673. 77. Coleman, K.S.; Coppe, M.; Thomas, C.; Osborn, J.A. Tetrahedron Lett., 1999, 40, 3723. 78. Minachev, Kh.M.; Antoshin, G.V.; Klissurski, D.G.; Guin, N.K.; Abadzhijeva, N.Ts. React. Kinet. Catal. Lett., 1979, 70, 163. 79. But see: Kirihara, M.; Ochiai, Y.; Takizawa, S.; Takahata, H.; Nemoto, H. Chem. Commun., 1999, 1387. 80. Muzart, J. Tetrahedron Lett., 1987, 28, 2133; Muzart, J. Chem. Rev., 1992, 92, 113. 81. Chen, J.D.; Lempers, H.E.B.; Sheldon, R.A. J. Chem. Soc., Faraday Trans., 1996, 92, 1807. 82. Sheldon, R.A.; Wallau, M.; Arends, I.W.C.E.; Schuchardt, U. Acc. Chem. Res., 1998, 31, 485; Sheldon, R.A.; Arends, I.W.C.E.; Lempers, H.E.B. Coll. Czech. Chem. Commun., 1998, 63, 1724. 83. Iwahama, T.; Sakaguchi, S.; Nishiyama, Y.; Ishii, Y. Tetrahedron Lett., 1995, 36, 6923. 84. Yoshino, Y.; Hanyashi, Y.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem., 1997, 62, 6810; Kato, S.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem., 1998, 63, 222; Sakaguchi, S.; Kato, S.; Iwahama, T.; Ishii, Y. Bull. Chem. Soc. Jpn., 1988, 77, 1. 85. Barak, G.; Dakka, J.; Sasson, Y. J. Org. Chem., 1988, 53, 3553. 86. Jacobsen, S.E.; Muccigrosso, D.A.; Mares, F. J. Org. Chem., 1979, 44, 921; see also Bortolini, O.; Campestrini, S.; Di Furia, F.; Modena, G. J. Org. Chem., 1987, 52, 5467. 87. Trost, B.M.; Masuyama, Y. Tetrahedron Lett., 1984, 25, 173. 65. 66.

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Bortolini, O.; Conte, V.; Di Furia, F.; Modena, G. J. Org. Chem., 1986, 57, 2661. Sato, K.; Aoki, M.; Takagi, J.; Noyori, R. J. Am. Chem. Soc., 1997, 779, 12386. Sato, K.; Takagi, J.; Aoki, M.; Noyori, R. Tetrahedron Lett., 1998, 39, 7549. Ishii, Y.; Yamawaki, K.; Yoshida, T.; Ura, T.; Ogawa, M. J. Org. Chem., 1987, 52, 1868. Ishii, Y.; Yamawaki, K.; Ura, T.; Yarnada, H.; Yoshida, T.; Ogawa, M. J. Org. Chem., 1988, 53, 3587. Yamawaki, K.; Nishihara, H.; Yoshida, T.; Ura, T.; Yamada, H.; Ishii, Y.; Ogawa, M. Synth. Commun., 1988, 18, 869. Yamawaki, K.; Yoshida, T.; Nishihara, H.; Ishii, Y.; Ogawa, M. Synth. Commun., 1986, 16, 537. Venturello, C.; Gambaro, M. J. Org. Chem., 1991, 56, 5924. Neumann, R.; Gara, M. J. Am. Chem. Soc., 1995, 117, 5066. Zauche, T.H.; Espenson, J.H. Inorg. Chem., 1995, 37, 6827. Espenson, J.H.; Zhu, Z.; Zauche, T.H. J. Org. Chem., 1991, 64, 1191. Masuyama, Y.; Takahashi, M.; Kurusu, Y. Tetrahedron Lett., 1984, 25, 4417. Kurusu, Y.; Masuyama, Y. Polyhedron, 1986, 5, 289. Kurusu, Y.; Masuyama, Y.; Saita, M. Bull. Chem. Soc. Jpn., 1985, 58, 1065. Kurusu, Y.; Masuyama, Y. J. Macromol. Sci. Chem., 1987, A24, 389. Kaneda, K.; Kawanishi, Y.; Jitsukawa, K.; Teranishi, S. Tetrahedron Lett., 1983, 24, 5009. Kanemoto, S.; Saimoto, H.; Oshima, K.; Nozaki, H. Tetrahedron Lett., 1984, 25, 3317. Kaneda, K.; Kawanishi, Y.; Teranishi, S. Chem. Lett., 1984, 1481. Choudary, B.M.; Vialli, V.L.K. J. Chem. Soc. Chem. Commun., 1990, 1115. Kozlov, A.; Kozlova, A.; Asakura, K.; Iwasawa, Y. J. Mol. Catal. A: Chemical, 1999, 137, 223. Arends, I.W.C.E.; Sheldon, R.A.; Wallau, M.; Schuchardt, U. Angew. Chem. Int. Ed. Engl, 1997, 36, 1144. Maspero, F.; Romano, U. J. Catal., 1994, 146, 476. Murahashi, S.I.; Naota, T.; Nakjima, N. Tetrahedron Lett., 1985, 22, 2361; see also Fung, W.H.; Yu, W.Y.; Che, C.M. J. Org. Chem., 1988, 63, 2873; Tanaka, M.; Kobayashi, T.; Sakakura, T. Angew. Chem. Int. Ed. Engl., 1984, 23, 518. Zondervan, C.; Hage, R.; Feringa, B.L. Chem. Commun., 1997, 419. Masutani, K.; Uchida, T.; Irie, R.; Katsuki, T. Tetrahedron Lett., 2000, 41, 5119.

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Chapter 4 Functional model oxygenations by nonheme iron complexes

Takuzo Funabiki Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

Abstract: Nonheme iron oxygenases are known to catalyze various types of highly selective oxygenations. Many types of nonheme iron complexes have been developed by mimicking structures and catalytic functions of oxygenases. Toward our goal to develop some selective oxygenation catalysts that are useful not only in laboratory but also industry and environmental fields, recent results showing positive activities on the enzyme-like oxygenations by nonheme iron complexes are summarised. The results are not limited to homogeneous systems, but some extended examples to the heterogeneous systems are also involved. Two types of oxygenations, dioxygenase-like and monooxygenase-like oxygenations, are reviewed separately. In each part, oxygenations by molecular oxygen and activated oxygens are summarised separately. Structural information of complexes and detailed discussions on mechanism of oxygenations are out of scope of this chapter, but attention was paid to discussions on radical and nonradical processes. Some recent important results on nonheme iron peroxo and oxo species are also involved in the final part. To avoid duplication with the contents in Vol. 19 of this series, most references are those from the last 5 years, but some types of model systems are described in the longer range with supplying references. Key words: nonheme iron complexes, functional models for oxygenases, dioxygenase-like and monooxygenase-like oxygenations, activation of molecular oxygen, radical and nonradical oxygenations, tridentate and tetradentate ligands

157 L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 157-226. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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1. INTRODUCTION Catalytic oxidations and oxygenations with molecular oxygen are among the most important topics in recent years in the large fields of chemistry and biology, and great efforts have been made for understanding mechanisms of reactions and development of chemical model systems in both fields of industries and academic laboratories. Insertion of oxygen to hydrocarbons as functional groups converts them to value-added materials that are used in the many different ways. However, molecular oxygen, that is a simple molecule and present in air, is the molecule that is hard to be controlled in the reactivity. This is based on the triplet state of oxygen in the ground state while the ground state of most materials is the singlet state. Once oxygen is activated, e.g. to superoxide ion, it reacts readily with various compounds in the way of out of control of the selectivity in some cases. Thus, various types of activate oxygen species, e.g., hydrogen peroxide, alkylperoxides, peracids, etc., have been used in place of molecular oxygen for the selective oxidations and oxygenations under the controlled conditions. These activated oxygen species, however, require special cares in handling and storage. Thus, people challenge to develop the selective reaction systems with using molecular oxygen. Various metal enzymes, on the other hand, are known to activate oxygen and to oxygenate materials with or without incorporation of molecular oxygen. Remarkably, oxygenases oxygenate various substances in the highest selectivity in spite of the usage of molecular oxygen. Interestingly most of these oxygenases involve iron or copper as an active center, that is common to other metalloproteins participating in the dioxygen transfer and storage (e.g. hemoglobin, myoglobin, hemocyanin, hemerythrin, etc.) and other catalytic processes by oxidases (cytochrome c oxidase, etc.), peroxidases (horseradish peroxidase etc.), catalases, and superoxide dismutases. Iron and copper are the most popular metals for mankind from ancient times, but these have not been used efficiently as artificial catalysts for catalytic oxygenations and oxidations. Chemists pose questions why metalloenzymes use these metals and how selectivity is controlled, and start to challenge to develop good catalysts that work not only in the similar fashion to enzymes, but also more efficiently to various substrates than enzymes. For this purpose, clarification of structures of active-center environments is important and a lot of heme and nonheme complexes have been synthesized as structural model complexes. No doubt information obtained from these complexes is great, but recent remarkable progress in X-ray crystallographic analyses has made possible to clarify directly structures of metalloproteins. Since X-ray crystallographic analyses are useless for structures of enzymes and complexes, various spectroscopic data are important for clarification of structures in solution. These structural model

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complexes are often too stable to function catalytic activity, that makes difficult to use the complexes for clarification of mechanisms and development of catalysis. Instead, functional model studies have attracted much attention in recent years. Most functional model studies are obliged to start before clarification of structures of metal centers, even the kind of amino acid residues that bind to metals. In case of heme protein models, various porphyrin complexes have been synthesized and their functions have been compared with enzymes. In case of nonheme proteins, combination of various ligands with N or O atom as a coordination site has been studied for development of function. Continuous efforts have been made for improvement of ligands, based on the electronic and steric effects of ligands on activity and selectivity of the reactions.

2. HEME AND NONHEME OXYGENASES Enormous amounts of research works have been reported on heme oxygenases and their model systems.1-5 Cytochrome P-450 monooxygenase is the most attractive research target in recent years, and keeping the position of the main topics in the field of bioinorganic chemistry. Characteristically the enzyme is ligated by porphyrin and cystein residue as an axial ligand. Formation of the highly oxidized iron porphyrin species, Fe(IV)=O porphyrin cation which is termed compound I, is thought to be involved cycle as a direct active species in the oxygenation.3, 6 In the cases of peroxidases and catalases, it became possible to detect compound I species, but not in the case of cytochrome P-450 probably because of the high reactivity.7 Prior to the O-O bond fission to form the compound I species, iron(III)-peroxo and/or hydroperoxo species is believed to be formed, but hard to be detected in the enzymatic systems. Contribution of model studies on these oxygen-bound iron species is great. Since Tabushi reported the first example,8 the systems of the reducing complexes have been used for the catalytic oxygenations of various organic substrates. The utilization of molecular oxygen is very attractive for development of catalytic processes and many different types of the reducing reagents and metalloporphyrins have been applied. However, the difficult points are encountered: the separation of the reductants from the active species to achieve the most efficient utilization of the reductants, development of the recycle process of the reductants, or the effective use of the oxidized products of the reductants. Instead of the reductive activation of molecular oxygen to form the compound I, various types of oxygen sources are used, e.g. hydrogen peroxide, alkyl peroxides, peracids, iodosylarene, sodium metaperiodate, etc. Strong supports for the ability of oxo-ferryl porphyrin radicals to carry out oxygen atom

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transfer to the organic substrates were first shown by Groves and co-workers.9, 10 Since then, numerous example of oxygen transfer to alkanes and alkenes from these oxygen donors and the spectroscopic characterization of a green oxo-ferryl porphyrin radical species have been reported.7, 11-22 This type of model systems can be applied to asymmetric or shape selective oxidations.6 The model complexes are placed in the different environments for achievement of specificity, e.g., in artificial membrane,23-26 on the polymers,27, 28 on the solid supports,29-36 in the dendolimers,37 etc. On the other hand, synthetic model complexes for peroxoiron(III) species, ferric porphyrin peroxo complexes, are known to catalyze epoxidation of olefins.38-45 Nucleophilic epoxidation reactions are accelerated by the presence of DMSO, which coordinates in the axial position.44 Compared with heme iron oxygenases, which function mostly as monooxygenases, various types of oxygenations are catalyzed by nonheme iron mono- and di-oxygenases.4 Types and numbers of amino acid residues as ligands depend on individual enzymes. Soluble methane monooxygenase (sMMO) has attracted much attention in recent years, and a great progress has been made in clarification of structures and reaction mechanisms of specific di-iron enzymes and model systems. In the other monooxygenases, pterin-dependent hydroxylases (phenylalanine hydroxylase, tyrosine hydroxylase, tryptophan hydroxylase) are interesting mono-iron enzymes, but many problems about enzyme structures and mechanisms are left unclear. Contribution of model studies for this type of enzymes must be said poor. Isopenicillin N synthase is also interesting mono-iron enzyme by which the C-C bond formation rather than oxygen insertion is catalyzed, but barrier is rather high for bioinorganic chemists to work on reactions that require treatment of organic substrates with complicated structures. The most popular dioxygenases in nonheme iron oxygenases are catechol dioxygenases that are the key enzymes in the metabolism of aromatic compounds. Two types of cleavage of the aromatic C-C bond, i.e. intradiol and extradiol cleavages, are catalyzed by two different types of oxygenases involving and respectively. -containing oxygenases have been extensively studied from both sides of enzyme and model in these two decades. -containing oxygenases are becoming a research target in recent years, but functional model studies by using -complexes should overcome a problem how the state is recycled after a catalytic turnover under the oxygenation conditions. acid-dependent dioxygenases are also interesting dioxygenases that insert only one atom of oxygen to substrates accompanied by the transfer of another atom of oxygen to acid. Chemical approach to this type of enzyme is poor, but the enzyme will be the next important object to be studied. Lipoxygenase is another important oxygenase. In

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spite of the extensive works on the enzymes including recent crystallographic analysis, the oxygenation mechanisms are left to be clarified. It is expected that model studies will give decisive information about reaction mechanisms. Characterization of active species in nonheme iron systems has been supposed more difficult than that in heme systems. However, in addition to isolation of stabilized species in the model systems, developments in the spectroscopic methods for detection of unstable species and in the quantum chemical methods have given a rich information about the structures and reactivity of nonheme iron oxygen species. High-valent iron-oxo species have been proposed in the nonheme systems to explain reactivity from analogy to heme species. No spectroscopic evidence for the unstable species has been obtained,46 but recent evidences for monoiron peroxo species and their recitivities4752 supported strongly the formation of the high-valent iron-oxo species. Formation of peroxo iron species is also highly probable in the methane monooxygenase systems. This chapter focuses mainly on functional model oxygenations by nonheme iron complexes and summarizes the progresses since 1996 to avoid repetition of the contents in Vol. 19 in the series in 1997.4 In the studies on dioxygenase-model oxygenations, a remarkable progress has been attained both in the intra- and extradiol cleavage of catechols by nonheme iron complexes with various types of ligands. Other dioxygenases such as acid-dependent dioxygenases and lipoxygenase have little studied for development of functional model oxygenations. In the studies on monooxygenase-model oxygenations, detection, isolation, and characterization of mono- and diiron nonheme iron-oxygen species, such as and are the highlight of the recent bioinorganic chemistry. Reactivities of these species for hydroxylation, ketonization, and epoxidation are also the most interesting topics.

3. FUNCTIONAL MODEL STUDIES ON NONHEME IRON DIOXYGENASES 3.1 Catechol Dioxygenases Catechol dioxygenases play a key role in the metabolism of aromatic compounds. They were first classified as dioxygenase in 1955 by Hayaishi who discovered the oxygen insertion into muconic acid in the oxygenative cleavage of pyrocatechol catalyzed by pyrocatechase.53 Two types of oxygenations are known, i.e., intradiol and extradiol cleaving oxygenations as shown in Fig. 1. The intradiolcleaving oxygenases involve as an active center, that is ligated by 2 His and 2 54-57 Tyr residues. The extradiol cleavage is mainly catalyzed by enzymes containing

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that are ligated by 2 His and 1 Glu residues,58, 59 but some enzymes are known also to catalyze the extradiol cleavage of some type of catechols.60-62 63,64 containing enzymes are known to catalyze the extradiol cleavage.

There is another group of catechol 1,2-dioxygenases, i.e. chlorocatechol dioxygenases. Compared with ordinary catechol dioxygenases (e.g. catechol 1,2dioxygenase, protocatechuate 3,4-dioxygenase), these enzymes exhibit the higher activities and affinities toward chlorinated catechols, the higher selective intradiol cleavage, and the higher affinities towards a wide range of substituted catechols bearing electron-donating and -withdrawing groups. The presence of high-spin Fe(III) in a rhombic environment with tyrosine coordination has been reported, but evidence for the histidine coordination has not been obtained.65, 66 Once chlorinated aromatic compounds are converted to chlorocatechols, these enzymes oxygenate them to chloromuconic acids that are dechlorinated to non-chlorinated acids by cycloisomerases. This indicates that the enzymes can be good targets of the model chemistry for development of catalysts that degrade halogenated pollutants.

3.1.1 Intradiol Cleavage Oxygenations Functional model oxygenations were performed by using activated oxygen species until early 80’s in the absence of metals, singlet 75-79 and peroxo compounds or in the presence of metals other

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than iron.77, 80-84 Strange to say, no nonheme iron complex has been used for cleavage of catechols with molecular oxygen so long time. In 1979, Funabiki et al. reported the oxygenative cleavage of 3,5-di-tertburylcatechol by a complex with molecular oxygen.85 The reaction was performed in anhydrous tetrahydrofuran at 25 °C under 1 atm by using pyridine and bipyridine as ligands. Though the yield of the oxygenated product was low compared with 3,5-di-tert-butyl-l,2-benzoquinone (DTBQ), it was first shown that iron complexes could be potential catalysts for the oxygenase-like oxygenations. In this first trial, a complex was used, considering the importance of interaction of the complex with oxygen, but soon later it was shown that the reaction is catalyzed by complexes similarly to -containing 86 oxygenases. The main oxygenation products are shown in eq. 1. Isolation of an intermediate monooxygenated compound, 2,4-di-tert-butylmuconic acid anhydride, and its conversion to dioxygenated product, 3,5-di-tert-butyl-5-(carboxymethyl)-2furanone, indicated that the oxygen insertion proceeds stepwise rather than in one step via a dioxetane intermediate.86 In addition to these intradiol cleavage products, extradiol cleavage products, i.e. 3,5-di-tert-butyl-5-(formyl)-2-furanone and 3,5-ditert-butyl-2-pyrone, were formed, 86 indicating that both intra- and extradiol cleavages occur in this iron system using simple ligands.

The detailed studies in seeking for the better system with the higher selectivity and catalytic activities revealed that in the series of mono- and bidentate ligands pyridine and bipyridine are the most effective for oxygenations. 87 The oxygenation proceeded catalytically in the presence of excess catechol, though the catalytic efficiency was low because of the formation of DTBQ. This problem was overcome by addition of 2,5-di-tert-butyl-hydroquinone that can convert DTBQ to In the studies on mechanisms, the insertion of two oxygen atoms into products was confirmed by experiments using in the anhydrous conditions. The insertion of two atoms of oxygen was also shown recently by Yamahara et al..88 They synthesized a tridentate ligand (L = N-(2-hydroxyphenyl)-N-(2-pyridylmethyl)-benzylamine) having two N and one O atoms for coordination and isolated catecholate complexes, and Oxygenation of these complexes with gave products with two atoms of

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different from the complexes having a tetradentate ligand, which are oxygenated with one atom of insertion of Importance of the vacant site for coordination of was suggested in the enzyme-like oxygenation. As for the intermediate species in the pyridine system, it was first shown that the reaction proceeds via a monocatecholatoiron complex, which exhibited characteristic bands at 550 and 980 nm in the electronic spectra. Later, these types of bands were found to be characteristic to most catecholatoiron complexes and assigned to the LMCT bands.89, 90 The crystallographic analysis of the complex clarified that the complex is dimeric in the solid state and monomeric in solvent (eq. 2).91

In 1982 and 1985, Weller and Weser reported that the catalytic oxygenation of could be performed by iron complex prepared in situ with and sodium nitrilotriacetate in aqueous borate buffer and MeOH or DMF.92, 93 However, the catalytic activity and selectivity reported were questioned afterwards for the lack of reproducibility of the results.94, 95 Nevertheless, this result could contribute greatly to the progress of the model chemistry of catechol dioxygenases, because the NTA ligand gave a basic idea for utility of the various tripodal ligands. In 1984, White et al. isolated a complex, which was crystallized for X-ray analysis. This complex reacted with oxygen to give oxygenated products in 80% yield, with (98%) when incorporation of one was used.96 Que et al. used a new ligand replacing one of the carboxymethyl arms of NTA by a 2-hydroxybenzy1 group (HDA).97 Effect of substituents on the 2-hydroxybenzyl group was studied with complexes and led the proposal that the yield of the oxygenated product decreases as the Lewis acidity of the metal center diminishes. The Lewis acidity of the metal center was correlated to the energy of the LMCT and the E° value. The similar results were obtained by using

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the analogous tripodal ligands L by the same research group: L = PDA (R = COO , R’ = 2-pyridyl), BPG (R = 2-pyridyl, R’ = COO-), HDP (R = 2pyridyl, R’ = 2-hydroxy-3,5-dimethylphenyl),98 TPA (R = R’ = 2-pyridyl).99 The high reactivity of the TPA complex towards was correlated with the stronger iron-catecholate interaction compared with other complexes, resulting in the enhanced covalency of the metal-catecholate bonds and low-energy catecholate LMCT bands.99 Some tripodal ligands are shown in Fig. 2.

Efforts were made to find a quantitative correlation between the reactivity of the catecholate ligand with and Lewis acidity of the iron center. However, the Lewis acidity which was originally introduced for the qualitative explanation is not suitable for the quantitative treatment even though it is predicted on the electrochemical (redox potential) or spectral data (LMCT band energies). Viswanathan and Palaniandavar have found a good correlation between and phenolate-to-iron(III) LMCT band energies in the series of complexes of [Fe(L)DTBC] prepared in situ in methanol by using L which have two nitrogen (amine, pyridine or imidazole) and one oxygen (phenol) for coordination (Fig. 3) : L = N-(pyridin-2-ylmethyl)salicylideneamine, (2-hydroxy-5-nitrobenzyl)(pyridine-2ylmethyl)amine, (2-hydroxy-5-nitrobenzyl)(2-pyridin-2-ylethyl)amine, N-(2-

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imidazol-4-ylethyl)salicylideneamine, N -(benzimidazole-2-ylmethyl)salicylaldimine, (benzimidazol-2-ylmethyl)(2-hydroxybenzyl)amine, (benzimidazol-2ylmethyl)(2-hydroxy-5-nitro-benzyl)amine100 or by using tripodal ligands involving one amine nitrogen atom, one or two oxygen of phenolate or nitrogen of pyridine or imidazole.101 A fairly good correlation was found between the catalytic activity of the oxygenation and the as well as redox potentials, but some innegligible exceptions were also found, e.g. increase in the yield by decreasing Lewis acidity on introducing one or two phenolate. The change of the rate-determining step, i.e. from the oxygen attack or ring opening to the product release, and steric effects were proposed for explanation of this result without any evidence. It seems rather difficult to explain the reactivities only by the electronic properties of the iron center and cateholate ligands.

Yamahara et al. synthesized the [Fe(L)(DTBC)+ complex (L = 2-hydroxy-3R’-5-R”-phenyl-bis(2-pyridylmethyl)amine, R’, R” = H, H; Me, Me; H, Cl), mimicking the enzymatic intermediate species ligated by one tyrosine and two histidine residues.102 The pseudo-first-order rate constants increased in the order of substituents on the phenolate ligand, Me, Me < H, H < H, Cl. Acceleration of the reaction by electron-withdrawing substituent is on the same line described above, that is, oxygenation is favored by the higher Lewis acidity of the iron center. Mialane et al. synthesized Fe(III) catecholate complexes which all contain aminopyridine ligands: N,N’-dimethyl-N,N ’-bis(2-pyridylmethyl)ethane-1 ,2-diamine, = N,N’-dimethyl-N,N’-bis(4-chloro-2-pyridyl-methyl)ethane-1,2-diamine, trispicMeen = N,N’-dimethyl-N,N’,N”-tris(2-pyridylmethyl)ethane-1,2-diamine, BQPA = bis(2quinolylmethyl)(2-pyridylmethyl)amine).103 These complexes react with to give quantitatively (90%) the intradiol cleavage product, 3,5-di-tert-butyl-5(carboxymethyl)-2-furanone in DMF. A correlation of the second-order kinetic

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constants with the optical parameters of the two LMCT O(DTBC) Fe(III) bands was found except for (probably for the reason of steric hindrance). The much lower activity of these complexes compared with was ascribed to importance of the asymmetric coordination of the DTBC ligand for the efficient intradiol cleavage. The effect of substituents on the reactivity and the electronic state of the catecholatoiron complexes were studied from different sides. Duda et al. have synthesized the tripodal ligands with one (BPIA) or two (BIPA) methylimidazole moieties (BPIA = bis[(2-pyridyl)methyl][(1-methylimiazol-2-yl)methyl]amine, BIPA = bis[(1-methylimidazol-2-yl)methyl][(2-pyridyl)methyl]amine).104-106 The reactivity of oxygenation (pseudo-first order rate constants) of the complex, decreased in the order of substituent on the catecholate ligand, indicating that electron-donating substituents on catechol result in a higher dioxygenase reactivity. The complexes exhibit the lower reactivity than the TPA complex, and the rate constant was correlated with the lower energy LMCT band (nm): (4.3, 865), (0.98, 842), (0.089, 729), (0.015, 718). Koch and Krüger developed a tetraazamacrocyclic ligand =N,N’ -dimethyl-2,11-diaza[3.3](2,5)pyridinophane) and reported the 54 turnovers in the oxygenation of DTBCH2.95 Mialane et al. synthesized an iron (III) complex, with a doubly positively charged ligand (bispic= N,N’-bis(2-pyridylmehtyl)-N-N’ -bis(5-trimethylammoniumpentyl)-1,2diaminoethane).107 The complex in DMF reacts with at 20 °C to afford selectively 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone (90%) after 48 h. The effect of the O and N atom for the ligand coordination is the recent topics to be clarified, because despite the Try and His ligands in enzymes no ligand having both N and O atoms for coordination gives the higher activity than those having only N atoms.100, 102 Weiner and Finke developed a very unique system using all-inorganic polyoxoanion as a ligand, that exhibits > 100,000 catalytic turnovers, e.g. by as a prototype precatalyst.108

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The functional model oxygenations above mentioned are all for catechol 1,2dioxygenases (CTD) or protocatechuate 3,4-dioxygenases (3,4-PCD). However, there is another group of catechol dioxygenases, chlorocatechol dioxygenases (CCD). These dioxygenases are characteristic of the low substrate specificity and catalyze the intradiol cleavage of not only mono or di-halogenated catechols, but also other various catechols.4 The function and structure of this type of enzymes are not well characterized,58, 59, 65, 66 but development of the functional model oxygenations is a very attractive object from both sides of basic and applied sciences, especially environmental sciences. Funabiki et al. was successful to develop the functional model oxygenation of 3- or 4-chlorocatechol by using TPA as a ligand (eq. 3).109

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Formation of chlorocatecholatoiron complexes and the oxygenative decomposition with molecular oxygen were monitored by electronic absorption spectroscopy and by product analysis. Interestingly dehydrochlroination of the initial products was accompanied to give the products without the chloride group. It was found that the addition of 2,6-lutidine as base is effective for promotion of the oxygenation. The cleavage of chlorocatechols by nonheme iron complexes indicated clearly that catechols for the model chemistry are not limited to 3,5-di-tert-butylcatechol and that iron complexes can oxygenate the catechols having electron-withdrawing substituents. From the viewpoint of dehalogenation of halogenated aromatic hydrocarbons, it is important to develop model systems that decompose multi-halogenated catechols, e.g. dichlorocatechol and tetrachlorocatechol. Apart from the model oxygenations, Sorokin et al. developed powerful catalysts for oxidation of tetrachlorocatechol to dichloromaleic acid by using ironphthalocyanine in the presence of or percarbonate as an oxidant.110 The catalyst oxygenates DTBC to give a furanone. A new functional model chemistry on catechol dioxygenases has been developed by Funabiki et al..111 Water-soluble ligands were prepared by sulfonation of tripodal ligands such as TPA and used for catalytic oxygenation of water-soluble catechols such as 4-tert-butylcatechol, 4-chlorocatechol, and protocatechuic acid (eq. 4).

Despite a typical substrate in the enzymatic system, protocatechuic acid was found to be oxygenated by model complexes not in organic solvents but in water. The highly selective and catalytic intradiol oxygenation of 4-tert-butylcatechol indicated that various types of catechols other than 3,5-di-tert-butylcatechol can be used as substrates in the model systems and that oxygenations by nonheme iron systems in water are attractive. The reactivity and selectivity are dependent on the substituent on catechols and of the solution.

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3.1.2 Extradiol Cleavage Oxygenations Structural studies on the catechol dioxygenases that function in the extradiol cleavage oxygenations have progressed remarkably in recent years. However, the functional model studies are far beyond those for the intradiol cleavage. This is because it is hard to isolate the intermediate model complexes, e.g. and even if isolated they are readily converted to the Fe(III) complexes under The first example of the extradiol cleavage by iron complexes was reported by Funabiki et al..86, 87, 112, 113 The extradiol cleavage products were obtained together with the intradiol cleavage products in the system (intra : extra = ca 2 : 1) (eq. 1). In this system, the presence of the species in solution is probable since the oxidation of DTBC to DTBQ as a side reaction is accompanied by the reduction of to Participation of the complex for the extradiol oxygenation was also shown by the different reactivity of from in the in situ system in the presence of 112 1:2 pyridine or water (intra : extra = 1 : 5 Similarly, Lin et al. found that compared with (TACN = 1,4,9-triazacyclononane) pyridine in methanol produced the higher yield of the extradiol cleavage product over the intradiol from pyrocatechol and 3methylcatechol: 1 : 6.7 and 1 : 3 from pyrocatechol.114 The rapid product formation was observed prior to the formation of the complex. The extradiol cleavage of a range of 3- and 4-substituted catechols with electron-donating substituents was observed.115 Different selectivity was observed when monosodium catecholate was used in the absence of pyridine, that may imply a requirement of a proton donor for the extradiol cleavage. Lim et al. synthesized a Fe(II) complex, (BLPA = bis(6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine, DTBCH = 3,5-di-tert-butylcatecholate monoanion), that reacts with to give the intradiol (65%) and extradiol (20%) cleavage products.116 The partial direct reaction of the Fe(II) complex with was assumed in addition to the reaction of the Fe(III) complex, which is formed rapidly under but the evidence for the participation of the Fe(II) complex is poor. Jo et al. also synthesized a series of Fe(II)-monoanionic catecholate complexes, 117

The crystal structure of showed that the DBCH ligand binds to the iron center asymmetrically. The complexes react with or NO to afford blue-purple Fe(III)catecholate dianion complexes, that react further with to give a high yield of cleavage products. The products are mainly derived from intradiol

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cleavage with a small extent of extradiol cleavage (89 : 3 or 78 : 12). Weiner et al. developed polyoxoanion-supported Fe(II) complex that produces the extradiol cleavage products (ca. 52%) together with DTBQ (40%).118 All-inorganic polyoxoanion as oxidation-resistant ligand forms a Fe(II)-DTBC species that binds with to form 1 : 1 : 1 species (volumetric titration) as shown in Fig. 5. The oxygen species decomposes very slowly to give oxygenated products. No explanation has been given to the very slow (7 days) oxygenation of DTBC, but the system gives an example of catalytic dioxygenase-like system without the need to worry about catalyst lifetime-limiting, ligand-oxidation side reactions.

Instead of the use of complexes, the extradiol oxygenation by complexes has been studied.90, 119, 120 This mimics the function of the containing enzymes that produce the extradiol oxygenation products from some catechols.60-62 Dei et al. synthesized tri- and tetra-azamacrocyclic ligands and studied the reactivity of the complexes with oxygen, and reported that the complex in acetonitrile yielded 3,5-bis(1,1dimethylethyl)-2H-pyranone (30%) and 4,6-bis(1,1-dimethylethyl)-2H-pyranone (5%) by the extradiol cleavage in addition to DTBQ (65%) (eq. 5).90

Ito et al. used (but not AgOAc) to remove the chloride from the same complex and found that the extradiol cleavage products, two isomeric di-tert-butyl2-pyrones, can be selectively produced (76 : 16).119 It is characteristic that the intradiol cleavage products are formed in only trace amounts. Ogihara et al. used the hydrotris(pyrazolyl)borate ligand to synthesize a five-coordinate

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catecholate complex from which the extradiol oxygenation products (pyrones: 39 + 28%) were obtained together with the intradiol oxygenation product (33%), but not with quinone.120 These results suggested that the presence of the vacant site, i.e. formation of the five-coordinate catecholatoiron species as an intermediate, is the more important for the extradiol cleavage oxygenations rather than the Fe(II) active center. However, the presence of the vacant site is not enough for explanation of the selectivity control by Fe(II) and Fe(III) centers. It is known that the five-coordinate intermediate gives selectively the intradiol cleavage product in the enzymatic system.121 Lim et al. reported formation of both the intradiol (75%) and extradiol (15%) cleavage products from the six-coordinate Fe(III) 122 complex,

Recently, Jo and Que reported the importance of the location of the vacant site for the extradiol oxygenation.123 They compared reactivities of a series of Fe(III) catecholate complexes, containing tridentate ligands that can coordinate to iron in a facial or meridional fashion ( 1,4,7-trimethyl-1,4,7-triazacyclononane, TPY = 2,2’ : 6’,2”terpyridine, BnBPA = N-benzyl N, N-bis(2-pyridylmethyl)arnine).123 The complexes react with in the presence of AgOTf to remove the Cl ligand. The products obtained indicated that the facial tridentate ligand favours the extradiol cleavage rather than the intradiol cleavage. In the proposed mechanism, the facial ligand allows and substrate to occupy the opposite face and form an intermediate that leads to the desired extradiol products (vide infra).

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Extradiol cleavage is performed by manganese-containing dioxygenases. Canseshi and Dei synthesized a manganese complex, [Mn(CTH)(DTBC)]Y which structure was analyzed by X-ray diffraction (CTH = (±)l5,7,7,12,14,14-hemxamethyl-1,4,8,11-tetraazacycloetradecane).124 It was found that the two complexes have different physical properties in the solid state, but the spectroscopic properties of their solutions are identical. In highly polar solvents such as DMSO, the spectra of the yellow solutions are consistent with the presence of an chromophore, whereas in weakly polar solvents such as toluene or acetone the spectra of the blue-green solutions are consistent with the presence of a chromophore. No detail has been reported, but interestingly the complex in polar solvents are stable under oxygen, while the complex in weakly polar solvents is quickly oxidized to give DTBQ (60%) and extradiol cleavage products, 3,5- or 4,6-bis(1,2-dimethylethyl)-2H-pyranones (36% + 4%). Funabiki et al. have synthesized a complex but it was converted selectively with molecular oxygen to an intradiol cleavage product.125, 126 3.1.3 Mechanisms of Oxygenations In the mechanism of the intradiol oxygenation, the first step is the activation of catechol by coordination to a Fe(III) center. The chelate coordination of catechols in the enzymatic systems (protocatechuate 3,4-dioxygenase, 3,4-PCD) has been clarified by the X-ray analysis.56, 57, l21, 127-132 In addition, the analysis indicated the binding of catechols to 3,4-PCD forms a five-coordinate complex with the axial Tyr dissociation from two Tyr and two His ligands (Fig. 7).

As described in 3.1.1, various types of catecholatoiron model complexes in the form of the chelate coordination have been isolated and found to give the oxygen insertion products by the reaction with molecular oxygen. Different from the enzyme structure, however, most of them are in the six-coordinate configuration.

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On the other hand, the probability of the monodentate coordination of the catecholate ligand has been proposed in the case of pyrocatechase.133-136 In the model system, Fujii et al.137 and Nishida et al.138, 139 reported supports for the monodentate dianionic catecholate species for the reaction with molecular oxygen based on the EPR spectroscopy and molecular orbital considerations, but no further evidence has been obtained. As described in 3.1.2, five coordinate catecholate complexes are formed in solution from [Fe(L)(DTBC)Cl] (L = TACN, after removing the chloride ligand by silver salt90, 119, l23 120 or by using a ligand in relevance to the extradiol cleavage. Interestingly, these five-coordinate complexes do not give selectively the intradiol cleavage oxygenation. Thus questions remain to be solved whether the five-coordinate intermediate for the protocatechuate 3,4-dioxygenase is specific or the oxygenation mechanisms are different between the enzymatic and model systems. The iron in enzymes retains the high-spin ferric state throughout oxygenation.140-142 However, it has been suggested that activation of the catecholate ligand in the dianionic chelate form to the radical form is important for the reaction with molecular oxygen.87, 143 This was first represented by equilibrium (eq. 6), but later by the radical character (eq. 7).

The radical character of the catecholate ligand has been supported by various spectroscopic data, e.g. UV/VIS, NMR, and XAFS. Characteristic phenolate-toiron(III) LMCT bands at ca. 500 and 900 nm, which were first observed by Funabiki et al.,86, 87, 94 are observed with various functional model catecholatoiron complexes and tried to correlate qualitatively with semiquinonate character of the catecholate ligands and quantitatively with the or DTBQ redox potentials.100, 101, l44 Recently the LMCT bands were observed both with the catecholatoiron complexes in the solid state and in solution, supporting the radical character of the catecholate ligand as represented by eq. 7.145 It was shown by the absorption edge values in the XANES spectra that the electronic states of iron of the catecholatoiron complexes are between those of and but very different 145 from that of Spin crossover from high- to low-spin states with decreasing

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temperature was observed with complex, that may be related to the intense LMCT band throughout the temperature range.145, 146 In the most mechanisms proposed for the intradiol cleavage oxygenations, the direct attack of molecular oxygen upon a carbon atom attached with a phenolate O atom is favored (substrate activation process, A in Fig. 8). The alternative mechanism prefers the direct attack of to the iron center (oxygen activation process, B in Fig. 8).

The A process is advantageous to explain the fact that even hexagonal catecholatoiron complexes such as react with This reason, however, is not applied to the enzymatic system since an open site becomes available for the direct coordination of to the iron center by dissociation of the Tyr ligand. One question arises whether the radical character of the aromatic carbon is strong enough for the direct attack of About this question, Funabiki et al. studied on the catecholatoiron complexes by Extended-Hückel148 and densityfunctional theory149 analyses. Catecholato- and 3,5-di-methylcatecholato(n = 3 or 4) were used as model complexes for convenience. Atomic charge and spin densities (spin, HOMO) of the dimethylcatecholate complex are followings. O1: -0.58 (0.29, 0.14); O2: -0.56 (031, 0.18); Cl: +0.14 (0.043, 0.088); C2: +0.043 (0.059; 0.12); C3: +0.0017 (0.023, 0.024); C4: +0.037 (0.056, 0.075); C5 +0.076 (0.049, 0.082); C6: +0.035 (0.026, 0.0006); Fe: +1.38 (3.91, 0.023). It is apparent that most of the negative and positive charges are located on O and Fe, respectively. Carbons are slightly positively charged. Most of the spin density is also located on

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O1 and O2, and the carbons have very low spin density. Recently, Wheeler et al. have studied on electronic structure of 3,5-di-tert-butylbenzosemiquinone (DTBSQ) by density functional theory analysis.150 Normal population analysis (NPA) atomic charge densities and spin densities in parenthesis (Mulliken net, NPA atomic) are followings. O1: -0.626 (+0.271, +0.272); O2: -0.662 (+0.245, +0.248); C1: +0.368 (+0.031, +0.038); C2: +0.362 (+0.075, +0.076); C3: -0.113 (+0.095, +0.087); C4: 0.257 (+0.116, +0.104); C5: -0.066 (+0.072, +0.075); C6: -0.306 (+0.111, +0.098). Most of the negative charge resides also on O1 and O2 and most of the positive charge resides on C1 and C2. Most of the spin density is also located on O1 and O2. The carbons attached to the tert-butyl groups (C3 and C5) have less spin density than those attached to hydrogens (C4 and C6). In both cases, the spin density on carbons, e.g. C2, is very low and does not link up with the direct attack of the molecular oxygen to C2 in the step of intradiol cleavage. By calculating the total energy change caused by approach to Fe or carbon, Funabiki et al. have proposed the probability of the B process (oxygen activation process) rather than the A process even in the intradiol cleavage.149 In this calculation, the configurational change of the catecholate ligand from chelate to monodentate form is assumed to open a coordination site for As for the reactivity or the semiquinonate ligand with Mialane et al. studied the structure and electronic properties of complex (L = N,N’bis(4-methyl-6-tert-butyl-2-methyl-phenolato)-N,N’-bismethyl-1,2-diaminoethane). 151 In spite of the coordination of the DTBSQ monoanion, the complex was found to be stable under supporting that the semiquinonate carbons do not bind directly with The complex is reduced not to but to that is also stable under Koch et al. also synthesized a complex, This complex was also found to be converted to with superoxide, but stable towards molecular oxygen. Since DTBSQ is proposed to react with as an intermediate in the oxygenative cleavage 153 of by the results indicate that the binding of DTBSQ to stabilizes the DTBSQ moiety against These results support the importance of the form for the C-C bond cleavage, but do not give any or positive support for the direct attack of oxygen to the semiquinonate carbon atom. Recently, experimental supports for the oxygen activation process (B) were reported to explain the extradiol oxygenation of the complex. 123 The process is favored by the complex having facial tridentate ligands rather than tetradentate ligands; in the former and substrate can occupy the opposite face and form an intermediate that leads to the extradiol products (Fig. 9). The A process was proposed for the intradiol oxygenations for the complexes with meridional

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tridentate or tetradentate ligands. However, this explanation is not enough for the preferential extradiol oxygenation over the intradiol oxygenation. If the radical spin density on aromatic carbon is similar among the complexes with facial or meridional tridentate ligands and tetradentate ligands, the probability of the oxygen attack to carbon in the B process may be similar. Since the radical character of carbon is different from that of iron in nature, we should be careful to ascribe the selectivity control only to the steric reason. If the ferric center is electron rich enough for binding with molecular oxygen, oxygen may bind first to iron for both intra- and extradiol cleavages, if necessary, with replacing a ligand.

The role of accessibility of to was also discussed on the comparable activity of the ligand system to the fast TPA systems = 2,11154 diaza[3,3](2,6)pyridinophane, Fig. 4). Different from the system, which produces only the intradiol cleavage products, the system produces both intra- and extradiol cleavage products in spite of the tetradentate ligand. The methyl group is thought to be effective to control the species. After the oxygen binding to either iron or aromatic carbon, formation of the peroxide species, has been proposed as an important species, from which one O atom is inserted between the C-C bond. There is little discussion on this process, but explained by the Baeyer-Villiger oxidation process or Criegee rearrangement. None of this type of oxygen adducts has been identified in the iron model systems. Stable rhodium and Iridium porphyrin complexes of the forms, 155 and Ir-O-O-DTBC,156, l57 were isolated, but the very low

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reactivity with is difficult to apply the result to iron systems. In the extradiol oxygenation, there are two possible ways of attachment of oxygen to aromatic carbon, C1 or C6. Funabiki et al. have first proposed the attachment of oxygen to C6 since the attachment at C1 has a possibility of both intra- and extradiol cleavages.87 However, Bugg et al. have preferred a common type of peroxo intermediate (an oxygen adduct at C1) for both intra- and extradiol cleavages.114, l l 5 , 158-163 Factors controlling the different migrations, i.e. acyl migration for intradiol cleavage and alkenyl migration for extradiol oxygenation (Fig. 10), should be clarified to support this mechanism.

Recently, Funabiki et al. have pointed out on the quantum chemical calculation study that the species if formed tends to be stabilized in its form rather than to break the O-O bond toward insertion of one O atom to DTBC.149 The alternative proposal is the attachment of an O atom of between a C-C bond to form an epoxide-like species (Fig. 11). The probability of this process should be discussed in future on the more elaborate calculations.

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Little has been studied on the mechanism of the extradiol oxygenations by enzymes and model complexes containing The first step was thought to be the 164 coordination of oxygen to a Fe(II) center, but now the coordination of catechol prior to oxygen is favored,165, 166 Two types of coordination fashion of catechols to have been proposed, i.e. bidentate59, 167-171 and monodentate.172 In the latter case, the loss of only one of the catecholate protons leads to highly asymmetric binding to This leads to charge distribution on the ring, which directs the oxygen attack of the iron-bound dioxygen to a position where extradiol cleavage must occur. Similarly to the intradiol oxygenation, carbon-oxygen bond formation between the semiquinone and superoxide to give an unsaturated lactone intermediate is postulated. Formation of this type of intermediate rather than a dioxetane intermediate was shown by the results of the incorporation by 2,3158 dihydroxyphenylpropionate 1,2-dioxygenase. As for the evidence for a semiquinone intermediate, Spence et al. have observed the cis-trans isomerization of a cyclopropyl radical trap in the oxygenation of cis- and trans-2-(2,3dihydroxyphenyl)cyclopropane-1-carboxylic acid by extradiol oxygenases.159 Formation of 85-95% trans product and 6-15% cis products during the oxygenation was explained by the reversible opening of the cyclopropyl ring of a semiquinone radical intermediate.

3.2 Dioxygenases other than Catechol Dioxygenases Lipoxygenase and dioxygenase are recent targets for model studies on nonheme iron dioxygenases other than catechol dioxygenases. Little progress has been attained in the functional model studies for these oxygenases, but some mechanistic works reported have been reported. As for lipoxygenase, Kim et al. synthesized an air-stable complex as a model for the iron(II) site of lipoxygenase.173, 174 The observed metal-centered transformation of this complex in the reaction with ROOH, i.e. was regarded to parallel the changes observed for lipoxygenase in its reaction with its product hydroperoxide. No application for peroxidation of substrate was studied. Goldsmith et al. studied the model reaction of (PY5 = 2,6bis(bis(2-pyridyl)methoxymethane)pyridine) with linoleic acid and hydrocarbons possessing weak C-H bond, for mimicking the rate-determining step, i.e. the hydrogen atom abstraction from the pentadiene subunit of the substrate by an active ferric hydroxide species to give a ferrous water species and an organic radical.175 It was found that reactivity scales best with the bond dissociation energies of

180 substrates, rather than (Scheme 1).

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supporting the hydrogen atom abstraction mechanism

acid-dependent dioxygenases were studied by Ha et al.,176 Funabiki et al.177, 178 and Que et al..179-185 It is proposed that the latter oxygenation involves the binding of to Fe(II), followed by substrate binding, and binding to afford a Fe(III) superoxide species as shown in Scheme 2. Model studies are mostly limited to characterization of complexes, which are inactive for oxygenation.

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Rieske dioxygenases catalyse enantioselective cis-dihydrokylation of arene and alkene double bonds (eq 8) and utilize a mono-nuclear nonheme iron active site.

Some nonheme monoiron complexes were found to catalyse the cis186-191 dihydroxylation with These catalysts are complexes of tetradentate N4 ligands such as TPA and BPMEN and capable of both epoxidation of olefins and cis-dihydroxylation. Recently Ryu et al. found that produces predominantly the cis-diol product with a diol : epoxide ratio of 3-4 under conditions of limiting as shown in Table 1..190 This is contrast to the preferential epoxide formation with It is reported that complexes forming low-spin Fe-OOH afford diols predominantly, while those forming hi-spin Fe-OOH afford epoxides (vide infra).

4. FUNCTIONAL MODEL SYSTEMS FOR NONHEME IRON MONOOXYGENASES Monooxygenases catalyze the incorporation of one atom of oxygen from molecular oxygen into substrates as shown in eq. 9.

Oxygenations proceed at nonheme mono- and diiron centers in the presence of electron and proton donors that are important for activation of molecular oxygen. The presence of the efficient electron and proton donor systems is characteristic of the enzymatic systems. The most popular nonheme iron monooxygenase is soluble

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methane monooxygenases (sMMO), that bear diiron centers and are able to catalyze monooxygenations of not only methane to methanol, but also other alkanes and alkenes to alcohols, ketones and epoxides.4 The other important types are monoiron enzymes, i.e. pterine-dependent hydroxylases; phenylalanine hydroxylase which catalyzes para-hydroxylation of an aromatic ring of phenylalanine, tyrosine hydroxylase which catalyzes ortho-hydroxylation of an aromatic ring of tyrosine to form a catechol moiety, and triptophan hydroxylase which catalyzes hydroxylation of an aromatic ring of triptophan. Isopenicillin N synthase is also an important monooxygenase, that catalyzes the ring closure reactions of (hydroxycarbonyl)pentanolyl]-L-cysteinyl-D-valine (ACV) without incorporation of oxygen into tiazolidine rings of isopenicillin N. Functional model chemistry for nonheme iron oxygenases has developed remarkably in recent years.5, 192, 193 Not only iron but also other metals were found to catalyze the oxygenase-like oxygenations. Examples shown here will be limited to those of nonheme iron model complexes. Enzymatic monooxygenations require molecular oxygen as an oxygen source, but both of molecular oxygen and activated oxygen, e.g. hydrogen peroxide, can be used in the functional model systems. Continuous challenging works have brought about new iron complexes that can incorporate one oxygen atom to substrates to give alcohols, ketones, epoxides, etc., and various types of ligands were designed for enhancement of activity, selectivity control, and clarification of mechanisms. In the oxygenation mechanisms, formation and structures of active iron-oxygen species are most interested. In the so-called metal-based mechanisms iron-oxo or iron-peroxo species take part in the oxygen transfer to substrates and participation of either the or manifolds has been discussed. In these efforts, and complexes have been isolated and their reactivities with substrates have been studied. It is well assumed that the postulated iron-oxo species and are too unstable to be detected or isolated, but nonspectroscopic evidence for these species came to appear in recent years. On the other hand, participation of free radicals should be considered in most cases of oxygenations. Both of carbon and oxygen centered radicals can take part in the oxygenation processes. Apart from the apparent autoxidation process, it is not easy to differentiate explicitly metal-based mechanisms from free radical mechanisms. Different types of the radical-clock reagents have been developed for detection of radicals of different lifetime, especially in the discussions on the radical-rebound mechanisms. One of important aims of functional model chemistry is to develop efficient oxygenation catalysts that are useful in industries and applied fields. Selective and

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efficient oxygenations with the use of molecular oxygen are highly desired. This is far beyond satisfactory, but very much promising.

4.1 Functional Model Oxygenations by Diiron Complexes 4.1.1 Monooxygenation by Diiron Complexes with Peroxides By mimicking soluble methane monooxygenases, monooxygenation of alkanes, alkenes, and arenes by various types of nonheme diiron complexes have been performed with utilization of activated oxygen species, e.g. mchloroperbenzoic acid (m-CPBA), hydrogen peroxide (TBHP), cumene hydroperoxide (CHP), hydrogen peroxide etc.. The reaction proceeds catalytically, but discussions focus on the oxygenation mechanisms, especially on the participation of radicals in the monooxygenation steps and similarity of mechanisms to that of enzymes. Followings are some of the recent examples. Kodera et al. synthesized a rigid complex with 1,2-bis[2-di(pyridyl)methyl-6-pyridyl]ethane that is a dinucleating hexapyridine ligand.194 The complex monooxygenates alkanes (cyclohexane, methylcyclohexane, adamantane) in the presence of m-CPBA with a large turnover frequency and number (TN ).195 Radical–rebound mechanism was suggested based on the reactivity with the additives and kinetic isotope effect. Likewise, the stable complex was activated in the presence of acid chloride RCOCl and DMF to oxygenate hydrocarbons such as cyclohexane.196 Payra et al. used diiron(III) bis(benzimidazole) complex for the quantitative and catalytic epoxidation of styrene in the presence of m-CPBA or NaOCl and N-methylmorpholine N-oxide as an important additives.197 In the absence of oxidant under anaerobic conditions, the reversible oxygen transfer was observed between the complex and triphenylphosphine. A complex with bidentate bipyridine (bipy) ligands was synthesized.198 The complex possesses Cl ligands that enable the coordination of substrates. This is modification of the model complexes with a variety of tridentate N-based ligands (L) that block the terminal binding sites for substrates.199-202 Using T B H P hydroxylation of ethane (TN: 1.2/3 days), oxygenations of propane (TN: 13/2 days) and cyclohexane (TN: 72/3 days) were performed, resulting in the reactivity sequence of Fish et al. have used TBHP and for oxygenation of cyclohexane, toluene, adamantane, propane, and ethane by another complex, (tmima = [{(1methylimidazol-2-yl)methyl}amine]), that is characteristic for the polyimidazole ligands.203 Formation of intermediate is suggested and the mechanism

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proposed involves the initiation of the reaction by the homolytic reaction with C-H bonds to form a carbon radical, followed by rapid trapping with to form the alkyl hydroperoxide. Decomposition of the peroxide is catalyzed by the initial FeOFe complex to form alcohol, aldehyde or ketone. A complex with an aqua ligand in place of the acetate ligand was synthesized and used for the alkane functionalization reaction.204 Compared with the complex, the complex resulted in the higher turnovers/h (approximately twice) (Table 2).

It was suggested that the loss of the terminal or ligand by TBHP displacement must be rate limiting in the formation of the active Fe oxidant complex. The radical clock experiment using trans-2-phenylmethycyclopropane indicated that the radical rearrangement of the cyclopropylcarbinyl to phenyl-but-3enyl radical is faster than trapping. This conflicts with the no ring-opened product formation by MMO, suggesting the nonradical formation in the MMOcatalyzed C-H functionalization reaction.205 The free radical process in the MMO alkane functionalization is also suggested by the formation of the partially racemized products from optically active (S)- or ethanes.206 Studies on the complex with the aqua ligand were extended to the reaction in aqueous solutions.207, 208 Complexes, (L = TPA or BPIA) and were prepared in situ using and used to the reaction of soluble substrates, i.e., alcohols, at pH 4.2 in the presence of It was shown that the complexes catalyze the homolytic decomposition of TBHP and the generated and radicals initiate homolytic C-H bond abstraction from the alcohol substrate to predominantly provide the aldehyde/ketone product. Reactivity of dinuclear iron complexes with simple bidentate ligands such as 2,2’-bipyridine and 1,10-phenanthroline for oxidation of alkanes, toluene, dimethyl sulfide, trans-stilbene and adamantane was studied in under Ar in the presence of oxidants (TBHP, CHP or ). The effect of the number of the bridging ligand was found remarkable: monobridged >

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dibridged >> tribridged (L: bidentate nitrogen ligand, X: potentially labile ligand).209-214 It is suggested that the first step of the reaction is the binding of the oxidant to the iron catalyst probably resulting in the cleavage of the dinuclear unit into monomers as a real active species. The preparation of highly stable, easy to handle, iron complexes with highly labile sites is thought to bring about the advantage of the dinuclear structure over the mononuclear one. As for the cleavage of the O-O bond within the iron-peroxo complex, both of the homolytic (with CHP) and heterolytic (with TBHP or cleavages are proposed. Reactivity of dinuclear iron complexes having tripodal ligands, represented by was investigated in the presence of TBHP under Ar215, 216 with varying the tripodal and the bridging ligands. It was shown that the ratio of (alcohol + ketone)/t-BuOO-adduct) increased as the ligands become more electron donating. Two mechanisms for the decomposition of TBHP have been proposed: a homolytic pathway is initiated by generation of and radicals that form and a heterolytic pathway is initiated by the dissociation of the bridging anion from one iron center to provide a site for the coordinating the alkyl peroxide ion. The latter metal-based mechanism involving an alkylperoxyiron(III) intermediate was supported by the selective alcohol formation 217 and the large kinetic isotope effect value 2-Methyl-1-phenyl-2-propyl hydroperoxide (MPPH) was used as a probe capable of distinguishing between free alkoxyl radical chemistry and radical-free (enzyme mimetic) chemistry.218-220 Miyake et al. studied the reaction of with MPPH and showed that MPPH breaks down by O-O bond homolysis, leading to the formation of the benzyl radical and a high-valent species (Scheme 3).221 The latter oxidizes exogenous substrates such as thioanisole, cyclohexanol, and cyclooctene.

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On the other hand, MacFaul et al. have demonstrated that the hydroxylation of cycloalkanes by involves freely diffusing 222 cycloalkyl radical (Scheme 4). The radical mechanism proposed by MacFaul et al. 220, 222 was also applied to selective oxidation of cyclohexane to cyclohexanol catalyzed by a diiron(II) complex and TBHP (L = 1,4,10,13-tetrakis(2-pyridyl)methyl-1,4,10,13-tetraaz-7,16-dioxacyclo-octadecane). 223

Hydrogen peroxide is no doubt a useful oxygen donor and decomposes to water, but it is usually treated as an aqueous reagent. Thus the effect of water should be considered. Functionalization of hydrocarbons with and using as the oxidant, was studied by Fish et al..224 The results obtained in the oxidation of cyclohexane was consistent with a free-radical chain mechanism in which an initially formed cyclohexyl radical is trapped by oxygen gas to give a cyclohexyl peroxy radical, which abstracts a hydrogen atom to give cyclohexyl peroxide. The selective abstraction of the tertiary hydrogen of adamantane was shown by the high values. Participation of hydroxyl radicals in the oxidation of toluene was also suggested. Similar reactivity was observed by the complex 212, 225 It is also shown that aromatic hydroxylation of the ligand is performed by with a diferric complex prepared in situ (L = N,N’-bis-(2,4,5-trimethoxybenzyl)ethylenediamine N,N’diacetic acid).213, 226 Meckmouche et al. have pointed out that the oxygenation of hydrocarbons utilizing hydrogen peroxide proceeds partly by the metal-based mechanism. 227 Using a diiron(III) complex with a chiral ligand, (pb = 4,5(-)pinene-2,2'-bipyridine), enantio-selective catalytic hydroxylation by hydrogen peroxide was first achieved. White et al. performed an efficient and selective epoxidation of decene and other alkenes with a mononuclear iron complex

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(mep : N , N '-dimethyl-N,N'-bis(2-pyridylmethyl)ethane, 1,2-diamine) in the presence of and acetic acid.228 Formation of the carboxylate-bridged diiron(III) as an active species was indicated.

4.1.2 Monooxygenation by Diiron Complexes with Molecular Oxygen Very little has been reported on the monooxygenation with in the presence or absence of reductants. Kitajima et al. reported the monooxygenation with by a dinuclear iron complex in the presence of and Zn powder229 and with Hfacac and Zn powder230 The latter exhibited the greater activity than the former for the dioxygen hydroxylation of alkanes (npentane, cyclohexane, adamantane) and arenes (benzene, toluene, chlorobenzene). Wang et al. have synthesized dinuclear Fe(II) macrocyclic complexes with two dinucleating ligands, [24]RBPyBc and [30]RBBPyBc, containing phenolate pyridine, bipyridine, and amino phenolate groups.231 The complexes react with molecular oxygen and catalyze the monooxygenation of cyclohexane and adamantane in the presence of a two-electron donor, Alcohols are the main product rather than ketones (CyOH/CyO = 1.06 - 1.94). The proposed mechanism involves the formation of a peroxo-bridged dinuclear Fe(III) complex that is converted to a high-valent iron species bridged by an group as a direct intermediate for the reaction with hydrocarbons.

4.2 Monooxygenation by

Diiron Complexes

Dioxygen diiron complexes have been synthesized by mimicking the probable intermediate structures in methane monooxygenase oxygenations. Examples are summarized in the recent review.232-235 One is the diamond core type and the other is the peroxo type. Recently, an unusual species has been suggested in the reaction of with carboxylatebridged diiron(II,II) paddlewheel complexes, but its role in the oxygenation has not been clarified.234, 236

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The type was proposed to correspond to the high-valent species in nonheme diiron enzymes.237, 238 Few examples of this type of complexes, however, are known and characterized by crystallographic analysis.239-241 The reaction between and forms quantitatively that is converted to In addition to the diiron(III) complex, a series of metastable complex with the diamond core was synthesized with using the tetradentate tripodal ligand (TPA and its methylated derivatives).237, 238, 243-245 Reactivity of the TPA complex demonstrated that a species could carry out oxidation reactions of hydrocarbons.246 Cumene is converted to cumyl alcohol and and ethylbenzene to 1-phenylethanol and styrene, but cycloheptane is not oxidized. Two-step mechanism has been proposed to explain the product distribution: formation of alkyl radical in the first and ratedetermining step by the abstraction of a hydrogen atom from the substrate followed by the reaction of the intermediate radical either with the diamond core species or Crystal structure of the high-valent complex with a diamond core was characterized with Resonance Raman evidence for an structure derived from the isomerization of the diamond core was obtained with the complex (Fig. 13).247 Recently, formation of the diamond core species from and TBHP has been shown.248

On the other hand, the reactivity of peroxo complexes toward a variety of substrates was investigated by LeCloux et al..249, 250 The nucleophilic/ basic complex (L = m-xylenediamine bis(Kemp’s triacid imide dinuleating ligand system) reacts with phenols and carboxylic acids to liberate hydrogen peroxide, but not with electrophilic reagents such as olefins or triphenylphosphine, or even with a weak hydrogen donors such as dimethyl benzylamine at -77 °C. At room temperature, the complex reacts with solvents such as THF, toluene, and cyclopentane, to form mixtures of alcohol and ketone products by a radical autoxidation pathway. The complex with

4. Functional model oxygenations by nonheme iron complexes less electron-rich pyridine analogue, inactive towards these substrates.

189 was found

4.3 Functional Model Oxygenations by Iron Species in the Polyoxometalate and Heterogeneous Matrix 4.3.1 Oxygenation by Iron Species in the Homogeneous System In the aim to overcome the catalytic instability towards activated oxygen species (particularly organic ligands), the use of the iron species without organic ligands has been tried for the catalytic system. One of these systems was developed by Zhang et al. who used and p-cyano-N,N-dimethylaniline N-oxide as an oxygen source251 and by Mizuno et al. who used polyoxometalates, specially di-iron-substituted silicotungstate, (Fig. 14) and as an oxygen source.252-254

Homogeneous oxidation reactions were carried out in the presence of 30 % in acetonitrile under Ar. It was shown that the efficiency of utilization to oxygenated products reached to ca. 100% for the oxygenation of cyclohexane. The catalytic system was applied to other alkanes such as n-hexane, n-pentane, and adamantane253 and alkenes such as cyclooctene, 2octene, 1-octene, cyclohexene, styrene and trans-stilbene.254 It is noteworthy that epoxides are selectively (or mainly) formed in the oxygenation of alkenes. Nonradical process is suggested by the fairly stereospecific epoxidation of cis-stilbene. Mizuno et al. found that a water soluble potassium salt of di-iron-substituted silicotungstate, catalyzes the conversion of methanol to methylformate (10.2%) > methanol (0.3%) > formic acid (0.2%) together with (16.6%).255 The work was attempted to demonstrate catalytic oxidation of methane with hydrogen peroxide in water. Interestingly, mono- and tri-iron-substituted silicotungstates is much less active and nonselective for the oxidation. The catalyst

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was used for the oxygenation of cyclohexane and adamantane under in as solvent without addition of reducing reagent or radical initiators.256 Alcohols and ketones were obtained catalytically by the radical-chain path. High turnover numbers were achieved in the selective epoxidation of alkenes with 1 atm in 1,2-dichloroethane/acetonitrile at 356 K. 257 Cyclooctene was converted to cyclooctene oxide (98%) under 82% converstion and 10000 TON (Table 3). Addition of 4-tert-butylcatechol as an alkyl radical inhibitor did not affect the reaction. IR spectra using and indicated the Fe=O band.

4.3.2 Oxygenation by Iron Species in the Heterogeneous System (L = TPA) was applied to the reaction in the heterogeneous system.258 The complex was embedded in an amorphous silicate surface modified by a combination of hydrophilic poly(ethylene oxide) and hydrophobic poly(propylene oxide). This assembly showed reactivity somewhat higher in comparison to an aqueous micelle system utilizing a surfactant. Functionalization of alkane and alkene in the biphasic catalytic system was performed in the different way by using perfluoroheptane-soluble catalysts.259 Immobilization of iron(III) on to the polymer matrix of cross-linked styrenedivinylbenzene matrix resulted in the epoxidation of cis-cyclooctene and styrene in the presence of TBHP.260 Likewise, oxygenation of cyclohexane was performed by dinuclear iron complexes and in hexagonal mesoporous solid (HPTB = N,N,N',N'-tetrakis(2-benzimidazolylmethy)-2-hydroxy1,3-diaminopropane, HPTP = N,N,N',N'-tetrakis(2-pyridylmethy)-2-hydroxy-l,3261 diaminopropane) in the presence of TBHP or Oxygenations of methane and benzene are thought to proceed by the reaction with that is formed on FeZSM-5 by the loading.262,263

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4.4 Functional Model Oxygenations by Mono-Iron Species 4.4.1 Oxygenation by Mono-Iron Complex/Activated Oxygen System Since Fenton first used a mixture of and hydrogen peroxide in 1894,264 the combination of the chemicals has been used as the powerful oxidizing reagents. However, the mechanism and the key intermediates in the Fenton chemistry are still under great discussions.265, 266 The chemistry is not so simple as explained by a free hydroxy radical as proposed by Haber and Weiss in 1930s.267, 268 Mechanism of the catalytic decomposition of hydrogen peroxide by nonheme Fe(II) complexes was studied in comparison with that of the heme system involving an oxoiron(IV) or ferryl species (Scheme 5) or that of a Fenton-like nonheme system involving the free hydroxyl radical (Scheme 6).

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The mechanism without formation of the hydroxyl radical was supported by the reactions of with ferrous ions complexed with diethylenetriamineN,N',N",N"-pentaacetate (DTPA),270 with nitriiotriacetate (NTA) or ethylenediamine-N,N'-diacetate (EDDA), 271 and with 8-methyl-1,4-bis(2pyridylmethyl)-l,4,8-triazacycloundecane and 1-methyl-5,9bis(2-pyridylmethyl)-1,5,9-triazacyclododecane Zhang et al. performed the reaction at neutral pH and scavenged a reactive intermediate by 2,2’azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), but not by bromide ion, indicating that the strongly oxidative intermediate is not the hydroxyl radical. Proposed mechanisms for the reactions of and complexes are shown in Scheme 7. This system was applied to hydroxylation of aromatic compounds: Zhang et al.269 and Dunforld283 has summarized the current state of mechanisms in Scheme 7.

In the biomimetic monooxygenations of alkane and alkene, Yamamoto and Kimura reported trans-epoxide formation from olefins with in 284 acetonitrile. Sugimoto and Sawyer performed epoxidation of and monooxygenation of some organic substances by a system. 285, 286 They found further efficient and selective epoxidation of various alkenes and monooxygenation of organic substrates including alkanes by a system in acetonitrile.287-291 The most effective catalyst systems were and or was proposed as the reactive intermediates, which may

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be transformed to in the presence of olefins.290 Non free radical process was also proposed for aromatic hydroxylation by Fe11 complexes The oxygenation by this system, i.e. the oxygenated Fenton chemistry, was summarized by Sawyer.293,294 Barton et al. studied the mechanism of alkane functionalization of alkanes (cyclohexane, adamantane, 3-ethylpentane, cyclododecane, etc.) by experiments of the catalytic oxygenation systems using and as iron species and and Geletii and Shilov used and for the oxidation of cyclohexane in the presence of pyridine 301 and compared the ketone/alcohol ratio with the Gif system.302 Barton et al. proposed the intermediate formation of the iron-carbon species and a as the reactive 295-300 and formation of alkyl hydroperoxide as an intermediate. 303, 304 The species formation of an intermediate containing an Fe-C bond was further supported by the efficient capture of the bond by iodine and iodide ion to give the corresponding alkyl iodide305 or by PhSeH.306 Using completely regiospecific oxidation of a methylene group of Binor S into a ketone was accomplished.307 Non radical mechanism in this system via an iron-alkyl species was suggested on the bromination of saturated hydrocarbons.308 Using and Barton et al. indicated that the oxygen atoms in the alkyl hydroperoxide do not arise directly from hydrogen peroxide but from formed in situ by the well-known iron(III)catalyzed decomposition of hydrogen peroxide.309 The radical trapping experiment with Tempo (2,2,6.6-tetramethyl-l-piperidinyloxy) also supported for the nonradical nature of the first intermediate in the reaction.309, 310 As an active species, formation of a complex from an isolated complex was proposed.311 The carboxylate ligands such as picolinate are thought important for the formation of a species as a key intermediate in the hydrocarbon activation chemistry. 312 Important role of the pyridine bases in the Fe(III) complexes for the efficient ketonization within the manifold 313 and of a suitable carboxylic acid as ligand within the manifold was reported.314 In the studies on the role of pyridine, the use of 4-tert-butylpyridine was found to permit the isolation of cyclohexanone and cyclohexanol by simple distillation.315 As an extended application of Gif systems, Barton et al. have developed the carboxylation of saturated hydrocarbons by in 316 pyridine-acetic acid or by These results in Gif Chemistry have been reviewed by Barton. 318, 319 Trapper et al.320 have reinvestigated a typical Gif system to lend support to the proposition of a preponderant, carbon- and oxygen-centered radical pathway rather than the nonradical pathway by Barton and Doller.321 The monomeric

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which was formed by dissolving or in pyridine, produced not only oxo products, but also 2- and 4-adamantylpyridines by the reaction with The formation of adamantylpyridines not only for the tertadamantyl position, but also for the sec-adamantyl sites, especially under Ar is thought to provide direct evidence for the generation of tert- and sec-adamantyl radicals.322 The same mechanistic aspect was obtained by using iron picolinate, It is suggested that the hydroxyl radical plays a key role in Gif oxygenation by (Ar), most likely coupled to substrate-centered alkoxyl radicals under The oxygen-centered radicals perform H-atom abstractions from Gif substrates to generate diffusively free carbon-centered radicals. Liu et al. also studied the hydroxylation of phenol by iron(II) phenanthroline, with Products are catechol > hydroquinone > p-benzoquinone and the efficiency of is 45-90% depending on the reaction condition. It is suggested that the coexistence of and is the key for the hydroxylation by the attack of the hydroxyl radical to phenol. Role of the hydroxyl radical as reactive oxygen species was also suggested in the oxidation of dichloroacetonitrile to phosgene.325 However, the non-radical process in the system was supported by the following results. Kim et al. reported stereospecific alkane hydroxylation with catalyzed by and an intermediate formation of was shown by electrospray ionization mass spectrometry (ESIMS).326 Chen and Que used In the oxidation of cyclohexane, the major product was cyclohexanol (TN: 5.6 within 0.5 h) compared with cyclohexanone (TN: 0.7). The system catalyzed the stereospecific hydroxylation of cis-1,2-dimethylcyclohexane, that supported formation of via (N4Py = N, N-bis(2-pyridylmethyl)-Nbis(2-pyridyl)methylamine) was also used for oxidation of alkanes such as cyclohexane, cyclohexene, adamantane, benzene to alcohols and ketones in up to 31% yield.328 It was proposed that that was detected as a transient purple species, reacts through homolysis of the O-O bond to afford two reactive radical species, and HO·. Two pathways of alkane hydroxylation were suggested in the system329: (A)when the TPA ligand has two or three a generated high-spin directly cleaves the C-H bond, forming the alkyl radicals, that are susceptible to radical epimerization and trapping by (B) when the TPA ligand has substituents, a generated low-spin intermediate derives an species by the heterolytic splitting of the O-O bond. The high-spin system

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strongly favors olefin cis-dihydroxylation in which both diol oxygen atoms derive from (Scheme 8).187,330 Asymmetric cis-dihydroxylation of olefins such as cyclooctene, cis-2-heptene and trans-2-octene was first performed by using and 82% ee has been 187 attained in the dihydroxylation of trans-2-octene. The low-spin system carries out highly stereoselective alkane hydroxylation, olefin epoxidation and olefin cis-dihyroxylation. 188, 190, 191, 330 It was shown that steric effects of 6-Methyl substituents on the ligands affect the spin state of the intermediate, which in turn influences the course of the oxidations.

The nonheme iron/peroxide system attracts attention in recent years for the potential use in the catalytic oxidation of environmental pollutants. Oxidative degradation of 2,4,6-trichlorophenol (TCP), a major pollutant produced by paper mills, was efficiently catalyzed by water-soluble iron tetrasulfophthalocyanine with to benzoquinone and ring cleavage products, mainly chloromaleic acid and carbon dioxide.331-334 Hemmert et al. obtained a nonheme iron complex using a new symmetric pentadentate ligand containing four pyridine groups, bis(di-2-pyridylmethyl)amine (BDPMA). The complex was found to catalyze the oxidative degradation of chlorinated phenols (TCP, 2,4dichorophenol (DCP), p-chlorophenol to corresponding benzoquinones with potassium monopersulfate as oxidant in a mixture of acetonitrile-water

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(1/3) at pH 2.335 The turnover rates were 27 (TCP), 22 (DCP) and 23 (pCP) cycles within 30 min. The proposed mechanism involves the iron(IV)-oxo active species, that abstracts one electron and one proton from the phenolic substrate to form a species and a phenolic radical as an intermediate. The formation of the ferryl species, in the initial step was supported by the MD simulation study.336 Horwitz et al. developed nonheme iron complexes with tetraamido macrocyclic ligands (TAML) that are efficient for the dye bleaching reactions with in water from neutral to basic pH. 337 338 Pinacyanol chloride as a reference, azo and quinone dyes were oxidatively degraded to show the bleaching effect. The Fe-TAML catalysts (Fig. 15) were applied to destroy pollutants pentachlorophenol (PCP) and TCPin water, in minutes, under ambient conditions of temperature and Substitution on the benzene ring with hydrophobic tail, pressure.339 produces micelle-forming catalysts. Substitution with the anionic group, increases the negative charge of the catalysts, that is effective to introduce a rate disincentive for the catalyst to attack substrates on negatively charged heterogeneous supports.340 Collins explains characteristics of the TAML activator/peroxide systems as follows: capability of more than 10000 turnovers per hour in certain applicants, weak catalase activity with highly efficient utility of peroxide, water-soluble catalysts that are easy to use and active under both neutral and basic conditions, rapid processes at mild temperatures and under ambient pressure.340 Bleaching of kraft pulp and color in plant effluents were reported as direct or potential examples of environmental applicability.

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The selective oxygenation of methane and light alkanes by the Fenton system was performed in the three-phase catalytic membrane reactor, enabling simultaneous reaction and product separation. Frusteri et al. reported that Nafion-based catalytic membranes catalyze the selective oxidation of methane to methanol,341 ethane to ethanol and acetaldehyde,342 and propane to acetone, propionic aldehyde, isopropanol, and n-propanol.343 The partial oxidation proceeds according to a radical mechanism which involves the activation of paraffin on superacid sites and the subsequent reaction of activated paraffins with OH radicals generated according to the Fenton reaction. Bianchi et al. developed a biphasic system (water/acetonitrile) for the selective oxidation of benzene to phenol, with the pyrazinecarboxylic acid derivatives as a ligand to in the presence of 344 trifluoroacetic acid. Tert -butyl hydroperoxide (TBHP) is no doubt an important and useful oxidant for monooxygenation of various hydrocarbons. Barton et al. developed oxidation systems which oxidizes cyclohexane and various hydrocarbons by TBHP with Fe(III) salts such as or a cyclohexane soluble iron complex, (TMA: trimethylacetate) in the absence or presence of picolinic acid.345347 A pathway, alkane alkyl hydroperoxide ketone or alcohol, was proposed. Schuchard et al. extended the reaction system to that under 15 bar of oxygen at 70 °C and found the increase in the conversion from 5% to 9% but the decrease in the selectivity.348 Oxidations of cyclohexadienes to aromatic products, anthracene to anthraquinone,349 and alcohols and allylic methylene groups to ketones even in the absence of pyridine/acetic acid350, 351 were reported. Nguyen et al. used a model ligand that mimics the metal-binding domain of the bleomycin for oxidation of cyclohexane to cyclohexanol and cyclohexanone (1 : 1 ratio, 1700% yield on the basis of catalyst concentration) and adamantane and 3-methylpentane. No oxidation of an aromatic C-H bond was observed.352 Low-spin species and were detected by ESR in the reaction of with TBHP and respectively. Formation of a perferryl intermediate from the Fe(III)-peroxo species were thought unlikely. The homolytic cleavage of the O-O bond in to form was thought to take part in the catalytic process by forming cyclohexyl and t-butoxy radicals. Spectroscopic properties and electronic structure, and reactivity for the homolysis of the O-O bond of low-spin (x = 1 or 2) have been studied in detail.353, 354 The role of and manifolds in the oxidation systems were studied by ionic trapping with chloride, azide, and other anions.351, 355, 356 However, since Minisci et al. have established that TBHP chemistry was best interpreted as

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radical,357-360 Barton et al. performed the radical trapping experiments 361-364 and led the conclusion that the results obtained in the system of or can be explained by radical chemistry and oxidation states of iron higher than are not involved.365 Oxygenation of nitroalkanes to the corresponding aldehydes or ketones in relevance to 2-nitropropane dioxygenase was performed by ferroxime(II) dimethylglyoxime)/TBHP in DMF or and both radical species are thought to be involved in the H-abstraction from nitroalkanes. Barton et al. used bis(trimethylsilyl)peroxide for the selective oxidation of alkanes to ketones using and The intermediate formation of which reacts with a methylenic carbon to form bond. By using this oxidant, it was found that unlike the system, no extra ligand was needed if was used.368 A carbon radical from the manifold was proposed to explain the results. Shyu et al. used sodium hypochlorite (NaOCl) for the epoxidation of cisstilbene with iron complexes with salicyladimine ligands, and N,N'-(1,1-dimethylethylene)bis(salicylaldimine), = N,N'-(1,1-dimethylethylene)bis(3-methoxysalicylaldimine). 369 The reaction proceeded readily to produce trans- and cis-epoxides in good yield: 40% yield with cis : trans = 20 : 80, 90% yield with cis : trans = 40 : 60.

4.4.2 Oxygenation by Mono-Iron Complexes/O2 System Monooxygenases catalyze incorporation of one atom of oxygen from molecular oxygen into a substrate accompanied with the formation of water. In this reaction the presence of electron and proton donors is required for activation of molecular oxygen (eq. 9). Thus, selections of electron and proton donors as well as iron complexes are key points for development of functional model Oxygenation systems. There are some results, however, reporting monooxygenation in the absence of electron and proton donors. These systems may not be regarded as functional model systems, but worthy to notify their reactivity. Usually, mechanisms are obscure in these systems. Shue et al. reported ketonization of methylenic carbons such as cyclohexane by (DPAH: 2,6-dicarboxylatopyridine)/ and the dioxygenation of acetylenes, aryl olefins, and catechols as a reaction mimics for dioxygenase.370 The simple system was used 371 Catalyst complexes were for Oxygenation of cyclohexene and methyl linoleate. prepared in situ by mixing or in MeCN with 2,2'-bipyridine, and was found the most reactive. Oxygenation of cyclohexene was performed by complexes in aqueous

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solutions.372 is suggested to be reactive with cyclohexene to produce its alcohol in the absence of oxygen. When is present, the oxygen atom transfer proceeds via which is formed from Hirao et. al. developed an efficient system for the catalytic epoxidation reaction of olefins stilbene) with molecular oxygen by the utilization of and the N-heterocyclic podand ligand, BIPA.373, 374 The reaction proceeds at room temperature and in the absence of a co-reductant. The coexistence of 4-ethoxycarbonyl-3-methyl-2-cyclohexen-l-one results in the facile epoxidation. The intermediate was assumed to be involved, but no direct support for the mechanism has been obtained. Iron(III)chloro complexes with salen ligands bearing electronegative substituents were used for the radical chain autoxidation of cyclohexane under 1 atm at 25 °C. About 2/3 of the activity of the best iron porphyrin catalyst was achieved.375 Duprat et al. used Fe(0) (iron rod) for the aromatic hydroxylation in acetic acid.376 Formation of that may be converted to as an active species, was postulated. Most functional model monooxygenations by nonheme iron complexes are performed in the presence of electron and proton donors. Udenfriend used ascorbic acid for monooxygenation with ferrous salts and molecular oxygen in the presence of EDTA as a ligand at neutral pH for aromatic hydroxylation.377, 378 The system was applied to oxygenations of salicylate,67 cyclohexane and cyclohexene,379, 380 naphthalene381 etc. As shown in following equations, ascorbic acid reactivates the catalyst. The active species may or may not involve a free hydroxyl radical. The reaction was improved in the presence of metallic iron which is 382 effective for elimination of the by-product oxalic acid.

Ullrich hydroxylated cyclohexane to cyclohexanol and cyclohexanone (6.9 : 2.8), toluene, acetoanilide, etc., by system in an aqueous media and compared the reactivity with cytochrome P-450.383 This was applied to the hydroxylation of naphthalene. 381 2-Mercaptobenzoic acid may act not only as a sulfur coordinating ligand but a reductant.381 Other thiols were used for hydroxylation of aniline and toluidine in aqueous acetone.384-386 Barton et al. have developed a new procedure for the oxidation of saturated hydrocarbons, e.g. adamantane, cyclohexane, pentane, etc. by using iron powder, carboxylic acids, and hydrogen sulfide in aqueous pyridine under 1 atm and compared the selectivity in the prim-, sec-, tert-C-H bonds.302, 387 Usefulness of as a reductant

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in the synergistic oxidation of cyclohexane388 and probability of the non-radical oxidation were reported.389 Barton et al. used zinc (Zn) as a reductant. They isolated a trinuclear iron complex, from a reaction system containing iron powder, carboxylic acids, and in aqueous pyridine under 1 atm and found that it oxidized alkanes in the presence of Zn, pyridine, and acetic acid under 1 atm (Gif system). Later, the Gif type oxygenations were performed mostly by using monoiron salts, e.g. Barton et al reported a number of experiments for application of this system to the more complicated compounds such as steroids391-398 and for clarification of mechanism, specially participation of the radical and nonradical processes.295, 296, 399-402 Intermediate formation of alkoxide403 and was proposed based on the effect of on the selectivity, cleavage of methylidene olefins into the ketone or aldehyde, or effect of additives such as thiophenols.300 Recently, Celenligil-Cetin et al. studied the reactivity of ferrous and ferric oxo/peroxo pivalate complexes and claimed that the peroxo species is not directly involved in catalytic Gif-type oxygenation, based on the low reactivity of the peroxo containing complexes in the oxidation of both cisstilbene and adamantane.405 Kitajima et al. used for oxygenation of alkanes and arenes in the presence of hexafluoroacetylacetone (Hhfacac) and Zn powder hydrotris-1-pyrazolylborate).229, 230 was isolated and assumed as an active complex. Balavoine et al. tried to use an electrode in place of zinc in the Gif system for the selective oxidation of saturated hydrocarbons.295, 406-408 Mimoun and Roch used hydrazobenzene (PhNHNHPh) for oxygenation of cyclohexane, cyclohexene, and toluene.409 The most active complex was formed from and carboxylic acid in the presence of hydrazobenzene. Davis et al. used and for the same reaction and proposed the hydroperoxide complex as an active species.410 Sheu et al. used complexes (PA: picolinato) for the monooxygenation of saturated hydrocarbons, especially ketonization of methylenic carbons. 370,411 This system was applied to the hydroxylation of aromatic hydrocarbons as reaction mimic for tyrosine hydroxylase.412 With phenol as a reactant, the dominant product was catechol. Funabiki et al. developed an monooxygenation system using hydroquinones with catecholatoiron(III) complexes in acetonitrile in the presence of pyridine. 413 First the system was applied to hydroxylation of aromatics.413-416 Catalytic activity depends greatly on the substituents on hydroquinones (R-HQ) and increases with the electron-donating property; In the oxygenation of

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anisole, selectivity to form either 4-MeOPhOH or PhOH depends greatly on the concentration of pyridine: selective formation of 4-MeOPhOH and PhOH at low and high concentrations of pyridine, respectively. In the oxygenations of p-D-toluene and p-xylene, NIH shifts of p-D and p-Me groups were observed in the fairly high values (55 and 35 %, respectively).415 The oxygenation of the methyl group to form benzyl alcohols and aldehydes was promoted by the high pyridine concentration. The system was used for the hydroxylation of tert-butylphenol to form selectively 4tert-butylcatechol as a model reaction for tyrosine hydroxylase.416 Alkane and alkene were also monooxygenated by the same catalytic system.417,418 Cyclohexane, cyclohexene, and n- and isoalkanes produced highly selectively alcohols at the low pyridine concentration (Table 4).417,418 This is very characteristic and different from other systems that produce ketones selectively, and supports the high-valent ironoxo species as an active species. The formation of ketone was promoted with the increasing pyridine concentration and became dominant in the pyridine solution. The effect of the pyridine concentration is due to the change of iron complexes from a catecholate iron complex to a pyridine iron complex, as supported by the same results with using Not only catalytic activity but also selectivity was greatly dependent on the substituent on hydroquinones. This suggests that coordination of hydroquinones to iron is involved in the oxygenation process416 417,4I8

Takai et al. developed an excellent catalytic system for epoxidation of olefins by tris(l,3-diketonato)iron(III) complexes and aldehydes as a reductant.419 A number of olefinic compounds including styrene analogues and olefinic alcohols were converted to corresponding epoxides in good to quantitative yields with combined use of molecular oxygen and an aldehyde (e.g. 2-ethylbutyraldehyde) at

202

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room temperature; e.g. styrene was converted to epoxide (89%) and aldehyde (10%) within 9 h in the 100% conversion. Murahashi et al. also used an aldehyde (heptanal) for aerobic oxidation of alkanes and alkylated arenes.420 Iron powder/acetic acid was used as the most effective catalyst, though and were also examined. Oxo-iron species, which may be formed via peracids from acetic acid, was assumed as an active species for hydrogen abstraction from alkanes and epoxidation of olefins. Kesavan et al.421 used nanostructured amorphous iron and alloy like for the oxidation of adamantane under 40 atm and at 28 °C in the presence of isobutyraldehyde and acetic acid. Only alcohols and ketone (l-ol : 2-ol : 2 -one = 17 : 2 : 1) were obtained in the 52 % conversion. Oxo-iron species was also assumed to explain the high selectivity for the tertiary alcohol. Ruiz et al. have used an iron(III)-carbonato complex of orthophenylenebis(oxamato) as a moderately efficient catalyst for the aerobic epoxidation of alkenes with co-oxidation of pivalaldehyde. 422 Characteristic results are the stereodependent epoxidation of cis- and trans-stilbene (conversion rate is trans >> cis) and the rate enhancement by the addition of N-methylimidazole. Iron(IV)acylperoxo or iron(v)-oxo species derived by oxygen-oxygen bond cleavage of the acylperoxo group was assumed as a probable candidate of active species. In this connection, Nam et al. demonstrated the formation of a metal-acylperoxo complex in the auto-oxidation of aldehyde.423

4.4.3 Mono-Iron Oxygen Species In the mechanism of cytochrome P450, it is assumed that the high-valent iron-oxo species is formed from that may derive from the oneelectron reduction of oxy adducts, probably via an It is well assumed that these types of iron-oxygen species are involved in the mechanisms of nonheme iron oxygenases. As an activated bleomycin (BLM), formation of a low spin by the reaction of with or 424, 425 by with was shown kinetically and by ESI-MS. As a synthetic high spin species, has been identified by resonance Raman and other spectroscopies.426,427 In recent years, formation of and its deprotonated form with different ligands have appeared. Kim et al. detected species of and by ESI-MS in the co-injection solution of , and TBHP.428 Bernal et al. detected a low-spin species by ESI-MS and ESR in the solution of and (L:N-methyl-N,N',N'tris(2-pyridylmethyl)ethane-l,2-diamine).429 Nguyen et al. detected a low-spin species and by ESR in the reaction of

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with TBHP and respectively, in acetonitrile/DMSO.352 Formation of a perferryl intermediate from the Fe(III)-peroxo species were thought unlikely, since the PMA- ligand framework is incapable of stabilizing the high-valent iron center and oxidation of both cyclohexene and norbornene yielded only the allylic oxidation products and the exo-epoxide, respectively. The homolytic cleavage of the O-O bond in to form was thought to take part in the catalytic process by forming cyclohexyl and t-butoxy radicals. deVries et al. used a ligand 2,6-bis[methoxybis(2pyridyl)methyl]pyridine (L) and observed by ESM the transient formation of a from Kim et al. proposed the structure and later for an intermediate detected in the solution of and It was shown by Raman spectroscopy that the O-O bond is significantly weakened as indicated by the lowest 790 Roelfes et al. synthesized pentadentate ligand N4Py with which the purple low-spin was generated by the reaction with and Fe(II) complexes.432 Mialane et al. also confirmed the formation of with L = N,N,N’-tris(2-pyridylmethyl)-N’-methyl-ethane-1,2diamine) by ESI-MS.433 Formation of high-spin blue species from low-spin complexes was reported by Jensen et al..49 They found that purple complex (bztpen: N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-ethane-1,2diamine) can be reversibly deprotonated to give transient blue species showing spectroscopic properties consistent with iron(III)-peroxide complexes, or others. Similarly, Simaan et al. reported the formation of a complex from a purple low-spin complex upon adding a base (L: N-methyl-N,N,N-tris(2-pyridylmethy])ethane-1,2-diamine).50 The complex was characterized by the UV/Vis change from 534 to 740 nm, by the ESR change from g = 7.5 and 5.9 to 9.3 and 4.3, and by ESIMS at m/z = 435. Ho et al. gave a resonance Raman evidence for the interconversion between and 632 and 790 for -1 and 495 and 817 cm for The former has activity for the hydroxylation of cyclohexane, but the latter looses it, supporting the hypothesis that protonation of the peroxo species activates it for participation in the oxygen activation mechanisms by iron enzymes. Similar results for identification of two peroxo complexesas shown in Fig. 16 were reported by Simaan et al..51,52

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As for the formation of high-valent oxo species of nonheme iron complexes, Lange et al. reported about evidence for a nonheme Fe(IV)=O species.434 No direct spectroscopic evidence for the species has been obtained, but the intramolecular hydroxylation of a ligand phenyl moiety was explained by its involvement in the mechanism. This supports the hypothesis that Fe(IV)=O species can be the active species responsible for substrate oxidation in the class of oxygen-activating nonheme iron enzymes. Wada et al. have isolated and characterized species from the reaction of with Reactions of the species with cyclohexene and cyclohexane in the stoichiometric oxidation gave alcohols as the main product with a trace amount of ketones. A higher efficiency was observed in oxidation of to rather than in the case of to indicating that the active species not only acts as a two-electron oxidant but also displays nucleophilicity. It is suggested that the homolytic O-O bond cleavage generates both of and HO, both of which cooperatively perform a C-H bond cleavage step with a subsequent C-O bond formation step in the hydroxylation.

5.

FROM FUNCTIONAL MODEL TO CATALYSIS

Goals for biomimetic chemistry on oxygenases are clarification of active species, e.g. Fe-OOH or Fe=O, involved in the enzymatic and model oxygenations, development of catalytic systems for the selective oxygenations using as the oxidant, and so on. Efforts for these topics in the last ten years have given rise to various types of nonheme iron complexes that stabilize iron-oxygen species and oxygenate substrates in the di- and mono-oxygenase-like fashion. The high-valent iron-oxo species became popular in the great development of cytochome P450 chemistry. These species was first thought characteristic to heme

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enzymes and their models, which involve porphyrin ligands suitable for stabilization of the high-valent state of iron. It is easy to assume and expect formation of the similar iron oxygen species in the nonheme iron systems, but at the same time we realize high barriers for characterization of these species spectroscopically or by isolation. Participation of these species in the oxygenations by nonheme iron complexes was first shown by selectivity of product, e.g. alcohol/ketone ratio or C2/C3 ratio in the alkane or adamantane oxygenations, but it was not necessary for us to wait so long before getting spectroscopic evidence for various types of nonheme iron-oxygen species. Some iron-oxygen species were characterized after isolation. However, we encounter paradoxical facts that stabilized iron-oxygen species exhibit poor reactivity. Many kinds of diiron complexes have been synthesized in relevance to methane monooxygenase. In general, the activities of functional model oxygenations are poor and the complexes are labile to be converted to monoiron species in solutions. The characteristic diiron structure, however, were applied to the diiron-substituted Keggin-type silicotungstate (Fig. 14), that exhibits a high catalytic activity under without addition of reducing reagent. On the other hand, some types of monoiron complexes have been developed to perform enzyme-like oxygenations producing alcohols selectively. In addition, some monoiron complexes are expected to become useful in oxidation of pollutants or bleaching of dyes, regardless of usage of molecular oxygen or activated oxygen such as hydrogen peroxide. Recently, however, reactivity of peroxo adducts of mono- and diiron complexes were compared for the oxidation of sulfides to sulfoxides. The dinuclear catalyst was found to be more reactive and (enantio)selective than its mononuclear counterpart, suggesting that a second metal site affords specific advantages for stereoselective catalysis.436 This result may be helpful for the design of future enantioselective iron catalysts. Nonheme iron oxygenases are characteristic for the abundant types of dioxygenation, compared with heme oxygenases. Progresses in chemistry of catechol dioxygenases are remarkable in these ten years, but little development has been achieved in chemistry of other types of dioxygenases. The selective cleavage of aromatic rings with molecular oxygen is an attractive reaction that converts aromatic to aliphatics. The mechanism of the selectivity control in the oxygen insertion process is an interesting subject in bioinorganic chemistry. It involves chemistry of Fe(III) and Fe(II) for activation of catechols and oxygen. The fruits in that chemistry will be applied to other types of dioxygenases. Recent great progress in the X-ray crystallographic analysis brings about structural information about metalloproteins directly. However, structural model

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investigations are no doubt important, especially for getting information about probable structures of unstable intermediates. Various types of iron-oxygen species isolated or detected contribute greatly to clarify mechanisms of oxygenations. On the other hand, functional model chemistry brings about more dynamic information about the mechanisms and lead to development of catalysis. Functional model investigations reported have been focused mostly on the mechanisms, but achievement of highly selective and highly active reactions with high TON is required for development of catalysts based on functional models. Molecular oxygen is more attractive as an oxygen source than activated species such as hydrogen peroxide. Water is also the more attractive solvent than organic solvents. The efficient catalytic systems of oxygenation using nonheme iron complexes are the challenges for the future.

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Chapter 5 Catalysts for selective aerobic oxidation under ambient conditions Thioether sulfoxidation catalyzed by gold complexes Eric Boring, Yurii V. Geletii and Craig L. Hill Department of Chemistry, Emory University, Atlanta, Georgia 30322

Abstract: Diversity-based methods for catalyst discovery coupled with the knowledge of lead systems for the catalysis of organic oxidation reactions has led to the development of new species that actually catalyze rapid and selective (non-radical-chain), reductant-free, oxidation under ambient conditions (room temperature and 1.0 atmosphere of air). The first process of focus is selective sulfoxidation of thioethers (organic sulfides). The principal work reviewed here involves homogeneous catalysis, but highly reactive heterogeneous formulations have already been identified. The stoichiometry is that characteristic of dioxygenase enzymes: (sulfoxide). Oxidative dehydrogenation, a less desirable net process, is not seen. Studies have primarily been conducted with 2-chloroethyl ethyl sulfide (CEES), which is both notoriously unreactive and a useful simulant for mustard. Extensive kinetics and product studies have identified the active catalyst, at least in acetonitrile solution, to be (1), and the rate limiting step to be reaction of 1 with another molecule of the thioether substrate. Reoxidation of the resulting Au(I) to Au(III) by is a fast subsequent step. The solvent kinetic isotope effect rate of sulfoxidation when Cl is replaced by Br, and multiparameter fitting of the kinetic data establish that the mechanism of the rate-limiting step itself involves a bimolecular attack of CEES on a Au(III)-bound halide and it does not involve Isotope labeling studies with indicate that and not or is the source of oxygen in the sulfoxide product. Interestingly, is consumed and subsequently regenerated in the mechanism. Despite the impressive (unique) reactivity attributes above, these recently developed catalytic systems have some limitations that include an induction period and inhibition by sulfoxide product. However, these two difficulties are eliminated in other solvents or in nontoxic developmentally attractive perfluoropolyether (PFPE) media. Another potential problem, is catalyst inactivation by precipitation of the Au as colloidal Au(0), but this can be largely avoided by use of appropriate reaction conditions. Finally, these Aucatalyzed aerobic sulfoxidation reactions can be co-catalyzed by some d-block ions. Cu(II) is particularly effective in this context resulting in substantial increases in reaction rate at low

227 L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 227-264. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Cu(II) concentrations. Co-catalysis by the d-block ions also results in elimination of the induction period in some cases.

Key words: Amino acids, Catalytic oxidation under ambient conditions, 2-chloroethyl ethyl sulfide, Co-catalysis by copper, Decontamination, Effect of ligands, Gold complexes, Mustard gas, Perfluorinated solvents, Solvent effect, Sulfoxide, Thioether

1. INTRODUCTION The selective reductant-free catalytic oxidation of organic compounds by has been a major goal in both fundamental and applied chemistry for years. Nearly all catalysts for oxidations, whether they be biological or abiological (synthetic/industrial) contain redox-active metal centers.1-7 “Selective” generally means that the radical-chain mechanisms that dominate the ubiquitous oxidations of nearly all organic and many inorganic materials (autoxidations) aren’t operable. While there are a handful of radical-chain oxidations that proceed with reasonably high selectivity, it is the nature of the substrate itself in these cases that ensures high selectivity and not the chemistry (control of the elementary processes in the mechanism). One of these rare examples is the oxidation of p-xylene to p-terephthalic acid, one of the largest scale commercial homogeneous catalytic processes globally. Despite proceeding by a homolytic mechanism (radical chains and many radical intermediates), the pterephthalic acid product is produced in a very high selectivity >99.9% primarily because its unusual structure makes it quite stable under the reaction conditions.8 The DuPont process for aerobic oxidation of cyclohexane (adipic acid manufacture) is more indicative. While this also remains a large scale process given its low cost, it proceeds with poor selectivity.9 One of the several reasons for the intrinsically low selectivity (and consequent generically low desirability) of conventional radical-chain oxidations, is that many oxygen-based and/or radical intermediates in such processes are capable of reacting simultaneously with the substrate. Unfortunately the metal centers in such systems, both catalytic and structural, typically exacerbate the problem by generating additional nonselective but kinetically potent reactive intermediates via both redox and Furthermore, the mechanistic electrophilic non-redox processes.10 complexity in conventional radical-chain oxidations, including the

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ensemble of freely diffusing time-dependent and concentration-dependent reactive intermediates, makes these oxidations difficult to control.9 Many of these limitations are avoided if a reducing agent is used. Indeed, a multitude of biological and abiological/industrial oxidation processes use a reductant because the resulting peroxo and other intermediate-redox-state forms of oxygen generally exhibit greater reactivity. More significantly however, these reactive oxygen species are frequently more selective and their chemistry more controllable, with or without the involvement of metal centers, than the chemistry of the active oxygen and oxygenated intermediate species operable during reductant-free oxidations. However, if a reducing agent is required, its cost must be factored into the overall economics, and this renders many an otherwise potentially attractive process unworkable practically. Given these points, it is no wonder that there has been and is now considerable interest in developing molecules and materials that catalyze the selective (non-radical chain) reductant-free oxidation of many classes of organic substrates using only There are just a handful of homogeneous catalysts for such processes, and despite the fact all of these have one or more significant (rate, selectivity and/or stability) limitations, each of these studies has garnered much attention.1-7 Furthermore, none of these systems functions effectively under ambient conditions (1 atm of air o and ~22 C). This is unfortunate because a major intellectual impetus as well as programmatic or developmental driver of such catalytic chemistry is the realization of materials (coatings, fabrics, cosmetics, others) that catalyze the degradation of the ubiquitous toxic agents in our environment without chemical or physical assistance (without the requirement for heat, light, water, solvents, activators, etc.).11-13 Such materials could economically function in a host of locations (the home, the workplace, the car, etc.) for a host of applications and thus benefit mankind. The toxic agents of relevance include sulfur compounds (thioethers, thiols and with mustard (formula: being one deleterious thioether of much current national and international concern because it is a widely prevalent chemical warfare agent.14-20 Several nitrogen compounds, including pyridine, nicotine, trimethyl amine, aldehydes, halogenated compounds and other volatile organic compounds (VOCs) also constitute everyday threats to human health.11-13,21 This chapter summarizes quite recent research on a new type of system based on the coinage metals that does in fact catalyze selective reducingagent-free oxidation of an important target substrate, 2-chloroethyl ethyl sulfide (henceforth CEES for convenience), a very effective simulant for mustard.22,23 The review addresses the genesis of this new type of system and its systematic experimental elaboration including a quite thorough analysis of mechanism. In this context, the selectivity (virtually quantitative

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for the desired minimally-toxic sulfoxide product, CEESO), the stoichiometry and the evidence for a non-radical chain mechanism are presented and discussed. This work represents a foray into “functionally smart” materials, materials that not only sense and adapt to ambient conditions but also execute important functions when appropriate. In this instance, the function is catalyzing the facile oxidative degradation of toxic sulfur compounds if and whenever they are present.

2. DISCOVERY OF CATALYTIC OXIDATION SYSTEM By combining heteropolyanions (polyoxometalates; POMs for convenience) and selected cations in acetonitrile, more than 150 combinations were assayed for their catalytic activity towards selective CEES oxidation to CEESO by dioxygen under ambient (room temperature and atmospheric pressure) conditions. The main criteria in choosing POMs were their ability to undergo reversible redox transformations and to catalyze homogeneous oxidations either by peroxides or other terminal oxidants. The cations chosen included redox-active transition metal ions or cations conventionally used as the counterions in POMs. In control experiments the chloride, nitrate or perchlorate salts of the same transition metal ions were also examined. The list of these catalytic systems and some selected results were recently published.22 Most of the screened combinations (catalysts) showed little if any catalytic activity. Only two catalysts exhibited considerable activity, both of which included The catalysts contained 1 equivalent of or and five equivalents of A mixing of with these POMs resulted in a formation of a white precipitate, which elemental analysis showed to be NaCl. It was hypothesized that removal of a chloride ligand from produced an active catalyst. To explore this idea Ag(I) a stronger halide abstractor in acetonitrile than Na(I), was used to remove a chloride anion from the Au center. The rate of CEES oxidation by this POM-free system based on Au and Ag was significant, and by varying the ratios of and the rate was increased several-fold. Interestingly, the Au(III)/Ag(I) system appeared to be inactive when was replaced with Subsequently, by varying the ratios of different Ag(I) salts it was established that the rate was very sensitive to the Au(III): ratio. A detailed study revealed that the most active catalyst was formed when and were combined in a 1 : 1 : 1 ratio, suggesting a formation of the complex. This catalytic system is significantly faster than the two most reactive catalysts in the literature for

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the

homogeneous oxidation of thioethers, namely and The Ru-based catalyst oxidizes tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) at elevated temperature and ~8 atm of with a rate of only 3.4 turnovers (at 110 °C). The latter catalyst is more active, producing 17.6 turnovers of THTO after 30 min, but the conditions are at elevated temperature and pressure (60 °C and 14 bar of Both these systems are completely inactive under ambient conditions. In contrast, our first unoptimized system was producing nearly comparable turnovers to the Cebased system (35.4 equivalents of CEESO per Au(III) after 4 hr) under ambient conditions. It’s worth mentioning that CEES is significantly harder to oxidize than THT, both thermodynamically and kinetically, since it is less reducing and less nucleophilic.16 The selectivity (CEESO formed based on consumed CEES) was also noteworthy.16 CEESO was the only product formed in the reaction, and further oxidation to did not occur. The simultaneous measurement of dioxygen and CEES consumption, as well as CEESO formation revealed the following reaction stoichiometry:

Given the unprecedented reactivity and selectivity of the discovered system (mixture of Au(III) and Ag(I) salts), we chose it to investigate the reaction kinetics in detail. The principal system of focus comprises 1, 0.75, and 1.25 equivalents of the and precursors respectively. The general features of this catalytic system, the complex rate law and other kinetic features of this reaction, and the reaction mechanism are reported below.

3. STOICHIOMETRIC Au(III) REDUCTION BY THIOETHERS Stoichiometric thioether oxidation by Au(III) complexes has been extensively studied in the literature. The major features of this reaction are the following: (1) is not reactive towards thioethers under ambient conditions; (2) halide and thioether ligands exchange in rapid pre-equilibria; (3) the rate-limiting redox step involves thioether reduction of Au(III) forming Au(I) and sulfoxide; (4) the formation constant for the Au(III) complex with 1 thioether ligand > the formation constant for the Au(III) complex with 2 thioether ligands (5) Au(III) complexes with 3 thioether ligands are not known; and (6) the lability of the transient Au(III) complexes renders them nonisolable.26-30 Our Au(III)-based catalytic

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system for oxidation shares all the above features but has additional components and therefore is more mechanistically complex.

4.

IN SITU CATALYST PREPARATION

As is the case with both Ag salt precursors, and by themselves are completely inactive as catalysts. Moreover, the addition of a soluble Ag(I) salt (e.g. or to does not result in precipitation of AgCl, as expected via eq 2:

However, the subsequent addition of CEES to this Au(III)/Ag(I) solution leads to the immediate disappearance of the yellow chromophore of Au(III), and solid AgCl forms (confirmed by elemental analysis after isolation). The stoichiometry for this catalyst preparation reaction was determined to be eq 3:

If solid AgCl is filtered off and the solvent is removed, the remaining solid contains Au, CEES and/or CEESO moieties but no Ag. Attempted recrystallization of this material was unsuccessful, and Au(III) was reduced to a catalytically inactive Au(0) colloid. It is observed that once Au(0) forms in the system, it is not reoxidized, leading to an irreversible inactivation of the catalyst. However, use of the correct ratios of precursors in the presence of or even ambient air, produces soluble and stable Aubased catalysts, some of which retain activity for at least one week. Most Au(I) complexes, with the exception of disproportionate in and several disproportionation equilibria have been measured.31-34 Although disproportionation is less favourable kinetically in solution, it is possible that the catalyst inactivates by hydrolysis of the Au(I) intermediate followed by its reduction to form Au(0).35 In acetonitrile, catalytic thioether oxidation may exhibit a short induction phase which is followed by the main reaction (Figure 1). This induction period depends on the concentrations of CEES, Au, and At higher concentrations the induction period is shorter (Table 1 shows the dependence of the induction period). During the induction period the system remains colorless, neither CEES nor are consumed, and no CEESO is formed. At the end of the induction period, the solution rapidly becomes yellow. The electronic absorption spectrum of this yellow species is fully

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consistent with a conventional square-planar Au(III) complexes.36 The concentration of Au(III) remains constant after the induction period, i.e. during the main reaction.

5. REACTION STOICHIOMETRY Once the catalyst is formed in situ (eq 1) and reoxidized by dioxygen during the induction period to the active Au(III) form, the main catalytic reaction then proceeds. The stoichiometry of the reaction, eq 1, was established by quantifying consumption, CEES consumption and CEESO formation. In general, other stoichiometries for thioether oxidations are also possible including the more common "oxidative dehydrogenation" process (eq 4 for substrate = These reactions are more favourable for

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easily dehydrogenated hydrocarbons.

substrates

including

alcohols

and

aliphatic

However, substrates with oxidizable lone pairs (such as thioethers, RSR) are usually oxidized through “oxygen atom transfer” mechanism, eq 5:

Peroxides, high-valent transition metal oxo species, or peroxo radicals are the typical “oxygen donor” reactive species, “O”, in sulfoxidation reactions.38-40 A conversion of to peroxo species or other forms of reduced oxygen requires a consumption of a 2-electron sacrificial reducing agent, The most frequently used reductants are NADH, NADPH (particularly in enzyme-catalyzed oxidation processes) or ascorbate and thiols. An exemplary “monooxygenase” stoichiometry of sulfoxidation by dioxygen is eq 6:

The "dioxygenase" stoichiometry, eq 1, is optimal because all of the oxidizing capacity of dioxygen is used and both oxygen atoms are accounted for in the desired product. For both stoichiometries, eq 1 and 6, the question arises whether the oxygen atom in the sulfoxide originates from dioxygen or from water, which is always present in our system in small amounts. The experiments with unambiguously showed that our Au-based catalytic oxidation always resulted in the formation of 100% labeled (the mechanism and details are discussed below). Determination of the stoichiometry also involves the product selectivity since sulfoxides can undergo further oxidation to the corresponding sulfones. This is particularly important in our work since mustard sulfoxide is much less toxic than sulfone and the sulfoxide is not a vesicant while both the sulfide and sulfone are.41 In our case, the selectivity for CEESO is exceptionally high and the sulfone, has never been detected. It is worth mentioning that a high selectivity is commonly observed at low conversions for most oxidation reactions. In acetonitrile, total CEES conversion was impossible to achieve due to the inhibition of the reaction by the CEESO product. However, the same catalytic system with trifluoroethanol as a solvent (see Section 15) can completely oxidized CEES, while the selectivity for CEESO formation was close to 100%.

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6. EMPIRICAL REACTION RATE LAW Typical kinetic curves of CEES consumption and CEESO formation are presented in Figure 1. The dotted, dashed and solid lines in Figure 1 arise from fitting of experimental data to the proposed mechanism and are discussed below. While it may not be readily apparent, the kinetics of the main reaction after the induction period does not obey any simple kinetic law. The reaction gradually slows down as a result of a mild inhibition by CEESO product. The dotted line is a simple exponential assuming no such inhibition takes place. The primary experimentally determined rate parameter was +d[CEESO]/dt, but -d[CEES]/dt and were also evaluated in many cases. These values are indicated henceforth as "rate". The reaction rate was determined as a function of the concentrations of CEES, Au(III), Ag(I), together in a constant 1:2:1 mole ratio, DMSO as a model for the CEESO product and

Figures 2-4 and Table 2 give the rate dependencies of the main reaction, eq 1, on the concentrations of CEES, total Au(III), and DMSO. The maximum rate is achieved at Au : : ratio 1 : 2 : 1 , suggesting that the dominant transition state complex contains one and two groups. Further, it was established that replacement of by or generated species with little or no catalytic activity. In contrast, replacement of by formed a more reactive complex, confirming a significant and specific role for (or in the active catalyst (Figure 2). The rate increases with the concentration of the thioether

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with saturation observed at high CEES , suggesting a replacement of a and/or ligand by CEES to form an inactive Au(III)-complex.

The presence of both and appear to be critical for catalytic activity. Since the activity of the catalysts depends on the ratio of Au : : this ratio was kept constant at 1 : 2 : 1 in the determination of the dependence. The rate increases quadratically with with the reaction rate order 1 < n < 2. The dependence (see Figure 7 in our recent publication22) is inversely hyperbolic, which is consistent with once again competing with and/or CEES for open coordination sites on the catalytically active Au center. This, in turn, decreases the concentration of the catalytically active Au complexes. The rate of the main reaction is independent of concentration (Table 1), implying that Au(III) reduction and not Au(I) reoxidation is rate limiting.

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7. RATE LIMITING STEP A zero order reaction rate with respect to dioxygen concentration suggests that the rate limiting step is Au(III) reduction by CEES, not reoxidation of Au(I) by dioxygen. For futher proof of the rate limiting step, we determined whether Au(I) and/or Au(III) was the dominant redox state of Au during the main reaction. This was assessed by measuring the absorbance for Au(III) after the induction period as function of CEES and concentrations, while all other reactants and conditions were kept constant. The electronic absorption spectrum (350 - 450 nm) of Au-complexes during the main reaction is very similar to that of conventional isolable square-planar Au(III) complexes for is colorless After the induction period, the observed spectra and their intensity remained unchanged and were independent of both reactants ([CEES] from 0.05–0.37 M and from 50100%) within experimental error. Thus, the aggregate Au species during catalytic CEESO formation was almost entirely in the form of Au(III). Based on the two definitive results described above, it became obvious that the reduction of Au(III) species and not the oxidation of Au(I) species is rate limiting in these aerobic oxidation reactions.

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It is worth mentioning that thioethers are catalytically oxidized to sulfoxides in nitromethane-aqueous nitric acid in the presence of with the rate-limiting step being the reoxidation of Au(I) to Au(III).28 This involves the use of an excess of relative to Au; however, in our systems and not is the terminal oxidant. This follows from the fact that only one equivalent of is used in our reactions, but 200 equivalents of CEESO product per equivalent of can be obtained.

8. PROPOSED REACTION MECHANISM As mentioned above, Figure 2 indicates that the most catalytically active species contains in a 1 : 2 : 1 ratio. Dimeric or oligomeric d8 square planar Au(III) complexes are very unlikely,43 therefore a monomeric Au(III) complex with 2 ligands and 1 ligand has the capacity to bind one more ligand strongly, most likely a CEES ligand. Five-coordinate Au(III) complexes are very rare. The only 5-coordinate Au(III) complexes with appreciable stability contain chelating ligands such as bromodicyano(l,10-phenanthroline)gold(III) isolated from dimethylformamide.44 If there is an interaction between a positive Au(III) center and a negative axial ligand counterion, this ligand would likely be because associates very weakly with Au(III) (and not at all with Au(I)) and probably can be ruled out in our case. A proposed mechanism that is compatible with the stoichiometric thioether-Au(III) reactions in the literature and all our data is given in Scheme 1. Literature data on the stoichiometric Au(III) reduction by thioethers provide evidence for the rapid exchange of all ligands on Au(III) prior to thioether oxidation.26,27,45-47 In our mechanism these pre-equilibria are summarized by eqs 7 and 8 in Scheme 1. Mixed Au(III) complexes are formed with and CEES: a complex with one CEES ligand (1), with formation constant and a complex with two CEES ligands (2), with formation constant These ligands drive eq 7 to the right, while and CEESO (or DMSO, data in Table 2), drive it to the left. Additionally, or other solvent molecules may also bind to Au(III) or shift both equilbria, eqs 7 and 8. Au(III) complexes with one thioether ligand, analogous to 1, are considerably more abundant in solution than complexes with two thioether ligands, such as 2 Au(III) complex with three thioether ligands are not considered because there is no data indicating these species form in solutions

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As discussed above, all the available information (section 7) is consistent with the rate limiting redox step being the simultaneous oxidation of thioether and reduction of Au(III). A mechanism involving the rate-limiting formation of the required Au(III) complex prior to undergoing redox chemistry is not supported by any of the literature studies. The kinetics and equilibrium investigations establish that in the oxidation of and by and of by the slowest step is the bimolecular reaction of thioether with Au(III) complexes containing one or two thioether ligands. Both these systems are stoichiometric; they do not undergo catalytic turnover. These systems differ from ours as they do not contain and are used in polar, protic solvent systems like 95:5 or 100% water.

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In our studies polar, aprotic solvents such as have primarily been used (see also sections 15 and 16). Nevertheless, the Au(III)-thioether coordination and the redox chemistry in the two systems are otherwise very similar. The two reactions depicted in eq 9 have the rate constants for 1 and for 2. Since 1 is the most reactive complex, eq 9 is written in terms of 1 and its redox products, and The subsequent steps, eqs 10 and 11, are fast and consequently kinetically inaccessible processes. Of the several possible mechanisms for the rate-determining redox step, eq 9, but one is more consistent with all available data (see the next section for details) and involves bimolecular attack of thioether on a coordinated chloride ligand of Au(III). This mechanism has been proposed for some stoichiometric thioether oxidations by Au(III).45,46,48 None of the data on our system contradicts this mechanism. It is also consistent with the increase in rate when is replaced by under otherwise identical conditions (see also section 12). Inner sphere ligand transfer redox processes, including reduction of Au(III), are known to proceed faster for compared with 46, 49 ligands. For example, is reduced by sulfite and hydrogen sulfite ca. 10 times faster than Interestingly, this kinetic preference is in the opposite direction from the reaction enthalpy since the metal centers with the have higher potentials and are stronger oxidants than their Since is large and then The equilibrium expressions from eqs 78, Scheme 1, and reaction mass balance expressions afford eqs 12 and 13.

Eqs 13 -15 can be used to obtain the expressions for [1] and [2], in eqs 16 and 17.

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Combining eqs 11, 16 and 17 gives eqs 18 and 19 for the reaction rate.

Eq 19 has been used to fit the experimental data, namely the dependencies of rate versus [CEES] and The best fit is a solid curve shown in Figure 4 with and The errors were estimated at the 95% confidence limit. Since the complex eq 19 simplifies to eq 20.

It is evident without curve fitting that and complex 2 shows a low catalytic activity towards CEES oxidation. At high [CEES] nearly all Au is present as Au(III), and eq 19 then simplifies to eq 21 (details are in Boring, et al.)22:

Figure 4 shows saturation kinetics at [CEES] > 0.4M, which clearly suggests that Thus, the CEES substrate slows the reaction by shifting eq 8 to the right in favor of the less reactive 2 by substituting CEES for and/or from the more reactive 1. Eq 20 also demonstrates how sensitive the overall reaction rate is to a change in and While the rate is linearly proportional to the dependence on is more complex and varies with and [CEES] as illustrated in Figures 3 and 4 by dashed and dotted lines computed at different values of The effect of solvents and ligand substitution on and is addressed below. The dependence of the rate on Figure 3, was also fitted to eq 20, the solid curve is with and which are in good agreement with those obtained from the fit of the rate vs. [CEES] data and Surprisingly, our value for is very similar to that for complexation of to in 5% aqueous methanol: However, in the latter system is ~ 12.27 While these two systems are very similar, they have different ligands (no and more in the latter one) and solvent.

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After the slow Au(III) reduction by thioether, the process involves conversion of the oxidized sulfur center to the sulfoxide. The first intermediate of this conversion is a chlorosulfonium ion (eq 9, Scheme 1). These species are known to hydrolyze rapidly to yield sulfoxides.50 The study with isotope labeled is entirely consistent with this mechanism (see below). As mentioned above, the reaction slows down with time due to inhibition by CEESO products. CEESO replaces or in the active complex 1 and gives an inactive complex analogous to 2. This inhibition has been modeled by DMSO, a molecule structurally and electronically similar to the CEESO product, and is addressed below (section 13). A full evaluation and nonlinear fitting of the kinetics of self-inhibition by CEESO is discussed in a recent paper.23 The result of this fitting is shown as a solid line in Figure 1, and the anticipated CEES consumption (an exponential decay with if there were no inhibition by CEESO is given as a dotted line (see below).

9. MECHANISMS RULED OUT Several other possible mechanisms for the rate-limiting redox step are discussed below and ruled out. The first one assumes that a slow formation of the required Au(III) complex (for example, 1) is the rate limiting step, and redox transformation is fast. However, this mechanism is not supported by the literature studies on stoichiometric Au(III) reduction by thioethers, nor by the kinetic data obtained in this research. Clearly the reduction of Au(III) is the slow step, but this in turn may proceed through different possible mechanisms. One mechanism for Au(III) reduction by thioethers involves the intramolecular reductive elimination of 1 and 2 to form a thioxonium salt (eq 9 in Scheme 1) and a Au(I) complex, eqs 22 and 23.

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This mechanism has been proposed for some thioether oxidations by Au(III)26,45,47 and the conversion of 2 to and CEESO can be 26 assisted by eq 24. However, this mechanism is also very unlikely in our catalytic system; analysis of the reaction kinetics unambiguously rules it out. If eqs 22 and 23 were rate limiting, then the overall rate would be eq 25.

Combining eqs 16, 17 and 25 gives eqs 26 and 27 for the reaction rate.

This rate law is incompatable with our experimental kinetic data. If then the reaction rate should increase quadratically with which is in agreement with Figure 3, while the reaction rate should decrease with increasing [CEES], which is inconsistent with our data (Figure 4). then the rate should increase with both and [CEES] finally reaching saturation with an observed reaction rate order < 1. This is in disagreement with the data presented in Figure 3. Thus, only one of the rate dependences, either that for [CEES] or that for can be explained by this mechanism, but not both simultaneously. Incorporation of eq 24 in the above analysis leads to the same expression as eq 27, where is replaced by Thus, the addition of eq 24 into the reaction mechanism does not affect the theoretical reaction rate law, eq 27. Additional evidence against the unimolecular collapse of 2, assisted by a water molecule, being the rate limiting step (eq 24) derives from a nonexistent kinetic isotopic effect when was used instead of The kinetics data are in accord with functioning as an inhibitor competing with or CEES for a coordination site in the Au(III) complexes and thus driving eq 7, Scheme 1, to the left. In principal, a radical-chain thioether oxidation initiated by Au(III) complexes may also take place. Such a mechanism has been proposed by Riley et al.25 for aerobic thioether oxidation catalyzed by Ce salts. This sulfoxidation reaction proceeds in at elevated temperatures (> 70 °C). The proposed mechanism includes the reduction of Ce(IV) by thioether to form the radical cation which is efficiently trapped by to give The latter radical, being a strong oxidant, reoxidizes Ce(III) to

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Ce(IV) and forms another reactive intermediate which in turn reacts with to produce 2 molecules of sulfoxide product. According to this mechanism, the oxygen atom in the sulfoxide should be incorporated from not Since in our system 100% oxygen-18 incorporation from was observed, such a radical-chain mechanism can be definitively ruled out. Additional evidence against this mechanism derives from a competitive oxidation of different thioethers (see section 18). Au(III)-based catalysts appear to be highly discriminatory with respect to thioethers when a combination of thioethers are oxidized together. The high-energy intermediate, proposed to be generated in the Ce-based systems, would likely not able to discriminate highly between different thioethers.25 Finally, another possible mechanism for aerobic sulfoxidation is proposed by Riley and co-workers for the reaction catalyzed by This chemistry proceeds at rather high temperatures (>100 °C) and requires the use of alcohols as solvents. The reactive species in this system is which is produced as an intermediate in the oxidation of Ru(II) to Ru(IV) by Thus, the Ru(IV) formed is reduced back to Ru(II) by alcohol, a solvent molecule. The labelling studies in this system show that no oxygen-18 incorporation from occurs, in direct and total opposition to our Au(III)-based catalytic system (100% incorporation from Additional evidence against such a mechanism derives from the stoichiometry itself (eq 1 in our case versus eq 6 in Ru-based system) and selectivity (~100% yield of CEESO at high CEES conversion, but further sulfoxide oxidation to sulfone in the Ru system studies.

10. ORIGIN OF OXYGEN IN SULFOXIDE PRODUCT; ROLE OF IN SULFOXIDATION The simplest procedure for sulfoxide synthesis is thioether oxidation by and acids are efficient catalysts for this reaction.51 In the presence of protons is a potential intermediate in Au(I) reoxidation by eq 28:

HC1 and are formed during the catalytic process, eq 9, and one of the reactants, is acidic. Thus CEES could be, at least partly, oxidized by formed during this process. To assess the role of and to address the formation of a thioxonium salt intermediate (by establishing the incorporation of oxygen-18 into CEESO from isotope labeling studies were done by replacing regular with In addition, CEES oxidation by in the presence of our Au(III)-catalysts was also assessed.

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Oxidation of CEES to CEESO by alone occurs very slowly (if at all), but the reaction is efficiently catalyzed by p-toluenesulfonic acid. In the presence of this strong acid or our Au(III)-based catalyst, the reaction proceeds upon mixing. Thus, if is formed as an intermediate in our system (eq 11), it should immediately react with CEES. In this case one half of the total CEESO formed in a catalytic reaction could derive from reaction of with CEES. However, oxygen-18 labeling experiments show that the CEESO produced in the early part of the reaction (up to 10 turnovers) is and The somewhat lower in the CEESO than in the used derives in part from water present in the initial This Au catalyst precursor compound is hygroscopic and very hard to dry. The percentage of in the CEESO also decreases with an increase of CEES conversion. For example, the incorporation into CEESO product was ~60 atom % after 60 turnovers. The dependence of incorporation percentage on the extent of CEES conversion is consistent with a consumption of labeled water during catalytic turnovers. The water concentration remains unchanged in the overall reaction: one water molecule is consumed in eq 9 but it is immediately regenerated in eq 11 resulting in a dilution of pool with Thus, CEESO is produced in the reaction involving exclusively but not or If alone were involved, the ratio would not exceed 50%.

11. REOXIDATION OF Au(I) BY DIOXYGEN. CATALYST PREPARATION FROM Au(I) COMPLEX The isotope labeling and kinetic studies of CEES sulfoxidation by reveals that reoxidation of Au(I) by in the presence of protons does not proceed via eq 28, and is not formed as an intermediate. As indicated above, this re-oxidation reaction is not a rate limiting process, therefore its mechanism can not be assessed kinetically. This reaction is also not precedented in the literature. Au(I) forms linear, trigonal planar or tetrahedral complexes, with the linear geometry being the most common.43 The complexes and are colorless (for lignad, easily prepared by reduction of in ethanol and isolable with bulky ligands 52, and stable in acetonitrile.33,34,42 The complex is of considerable commercial importance, since it is used in the extraction of gold from its ores and in gold electroplating applications. It is polymeric with a linear Complexes of Au(I) with thioethers of formula are usually prepared by reduction of in the presence of appropriate dialkylsulfide, eq 29.29,43,54

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Alternatively, the complexes can be prepared by direct addition of the thioether to In solutions with an excess thioether, the thioether ligand is in rapid exchange with In (1:1) the complex, is formed after the addition of 2 eq of thioether to 1 eq of This suggests that thioethers bind strongly to Au(I). The equilibrium constants and have been determined for the displacement of in by and other ligands33,42, and log is 10.6 for the latter one. Because Au(I) is a 2-electron reducing agent while is a 4-electron oxidizing agent, the reaction between them requires either formation (in protic media) or the formation of other intermediates. So far no experimental data on this re-oxidation reaction are available, therefore only a highly speculative mechanism could be proposed, Scheme 2:

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Based on structural properties of Au(I) complexes, the formation of a binuclear complex 4 bridged by two Cl-ligands with a linear moiety is reasonable, eq 30, Scheme 2. Dioxygen binding to this complex promoted by the presence of two thioether ligands results in the formation of a mixed Au(III)/Au(I) complex 5, with a square-planar peroxo-Au(III) and linear Au(I) moieties. Mixedvalent Au-complexes are precedented in the literature. The vibrational spectra of some dialkyl sulphide complexes of gold (III) and gold (I) halides have been reported.56 A mixed-valence complex of gold with dimethyl sulfoxide has been reported.57,58 The unimolecular rearrangement of 5 results in the formation of a complex 6, eq 33. Transfer of terminal oxygen atom from the peroxo-group to the sulfur atom of the thioether, eq 32, can be definitively ruled out since it would result in only 50% incorporation of oxygen-18 from labeled water during the catalytic reaction. A heterolytic cleavage of O-O bond in 6, assisted by protons, would result in the formation of the catalytically active complex 1, eq 34, without intermediate formation. Scheme 2, while highly speculative, is compatible with the structural properties of Au(I) and Au(III) complexes in the literature and our current experimental data. Scheme 2 suggests that an active catalyst can be prepared starting from a Au(I) complex and proper amounts of nitrate and chloride salts in the presence of a strong acid. We attempted to prepare such a catalyst by mixing p-toluenesulfonic acid, and CEES in acetonitrile under oxygen and have found that the rate of catalysis increases with increasing acid concentration. For example, the reaction occurs very slowly if at all when no acid is present, but reactivity is clearly visible when one equivalent of p-toluenesulfonic acid acid is present. When three equivalents are used, the reactivity is comparable to the system.

12. EFFECT OF LIGANDS ON REACTIVITY As described above and in recently published mechanistic studies22 a ratio of catalyst components of 1 produced the most reactive catalyst, indicating that is necessary for high catalytic activity. To assess this necessity, was systematically replaced with other ligands and Surprisingly, the stoichiometric substitution of with produced a catalyst with ~2fold higher activity than in the case of (Figure 2). However, this new system was not thoroughly investigated because is more toxic than and would be less desirable for many applications. Other ligands led to

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completely inactive catalytic systems. Thus, or are definitely necessary for high catalytic activity. Above it was noted that the replacement of with increases the initial reaction rate. This increase in rate is consistent with the involvement of a ligand transfer reaction in the rate limiting step, eq 9 in Scheme 1, in the proposed mechanism (for detailed discussion see section 8 and 9). However, there is another factor, which considerably affects the overall reaction rate. In acetonitrile at [CEES] > 0.1 M a considerable part of total Au(III) is in the form of the inactive complex 2, which results in a saturation of the initial rate with increasing CEES concentration (Figure 4). Eq 20 perfectly describes this complex experimental dependence (solid line on Fig 3). Because a ligand binds more strongly than to Au(III)58, (eq 8, Scheme 1) is lower for the Br-Au complex than the Cl-Au complex. Nitrite is a softer ligand than nitrate, and subsequently it also binds stronger to Au(III) (which prefers to complex with soft ligands) than nitrate.59 Several gold (III) nitrite complexes have been identified60-62, but none have been isolated. Thus, both and evidently drive eq 8 to the left thereby decreasing Changing the value of has a dramatic effect on the reaction rate according to eq 20. The magnitude of this effect also depends on CEES and concentrations and is demonstrated by dashed and dotted lines in Figures 3 and 4. These theoretical dependencies are computed using different values for It is clearly seen that a decrease in the equilibrium constant, results in a considerable increase in the reaction rate. Thus, higher activity for and is at least partly explained by a decrease of for these ligands. A more thorough investigation shows that this ligand replacement also reduced the induction period and decreased the inhibition of the overall reaction by CEESO product (Figures 2). As a consequence, a significantly higher conversion of CEES to CEESO can be achieved in the system with a ligand. Inhibition of the reaction by CEESO product arises from formation of the inactive complex 2’ with the sulfur ligands, analogous to 2, but with one of the CEES ligands replaced by a CEESO22 ( see Scheme 3 and section 13 below). The analogous complex with two sulfoxide ligands, 2”, is also inactive. A replacement of bromide or nitrite with CEESO in a complex similar to 1 can be described by equilibrium constants and which are analogous to Because a replacement of bromide or nitrite by CEESO is less favourable is lower for bromide or nitrite than for chloride and nitrate), product inhibition is less pronounced. An induction period in CEES oxidation is likely to be the result of slow catalyst formation during the reoxidation of Au(I) by dioxygen at the beginning of the reaction.22 However, since reoxidation is not a rate limiting step during the main process, a kinetic evaluation of the ligand effect on the rate of Au(I) reoxidation is not possible.

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13. PRODUCT INHIBITION (DMSO EFFECT) As noted above, the reaction slows down as product CEESO accumulates (Figure 1). Inhibition by CEESO product has been proven by using DMSO as a model and also by nonlinear least squares fitting of kinetics of CEES consumption.22 However, a more thorough evaluation of the DMSO dependence reveals that at low DMSO concentrations (< 0.03 M), the initial reaction rate increases to a maximum and then subsequently decreases with increasing [DMSO]. A maximum rate is observed at ~10 mM DMSO (Table 2). A more complex model compared to the one previously described22 to interpret this dual effect of DMSO (initial acceleration followed by inhibition) is given in Scheme 3.

Equilibria for the reaction of DMSO with Au(III) are similar to those of CEES with Au(III). DMSO forms complexes 7, 2’ and 2”. Complex 7 is an

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analogue of 1, while 2’, 2” and 3’ are analogues of 2 and 3, respectively. Complexes 3 and 3’ are exchanging CEES and CEESO ligands in fast equilibria.

Equilibria and mass balance expressions lead to eqs 39-40 for the concentrations of 1 and 723:

where Eqs 39 and 40 give eq 41 for the reaction rate law:

Nonlinear least squares fitting of initial rate versus [DMSO] dependencies at different initial CEES concentrations (some data are given in Table 2) is in agreement with the experimental data (Figure 4 in Boring et al.)23. The results of this fitting show that is roughly three orders of magnitude higher than In summary, two DMSO properties explain its dual effect on the reaction rate: its stronger binding to Au(III) compared with thioethers and the higher reactivity of Au(III)-sulfoxide complex towards thioether oxidation than the analogous Au(III)-thioether complex DMSO is a stronger electron withdrawing ligand compared to CEES. Therefore, electron density on the Au center in complex 7 is lower resulting in a higher potential than for 1, consequently Indeed, a study of the square planar complexes (where L are different thioethers and their sulfoxides), which is isoelectronic to Au(III), reveals that the potentials of the sulfoxide complexes are 200-250 mV higher than those the corresponding thioether complexes.63 Crystal structure studies of and provide additional evidence. The structures indicate that a covalent bond between DMSO and Pt(II) is stronger than between DMS and Pt(II). Steric hindrance may be another factor in the preferential binding of DMSO relative to CEES (CEES is more bulky). Because sulfoxides have a stronger affinity to Au(III), they form inactive 2’ and 2” complexes at lower concentrations and thus the inhibition is observed when [DMSO] < [CEES]. CEESO, the product of CEES oxidation, forms a similar sulfoxide complexes 2 ’” ( eq 42).

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At a higher conversion of CEES, more inactive Au(III)-complexes are formed. As a result the reaction slows down faster than expected accounting only for CEES consumption. In other words, the reaction is self-inhibited by the reaction product, CEESO. This self-inhibition is clear in Figure 1, a line with (+) symbols represent the kinetics of CEES consumption assuming that no selfinhibition takes place (exponential kinetics). The extent of self-inhibition is evidently dependent on the binding properties between sulfoxides and Au(III), represented by the equilibrium constant (eq 42). A stronger binding of the sulfoxide to Au(III) (higher results in a stronger self-inhibition. Dashed and dotted lines in Figure 1 represent the theoretical curves describing CEES consumption using different values of The best fit is obtained when (a solid line in Figure 1). The formation of inactive complexes is accompanied by a complexation of a nitrate anion and a large Au(III) cation, 2’,2” or 2’” . The formation of these charged species should be more favourable in a polar solvent such as This explains why self-inhibition is more pronounced in than in (for additional discussion see section 15). Au(III)-Br complexes are larger than similar Au(III)-Cl complexes because the ionic radius of in crystals is higher than that of 1.96A and 1.81, respectively. The enthalpy of solvation also decreases in the order implying that the formation of the analogous bromide complexes, 2’ and 2” is less favourable. Therefore, as mentioned in section 12, product inhibition is less severe when a Cl-ligand is replaced with Br.

14. CO-CATALYSIS BY TRANSITION METAL IONS Since the reduction of a Au(III) complex by CEES is the rate limiting step, it was speculated that transition metal ions in high oxidation states might catalyze the reduction of Au(III). Both Fe(III) and Cu(II) were screened for their activity in acetonitrile with and ligands. Both these metals eliminate induction period (Figure 5), but only Cu(II) gave a rate enhancement (i.e. co-catalyst activity), particularly in the presence of (see Figure 2 in recent article).23 For example, the initial oxidation rate at ~3.0 M CEES is approximately 2 times faster for than for while is 8 times faster than It is important to note that addition of Cu(II) salts to the system eliminates the inhibition effect of the sulfoxide product, allowing for almost the complete oxidation of CEES to CEESO. Co-catalysis by a copper salt is even more

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pronounced in heterogeneous systems if perfluorinated polyethers (PFPE oils) are used as a solvent (see section 16). This co-catalysis by Cu(II) can be attributed to the formation of Au(III)-HalCu(II) complexes, where Hal is or The oxidation of CEES by this Au(III)/Cu(II) is more facile due to the additional withdrawal of electron density from Au(III) by Cu(II) (a more electropositive Au center results in an increase in the overall reaction rate).

Au(III)-Hal-Cu(II) moieties are precedented in the literature. For example, a copper(II) chloroaurate(III) complex with a Au(III)-Hal-Cu(II) unit is formed by the neutralization of with and has been characterized (GMELIN registry number 177659). Additionally, a withdrawal of electron density from Au(III) by Cu(II) makes the Au(III)ionic bond stronger, resulting in a less favourable ligand substitution of nitrate by CEESO, and consequently less product inhibition.

15. SOLVENT EFFECTS Acetonitrile is a polar organic solvent and therefore many inorganic salts and organic compounds are reasonably soluble in it. Additionally it is extremely robust towards oxidation and is aprotic. Therefore most of the research reviewed here involved the use of acetonitrile as the solvent. However, it is rather toxic (40 ppm is the practical exposure limit (PEL), http://www.msdsonline.com) limiting its use in personal care products including topical skin protectants (TSPs) and other skin creams. Another

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disadvantage of acetonitrile is that when it is used as a solvent, an induction period can occur during the catalytic reaction. For these reasons we looked at the possibility of using other solvents. Figure 6 shows two of the four solvents (trifluoroethanol and acetonitrile) in which catalysis of CEES oxidation to CEESO occurs. Three of the four solvents, trifluoroethanol, nitromethane and 1,2-dichloroethane, do not exhibit a measurable induction period. The highest rate is observed in trifluoroethanol. It is also noteworthy to mention that a complete conversion of CEES to CEESO can be achieved in trifluoroethanol. When ethanol or acetone is used, the Au(III) is immediately reduced to colloidal Au(0), which cannot be reoxidized by In tert-butanol, Au(III) is reduced to Au(I) but reoxidation of Au(I) to Au(III) is not observed. In pyridine, a mixing of and CEES does not result in precipitation of AgCl indicating that the active catalyst is never formed. In perfluoropolyether (PFPE) solvents at least one component is insoluble, so these reactions are not entirely homogeneous. These systems are classified as heterogeneous and are discussed below.

Binding of solvent molecules to Au(III), eq 7 in Scheme 1, as well as a shift of the equilibrium between the cation 2 and neutral 1 (eq 8) should be considered. For example, acetonitrile binding to both Au(III) and Au(I) complexes is well precedented35,69,70, and the solvolytic equilibrium, eq 43, has been studied in (95:5, v/v) by Canovese et al.71

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The value of was found to be implying that 26% of the total Au(III) (at 5 mM) is in the form In contrast, trifluoroethanol, nitromethane, and 1,2-dichloroethane would coordinate weakly if at all with the Au complexes. The equilibrium constants are likely to be very sensitive to the nature of the solvent. For example, the equilibrium constant for the displacement of heterocyclic amines (for example pyridine, py) by from Au(III), eq 44, changes from 25 in to 0.085 in

The equilibrium (eq 8 in Scheme 1) between a neutral and positively charged complex, 1 and 2 respectively, depends on the solvent polarity. The more polar the solvent, the more eq 8 is driven to the right, which increases and thereby decreases the concentration of the active complex 1 and the overall reaction rate (see discussion on the effect of in section 12). However, the solvent effect on the reaction rate is more complex. The overall rate is controlled not only by the concentration of the reactive complex 1, but also by the rate of the reduction of 1 by CEES, eq 9 in Scheme 1. In this rate limiting step two charged species, chlorosulfonium and nitrate ions, are formed. Their charge separation should be more favourable in polar solvents. Thus, predicting the effect of the solvent on the overall rate could be difficult a priori because and have dependencies in opposite directions. The highest oxidation rate is observed in trifluoroethanol, a less polar solvent than acetonitrile. This higher rate is attributable to a shift of eq 8 to the left This is also consistent with the weak product inhibition observed in trifluoroethanol. Formation of the inactive 2’” cation and nitrate anion, eq 42, is not as favourable in lower polarity solvents resulting in less self-inhibition. For that reason, CEES oxidation to CEESO proceeds to almost completion in trifluoroethanol, but significant inhibition occurs in acetonitrile (Figure 6). It has also been observed that an induction period is not significant, if it exists at all, in solvents other than acetonitrile. The length of this induction period depends on dioxygen concentration and therefore is likely to depend on the rate of Au(I) oxidation by dioxygen.22 Since this reaction is not rate limiting in the catalytic process it is impossible to assess a solvent’s effect on the rate of Au(I) reoxidation.

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16. HETEROGENEOUS SYSTEMS Since it was found that the reaction proceeds faster, with no induction period and is less self-inhibited in trifluoroethanol, the next logical step was to replace trifluoroethanol with high molecular weight perfluorinated polyethers (PFPEs). The PFPEs are the principal components of TSPs and are effectively nontoxic. The systems appears to be active for selective aerobic sulfoxidation of CEES in the PFPE oil Galden D® 02 , (17 turnovers per hour, see Table 1 in a recent article)23 and highly active in Fomblin MF-300®,a PFPE surfactant with terminal carboxylic acid functional groups (1170 turnovers per hour, Figure 7).

However, both these systems are heterogeneous because is ® only partially soluble in Fomblin MF-300 and completely insoluble in Galden D-02® (perfluorinated oil). In Fomblin MF-300® the catalyst partially dissolves when CEES is added because the Au(III) precursor, is soluble. With the co-catalyst activity of Cu(II) in acetonitrile in mind, the sulfate salts of Fe(III), Cu(II), Mn(II), V(IV), Ti(IV), Co(II), and Ni(II) were also evaluated in Fomblin MF-300®. A synergistic effect is observed when all of these redox active metals are added to the

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system (see Table 2 in a recent paper).23 The most active co-catalyst is Cu(II). When 2 equivalents of is combined with 1 equivalent of the catalyst is 3.8 and 6.5 times more effective after 10 minutes of reaction time than when one of the components is omitted and respectively). It is noteworthy that is catalytically inactive without a feature that is present in fluorinated media as well as in acetonitrile. Also, the recently reported data (Figure 6 in a recent paper)23 indicate that inhibition by product sulfoxide is less pronounced in the system. Importantly, product selectivity in the heterogeneous systems is the same as in acetonitrile, eq 123, namely that no sulfoxide overoxidation to sulfone is observed in these systems within the limits of instrumental detection. This is very important for mustard gas (HD) oxidative detoxification, because the sulfoxide, “HD(O)”, is significantly less toxic than the sulfone, Since these systems are heterogeneous, their full kinetic evaluation is not possible. However, they are very similar to homogeneous ones because the same product (CEESO) and selectivity (~100% CEESO) are observed; both halide and are required for activity; and Cu(II) is a highly active cocatalyst. These similarities suggest that the key features of the mechanisms in the two types of media are also very similar,

17. EFFECT OF AMINO ACIDS A key goal of this work is to develop a catalytic system that could be incorporated into TSPs for the oxidative detoxification of mustard gas. Cornified layers of skin (epidermal) cells contain different amino acids which could bind to active Au(III) catalytic complexes and thus could reduce or eliminate their activity. Therefore, the effect of amino acids containing such functions as alkyl, amide, amine, carboxylate, imidazole, indole, alcohol, phenol, disulfide, thioether, and guanidino side chain groups on the catalytic activity for aerobic CEES oxidation were evaluated in heterogeneous systems using Fomblin MF-300® as a solvent. These Au(III)based catalysts remain active in the presence of most amino acids. Only a few amino acids exhibit moderate inhibition of the reaction. The inhibitory effect is as follows: tryptophan (indole) (most inhibiting) > methionine (thioether) > tyrosine (phenol) > leucine (alkane) > histidine (imidazole) > arginine (guanidine) > asparagine (amide) > serine (alcohol), aspartate (carboxylate) > cystine (disulfide) (least inhibiting).23 Thus, if Au(III) centers in the suspended Au(III)-based catalysts in a deployed TSPs have direct molecular contact with the amino acids in the skin, then the epidermal

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polypeptides would probably have little impact on the catalytic aerobic oxidative decontamination (sulfoxidation) of HD.

18. OXIDATION OF THIOETHERS OTHER THAN CEES The majority of this comprehensive study has been concentrated on the chemistry of CEES oxidation, since it closely resembles one toxic target, mustard gas. However, the Au-based catalysts reported here are able to oxidize a variety of thioethers, as well as disulfides. In fact, of the six organosulfur compounds tested, CEES under optimized catalytic conditions is actually oxidized at a much lower rate and shows more significant inhibition than the other substrates. For comparison, when THT was evaluated, the optimized Au-based catalyst under ambient conditions yields ~27 turnovers in 30 min, while Riley’s Ce-based system yields 17.6 turnovers at elevated temperature and pressure over the same time period.25 Table 3 gives the substrates that were evaluated at similar concentrations, the induction period associated with each substrate, as well as the initial rates of oxidation. The thioether oxidations were found to be completely selective in that only sulfoxide was formed, while the products arising from dimethyl disulfide oxidation have not yet been characterized (cleavage of S-S bond is suspected). The thioethers thus far studied can be divided into two groups. The first group are those that exhibit no visible induction period (Figure 8), and the second group are those that do exhibit a visible induction period (Figure 9). Thioethers with higher initial oxidation rates, tend to have shorter induction periods.

An interesting feature of our Au-based catalysts is the high substrate selectivity during competitive thioether oxidation (Figure 10). When equal amounts of two thioethers (0.36 M total thioether concentration), THT and CEES, are added simultaneously to the system, the initial rates of oxidation of the two substrates are and

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respectively. Under the same conditions, the rates of oxidation for the same two substrates tested separately are and Thus while oxidation of THT in the presence of CEES is roughly twice slower than when THT is tested alone, the oxidation rate of CEES in the presence of THT is 52 times slower than when CEES is tested alone.

Thus while oxidation of THT in the presence of CEES is roughly twice slower than when THT is tested alone, the oxidation rate of CEES in the presence of THT is 52 times slower than when CEES is tested alone.

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Importantly, the ratio of initial rates is 180 in competitive oxidation, while it is only 7 when THT and CEES are oxidized separately. An additional point to be made is that in the competitive oxidation, CEES starts to be oxidized with a much higher rate only when almost all (97%) THT is consumed. This rate is similar to that of CEES oxidation when it is tested alone, indicating that in the mixed substrate system the catalyst has extremely high substrate selectivity and that THTO has very little if any inhibitive effect on the oxidation of CEES. Thus, this study shows that thioethers other than CEES can also be oxidized in this system with efficiencies even higher than that for CEES. Competitive oxidation reveals that this catalyst discriminates between different thioether substrates with considerable efficiency.

19. EXPERIMENTAL DETAILS General procedures. Quantification of thioethers and sulfoxides was performed on an HP5890 Gas Chromatograph equipped with a FID detector and a 5% phenyl methyl silicone capillary column. As an internal standard for GC analyses, 1,3-dichlorobenzene was used. Identification of products was performed using a HP 5890 GC with a 5% phenyl methyl silicone capillary column and a 5971A Mass Selective Detector. Quantification of Au(III) concentration was performed using a HP 8452A Diode Array Spectrophotometer. The oxygen concentration was varied using a Series 810 Mass Trak flow-meter with dried argon as the other gas.

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General procedure for sample preparation in acetonitrile. Once the discovery had been made that was highly active for thioether oxidation, stock solutions of each component were prepared in 20-mL vials using anhydrous and exposure to light was prevented by wrapping each vial with aluminum foil. can replace as a source that acts as a poor halide abstractor. All reagents were dried using activated molecular sieves after they had been dissolved in anhydrous Appropriate amounts of each stock solution were added via syringe to a 20-mL glass vial fitted with a PTFE septum that was first purged with The atmospheric pressure was adjusted to 1.0 atm and the reactions were carried out at In all cases, thioethers or sulfoxides were the last component to be added. The same procedure as above was used except that the total volume was adjusted using a solvent other than acetonitrile (stock solutions of each components were prepared in acetonitrile). Determining the stoichiometry of sulfoxide with respect to The stoichiometry of sulfoxide with respect to was determined by measuring consumption using a volumetric method and sulfoxide formation by GC. Evaluation of the reaction kinetics. The reaction kinetics were evaluated and curves fit using the Solver subprogram in Microsoft Excel. Rate laws were determined by varying one component of the system while keeping all others constant. In order to determine the effect on reaction rate of the product sulfoxide, DMSO was used as the model for CEESO. Determining the source of oxygen in product sulfoxide. The source of oxygen of the product sulfoxide was determined by using and monitoring the mass abundances by GC-MS. In a separate experime,t concentrated was added after the addition of thioether. Substitution of and with other anions. In separate experiments, was replaced with equimolar amounts of TMAOH. TBA and TMA are abbreviations for tetra-n-butylammonium and tetramethylammonium cations, respectively. In a separate experiment was replaced with an equimolar amount of Measuring product selectivity using DMSO. Using similar reaction conditions as above, DMSO was added after the addition of CEES and the quantity of DMSO and were monitored by GC. Determining the effect of DMSO (a product sulfoxide model) on the rate of catalysis. Varying amounts of DMSO were added to the normal reaction components to assess the effect on induction period, rate of catalysis, and self-inhibition. Assessment of Cu(II) and Fe(III) on rate of CEES oxidation by Using similar reaction conditions as above in two separate experiments, two equivalents of and were added before the addition of CEES.

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CEES oxidation in PFPE media. Using as the Au source, as the source, and transition metal sulfates as the transition metal source, reactivity towards CEES oxidation was assessed in two types of PFPE media: Galden D-02®, a perfluorinated polyether oil, and Fomblin MF-300®, a perfluourinated polyether surfactant. was synthesized using a literature procedure.36 TEA is an abbreviation for tetran-ethylammonium cation. Effect of amino acids on the rate of CEES oxidation. Using the same procedure as in the previous section, the inhibitory effect of various amino acids were evaluated in Fomblin MF-300®, a PFPE surfactant. Competitive oxidation between CEES and THT using Using similar reaction conditions as above, two different thioethers, GEES and THT, were added simultaneously in equimolar amounts and the reactivity was compared to that of CEES alone.

20. CONCLUSIONS Diversity-based methods combined with mechanistic information on currently successful catalytic systems for oxidations have led to the discovery of catalysts that addresses two of the holy grails in catalysis and oxidation: complexes that catalyze selective (non-radical-chain) oxidation by without the requirement of a sacrificial reductant, and complexes that catalyze rapid reactions with the ambient environment (room temperature and 1.0 atmosphere of air). The reaction studied here is thioether (organic sulfide) sulfoxidation via the “dioxygenase stoichiometry”: (sulfoxide), and the principal catalyst in homogeneous acetonitrile is (1). The selective sulfoxidation of thioethers is of interest in decontamination (mustard destruction) and organic synthesis. Extensive kinetics, product, spectroscopic studies have established all the elementary processes in the mechanism and the detailed aspects of the rate-limiting step which involves reaction of 1 with a molecule of the thioether substrate. The limitations exhibited by this initial system in acetonitrile solution, namely the presence of a significant induction period and inhibition by the sulfoxide product, are eliminated by the use of other solvents or developmentally attractive and nontoxic perfluoropolyether (PFPE) media. These aerobic selective catalytic sulfoxidation reactions are also co-catalyzed by some d-block ions. All the mechanistic and energetic information established in this initial research reviewed here augurs well for further logical development of this new environmentally attractive (green) catalytic chemistry.

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References 1. Groves, J. T.; Quinn, R. J. Am. Chem. Soc. 1985,107, 5790-5792. 2. Hill, C. L.; Khenkin, A. M; Weeks, M. S. ACS Symp. Ser. 1993,523, 67-80. 3. Neumann, R.; Dahan, M. Nature 1997,388, 353-355. 4. Hill, C. L.; Weinstock, I. A. Nature (London) 1997,388, 332-333. 5. Neumann, R.; Dahan, M. J. Am. Chem. Soc. 1998,120, 11969-11976. 6. Hill, C. L. Nature 1999,401, 436-437. 7. Brink, G.-J. ten; Arends, I. W. C. E.; Sheldon, R. A. Science 2000,287, 1636-1639. 8. Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis. The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes; 2nd ed.; Wiley-Interscience: New York, 1992. 9. Tolman, C. A.; Druliner, J. D.; Nappa, M. J.; Herron, N. Alkane Oxidation Studies in Du Pont's Central research Department; Hill, C. L., Ed.; Wiley: New York, 1989, pp Chapter 10. 10. Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. 11. Xu, L.; Boring, E.; Hill, C. J. Catal. 2000,195, 394-405. 12. Fukumoto, K.; Onoda, S.; Sugiura, M.; Horii, M.; Hayashi, H. Material for Removing Offensive Odor; Kabushiki Kaisha Toyota Chuo,Kenkyusho, Aichi-ken, Japan: United States, 1997. 13. Dimotakis, E. D.; Cal, M. P.; Economy, J. Environ. Sci. Technol. 1995,29, 1876-1880. 14. Menger, F. M.; Elrington, A. R. J. Am. Chem. Soc. 1990,112, 8201-8203. 15. Yang, Y.; Szafraniec, L. L.; Beaudry, W. T.; Davis, F. A. J. Org. Chem. 1990,55, 36643666. 16. Gall, R. D.; Faraj, M.; Hill, C. L. Inorg. Chem. 1994,33, 5015-21. 17. Hill, C. L.; Gall, R. D. J. Mol. Catal. A: Chem. 1996,114, 103-111. 18. Johnson, R. P.; Hill, C. L. J. Appl. Toxicol. 1999,19, S71-S75. 19. Koper, O.; Lucas, E.; Klabunde, K. J. J. Appl. Toxicol. 1999,19, S59-S70. 20. Reviews of decontamination: a) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729-1743, b) Organic Indoor Air Pollutants: Occurrence - Measurement Evaluation. Tunga Salthammer, (Ed.), Wiley-VCH: Weinheim, 1999; c) Cheremisinoff, P. N.; Abraham, J. Encyclopedia of Environmental Control Technology: Work Area Hazards; Gulf Publishing Co.: Houston, 1995; Vol. 8, p.51, d) Harrison, R. M. In Air Pollution and Health, R. E. Hester and R. M. Harrison, (Eds.), Royal Society of Chemistry: Cambridge, 1998; Vol 10, pp. 101-126. 21. Ashley, D. L.; Bonin, M. A.; Cardinali, F. L.; McCraw, J. M.; Wooten, J. V. Environ. Health Perspectives 1996,104, 871-877. 22. Boring, E. A.; Geletii, Yu. V.; Hill, C. L. J. Am. Chem. Soc. 2001,123, 1625-1635. 23. Boring, E. A.; Gueletii, Yu. V.; Hill, C. L. J. Mol. Catal. A, 2001,176, 49-63. 24. Riley, D. P. Inorg. Chem. 1983,22, 1965-1967. 25. Riley, D. P.; Smith, M. R.; Correa, P. E. J. Am. Chem. Soc. 1988,110, 177-180. 26. Ericson, A.; Elding, L. I.; Elmroth, S. K. C. J. Chem. Soc., Dalton Trans. 1997,7, 11591164. 27. Annibale, G.; Canovese, L.; Cattalini, L.; Natile, G. J. Chem. Soc., Dalton Trans. 1980, 1017-1021. 28. Gasparrini, F.; Giovannoli, M.; Misiti, D.; Natile, G.; Palmieri, G. Tetrahedron 1983,39, 3181-3184. 29. De Filippo, D.; Devillinova, F.; Preti, C. Inorg. Chim. Acta 1971,5, 103-108. 30. Natile, G.; Bordignon, E.; Cattalini, L. Inorg. Chem. 1976,15, 246-248.

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31. Schmid, G. M.; Curley-Fiorino, M. E. Encylopedia of Electrochemistry of the Elements; M. Dekker: New York, 1975; Vol. 4. 32. Skibsted, L. H.; Bjerrum, J. Acta Chem. Scand. A 1977,31, 155-156. 33. Goolsby, D.; Sawyer, D. T. Anal. Chem. 1968,40, 1978. 34. Fenske, G. P.; Mason, W. R. Inorg. Chem. 1974,13, 1783-1786. 35. Kissner, R.; Welti, G.; Geier, G. J. Chem. Soc., Dalton Trans. 1997,10, 1773-1777. 36. Mason, W. R.; Cray, H. B. Inorg. Chem. 1968,7, 55-58. 37. Chambers, R. C.; Hill, C. L. J. Am. Chem. Soc. 1990,112, 8427-33. 38. Mata, E. G. Phosphorus, Sulfur Silicon Relat. Elem. 1996,117, 231-286. 39. Mashkina, A. V. Catal. Rev. 1990,32, 105-161. 40. Capozzi, G.; Drabowicz, J.; Kielbasinski, P.; Menichetti, S.; Mikolajczyk, M.; Nativi, C.; Schank, K.; Schott, N.; Zoller, U. The Syntheses of Sulphones, Sulphoxides and Cyclic Sulphides: Updates from the Chemistry of Functional Groups; John Wiley & Sons: Chichester, 1994. 41. Marrs, T. C.; Maynard, R. L.; Sidell, F. R. Chemical Warfare Agents: Toxicology and Treatment; Wiley & Sons: Chichester, New York, 1996, pp 141. 42. Roulet, R.; Lan, N. O.; Mason, W. R.; Fenske, J., G. P. Helv. Chim. Acta 1973,56, 24052418. 43. Puddephatt, R. J. Comprehensive Coordination Chemistry: The Synthesis, Reactions, Properties, and Applications of Coordination Compounds; 1 ed.; Wilkinson, G., Gillard, R. D. and McCleverty, J. A., Ed.; Pergamon Press: Oxford, 1987; Vol. 5. 44. Marangoni, G.; Pitteri, B.; Bertolasi, Y.; Gilli, G.; Ferretti, V. J. Chem. Soc., Dalton Trans. 1986, 1941. 45. Elding, L. I.; Skibsted, L. H. Inorg. Chem. 1986,25, 4084-4087. 46. Berglund, J.; Elding, L. I. Inorg. Chem. 1995,34, 513-519. 47. Elmroth, S.; Elding, L. I. Inorg. Chem. 1996,35, 2337-2342. 48. Elding, L. I.; Olsson, L. F. Inorg. Chem. 1982,21, 779-784. 49. Kochi, J. K. Oxidation-Reduction Reactions of Free Radicals and Metal Complexes; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 1, pp 591-685. 50. Smith, S. G.; Winstein, S. Tetrahedron 1958,3, 317. 51. Drabowicz, J.; Kielbasinski, P.; Mikolajczyk, M. Synthesis of sulphoxides; Patai, S. and Rappoport, Z., Ed.; John Wiley & Sons: Chichester, 1994, pp 529-648. 52. Braunstein, P.; Clark, R. J. H. J. Chem. Soc., Dalton Trans. 1973, 1845. 53. Puddephatt, R. J. The Chemistry of Gold; Elsevier Scientific: New York, 1978. 54. Uson, R.; Laguna, A.; Vicente, J. J. Organomet. Chem. 1977,131, 471-475. 55. Dash, K. C.; Schmidbaur, H. Chem. Ber. 1973,106, 1221-1225. 56. Allen, E. A.; Wilkinson, W. Spectrochim. Acta 1972,28A, 2257-2262. 57. Schoenfelner, B. A.; Potts, R. A. J. Inorg. Nucl. Chem. 1981,43, 1051-1053. 58. Elding, L. I.; Groening, A.-B. Acta Chem. Scand. 1978,32, 867-877. 59. Douglas, B. E. Concepts and Models of Inorganic Chemistry; 3rd ed. ed.; Wiley: New York, 1994. 60. Cattalini, L.; Tobe, M. L. Inorg. Chem. 1966,5, 1145-1150. 61. Cattalini, L.; Orio, A.; Tobe, M. L. J. Am. Chem. Soc. 1967,89, 3130-3134. 62. Cattalini, L; Orio, A.; Tobe, M. L. Inorg. Chem. 1967,6, 75-78. 63. Davies, J. A.; Hasselkus, C. S.; Scimar, C. N.; Sood, A.; Uma, V. J. Chem. Soc., Dalton Trans. 1985,1985, 209-211. 64. Kapoor, P.; Lövqvist, K.; Oskarsson, Å. J. Mol. Struct. 1998,470, 39-47. 65. Horn, G. W.; Kumar, R.; Maverick, A. W.; Fronczek, F. R.; Watkins, S. F. Acta Cryst. C 1990,C46, 135-136. 66. Melanson, R.; Rochon, F. D. Can. J. Chem. 1975,53, 2371-2374.

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67. For data in water, see Atkins, P. W. Physical Chemistry; W. H. Freeman: New York, 1990. 68. Mylius, F. Z Anorg. Chem. 1911,70, 203-231. 69. Bravo, O.; Iwamoto, R. T. Inorg. Chim. Acta 1969,3, 663-666. 70. Tsvelodub, L. D.; Malkova, V. I. Sib. Khim. Zh. 1991,3, 72-77. 71. Canovese, L.; Cattalini, L.; Tomaselli, M.; Tobe, M. J. Chem, Soc., Dalton Trans. 1991,1991,307-314. 72. Cattalini, L.; Ricevuto, V.; Orio, A.; Tobe, M. L. Inorg. Chem. 1968,7, 51-55.

Chapter 6 Catalytic oxidations using cobalt(II) complexes

László I. Simándi Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary

Abstract: The reactivity of cobalt(II) complexes toward dioxygen has long been recognized. Synthetic oxygen carriers reversibly form mononuclear superoxo and dinuclear complexes. The bound can be removed by pumping or flushing with an inert gas and this cycle can be repeated many times over. However, there is always some loss of reversibility, leading to cobalt- or ligand-centered oxidation. Dioxygen complexes are generally regarded as the source of activity in cobalt-catalyzed oxidations. The term dioxygen activation is used to describe the oxidation of added oxidizable substances via interaction with intermediate dioxygen complexes or their conversion products. Observations of this behavior have prompted extensive research into the vast area of homogeneous catalytic oxidation using cobalt complexes. In this review the progress made in the study of various cobalt-based catalyst systems in the last decade is surveyed. Catalytic oxidations by different classes of cobalt(II) complexes with salen, porphyrin, phthalocyanin, dioxime, amine, pyridine, cyclidene, peptide and carboxylato ligands are discussed with specific reference to the products formed and the underlying reaction mechanisms. For each catalyst type the oxidation of various substrates is reviewed, including substituted phenols, lignin phenolics, catechols, anilines, thiols, alcohols, diols, and alkenes. Oxygen insertions, NO oxidation and oxidations via alkylperoxo complexes are treated. Obviously due to the paramagnetic nature of cobalt(II) complexes, free-radical mechanisms are predominant in cobalt-catalyzed oxidations, permitting insight into the nature of reaction intermediates by the ESR technique. Key words: Catalytic oxidation, homogeneous catalysis, dioxygen activation, dioxygen complexes, biomimetic oxidation, functional metalloenzyme models, oxidation mechanisms, oxidative dehydrogenation, oxygen insertion, alkene epoxidation, catecholase reaction

265 L.I. Simándi (ed.), Advances in Catalytic Activation of Dioxygen by Metal Complexes, 265-328. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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1. INTRODUCTION Various cobalt (II) complexes, usually referred to as synthetic dioxygen carriers, are known to interact reversibly with dioxygen under ambient conditions, affording predominantly dioxygen complexes of different types, which have been extensively reviewed 1-6. The ranges of O-O bond lengths in the most common structural types are shown in Table 16.

In its triplet ground state the reactivity of dioxygen, having two unpaired electrons in its degenerate antibonding orbitals, is controlled by the rules of spin conservation. The kinetic barrier imposed by these conditions is sufficiently large to ensure survival of all living organisms despite their thermodynamic instability toward oxidation in an atmosphere containing dioxygen. The kinetic inertness of dioxygen can be overcome by coordination to a metal center, which eliminates the restrictions of spin conservation. The interaction of with a transition metal ion surrounded by suitable ligand(s), i.e. dioxygen complex formation, can be broadly regarded as of two major types, viz., (i) reversible binding (oxygenation), and (ii) activation of dioxygen. Reversibly bound can be removed from the complex by pumping or flushing with an inert gas ( Ar), and this cycle can be repeated many times over. However, there is always a gradual loss of reversibility, as demonstrated for cobalt(II)-based synthetic dioxygen carriers. This is due to irreversible oxidation of the central metal ion and/or the ligand(s) surrounding the metal. In the presence of a suitable external substrate, its catalytic oxidation may take place, leading to a catalytic cycle, in which the activated dioxygen species is the key intermediate. Substrate oxidation and catalyst regeneration should occur in the successive cycles. The catalytic activity usually decreases with time and is ultimately lost due to metal centered and/or ligand based irreversible oxidations.

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2. COBALT DIOXYGEN COMPLEXES The majority of cobalt dioxygen complexes can be classified as of the mononuclear superoxo or dinuclear type7. 1-7 For other types the reader is referred to the review literature . Dioxygen complexes are often formed reversibly and can be characterized by equilibrium constants. In catalytic systems usually enters the catalytic cycle via equilibria of the types (1) and (2):

These reactions can be regarded as autoxidation of although both steps (1) and (2) are substitution (complex formation) reactions. True autoxidation with loss of an electron by occurs on protonation of the dimer, leading to the formation of as in the case of

The cobalt-bound (activated) dioxygen exhibits higher reactivity toward certain substrates than does free In kinetic studies the above equilibria may be treated as rapid pre-equilibria, which maintain a near constant concentration of the active intermediate(s). The rate constants for binding are usually very large and kinetic studies require the stopped-flow technique, in which a solution of the cobalt complex prepared under an inert gas is rapidly mixed with an solution. The reaction is then monitored spectrophotometrically. The problem of inert storage was eliminated in the case of cobaloxime(II), by preparing the cobalt complex in the mixing chamber from and cobalt perchlorate9. This was made possible by the fact that the formation of is much faster than its subsequent reaction with Recently, a new method has been reported for studying rapid biological reactions involving dioxygen10. It was used to investigate the reduction of dioxygen to water by cytochrome c oxidase. Photolysis of a synthetic caged dioxygen carrier, produces dioxygen in situ on a nanosecond or faster time scale. This avoids complications due to the fate of photodissociated CO in a conventional CO flow-flash experiment. The kinetics of dioxygen binding to the cobalt(II) complexes of

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1,4,7,10-tetraazacyclododecane and 1,4,8,11-tetraazacyclotridecane has recently been studied11. The general features of oxygenation equilibria and autoxidation involving cobalt-based lacunar cyclidene dioxygen carriers have been extensively studied by Busch and coworkers with special reference to dioxygen affinities and their correlation with structural features 3,4,12,13. Lacunar systems have considerable activities for reversible dioxygen binding in their cavities under ambient conditions.

The autoxidation of dioxygen carriers may involve both metal-centered oxidation to and ligand oxidation, leading to ligand destruction. Typical ligand oxidations involve oxidative dehydrogenation of to imine bonds as in the case of cobalt (II) complexes of Pydien14, and to carbonyl group oxidations in macrocyclic chelates15,16:

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Recent progress in bimetallic dioxygen complexes containing compartmental ligands has been reviewed.17

3. OXIDATIONS CATALYZED BY Co(salen) COMPLEXES The cobalt(II) complexes of salen type ligands have long been used as catalysts for the oxidation of substituted phenols. The subject has been extensively reviewed22,23. In this chapter emphasis is placed on recent advances.

3.1 Oxidation of substituted phenols 3.1.1 2,6-di-tert-butylphenol

The properties of oxygen adducts of complexes (Figure 3) have been studied by cyclic voltammetry, IR, electronic and ESR spectroscopy. The complexes exhibit oxygen-binding ability in the presence of an axial ligand (py), and end-on superoxocobalt type dioxygen adducts are formed18. Electron-donating substituents gave higher concentrations of the superoxo complex. The Meand derivatives give superior catalytic activity in the oxygenation of 2,6-di-tert-butylphenol to the corresponding quinone.

The oxidation of 2,3,6-tri-tert-butylphenol and 2,4,6-tri-tert-butylaniline

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with molecular oxygen and tert-butylhydroperoxide was investigated using biomimetic Mn-, Fe- and Co-complexes as catalysts19. The catalytic activity and product distribution were determined and compared with those observed in the reactions of the well-known Co(salen) complex. Supercritical is gaining importance as a reaction medium. The oxidation of substituted phenols by O2 has been studied in supercritical using [{N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato(2- )}cobalt(II)] as catalyst20 Both oxidase and oxygenase activities are observed with 2,6-di-tert-butylphenol as substrate at 70°C and 207 bar total pressure, at an O2:phenol:catalyst ratio of 1500:20:1, and a methylimidazole to catalyst ratio of 1.28. Total conversion to 2,6-di-tert-butyl-1,4-benzoquinone and 3,5,3’,5’-tetra-tert-butyl-4,4'-diphenoquinone took place in 21 h.

3.2 Oxidation of 3,5-di-tert-butylcatechol Binuclear Co(II) complexes derived from 2,6-diformyl-4-methylphenol and various aromatic monoamines have been prepared and investigated.21 The Co(II) complexes have the composition where L represents the organic ligand. The complexes are active catalysts in the oxidation of 3,5-di-tert-butylcatechol.

3.3 Oxidation of lignin phenolics Earlier work on the Co(salen)-catalyzed of p-substituted phenols to p-benzoquinones22,23 has been extended to include substrates that serve as models for lignin phenolic subunits. Lignin is a renewable source of carbon and its oxidation to p-benzoquinone derivatives would allow conversion to useful intermediates. Certain p-substituted phenolics can be oxidized to p-benzoquinones with dioxygen using the Co(salen) complexes A and B in Figure 4 as catalysts24.

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These 5-coordinate cobalt complexes form mononuclear superoxo and bridged dinuclear complexes, which are reactive toward phenolic substrates, thereby initiating catalytic oxidation cycles23:

Typical reaction conditions are shown in Figure 5 for syringyl alcohol as substrate. In the presence of the catalysts in Figure 4, 2,6-dimethoxybenzoquinone is formed in 71 and 88% yield, respectively.

The p-substituted phenolic substrates and the yields of their oxidation products are listed in Figure 6. The proposed mechanism of oxidation is shown in Figure 7 25,26. The superoxo complex abstracts the phenolic hydrogen, affording a phenoxy radical, which is trapped by a second complex or dioxygen, giving intermediate (I) . When phenol oxidation is carried out stoichiometrically with type systems, structural analogs of I have been isolated and characterized27. Elimination of formaldehyde from I produces quinone and, when a catalytically active Co-hydroxy 28 species . The complex formed in the H-abstraction step breaks down to regenerate the starting catalyst26. The catalytic effect depend on the ease of removal of the phenolic hydrogen, the rate decreasing in the following order of p-substituents: MeO (123), Me (28) CN (1). The propenoidic phenols, E-methyl ferulate (1), E-4-hydroxycinnamic acid methyl ester (4) and E-3-chloro-4-hydroxycinnamic acid methyl ester (7), can be catalytically oxidized with dioxygen in the presence of [Co(salen)] (Figure 8)29. The yields depend on the solvent and the phenyl substituents. EPR and electronic spectra suggest the involvement of a coordinated o-benzosemiquinone type radical as the active intermediate. Oxidative processes are involved in both the polymerization of phenylpropendioic phenols to lignin and the degradation of lignin in the environment30,31. Model systems for the oxidative degradation of phenolic phenylpropenoids provide mechanistic information about the biological cycle of lignin32. Co(salen) was found to catalyze the oxidation of the lignin

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model E-ferulic acid to vanillic aldehyde, vanillic acid methyl ester and homovanillic aldehyde (Figure 9) at 1 bar of and

These transformations correspond to double bond cleavage with O-atom insertion and aromatic hydroxylation. ESR studies suggest an organometallic radical intermediate in chloroform and methanol and a superoxocobalt(III) species in pyridine.

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3.4 Nitrogen monoxide reacts with nitrogen monoxide to give the nitrosyl complex where is a dinucleating macrocycle, having a salen- and a saldien-like metal binding site (Figure 10). The Co occupies the salen and the Pb the saldien site (Figure 11)34.

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The NO coordinates to the Co at the axial site trans to the bridging dmf oxygen, providing a six-coordinate geometry at the metal.

The nitrosyl complex is oxidized with molecular oxygen to the nitro complex

3.5 Oxidative dehydrogenation In -saturated dmf solution, the Co(II) complex of the tetrahydrosalen ligand undergoes slow oxidative dehydrogenation at both C-N bonds and the cobalt(II) salen complexes (CoL) are formed35 (Figure 12). The overall process is autocatalytic and involves reduction of to water, but is probably an intermediate.

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3.6 Oxidation of quercetin Quercetinase is a dioxygenase, catalyzing the insertion of into quercetin and 3-hydroxyflavones, leading to oxidative cleavage of the heterocyclic ring, affording the corresponding depsides and carbon monoxide (Figure 13)36. Model studies of oxygenations have been reported using copper(II) complexes37. Co(salen) exhibits a remarkable activity in the cleavage of model substrate (Figure 14)38. Cyclic voltammetry of the substrate anion-catalyst binary intermediate complex indicates that it is the substrate anion involved in an ion-pair complex with the cationic that inserts dioxygen, yielding the product depside39 HL2.

3.7 Mercaptoethanol Co(salen) immobilized on silica, layered double hydroxides and NaX zeolite act as photocatalysts for the oxidation of 2-mercaptoethanol and sodium thiosulfate40.

3.8 Alkenes and alcohols In the presence of cobalt(II) Schiff-base complexes some ketoesters and

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aldehydes promote the oxidation of alkenes and alcohols with The added carbonyl compounds assist in the formation of dioxygen complexes and act as reducing agents during oxygen atom transfer to the organic substrates. The cobalt(II) Schiff-base complexes shown in Figure 15 act as catalysts for the oxidation of a wide range of organic substrates (e.g. alkenes, alcohols, benzylic compounds and aliphatic hydrocarbons) with dioxygen in the presence of aliphatic aldehydes, ketones or ketoesters.

EPR studies in acetonitrile at room temperature have shown that aliphatic carbonyl compounds promote the formation of superoxocobalt(III) 53 complexes, which are responsible for the catalytic effect . According to the general procedure for catalytic oxidation of alkenes, benzylic compounds and alcohols, the mixture of 5 mmol substrate, 5 mol% of cobalt(II) complex, 10 mmol were stirred under 1 atm for 20-35 hours at room temperature or 50-60°C. The observed types of reactions include alkene epoxidation, allylic and benzylic oxidation, and alkane hydroxylation. The ligands on the cobalt atom control the chemoselectivity of oxidation. The proposed reaction mechanism is shown in Figure 16. The catalytic activity is due to the superoxocobalt(III) complex which binds a ester or aldehyde (left and right end of the horizontal row, respectively). Intramolecular H-atom transfer affords an enolato-cobalt complex (left) or a coordinated acyl radical (right). Both of these release an

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oxocobalt(IV) species, which is regarded as the active intermediate responsible for the oxidation products formed via O-atom transfer. These are shown above and under the horizontal row. Oxometal species have been variously invoked as intermediates in Otransfer reactions, especially in connection with single oxygen atom donors, such as iodosylbenzene, persulfate, N-oxides and peroxy compounds54. Oxocobalt(IV) was proposed as the active intermediate in the cobaloxime(II)-catalyzed oxidation of hydrazobenzene with

3.9 Alkene epoxidation In the presence of methyl, tert-butyl, or (-)-menthyl esters of 2oxocyclopentanecarboxylic acids, the cobalt(II)-salen type complexes and Jacobsen-type manganese(III) complexes shown in Figure 17 are active catalysts for alkene epoxidation with dioxygen56. In these epoxidations (Figure 18), alkyl-1-hydroxy-2-oxocyclopentanecarboxylates and 1-alkyl-2-oxo-hexanedicarboxylic acids are formed as cooxidation products.

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The (-)-menthyl/cobalt system is selective for epoxide formation but the products are racemic in line with radical epoxidation in solution rather than

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at the cobalt complex. The experimental results are consistent with chain reactions involving the free radicals shown in Figure 19. The Jacobsen-type manganese complex gives lower yields of epoxides (40-60%), but for 2,2-dimethylchromene and styrene the epoxides are optically active (12-60% ee).

3.10 Primary amines Bis[3 -(salicylideneimino)propyl] methylaminecobalt(II) (CoSMDPT) catalyzes the oxidation of primary amines with dioxygen57. Benzylamine, 4methyl-benzylamine and 4-methoxybenzylamine gave aldehyde intermediates, which were converted to Schiff bases with the starting amines. The oxidation of 1-phenyl-ethylamine gave 2,4,6-triphenyl-3-aza-3,5-heptadiene via disproportionation of the intermediate Schiff base.

4. OXIDATIONS CATALYZED BY COBALOXIMES The cobaloxime(II) derivatives where H2dmg is dimethylglyoxime, and L is py and catalyze the oxidative dehydrogenations (Figure 20) and oxygen insertions of some organic substrates at room temperature and atmospheric dioxygen pressure58-61.

where Q – p-benzoquinone, AB – azobenzene. In the following discussion and stand for cobaloxime(II) and cobaloxime(III), i.e. and respectively, and L is py, or In the reaction mechanisms proposed for cobaloxime-catalyzed oxidations, the superoxocobaloxime(III), formed directly from cobaloxime(II) and is the key intermediate. It is also the source of several

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other active intermediates reactive toward different substrates. Thus, formed from is the source o f hydroxocobaloxime(III), and of oxocobaloxime(IV), The complex is capable of both H-atom and electron abstraction, both possibilities being feasible routes for oxidative dehydrogenations. Oxygen insertion reactions were in some cases interpreted in terms of transient oxocobaloxime(IV). The formation routes and reactions of these active oxygen species are summarized below. Dioxygen complex formation

Oxidative dehydrogenation

where

is

or

and

is a free radical.

Electron and proton transfer

Oxygen insertion

where S is

, RNC, or PhNO.

4.1 Oxidation of o-phenylenediamine The cobaloxime(II) derivatives and where is dimethylglyoxime, are catalysts for the oxidation of ophenylenediamine (OPD) by atmospheric at ambient temperature63. In acetone, methyl ethyl ketone or cyclohexanone as solvents, cyclization to 2,2-disubstituted dihydrobenzimidazoles (DHB) is followed by

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dehydrogenation to 2,2-disubstituted 2H-benzimidazoles (2HB) (Figure 21). Acetaldehyde affords 2-methylbenzimidazole in a similar way62 (Figure 22).

4.2 Oxidation of 2-aminophenol Cobaloxime(II) derivatives where and catalyze the oxidative dehydrogenation of 2-aminophenol (ap) at room temperature and 1 bar in The reaction product is 2-aminophenoxazine-3-one (apx) formed in quantitative yield according to the stoichiometric equation of Figure 23. This system can be regarded as a functional model of Phenoxazinone synthase, which is involved in the biosynthesis of Actinomycin D (AD), a

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naturally occurring antineoplastic agent, from the 2-aminophenol derivative A (Figure 24).

The kinetics of oxidation of 2-aminophenol (ap) in the presence of [Co has been interpreted64 in terms of a mechanism consisting of the steps shown in Figure 25. Upon dissolution of the catalyst precursor in MeOH one of the axial ligands dissociates, generating the active pentacoordinate catalyst, denoted by The key intermediate is the superoxocobaloxime derivative which abstracts an H-atom from the ap substrate via an H-bonded species X. The aminophenoxyl radical produced is further oxidized to -benzoquinone monoimine (bqmi), which is an intermediate on the path to apx formation. EPR studies provide evidence for formation of the free radical as intermediate.65

4.3

Oxidation of 3,5-di-tert-butylcatechol

The oxidation of 3,5-di-tert-butylcatechol by to the corresponding 1,2-benzoquinone (DTBQ) is catalyzed by the cobaloxime

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in methanol66 and benzene.67 The overall stoichiometry is given by the equation:

An intermediate catecholatocobaloxime (III) has been isolated from the reacting mixture. Its molecular structure determined by x-ray diffraction66 revealed an axially bonded unidentate catecholato ligand, which is formed via free-radical coupling between the cobaloxime(II) and the semiquinone anion radical The latter has been detected by EPR spectroscopy during the reaction together with its cobaloxime(III)-bonded derivative which indicates a free-radical mechanism for catechol oxidation. The mechanism proposed for the catalytic oxidation on the basis of the detectable intermediates is shown in Figure 26. In this scheme stands for also referred to as cobaloxime(II). The double catalytic cycle is joined together by and -activation takes place via formation of superoxocobaloxime(III), followed by Hatom abstraction from affording During the catalytic reaction, and catechol oxidation are taking place according to the lower cycle, while the cobaloxime species in the upper cycle are present at steady state or equilibrium concentrations.

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285

A detailed kinetic study68 of the cobaloxime(II) catalyzed oxidation of DBCatH2 has led to the kinetic equation (4).

which is consistent with the reaction mechanism:

The kinetic behavior requires the formation of intermediate X (Figure 27), which decomposes in the rate-determining H-atom abstraction step. In the steady state following the fast initial phase the overall stoichiometry requires that be reduced to rather than therefore, the hydroperoxocobaloxime formed in step (7) should undergo disproportionation, regenerating one half of the absorbed and producing the hydroxocobaloxime (step 10). Also, for a sustained catalytic cycle to occur in the steady state, a route to the product DTBQ and a path regenerating the catalyst are necessary. These requirements of a catalytic cycle are fulfilled by addition of electron transfer steps (11) and (12) to the mechanism. Formation of the cobaloxime derivative exhibiting the 8-line ESR signal observed during the catalytic oxidation can be explained by reaction (12). It is also involved in equilibrium (13), which has been demonstrated by reacting cobaloxime(II) with DTBQ under when the same ESR spectrum was obtained. A remarkable feature of mechanism (5) - (13) is that the dioxygen activation steps (5) and (6) are involved in both the rapid initial phase and the steady state. They control the rate of dioxygen uptake by the reacting solution, which, however, differs very strongly in these two phases. This

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implies that as the system reaches the steady state very shortly after mixing the reactants, the concentration of drops to a fraction of the starting value, so that the consumption rate falls from the stopped-flow level9 to values susceptible to monitoring by gas volumetry.

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4.4 Oxidative cleavage of a stilbene derivative In the presence of the cobaloxime(II) catalyst at room temperature and 1 atm or air, undergoes oxidative dehydrogenation to the corresponding stilbenequinone (StQ), and parallel oxidative cleavage at the C=C double bond to afford 2,6-di-tert-butyl-4-hydroxybenzaldehyde (Ald, Figure 28)69. The relative contribution of these two stoichiometries depends on the degree of conversion.

In the steady state the relative rate of formation of StQ and Ald is approximately constant throughout a given run, indicating that they are formed in competitive reactions, via a common free-radical intermediate (Figure 29) detected by EPR spectroscopy during the reaction.

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The kinetics of the catalytic oxidation has been studied by a volumetric technique at constant pressure. The observed kinetic behavior was consistent with the reaction mechanism (14) - (20), where represent the moiety, respectively.

The rate law corresponding to the proposed mechanism can be derived by assuming that steps (14) and (15) are reversible and step (16) is ratedetermining. Disproportionation (17) can be regarded as fast9, consequently, the concentration of is negligible. As the kinetics were determined from initial rates and the formation of Ald is delayed relative to that of StQ, steps (19) and (20) can be disregarded and the concentration of CoOOR- is also negligible. These considerations lead to rate equation (21), which is consistent with the kinetic behavior.

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Formation of the aldehyde product requires a reaction between and in which the latter is attacked in its mesomeric form with the unpaired electron at one of the olefinic carbon atoms. The alkylperoxocobaloxime(III) formed will decompose to the aldehyde probably via a dioxetane intermediate. The required protons are derived from the water present in ordinary benzene. The proposed reaction mechanism is depicted in Figure 30

4.5 Oxygen insertions 4.5.1 Oxygen insertion to terminal P, C and N atoms

Cobaloxime(II) catalyzes formal O-atom insertion into triphenylphosphine, alkyl isocyanides and nitrosobenzene 58,59:

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Similarly, O-atom insertion was also observed in the case of benzimidazole, which was converted to bis(N-oxide) in the presence of cobaloxime(II) (cf. Figure 21)63. Double oxygen insertion takes place in the oxidative bond cleavage of (cf. Figure 28)69: the olefinic bond is split, affording two substituted benzaldehydes. 4.6.2 Insertion of

into alkylcobaloximes

Upon photoirradiation of alkylcobaloximes under dioxygen, the insertion of into the Co-C bond is observed70,71. The (alkyldioxy)cobaloxime formed is then degraded into a carbonyl compound or an alcohol by reduction or heating (Figure 31)72-75.

Irradiation of alkylcobaloximes 1a-h under oxygen in chloroform afforded a carbonyl compound and an alcohol as main organic products (Figure 32)76. Chlorocobaloxime was isolated after the reaction in all cases. The reaction proceeds through a stable intermediate (alkyldioxy)cobaloxime. The alkyl ligand controls the selectivity to the oxygenated organic products.

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291

The reactions of molecular oxygen with photoproduced cobaloximes(II) from the cobalt(III) complexes and (where is dimethylglyoxime and py is pyridine) have been investigated by flash photolysis methods77. In the first step mononuclear superoxocobalt(III) species are formed, which then react with photoproduced alkylcobaloxime(III) to yield Cobalt(II) complexes of 9,10-phenanthrenedionedioxime and benzoquinonedioxime were prepared and investigated by EPR in the solid state78. Dioxygen adducts are presumably formed as intermediates, affording ultimately the tris-(oximato)cobalt(III) chelates. A detailed kinetic study of dioxygen insertion into 27 organocobaloximes derived from dimethyl-, dicyclohexyl- and diphenylglyoxime has been performed under thermal and photochemical conditions79. The rate of insertion on the nature of the equatorial glyoxime, the axial ligand, the organic group and the solvent.

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5. OXIDATIONS CATALYZED BY COBALT(II) PORPHYRINS Metalloporphyrins are versatile oxidation catalysts. The field is dominated by iron porphyrins, which serve as synthetic models for heme type oxygenases. They have been extensively studied as witnessed by the number of review articles and books on the subject. It is only possible here to cite a few items of the vast selection80-88. Cobalt(II) porphyrins bind dioxygen reversibly and thereby mediate porphyrin degradation89,90, which can be regarded as a catalytic oxidation. In poorly coordinating solvents (dichloromethane and THF) cobalt(II) octaethylporphyrin is converted to cobalt(III) octaethyloxaporphyrin dichloride, a cobalt verdoheme analog, containing an oxidized porphyrin ring. of a type complex are formed91,92 (Figure 33).

In an extensive series of studies Lyons and coworkers have shown that halogenated metalloporphyrins are exceptionally active catalysts for the selective reaction of alkanes with molecular oxygen93-100. The greater the degree of halogenation of the ring, the greater is the catalytic activity of the metal complex. Complexes of iron are generally more active than those of cobalt, manganese, or chromium101. The product profile of isobutane oxidation is characteristic of radical reactions but also sensitive to the nature of the metal center. The selectivity to tert-butanol is about 90% or better. Among cobalt complexes catalyzing the oxidation of alkanes (isobutane or propane), is the most active101, surpassing Co(acac)2 and Co(BPI)(OAc), or (BPI = bispyridyliminoisoindoline)102,103.

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Besides being very active catalysts for alkane oxidation by the iron perhaloporphyrins are also most active in the decomposition of alkyl hydroperoxides. The nature of products formed depends on the structure of the aliphatic substrate and can be rationalized by a catalytic pathway very efficiently generating alkyl and alkoxy radicals at low temperatures. Obviously, similar considerations apply also to the mechanism of oxidation catalysis by (Figure 34). Cobalt(II)-porphyrins a-d (Figure 35) are versatile catalysts promoting the oxidation of the organic substrates listed in (Figure 36) by a combination of molecular oxygen and 2-methylpropanal under ambient conditions104. Typically 10 mmol of hydrocarbon and 20 mmol of the aldehyde are stirred in an autoclave in 15 mL acetonitrile for 12-15 hours under at room temperature. Although not stated explicitly by the authors, acyl free radicals are obviously the key intermediates, converted by to acylperoxy radicals responsible for the oxidation.

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Alkenes, allylic or benzylic substrates and alcohols are also converted to the corresponding oxidized products at ambient conditions (Figure 37)105.

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295

The approach of fluorous biphase systems106 utilizes the low miscibilities of fluorocarbons with most organic solvents, which allow easy separation of a catalyst soluble in the fluorocarbon phase from the product soluble in the organic phase106,107. The fluorocarbon-soluble cobalt complex of the tetraarylporphyrin ligand in Figure 38 catalyzes the epoxidation of alkenes

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under fluorous biphase conditions in the presence of 2-methylpropanal (Table 2). Advantages of this approach include higher substrate to catalyst ratios (1000:1), facile product separation and recycling of the expensive fluorous solvent.

and its fluorinated analog catalyze the oxidation of ethane, propane and cyclohexane by (100 psi, 85°C, 20-24 hours) to the corresponding alcohols in the presence of CO, in a mixture of trifluoroacetic acid and water. Further oxidation to aldehydes and acids also takes place108. It is proposed that CO converts the known superoxo species to an oxocobalt(IV) complex, which is capable of abstracting an H-atom from the alkanes (eq. 23). This possibility has been suggested earlier for cobaloxime(II)-catalyzed oxidations60,61.

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Remarkably, primary C-H bonds are more reactive than the weaker secondary C-H bonds or C-H bonds in to an alcohol functionality. Under illumination by a 450 W high pressure mercury lamp at 30°C and 1 atm benzaldehyde and p-chlorobenzaldehyde undergo oxidation by molecular oxygen in the presence of Co(II) tetraphenylporphyrin, Co(II)[meso-tetra (benzoyloxy-phenyl)porphyrin] (CoTBCOPP) and Co(II)[meso-tetra(benzenesulfonyloxy-phenyl)]porphyrin (CoTBSOPP)109. The activity of the photocatalyst decreases in that order. An induction period in uptake was observed and accelerates the oxidation. The proposed reaction mechanism involves a free-radical chain process intiated by a superoxocobalt(III) porphyrin species, which abstracts an H-atom from the aldehyde, generating an radical. The latter participates in the following propagation and termination steps: Initiation

Propagation

Termination

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Step (26) differs from what the authors have proposed. The effect of light is assigned to generation of free radicals, which act as initiators and shorten the induction period. The oxidation of alkylaryl sulfides with dioxygen/2,2-dimethypropanal is catalyzed by Co(II)-tetraarylporphyrin and a perfluoroalkyl-substituted Co(II)-phthalocyanine in a fluorous organic biphasic system110. Sulfoxide was generally obtained as the main product, together with variable quantities of sulfone (0-100%), depending on the nature of the substrate. The reaction probably proceeds through a free-radical oxidative process initiated by the Co(II)-porphyrin. The catalysts used are decomposed under the free-radical reaction conditions.

6. OXIDATION WITH COBALT(II) PHTHALOCYANINES The interaction of cobalt(II) phthalocyanine with ammonia and dioxygen has been studied by ESR spectroscopy. Both 1:1 and 1:2 adducts with are formed. The phthalocyanine ring is oxidized to a cation radical when the binunlear adduct is formed111. Cobalt(II) phthalocyanines are efficient oxidation catalysts112. They have been used for the oxidation of ascorbic acid113, cysteine114, mercaptoethanol115, hydrazine116, hydroxylamine117 and sulfite118. In pyridine solution the nitroso derivative PcCoNO can be converted under an atmosphere (50 atm) to the nitrito derivative which oxidizes triphenylphosphine or 1-octene by O-atom transfer119,120. The tetra-tert-butylphthalocyanine complex of cobalt(II) catalyzes the oxidation of styrene to 1-phenylethanol with at room temperature in the presence of (Figure 39). The analogous reactions with the Mn(III) and Fe(III) complexes are inhibited by the freeradical scavenger TEMPO, indicating a free-radical mechnism. The lack of inhibition of the cobalt(II)-catalyzed oxygenation points to the involvement of a intermediate. The zinc complex is catalytically inactive. In the presence of water-soluble cobalt phthalocyaninetetra(sodium sulfonate) (CoPcTS), 3,4-dimethoxybenzyl alcohol (a lignin model) is catalytically oxidized by dioxygen to 3,4-dimethoxy-benzaldehyde123-127. Typical reaction conditions are 1 atm and 85°C, 12 hours at pH 11, yield 100%.The product yield decreases with decreasing pH; it is 84% at pH 10 and 18% at pH 8. A cationic latex particles had little effect on the yield or rate of oxidations. PcTS complexes of Fe(II), Cu(II) and Ni(II) gave less than 1% yield under comparable conditions.

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The kinetics and mechanism of the oxidation of 2-aminophenol (ap) to 2-aminophenoxazin-3-one (apx) under ambient conditions, catalyzed by the recently synthesized tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)-dodecachlorophthalocyaninatocobalt(II), (Figure 41) have been studied by spectrophotometry128,129. The rate of ap formation is first-order in and obeys Michaelis-Menten type kinetics with respect to [ap]. The suggested mechanism involves rate-determining inner-sphere electron transfer from coordinated ap to coordinated in the superoxo complex. Cobalt(II)-phthalocyanine [Co(II)pc] supported on active carbon exhibits catalytic activity in the oxidation of sulfide ions to elemental sulfur by dioxygen in aqueous solution131. Hydrophobization of the carbon surface facilitates activation of dioxygen by adsorption. Sulfide ion activation occurs mainly on the supported Co(II)Pc. Water soluble cobalt(II) 2,9,16,23-tetrasulfophthalocyanine, zinc(II)2,9,16,23-tetrasulfo-phthalocyanine, zinc(II)tetracarboxyphthalocyanine, and non-metallic sulfophthalocyanine complexes are catalysts and photocatalysts for the oxidation of sulfide, sulfite and thiosulfate ions by dioxygen132. Typical conditions are 293 K, 1 atm and pH 9.24 in aqueous solution.The cobalt phthalocyanine complexes show high catalytic activity only in the

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oxidation of sulfide, but oxidation is incomplete and not enhanced by irradiation with visible light.

Zinc phthalocyanine complexes show good catalytic activity only upon simultaneous irradiation with visible light132. Interaction between dioxygen and the long-lived triplet state of these complexes produces reactive singlet dioxygen, which also interacts with compounds containing sulfur in various oxidation states. Immobilized cobalt(II) and zinc(II)phthalocyanine anchored on silica or intercalated in the galleries and cavities of layered double hydroxides (hydrotalcite) and NaX zeolite catalyze or photocatalyze the oxidation of 2mercaptoethanol and sodium thiosulfate133. The activity of the immobilized catalysts for oxidation and photooxidation is lower than that of the complexes in homogeneous phase. This is due to the hindered diffusion of dioxygen and sulfur-containing compounds to the active catalyst sites. The six-coordinate low-spin cobalt(III) complexes and (pc = phthalocyaninate, py = pyridine, dce = 1,2-dichloroethane and thf = tetrahydrofuran) oxidize terminal olefins to the corresponding methyl ketones134. Dioxygen activation and oxygen-atom transfer reactions are possible in terms of the redox couple135. Particulate and solubilized cobalt(II) phthalocyanines (PCs) exhibit catalytic activity in the oxidative decomposition of erythrosine with hydrogen peroxide136.

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7. OXIDATIONS CATALYZED BY COBALT(II) AMINE COMPLEXES 7.1 Catalytic oxidation of substituted anilines In the presence of cobalt(II) salts, o-phenylenediamine (OPD) undergoes facile and selective catalytic dehydrogenation by dioxygen137,138. The organic products are 2,3-diaminophenazine in methanol, and 2Hbenzimidazoles in acetone and other aldehyde or ketone type solvents (Figure 42).

Kinetic studies have shown that the catalytic activity is due to the species, which is reactive toward dioxygen139. Under oxidative conditions, the square-planar o-benzosemiquinonediimine (s-BQDI) complex and the square-pyramidal are formed; they have been characterized by X-ray diffraction.140 The complexes (X = As, Sb, P) have been isolated from the system and analyzed by X/ray diffraction141. Triphenylphosphine added to the system is oxidized to indicating that there is an active superoxo or intermediate in the 142 reacting mixture . Solutions of 2-aminothiophenol (HAT) in methanol or THF, containing cobalt(II) perchlorate rapidly absorb dioxygen at room temperature and atmospheric pressure143, producing 2,2’-diaminodiphenyl disulfide (DADS) in 90% yield (Figure 43). The rate of the catalytic reaction as a function of the 2-aminothiophenol concentration shows a maximum at a cobalt(II) to HAT ratio of 1:2, indicating that a complex of the composition is the major catalytic species. The proposed reaction mechanism for HAT oxidation is shown in Figure 44.

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Under similar conditions 2-aminophenol (ap) is catalytically converted to the mixture of products shown in Figure 45143,144. The catalytic activity can be ascribed to the complex, which binds dioxygen to form the superoxo species An intramolecular redox reaction then produces the o-benzoquinone monoimine (bqmi) intermediate, which undergoes oxidative dimerization to 2-aminophenoxazine-3-one (apx) and 2,2’-dihydroxyazobenzene (dhab).

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N-benzylidene-2-hydroxyaniline (Bha) and its related derivatives (a-f, Figure 47) can be catalytically oxidized by in the presence of the hydroxo-bridged dicobalt complex (Figure 46) in DMF at 90°C145.

The oxidation products were the corresponding 2-substituted benzoxazoles (Box), formed in yields higher than 90% (Figure 47). Hydrolysis of the starting compound with water produced 2-aminophenol in a side reaction. It was oxidized to 2-aminophenoxazine-3-one. Galvinoxyl radical did not affect the rate of oxidation, therefore no free radical reactions seem to be involved.

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Kinetic studies reveal first order dependence on both the catalyst concentration and dioxygen pressure. The dependence of the initial rate on the substrate concentration shows a saturation type behavior, indicating reversible initial coordination of the substrate to (A), with subsequent coordination of to the catalyst, producing intermediate X, which is oxidized in the subsequent rate-determining step (Figure 48).

The proposed structure of intermediate X is shown in Figure 49.

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In an atmosphere of dioxygen, the catalytic oxidation of 1,2diaminobenzene takes place in the presence of the dinuclear cobalt complex of the 24-member macrocyclic ligand OBISDIEN 146. In water at 25°C the only oxidation product is 2,3-diaminophenazine, which is formed via a complex mechanism involving o-benzoquinonediimine and a complex as intermediates. The mechanism shown in Figure 50 has been proposed to interpret the observed kinetic behavior.

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Oxidation of miscellaneous substrates

The derivatives of the dicobalt(III) OBISDIEN complex, formed under O2 atmosphere may also bind a bridging substrate, such as oxalic147,148 or ketomalonic149 acid and subsequently undergo intramolecular redox reaction with substrate oxidation. Other substrates studied in the OBISDIEN-dicobalt-dioxygen system included phosphonoformic acid, malonic acid, ethylenediamine, glycine, catechols and others150. The size-fit relationship is important for the formation of the dioxygen complex containing the bridging substrate and for the redox reaction to occur. The substrate becomes oxidized only after it forms the reactive intermediate complex.

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Phosphite ion coordinated to dinuclear complex is oxidized to phosphate via apparent O-atom insertion into the P-H bond, which resembles a hydroxylation reaction151. The proposed mechanism (Figure 51) of phosphite to phosphate oxidation within the complex involves the intermediate in Figure 52.

8. OXIDATIONS CATALYZED BY COBALT(II) PYRIDINE COMPLEXES 8.1

epoxidation

In the presence of t-BuOOH and or as catalyst, undergoes oxidation by to pinene oxide (PO), trans-verbenol (Vol) and verbenone (Vone) (Figure 53)152-154. Typically 8-12 mol% t-BuOOH and 0.15 mol% catalyst are used in at 60-100°C. In reaction times of 24 hours, a maximum of 60% verbenone is obtained. Initial Co(II) to Co(III) oxidation of the catalyst by t-BuOOH affords butoxy and butylperoxy radicals. The former abstracts an allylic hydrogen from αpinene, producing an allyl radical, which reacts with yielding transverbenol and verbenone. The butylperoxy radical adds to the double bond of The resulting alkylperoxy radical decomposes to pinene oxide.

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An efficient catalytic system for oxygenation of phenols to p- or oquinones with molecular oxygen was achieved by utilization of a catalyst consisting of and the multidentate N-heterocyclic podand ligand, N,N'-bis-2-(2-pyridyl)ethyl)-2,6-pyridinedicarboxamide, 2-BPEPA (Figure 54)155.

In acetonitrile the bipyridinecobalt(II) complex activates dioxygen via the reversible formation of a complex156, which dehydrogenates (oxidizes) N-methylanilines, benzyl alcohol, and aldehydes (with subsequent autoxidation). In the absence of substrate, the complex reduces dioxygen via residual water in the solvent to generate HOOH.

9. COBALT-FENTON SYSTEMS According to recent reports157, Fenton chemistry involves the formation of hydroperoxide (ROOH) adducts (A) of reduced transition metal ions [iron(II), copper(I), and cobalt(II)] via nucleophilic addition, e.g.

where B = py or These reactive intermediates (A) bind dioxygen to form adducts of the type (B), which react selectively with methylenic carbon centers of hydrocarbons to form ketones:

and with arylolefins to form dioxygenation products:

6. Catalytic Oxidations using Cobalt(II) Complexes

This phenomenon has been termed oxygenated Fenton chemistry. It has also been observed with in 4:1 MeCN/py, which effects ketonization of the methylenic centers of cyclohexane, cyclohexene and ethylbenzene158-160.

10.

CATALYZED OXIDATIONS

10.1 Under Mukaiyama’s conditions161, aromatic (benzoin, 4,4’-dimethylbenzoin and 4,4’-dimethoxybenzoin [anisoin]) are readily oxidized by dioxygen at room temperature in the presence of excess aldehyde or aldoacetal with catalytic amounts of or under homogeneous conditions162. The stoichiometric equation is shown in Figure 55.

Typical reaction conditions are: 1.2 mmol anisoin, 0.020 mmol 4 mL 1,2-dichloroethane, 3.6 mmol iso-butyraldehyde (0.9 mmol added initially, the remainder added dropwise over 1 hour at room temperature). Yield of diketone 94%.

10.2 Substituted phenols The oxidation of substituted phenols has both biological and synthetic relevance163-166. catalyzes the oxidation of substituted phenols Pac by in the presence of 3-methylbutanal at 40°C in 1,2-dichloroethane (Figure 56). No oxidation takes place in the absence of a catalyst. The major oxidation product of 2,6-dimethylphenol is the corresponding diphenoquinone DQa, whereas 2,6-di-t-butylphenol affords comparable amounts of benzoquinone BQb and DQb. and act similarly.

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3-Methylbutanal is the source of the corresponding hydroperoxide, which is the active oxidant in the reaction.

10.3 Pinanediols In the presence of a 3-fold excess of sacrificial 2-methylpropanal, effects the aerobic oxidative cleavage (Figure 57) of (+)- and (-)pinanediols (PDa,b) to the enantiomerically pure (+)- and (-)-cis-pinonic acids (PAa,b)167. In 1,2 dichloroethane at room temperature, 88% yield was observed in a reaction time of 6 hours.

A variety of secondary and benzylic alcohols can be oxidized by in the presence of 2-methylpropanal, a sacrificial aldehyde168,169 both in the presence and absence of a metal catalyst in a homogeneous phase. The results are compared in Figure 58. is the most efficient catalyst as compared with Cu, Ni, Pd, Fe, Mn acac complexes. Depending on the substrate, yield enhancements by a factor of 1.5 - 24 are observed for

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311

identical reaction times.

Similar yields were obtained with a heterogenized cobalt catalyst prepared by copolymerization of [2-(acetoacetoxy)ethyl methacrylate(1-)] with N,N-dimethylacrylamide and N,N’-methylenebis(acrylamide)168.

11. OXIDATIONS VIA ALKYLPEROXOCOBALT COMPLEXES The butylperoxocobalt(III) complex of the pentadentate ligand N,N-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide Figure 59) oxidizes alkanes upon thermal decomposition170. The complex can be prepared by reacting with in dichloromethane.

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When cyclohexane is used as the substrate, cyclohexanol, cyclohexanone and cyclohexyl chloride are the products. In single turnover oxidation of cyclohexane at the optimum temperature of 80°C, a maximum yield of 59% of the oxidized products is obtained. The mechanism of cyclohexane oxidation involves homolytic scission of the 0-0 bond exclusively. The radicals generated abstract an Hatom from cyclohexane to afford cyclohexyl radicals, which in turn react with dioxygen and produce cyclohexanol (CyOH) and cyclohexanone (CyO) presumably via the Russell-type termination reaction171:

The oxidation of cyclohexane can be either stoichiometric or catalytic. In the presence of excess TBHP, higher yields of oxidized products are obtained, indicating multiple turnovers. Twelve additional Co(III)-alkylperoxo complexes have been synthesized, including those of the ligand N,N-bis[2-(1-pyrazolyl)-ethyl]-pyridine2,6-dicarboxamide (Figure 60), with various primary, secondary, and tertiary R groups172. When the various complexes are warmed (60-80°C) in dichloromethane in the presence of cyclohexane(CyH), the formation of cyclohexanol (CyOH ) and cyclohexanone (CyO) is observed in good yields. Homolysis of the O-O bond in the complexes generating radicals is responsible for the alkane oxidation. Since species are converted into complexes at the end of a single turnover in stoichiometric oxidations, catalytic systems can be generated by the addition of excess ROOH to the reaction mixtures. Catalytic oxidations proceed at considerable rates at moderate temperatures

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313

and involve species as a key intermediate. Turnover numbers in excess of 100 and ca. 10% conversion of CyH to CyOH and CyO are achieved in 4 h in most catalytic oxidations.

12. OXIDATIONS WITH Co-CYCLIDENE COMPLEXES Busch and coworkers173-175 have synthesized a series of remarkable vaulted and lacunar cobalt(II) cyclidene complexes, which reversibly bind dioxygen and undergo autoxidation to cobalt(III) species. In some cases they exhibit catalytic properties in the oxidation of phenol derivatives. The totally synthetic superstructured cobalt(II) cyclidene complexes (Figure 61) CoA (vaulted), CoB (unbridged) and CoC (lacunar), function as both oxidase and oxygenase models in oxygenation of substituted phenols176. The vaulted complex CoA catalyzes the oxidation of 2,6-di-tert-butylphenol in acetonitrile solution. Typically, at 25°C and 1 atm a conversion of 37% is obtained in 24 hours, the products being 5% 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and 22% 3,5,3’,5’-tetra-tertbutyl-4,4’-diphenoquinone (TTBDQ ). Irreversible loss of the catalyst occurs due to autoxidation. The product distribution is determined by the competing reactions shown in Figure 62. Previous studies on the catalytic oxidation of phenols led to the conclusion that the phenoxyl free radical required for interpretation of the observed product pattern and kinetic behavior is formed via H-atom abstraction by the omnipresent superoxocobalt species. In this work an alternative mechanism is proposed, involving electron transfer between the same pair of reactants, followed by the loss of a proton, as described in Figure 63. Electron transfer is pictured to occur through the delocalized system of the phenol, which is supported by the results of molecular mechanics studies173.

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The detailed reaction mechanism proposed is shown in Figure 64. The main mechanistic features of the catalytic oxidation/oxygenation of 2,6-di-tert-butylphenol are analogous to those described earlier177. The phenoxy radical generated by one-electron transfer can be attacked by either the superoxo complex (oxygenase action) or another phenoxy radical to form either a adduct or TTBDQ, respectively. The adduct releases benzoquinone and yields the species; the latter abstracts an electron from another phenol molecule, releasing water and regenerating the catalyst.

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13. OXIDATIONS WITH COBALT PEPTIDE COMPLEXES Recently, there has been increasing interest in metallopeptides as nucleic acid modification reagents178 that can act as affinity cleavage appendages to nucleic acid binding domains179,180. The development of site-selective DNA strand scission agents may involve the substrate oxidation reactions of metallo-Gly-Gly-His and its derivatives181. Photirradiated mixtures of Co(II) and under ambient totally converted form I DNA to form II182. An oxygenated Co(III)-peptide complex is presumably formed upon photoirradiation, which is capable of inducing DNA strand scission by OH radical generation via His-Co coordination in close proximity to DNA.

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The stereoisomerism and equilibrium properties of dioxygen carrying cobalt(II) complexes of histamine and its derivatives have recently been reinvestigated183. The Co(II) - glycyl-L-histidine-imidazole Co(II)-L-histidylglycineimidazole and the corresponding L-histidine and glycylglycine systems have been studied184. Potentiometric, gas-volumetric and spectroscopic (UV/VIS, near IR, ESR) measurements indicated the presence of two types of ternary species: with parent complexes containing a deprotonated amide group (as for glycylglycine and similar dipeptides) and with parent histidinelike active complexes. The overall stability constants of the ternary complexes have been determined. An increase in reversibility of dioxygen uptake was found in both cases relative to the systems without imidazole. The oxygen uptake by Co(II) complexes with a group of diastereoisomeric dipeptides, consisting of alanine and leucine in various chiral forms, has been studied in aqueous solution185.

15. CARBOXYLATOCOBALT COMPLEXES AND SALTS Cobalt(II) ion has been found to catalyze the dioxygen driven oxidation of N-(phosphonomethyl)iminodiacetic acid (PMIDA ) to N-(phosphonomethyl)glycine (PMG ) in aqueous solution186. Additional products are and formic acid. This homogeneous catalytic conversion is novel and represents, in effect, an oxidative dealkylation of one carboxymethyl moiety, yielding the Nsubstituted glycine. The reaction is selective to the desired product PMG when carried out at the natural pH of the free acid substrate (approximately 1-2) and when carried out at substrate loadings less than 5% by weight. PMG is the active agent in the herbicide Roundup187.

Kinetic studies on dilute systems have been carried out. The reaction is first order in substrate and The oxygen pressure dependence exhibits saturation kinetics, while the selectivity increases as the oxygen pressure increases. The rate is also inversely proportional to The proposed mechanism consists of the reaction sequence (a) – (d), involving complexation of Co(II) and generation of the active oxidant via oxidation by Its oxidative dealkylation to the product PMG is shown in Figure 65.

318

L.I. Simándi

In the presence of diacetyl, the acetylation of adamantane (Ad) is catalyzed by in acetic acid at 60°C under 1 atm of (Figure 66). After 2 hours of reaction, the products are 1-acetyladamantane (AAD) (47%) 1,3-diacetyl-adamantane (20%) and 3-acetyladamantan-1-ol (6%)188.

6. Catalytic Oxidations using Cobalt(II) Complexes

No reaction takes place in the absence of with Co(II) or in the presence of with Co(III), which points to a cobalt-dioxygen complex as key intermediate. It reacts with diacetyl to produce acetyl free radicals, which in turn are converted to acetylperoxyl radicals via reaction with The latter are capable of abstracting an H-atom from adamantane, which constitutes the hydrocarbon activating step (Figure 67).

Cobalt salts have been extensively used as catalysts for the oxidation of monoterpenes with for the flavor and fragrance industry189, 190,191. The addition of NaBr to increases the conversions of limonene (L), (aP) and (bP) in autoxidations, by dioxygen (Figure 68)192,193.

319

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L.I. Simándi

The best selectivities to various allylic oxygenated products (11 – 24 %) were achieved at a ratio of 1. Substitution of Co(II) by Mn(II) up to 30% had no effect on the conversion, but shifted the selectivity significantly towards alcohols and acetates. This was ascribed to a decrease in the overall redox potential, which suppressed further oxidation and side reactions of the primary products. Cobalt(II) acetate is slightly soluble in boiling nonane and decane and promotes autoxidation to ketones as major products It is proposed that the reaction takes place via a cobalt(III)-peroxy species which reacts with the alkanes to form cobalt(II)-alkylhydroperoxide complexes. These complexes decompose to form preferentially ketones, with concomitant regeneration of cobalt(II) acetate194.

16. MISCELLANEOUS COBALT CATALYSTS N-hydroxyphthalimide, a radical catalyst, has been reported to promote the oxidation of various organic substrates such as diols, alkylbenzenes, cycloalkanes and adamantanes by dioxygen under mild conditions195-199. The oxidation takes place both in the absence and presence of or Isobutane is converted to t-butyl alcohol with high selectivity at 100°C and an air pressure of 10 atm in benzonitrile over 8 hours200. Acetone and tbutyl hydroperoxide are formed in smaller amounts, the latter being an intermediate of the reaction as indicated by the maximum observed in its concentration. The catalyst used is a combination of N-hydroxyphthalimide (NHPI ) and or may also be used as the active cobalt species, the former being the most active. NHPI is a source of phthalimidoxyl free radicals (PINO), produced by H-atom abstraction from NHPI by the superoxocobalt(III) species or the complex PINO in turn abstracts an H-atom from isobutane to produce an isobutyl radical, which is trapped by dioxygen to afford the tbutylhydroperoxy radical (A) and ultimately t-butyl hydroperoxide. The

6. Catalytic Oxidations using Cobalt(II) Complexes

321

latter is readily decomposed by metal ions to the t-butoxy radical, which is then cleaved to acetone (see Figure 69).

In addition to isobutane, 2-methylbutane, 3-methylpentane and 2,3dimethylbutane also undergo oxidation in the presence of the system200. The exposure of solid [Tp”Co(CO)] (Tp” = hydrotris(3-isopropyl-5methylpyrazolyl)-borate) to excess gas afforded the dioxygen complex A close analog of this paramagnetic complex (Tp’ = hydrotris(3-tert-butyl-5-methylpyrazolyl)-borate)202 decomposes in solution to yield the doubly bridged and more reactive transient intermediate which subsequently abstracts H-atoms from the ligand (Figure 70)203.

322

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The chemistry of cobalt-dioxygen complexes of hydrotris(pyrazolyl)borate ligands (Tp(R)) has been reviewed204 with special reference to the formation of low-valent metal-peroxo and high-valent metal-oxo species, such as Co(II)-superoxo, alkylperoxo and dinuclear complexes. In the hydrotris(3,5-diisopropyl-1-pyrazolyl)borate ligand system oxygenation of the proximal isopropyl substituents on is mediated by the and the alkylperoxocobalt species.

17. CONCLUSIONS The cobalt(II) species most widely used as catalysts in oxidations by are complexes with salen, porphyrin, phthalocyanine, acac, dimethylglyoximato, amine, pyridine, cyclidene and carboxylato ligands. Low-spin cobalt(II) complexes are inherently reactive toward dioxygen due to their d7 electron configuration. Many of the binding reactions are very fast, therefore, few rate constants have been determined and special techniques such as stopped-flow or flash photolysis had to be employed in kinetic work. The superoxocobalt(III) species formed are in most cases the active intermediates in catalytic reactions, which occur readily with substituted phenol or aniline derivatives, producing quinone type dehydrogenation products. The key step in catalytic cycles is often H-atom abstraction or electron transfer to the superoxo complex from the substrate, which is converted to a free radical, possibly reacting further with cobalt(III)

6. Catalytic Oxidations using Cobalt(II) Complexes

323

derivatives formed via cobalt-centered oxidation. This is a pathway for the regeneration of cobalt(II) for further catalytic cycles. Alternatively, the intermediate phenoxyl radical may be attacked by the superoxo complex at the para-position, leading to oxygen insertion products. A procedure widely applied for effecting O-atom insertions (olefin epoxidation, hydrocarbon hydroxylation or ketonization) with cobalt(II) catalyts is the addition of sacrificial 2-methylpropanal. It is converted via Hatom abstraction to an acyl radical, which upon reaction with produces an acylperoxyl radical. H-atom abstraction by the latter leads to a hydroperoxide, which is capable of effecting O-atom insertions via freeradical chain reactions. The sacrificial aldehyde is lost via oxidation to an acid or an ester.

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Acknowledgement. The author’s work described in this review was supported by the Hungarian Science Fund (OTKA Grant No. T029036).

329

Subject Index 3,5-di-tert-butyl-5-(formyl)-2-furanone

1

163 184

1,10-phenanthroline

2 2,2’-bipyridine 2,3,6-tri-tert-butylphenol

184

269 oxidation of 2,3-dihydroxyphenylpropionate 1,2179 dioxygenase 195 2,4,6-trichlorophenol 2,4,6-tri-tert-butylaniline 269 oxidation of 2,4-di-tert-butylmuconic acid anhydride

3,5-di-tert-butylcatechol oxidation of 3-ethylpentane 3-methylcatechol 3-methylpentane

3 3,3',5,5'-tetra-tert-butyl-4,4'dihydroxystilbene 287, 290 168,173 3,4-PCD 3,5-di-tert-burylcatechol 163 3,5-di-tert-butyl-l,2-benzoquinone 163 3,5-di-tert-butyl-2-pyrone 163 3,5-di-tert-butyl-5-(carboxymethyl)-2163, 167 furanone

193 170 197

4 4,6-bis(1,1 -dimethylethyl)-2H-pyranone

171 4-chlorocatechol 4-tert-butylcatechol

168,169 169,201

5

163 2,6-di-tert-butylphenol 269, 270, 313, 314 oxidation of 2-aminophenol oxidation of 282, 283, 284,299, 302, 303 2-aminothiophenol 301, 302 oxidation of 2-chloroethyl ethyl sulfide.. 227,228,229 199 2-mercaptobenzoic acid 276, 300 2-mercaptoethanol 2-methyl-1-phenyl-2-propyl 185 hydroperoxide 2-methylpropanal 293, 294, 310, 311, 323

270, 283, 286

181

6 195 179, 188

A ABTS acetonitrile

192

227, 230, 245, 247, 251, 252, 253, 260, 261 activation of dioxygen 266, 299 acyl radical 277, 323 adamantane 183, 184, 189, 190, 193, 197, 199, 200, 202, 205 acetylation of 318, 319 alcohols 182, 194, 197, 201, 204, 205 oxidation of 265, 276, 294, 296, 310, 311, 320 198, 201 aldehydes alkanes 160, 182, 187, 189, 192, 197, 198, 200, 202 alkene epoxidation 265, 277, 278

330 160, 182, 189, 190, 202, 265, 77, 295, 296 184, 193, 197 alkyl hydroperoxide alkyl isocyanides 289 oxidation of 158 alkylperoxides 289 alkylperoxocobaloxime(III) 185 alkylperoxyiron(III) 2, 60, 61 amidation 84, 87, 102, 103 amine oxidase 256, 261 amino acid 166 aminopyridine ligands 202 amorphous iron 199 aniline 197 anthracene 82 ascorbate oxidase 199 ascorbic acid 160, 167, 179 asymmetric 267 autoxidation of 2, 59, 60 aziridination

alkenes

B Baeyer-Villiger oxidation BDPMA BIPA bipyridinecobalt(II)

177 195 167, 184, 199 308

167 166 166 bleomycin BLPA BnBPA BPG BPIA BPMCN BPMEN BQPA Busch, D.H bztpen

197, 202 170, 172 172, 174

165 167, 184 188, 195 181, 194

166 268, 313, 323, 328

203

C C=C cleavage

2

186 values carbon radical 184, 198 100, 118, 121 Casella, L catalases 158, 159 catalyst inactivation 227 162 catechol 1,2-dioxygenase catechol 1,2-dioxygenases 162, 168 160, 162, 168, catechol dioxygenases 169, 170, 179, 205 Catechol dioxygenases 161, 162 catechol oxidases.81, 82, 83, 84, 100, 123 CCD

168

227, 229, 235, 240, 245, 247, 250, 252, 259, 261 CEESO 230, 233, 238, 242, 248, 250, 253, 260 chiral porphyrins 1, 2, 29, 30, 62 162, 168 chlorocatechol dioxygenases chloroperbenzoic acid 183 242 chlorosulfonium ion CHP 183, 184 cis-1,2-dimethylcyclohexane 194 cis-cyclooctene 190 cis-dihydroxylation 195 292, 309, 310, 311, 320 280, 282 Co(III)-alkylperoxo oxidation via 312 Co(salen) 269, 270, 271, 273, 276 Co(salen) complexes 270 267, 278, 284, 287, cobaloxime(II) 290, 296 cobalt(II) complexes 265, 267, 269, 317,

CEES

322 cobalt(II) cychdene complexes 313,314 cobalt(II) OBISDIEN 305, 306 cobalt(II) octaethylporphyrin 292 298, 300 cobalt(II) phthalocyanine cobalt(II) porphyrins 292 co-catalysis 252 co-catalyst 251, 255, 256 colloidal Au(0) 227, 253

331 244, 259 competitive oxidation 159 compound I 228, 251, 252 copper 104, 123 aliphatic hydroxylation 98, 99 aromatic hydroxylation

87, 91, 94, 96, 100, 105 84, 102, 123 cofactor biogenesis 79 dioxygen activation 79, 84, 93, 94, 95, 100, 105, 108, 123 79, 114, 116 DNA cleavage 93, 108 hydroperoxo 80, 123 industrial processes 79, 100 model compounds 79, 82, 91, 98, 104 monooxygenase 99, 104 N-dealkylation 81 nitrite reductases ortho-oxygenation of phenols 79, 83 108, 123 oxidase models 80, 82, 97, 102 oxidative coupling 81 oxygenases 87, 88, 90, 93, 94, 97 peroxo phenanthroline DNA oxidation 112, 123 87 phenoxide bridged 79, 110 phenoxyl radical 82 proteins (table) 87, 88, 90 superoxo 82 copper methane monooxygenase Criegee rearrangement 177 168 CTD 173 CTH dioxygen adducts

318 catalysis by 2 Cu-containing proteins 183 cumene hydroperoxide 188 cumyl alcohol 186 cycloalkyl radical 188 cycloheptane 183, 189, 190, 197, 198, cyclohexane 199, 200, 203, 204 185, 186, 193, 197, 199 cyclohexanol

cyclohexene

189, 194,198 186 cyclohexyl peroxide 185, 189, 190, 195 cyclooctene 179 cyclopropyl radical 158 cytochrome c oxidase cytocnrome c oxidases 86, 87, 123 cytocnrome P-450 2, 6, 7, 8, 52 ,62, 159

D 195 DCP dehydrochlroination 169 dehydrogenation of phenols 2 175 density-functional theory 276 depsides 247 dialkyl sulphide 187, 188 diamond core dichlorocatechol 169 257 dimethyl disulfide dimethyl sulfide 184 247 dimethyl sulfoxide dinuclear iron complexes 184, 185, 190 dioxoruthenium(VI) 2, 29 dioxygenase 227, 234, 261 161 dioxygenase-model oxygenations dioxygenases 2 DNA cleavage 114, 116 dopamine 79, 82, 104 198 DPAH DTBC 164, 165, 167, 169, 170, 174, 178 163, 164, 166, 167, 174, 176 DTBSQ 173, 176, 177

E EDDA 192 effect of solvent 241 E-methyl ferulate 271 epoxidation l, 2, 9, 21, 26, 32, 41, 5 9 , 6 2 , 160, 183, 186, 189, 192, 195, 198, 201 epoxides 182, 189, 198, 201 ESI-MS 194, 202

332 ethylbenzene 188 Extended-Hückel 175 extradiol 160, 161, 163, 170, 173, 176, 179

F facial ligand Fe(IV)=O

172 159,204 176 176 161, 194, 204 161, 182, 185, 193, 196, 204 Fenton 191, 193 161, 182, 194, 200, 201 193 FeZSM-5 190 fluorous biphase systems 295

G galactose oxidase Gif system Glaser process gold complexes Gorun, S.M.

2,11, 82, 85, 108 193,200 80 227 94,120

H 204 198 162, 169, 184, 189, 193, 195,204 Haber-Weiss reaction 2,4 halogenated metalloporphyrins 292 halogenated porphyrins 2 187, 200 HDA 164 HDP 165 heme protein models 159 heme-copper oxidases 86 hemerythrin 158 hemocyanin 81, 82, 84,123,158 hemoglobin 2,6,65,158

high-valent histidine HPTB HPTP hydrazobenzene hydrocarbons oxidation of hydrogen peroxide

161 162,166 190 190 200, 278, 280

277, 308 158, 182, 183, 186, 191, 193, 205 hydrogen sulfide 199 hydroquinones 200 hydrotris(pyrazolyl)borate ligand 171 hydroxy radical 191 hydroxylation 1, 2, 21, 29, 33, 62, 161, 182, 186, 192, 199, 200, 204

I immobilized Co(II) phthalocyanine 300 induction period 227, 232, 237, 248, 251, 253, 257, 261 inhibition 227, 234, 242, 248, 251, 254, 261 intradiol 160, 161, 166, 168, 170, 171, 172, 178, 179 iron(III)-peroxo 159 iron-oxo 161, 182, 201, 202, 204 iron-peroxo 182, 185 isopenicillin N synthase 160, 182

K Kemp’s triacid imide 188 ketones 182, 187, 194, 197, 201, 204 ketonization 161, 193, 198, 200 kinetic isotope effect 183, 185 kinetics of dioxygen binding 267 Kitajima,N. 89, 94, 119, 120 Klinman,J.P 84, 102, 118 Knowles,P.F 84, 118 Kodera,M. 93, 107, 120, 121

333

L

N

laccase 82 lacunar cobalt(II) cyclidene complex. 313 Lewis acidity 164, 166 lignin phenolic subunits 270 lignin phenolics oxidation of 270 lipoxygenase 160, 179 LMCT 164, 165, 167, 174 177

167, 176 81 197 292, 325, 326

Lou Gehrig’s disease low-spin Lyons, IE.

M macrocylic cobalt(II) complexes 268 Masuda, H. 93, 120 m-CPBA 183 172, 174 membrane reactor 197 mep 187 meridional 172, 177 metal-based mechanisms 182 metalloenzymes 158 metalloporphyrins 2, 8, 9 methane 160, 161, 182, 187, 97, 205 methane monooxygenase 160, 161, 187, 205 methanol 170, 182, 189, 197 methyl linoleate 198 methylcyclohexane 183 MMO 184 molecular oxygen 158, 159, 163, 169, 173, 181, 187, 201, 205 monooxygenases. 2, 3, 160, 181, 182, 198 monoterpenes oxidation of 319 MPPH 185 mustard gas 2, 256, 257 myoglobin 2, 6, 12, 65, 158

N4Py 194, 203 NaOCl 183, 198 N-benzylidene-2-hydroxyaniline oxidation of 303 NIH shifts 201 nitrate 230, 247, 248, 251, 252, 254 nitrene complexes 2 nitrene transfer 2, 55 nitrilotriacetate 164, 192 nitrite 248 nitrogen monoxide oxidation of 274 nitrosobenzene oxidation of 289 nitrous oxide 2, 34 N-methylmorpholine N-oxide 183 nonlinear fitting 242 NTA 164, 192

O O-atom donors 1, 2, 9, 11, 29, 63 O-atom insertion .273, 289, 290, 307, 323 o-benzosemiquinone-diimine 301 olefins oxidation of 2, 9, 14, 21, 24, 26, 30, 34, 38, 58, 62 O-O-bond lengths 266 o-phenylenediamine oxidation of 281, 301 organocobaloximes 291 organosulfur compounds 257 ortho-hydroxylation 182 O-transfer 278 oxidases 2, 3, 11 oxidations 158, 197 oxidative dehydrogenation of amines 2 of alcohols 2 oxidative dehydrogenations 280, 281 oxidative detoxification 256 oxocobalt(IV) 278, 296

334 oxo-ferryl 159 oxygenases158, 159, 160, 161, 163, 179, 182, 202, 204, 205 oxygenated Fenton chemistry 309 oxygenations 158, 163, 168, 170, 172, 175, 179, 182, 187, 199, 200, 206

P PA

167, 192, 198, 200

pb 186 pCP 195 PCP 196 PDA 165 pentadentate ligand 195, 203 peptidylglycine monooxygenase 79, 82, 104 peracids 158, 159, 202 perfluorinated polyethers 251, 255 perhaloporphyrins cobalt(II) 293 peroxidases 158, 159 peroxo-iron(III) 160 phenol oxidation 79, 83 phenylalanine hydroxylase 160, 182 phosphines oxidation of 1, 2, 14, 16, 55, 58, 61 phosphite oxidation 307 pivalaldehyde 202 PMAH 197 pollutant 195 pollutants 162, 195, 205 polyoxoanion 167, 171 porphyrins 1, 2, 8, 9,10,14, 21, 27, 28, 37, 62 primary amines oxidation of 280 propane 183, 197 propenoidic phenols oxidation of 271, 273 protocatechuate 3,4-dioxygenasel62, 173, 174 protocatechuic acid 169

p-substituted phenols oxidation of 270, 272, 273 pterin-dependent hydroxylases 160 p-xylene 201 pyridine 163, 165, 170, 183, 187, 189, 197, 199, 200, 203 pyrocatechase pyrocatechol

2, 4, 161, 174 161, 170

Q quantum chemical calculation quantum chemical methods quercetin oxidation of quercetin model

178 161 276 276

R radical autoxidation 188 radical cation 243 radical character 174, 175, 177 radical-chain mechanism 228, 244 radical-clock reagents 182 radical-rebound mechanism 183 rate constants for binding 267 rebound mechanism 2 reductive activation of 2 reductive elimination 242 Reedijk, J. 89, 117, 119 Reglier, M. 106 reoxidation 236, 238, 244, 245, 248, 253, 254 reversible binding 266 Rieske dioxygenases 181 ruthenium imine/amine complexes of 2 ruthenium amido complexes 2 ruthenium porphyrins 1, 2 ruthenium(IV) disproportionation of 2

335

S salicylate 199 saturated hydrocarbons oxidation of 1, 2, 19, 28, 41, 63 shape selective 160 singlet state 158 sMMO 160, 182 solvent kinetic isotope effect 227 spin density 175, 177 Stack, T.D.P. 92, 96, 109, 120, 121, 122 steroid epoxidation 2 structural model complexes 158 Structural type 266 substituted phenols oxidation of265, 269, 270, 309, 310, 313 sulfone 234,244,256 sulfoxidation 227, 234, 255, 257, 261 sulfoxide 227, 230, 234, 242, 244, 247, 256, 257, 261 superoxide dismutase 82 superoxide dismutases 158 superoxide ion 158 superoxocobaloxime(III) 280, 284 supported catalysts 2 synergistic effect 255 synthetic dioxygen carriers 266 syringyl alcohol oxidation of 271

T TACN TAML TBHP

170, 171, 172, 174 196 183, 184, 188, 190, 197, 202 184, 185, 198 184, 185 TCP 195, 196 Tempo 193 tert-butylphenol 201 tetera-chlorocatechol 169 tetraamido macrocyclic ligands 196 tetraazamacrocyclic ligand 167

tetradentate ligands 168, 176 tetrahydrosalen dehydrogenation of 275 tetrahydrothiophene 231 thioanisole 185 thioether 227, 235, 238, 242, 243, 244, 246, 249, 250, 256, 259, 261 thioethers oxidation of 1, 2, 14, 17, 19 TMA 197 tmima 183, 186 Tolman, W.B. 91, 99, 119, 120, 121 toluene 173, 183, 184, 186, 199, 201 toluidine 199 topical skin protectant 252 TPA 165, 167, 168, 170, 175, 177, 188, 190, 194, 197, 202 171, 174 TPY 172, 174 trans-2-octene 195 trans-2-phenylmethycyclopropane 184 trans-stilbene 184, 189, 202 tridentate ligand 163, 172 trifluoroethanol 234, 253, 254, 255 triplet state 158 tripodal ligands 164, 165, 167, 169, 185 trispicMeen 166 tryptophan dioxygenase 2, 4 tryptophan hydroxylase 160 tyrosinases79, 81, 82, 83, 86, 98, 100, 102, 104, 123 tyrosine 160, 162, 166, 182, 200, 201 tyrosine hydroxylase 160, 182, 200, 201

W Wacker process water-soluble Water-soluble ligands Wieghardt, K

80 169, 195 169 110, 121

X XANES

174

336

Z zinc

200

acid 160, 161, 180 acid-dependent dioxygenases 160 188

diferric complex diiron(H)

203 203 186 186

cation

159

Catalysis by Metal Complexes Series Editors: R. Ugo, University of Milan, Milan, Italy B.R. James, University of British Colombia, Vancouver, Canada

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K. Kalyanasundaram and M. Grätzel (eds.): Photosensitization and Photocatalysis Using Inorganic and Organometalic Compounds. 1993 ISBN 0-7923-2261-4

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P.A. Chaloner, M.A. Esteruelas, F. Joó and L.A. Oro: Homogeneous Hydrogenation. 1994 ISBN 0-7923-2474-9

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G. Braca (ed.): Oxygenates by Homologation or CO Hydrogenation with Metal Complexes. 1994 ISBN 0-7923-2628-8

17.

F. Montanari and L. Casella (eds.): Metalloporphyrins Catalyzed Oxidations. 1994 ISBN 0-7923-2657-1

Chemistry. 1983

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T. Funabiki (ed.): Oxygenases and Model Systems. 1997

20.

S. Cenini and F. Ragaini: Catalytic Reductive Carbonylation of Organic Nitro Compounds. 1997 ISBN 0-7923-4307-7

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A.E. Shilov and G.P. Shul’pin: Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes. 2000 ISBN 0-7923-6101-6

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