Florencio Zaragoza Dorwald
Metal Carbenes in Organic Synthesis
@ W ILEY-VCH Weinheim - New York Chichester Brisbane -...
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Florencio Zaragoza Dorwald
Metal Carbenes in Organic Synthesis
@ W ILEY-VCH Weinheim - New York Chichester Brisbane - Singapore Toronto
This Page Intentionally Left Blank
Florencio Zaragoza Dorwald
Metal Carbenes in Organic Synthesis
@ WILEY-VCH
This Page Intentionally Left Blank
Florencio Zaragoza Dorwald
Metal Carbenes in Organic Synthesis
@ W ILEY-VCH Weinheim - New York Chichester Brisbane - Singapore Toronto
Dr. Florencio Zaragoza Dorwald Novo Nordisk A/S MedChem Research Novo Nordisk Park DK-2760 Mbl0v - Denmark
This book was carefully produced. Nevertheless, author, editors and publisher do not warrant the information contained therein to be free of errors. Readers are adviced to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No. applied for.
A catalogue record for this book is available from the British Library.
Deutsche Bibliothek Cataloguing-in-Publication Data: Zaragoza Dorwald, Florencio: Metal carbenes in organic synthesis / Florencio Zaragoza Dorwald. Weinheim ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH, 1999 ISBN 3-527-29625-5
OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany). 1999 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Fa. Richarz Publikations-Service GmbH., D-53734 Sankt Augustin. Printing: betz-druck GmbH, D-64291 Damstadt Bookbinding: J. Schaffer GmbH & Co. KG., D-67269 Grunstadt Printed in the Federal Republic of Germany.
Foreword
In the period between the discovery in the late 1950s that copper catalyzed the addition of diazo compounds to olefins to yield cyclopropanes and the recent introduction of olefin metathesis into the synthesis of fine chemicals, a wide variety of useful organic transformations that involve metal carbene intermediates has been discovered. Since most of the reactions of metal carbenes result in the formation of carbon-carbon bonds, the reactions have played a major role in the organometallic revolution in organic synthetic technology during the past few years. Zaragoza-Dorwald has assembled this large array of reactions into a system that allows the common feature of the reactions to be recognized. After outlining the bonding description of the types of complexes, he uses the simple structure types to divide an amazing array of reactions that range from cyclopropanation to the olefination of carbonyls into groups that follow a general mechanistic pathway. The book follows a consistent outline that efficiently provides the active chemist with the information needed to follow up a reaction type. Each section starts with a definition of the metal carbene structure along with a general mechanistic scheme for the preferred reactions of that type of carbene. A few specific examples are discussed to demonstrate the key features, and then a large number of examples are tabulated. At the end of each section an experimental procedure is presented that provides the details of a representative reaction. I found the progression from general to specific examples an extremely efficient presentation of important ideas without excessive details. For example, his treatment of the Dotz and related reactions allowed him to introduce many of the subtle features of this complex reaction in a concise fashion. This approach also allows the book to be read at many levels. The brief introduction and the array of tables allow for a rapid overview of the field, while the extensive references and experimental procedures provide ample detail for an in-depth study of specific reactions. Along a number of reviews have been written on specific aspects of metal carbene complex chemistry, this book describes in one place the array of reactions involving these intermediates and provides the structural basis that ties them together. Although some parts of the book might quickly become out of date, this concise presentation of all the aspects of the use of carbene complexes in synthesis will help provide the impetus for even more rapid developments in this field of research. Pasadena, October 1998
Robert H. Grubbs (California Institute of Technology, Pasadena)
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Preface
To the memory q f m yfriend and colleague Lutz Richter
In recent decades there has been an exponential increase in the use of transition metals in organic synthesis. Among the different types of transition-metal-based reagent described, carbene complexes are among the most versatile. The applications of carbene complexes include both their use as catalysts for a number of important synthetic transformations and their utilization as stoichiometric reagents. The aim of this book is to give the reader a well-structured overview of the most important applications of carbene complexes in organic synthesis. Special emphasis has been given to recent innovations, in an attempt to pinpoint new and promising research areas. Hopefully this will give plenty of inspiration for the development of new research projects. As an organic chemist I consider reaction mechanisms of crucial importance, both for the classification of reactions and for synthesis-planning. For this reason mechanisms are proposed for almost all the reactions described herein. Most of these mechanisms have not yet been rigorously proven, however, and should be considered as preliminary. The subject of this book has been organized in three main sections: preparation and applications of heteroatom-substituted carbene complexes (Fischer-type carbenes), non-heteroatom-substituted carbene complexes, and acceptor-substituted carbene complexes. In each section the different types of reaction have been ordered either according to the mechanism or according to the type of product. In addition to a selection of illustrative examples, several experimental procedures have been included. These were chosen taking into account safety, availability of starting materials, relevance of the products, and general interest. I would like to thank my colleagues and supervisors at Novo Nordisk A/S, in particular Jesper Lau and Behrend F. Lundt, for their support and encouragement. It is also a pleasure to acknowledge Kilian W. Conde-Friboes, Robert Madsen, and Thomas Redemann for proofreading various sections of the manuscript and for their helpful comments and suggestions.
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Table of Contents
Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XI11
Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . .
XV
1 1 .1 1.2
1.3 1.4
2 2.1 2.1.1 2.1.1.1 2.1.1.2 2.1.1.3 2.1.2 2.1.3 2.1.4 2.1.5 2.1.5.1 2.1.5.2 2.1.6 2.1.7 2.1.8 2.1.9 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5
. . . . . . . . . . . . . . . . Reactivity of Carbene Complexes . . . . . . . . . . . . . . . . .
1
Fischer-Type and Schrock-Type Carbene Complexes: Theoretical Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olefin Metathesis and Olefin Cyclopropanation . . . . . . . . . . Characteristic NMR Data . . . . . . . . . . . . . . . . . . . . .
3 5 9
Heteroatom-Substituted Carbene Complexes . . . . . . . . . . Generation of Heteroatom-Substituted Carbene Complexes . . . . From Acyl Complexes . . . . . . . . . . . . . . . . . . . . . . From Acyl Complexes Generated from Carbonyl Complexes . . . From Acyl Complexes Generated from Metallates . . . . . . . . From Acyl Complexes Generated by Other Methods . . . . . . . From Isonitrile Complexes . . . . . . . . . . . . . . . . . . . . From a-Haloiminium Salts and Metallates . . . . . . . . . . . . From Carboxamides and Metallates . . . . . . . . . . . . . . . . From Vinylidene Complexes . . . . . . . . . . . . . . . . . . . From Vinylidene Complexes Generated from Alkynes . . . . . . From Vinylidene Complexes Generated from Alkynyl Complexes From Carbenes and Carbenoids . . . . . . . . . . . . . . . . . . From Alkyl Complexes by a-Abstraction . . . . . . . . . . . . . From Carbyne Complexes . . . . . . . . . . . . . . . . . . . . . Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
13 14 14 15 18 19 20 20 21 25 25 25 27 29 32 33
Synthetic Applications of Heteroatom-Substituted Carbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Considerations . . . . . . . . . . . . . . . . . . . . . . Demetallation and Formation of Acyclic Products . . . . . . . . . Photochemical Transformations . . . . . . . . . . . . . . . . . . Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Benzannulations . . . . . . . . . . . . . . . . . . . . .
34 35 37 41 45 49
The Carbon-Metal Double Bond
1
X
Table o j Contents
2.2.5. I 2.2.5.2 2.2.6 2.2.6.1 2.2.6.2 2.2.6.3 2.2.6.4 2.2.7 2.2.8
The Dotz Benzannulation Reaction . . . . . . . . . . . . . . . . 49 Other Thermal Benzannulations . . . . . . . . . . . . . . . . . . 5.5 Formation of Five-Membered Rings . . . . . . . . . . . . . . . . 56 Cyclization of (1,3.Butadien. 1.yl)carbene Complexes . . . . . . . 56 Cyclization of Functionalized Carbene Complexes . . . . . . . . 63 Rearrangement of Ammonium Ylides . . . . . . . . . . . . . . . 64 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Formation of Six.Membered. Non-Aromatic Carbocycles and Six-Membered Heterocycles . . . . . . . . . . . . . . . . . . . . 66 Formation of Seven-Membered Rings . . . . . . . . . . . . . . . 70
3
Non-Heteroatom-Substituted Carbene Complexes
3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.3 3.1.3.1 3. I .3.2 3.1.4 3.1.4.1 3.1.4.2 3.1.5 3.1.6 3.1.7 3.1.8
Generation of Non-Heteroatom-Substituted Carbene Complexes . a-Abstraction of Electrophiles (Nucleophilic Abstraction) . . . . . a-Abstraction of Nucleophiles (Electrophilic Abstraction) . . . . . a-Abstraction of Hydride . . . . . . . . . . . . . . . . . . . . . a-Abstraction of Oxygen-Bound Leaving Groups . . . . . . . . . a-Abstraction of Thioethers . . . . . . . . . . . . . . . . . . . . a-Abstraction of Halides . . . . . . . . . . . . . . . . . . . . . From Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . . From Diazoalkanes . . . . . . . . . . . . . . . . . . . . . . . . From Other Ylides . . . . . . . . . . . . . . . . . . . . . . . . From Carbyne Complexes . . . . . . . . . . . . . . . . . . . . . Nucleophilic Additions to Carbyne Complexes . . . . . . . . . . Electrophilic Additions to Carbyne Complexes . . . . . . . . . . From Alkynyl and Alkenyl Complexes . . . . . . . . . . . . . . From Alkyne and Cyclopropene Complexes . . . . . . . . . . . . By [2 + 21 Cycloreversion . . . . . . . . . . . . . . . . . . . . Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
76 78 82 83 84 87 89 90 90 93 93 94 96 98 98 00 101
3.2
Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Considerations . . . . . . . . . . . . . . . . . . . . . . Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . . Stoichiometric Cyclopropanations . . . . . . . . . . . . . . . . . Catalytic Cyclopropanations with Diazoalkanes . . . . . . . . . . Catalytic Cyclopropanations with Other Carbene Precursors . . . C-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . C-H Insertions of Nucleophilic Carbene Complexes . . . . . . . C-H Insertions of Electrophilic Carbene Complexes . . . . . . . Carbonyl Olefination . . . . . . . . . . . . . . . . . . . . . . . Carbonyl Methylenation . . . . . . . . . . . . . . . . . . . . . . Carbonyl Olefination with Higher Alkylidenes . . . . . . . . . . Olefin Metathesis . . . . . . . . . . . . . . . . . . . . . . . . . General Considerations . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . .
103 103 10.5 106 114 116 119 119 122 125 125 129 134 135 138
3.2.1 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.2.3 3.2.3.1 3.2.3.2 3.2.4 3.2.4.1 3.2.4.2 3.2.5 3.2.5.1 3.2.5.2
.......
75
XI
Tuble of Contents
3.2.5.3 Homogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . . 140 3.2.5.4 Scope and Limitations of Molybdenum- and Ruthenium-Based Homogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . . 143 3.2.5.5 Ring-Opening Metathesis Polymerization (ROMP) . . . . . . . . 144 3.2.5.6 Ring-Closing Metathesis (RCM) . . . . . . . . . . . . . . . . . 148 3.2.5.7 Cross Metathesis . . . . . . . . . . . . . . . . . . . . . . . . . 161 3.2.5.8 Ring-Opening Cross Metathesis . . . . . . . . . . . . . . . . . . 168 Other Applications of Non-Heteroatom-Substituted Carbene 3.2.6 Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
. . . . . . . . . . . 171
4
Acceptor-Substituted Carbene Complexes
4.1 4.1.1 4.1.1.1 4 .1. 1.2 4.1.2 4.1.3
Generation of Acceptor-Substituted Carbene Complexes . . . . . From Acceptor-Substituted Diazomethanes . . . . . . . . . . . . Preparation of Acceptor-Substituted Diazomethanes . . . . . . . . Catalysts for Diazodecomposition . . . . . . . . . . . . . . . . . From Other Ylides . . . . . . . . . . . . . . . . . . . . . . . . From Other Carbene Complexes . . . . . . . . . . . . . . . . .
171 172 172 175 176 176
4.2
Synthetic Applications of Acceptor-Substituted Carbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Considerations . . . . . . . . . . . . . . . . . . . . . . C-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . Scope and Limitations . . . . . . . . . . . . . . . . . . . . . . . Stereoselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . Intramolecular 1,2.CH . Insertions . . . . . . . . . . . . . . . . Intramolecular 1,3. CH . Insertions . . . . . . . . . . . . . . . . Intramolecular 1,4. CH . Insertions . . . . . . . . . . . . . . . . Intramolecular 1 ,5. CH . Insertions . . . . . . . . . . . . . . . . Intramolecular 1,6. and 1,7. C-H Insertions . . . . . . . . . . . Intermolecular C-H Insertions . . . . . . . . . . . . . . . . . . Si-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . C-C Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . X-H Insertions (X: N. 0. S) . . . . . . . . . . . . . . . . . . N-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . O-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . S-H Insertions . . . . . . . . . . . . . . . . . . . . . . . . . . Y lide Formation . . . . . . . . . . . . . . . . . . . . . . . . . . Ammonium Ylides . . . . . . . . . . . . . . . . . . . . . . . . Azomethine Ylides . . . . . . . . . . . . . . . . . . . . . . . . Nitrile Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxonium Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonyl Y lides . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfonium Ylides . . . . . . . . . . . . . . . . . . . . . . . . . Thiocarbonyl Ylides . . . . . . . . . . . . . . . . . . . . . . . . Other Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclopropanation . . . . . . . . . . . . . . . . . . . . . . . . .
178 178 179 179 180 180 181 181 182 189 189 192 193 193 194 196 197 198 198 202 203 205 206 213 216 217 218
4.2.1 4.2.2 4.2.2.1 4.2.2.2 4.2.2.3 4.2.2.4 4.2.2.5 4.2.2.6 4.2.2.7 4.2.2.8 4.2.3 4.2.4 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3 4.2.6 4.2.6.1 4.2.6.2 4.2.6.3 4.2.6.4 4.2.6.5 4.2.6.6 4.2.6.7 4.2.6.8 4.2.7
. . . . . . .
XI1
Table of Contents
4.2.7.1 4.2.7.2 4.2.7.3 4.2.7.4 4.2.8
Scope and Limitations . . . . . . . . . . . . . . . . . . . . . . . 218 219 Stereoselectivity . . . . . . . . . . . . . . . . . . . . . . . . . . Intramolecular Cyclopropanations . . . . . . . . . . . . . . . . . 220 Intermolecular Cyclopropanations . . . . . . . . . . . . . . . . . 224 Formal 1,3-Dipolar Cycloadditions of Acyl- and Vinylcarbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Ring Fission of Pyrroles and Furans . . . . . . . . . . . . . . . 231 Other Synthetic Applications of Acceptor-Substituted Carbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
4.2.9 4.2.10 5
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . .
233
Index
...................................
267
Abbreviations
acetyl acetylacetonato acyclic diene metathesis ally1 2,6-di(tert-butyl)-4-methylphenol benzyl tert-butyloxycarbon y 1 boiling point butyl ceric ammonium nitrate, (NH4),Ce(N0,), caprolactam benzyloxycarbonyl 1,5-~yclooctadiene 1,3,5-cyclooctatriene $-cyclopentadienyl pentamethyl-qs-cyclopentdienyl cy clohex y 1 1,2-dichloroethane diastereomeric excess decomposition dimethyl acetylenedicarboxylate 1,2-dimethoxyethane N,N-dimethy lformamide 4-(dimethy1amino)pyridine dimethyl sulfoxide 1,2-bis(diphenylphosphino)ethane enantiomeric excess equivalents hexyl hexafluoroacetylacetonato hexamethylphosphoric triamide ligand metal M MCPBA 3-chloroperoxybenzoic acid methox ymethy 1 MOM
Ac acac ADMET All BHT Bn Boc bP Bu CAN Cap Cbz COD COT CP CP’ CY DCE de dec . DMAD DME DMF DMAP DMSO dPPe ee eq. Hex hfacac HMPA
XIV
Abbreviations
MS mP MTO mwt NP Ns Pht piv PMP PNB Pol Pr PYr RCM ROMP salen TBAF TBS TES Tf TFA THF THP TIPS TLC TMEDA TMS To1 TP’ Tr Ts
molecular sieves melting point methylrhenium trioxide, CH3Re03 molecular weight neopentyl, 2,2-dimethylpropyl (trimethylsily1)methyl phthaloy 1 pivalate, 2,2-dimethylpropionate 4-methoxyphenyl 4-nitrobenz y l polymeric support ProPYl p yridine ring-closing metathesis ring-opening metathesis polymerization bis(salicyla1dehyde)ethylenediimine tetrabutylammonium fluoride tert-buty ldimethylsily 1 trieth ylsil yl trifluoromethanesulfony1 trifluoroacetic acid tetrahydrofuran 2-tetrahy dropyrany l triisopropylsily 1 thin-layer chromatography N,N,N’,N’-tetramethylethy lenediamine trimeth ylsilyl 4-methylpheny l hydrotris( 3,5-dimethyl- 1-pyrazolyl)borato trityl p-toluenesulfonyl, tosyl
Experimental Procedures
Experimental Procedure 2.1.1. Preparation of a Chromium Carbene Complex from Chromium Hexacarbonyl: [Methoxy(methyl)carbene]pentacarbonylchromium
MeLi
Cr(CO),
OLi
[Me,O][EF,]
OMe
(CO),Cr 92%
15
Experimental Procedure 2.1.2. Preparation of a Chromium Carbene Complex from a Carboxamide: { [(4S)-2,2-Dimethyl-4-phenyloxazolidin-3-yl]me~ylene} pentacarbonylchromium 1. Na,[Cr(CO),] 2. Me,SiCI 3.AI,03
*
(c0)5cL
93% Ph
Ph
23
Experimental Procedure 2.1.3. Preparation of an Iron Carbene Complex by &Abstraction of Hydride: Dicarbonyl(~5-cyclopentadienyl)(phenylthiocarbene) iron hexafluorophosphate
1. NalHg 2. PhSCH,CI
[Cp(CO),Fe],
51%
OC SPh Cp-kd
I
oc
[Ph&l[pF~l
OC,
78%
29
XVI
Experimental Procedures
Experimental Procedure 2 2-Benzyl-4-benzyloxy-4din-3-one
Photolysis of a 1-2,3,4,4a,7,7a-he
45 Experimental Procedure Complex: Diethyl trans-3OMe
(co)p=(
+
E
Ph
48
Experimental Procedure 2.2.4. Benzannulation with a Chromium Furylcarbene Complex: 5,6-Diethyl-4,7-dihydroben Cr(CO),
@OMe
+
54
Experimental Procedures
XVII
Experimental Procedure 2.2.5. Cyclopentannulation with a Tungsten Alkynylcarbene Complex: 1-(1-Ethoxy-3-phenyl-5,9b-dihydro-4H-cyclopenta[u]naphthalen-9-y1)pyrrolidine
59
Experimental Procedure 2.2.6. Cyclopentannulation with a Molybdenum Arylcarbene Complex: 3-Hexyl-5-methyl- 1-indanone MOW),
1.9ooc 2. CAN +
Hex
61
Experimental Procedure 2.2.7. Diels-Alder Reaction of a Tungsten Vinylcarbene Complex: Methyl 2-(2-Furyl)-5-methyl-4-oxo-1-cyclohexanecarboxylate
70
XVIII
Experimentul Procedures
Experimental Procedure 2.2.8. [4 + 31 Cycloaddition of a Chromium Vinylcarbene Complex to a l-Azadiene: truns-4-(2-Furyl)-2-methoxy-5-methyl-4,5dihydro-3H-azepine
71
Experimental Procedure 2.2.9. [4 + 31 Cycloaddition of a Chromium Carbene Complex to a 2-Aminodiene: 6-(2-Furyl)bicyclo[5.4.0]undecan-2,4-dione
73
Experimental Procedure 3.1.1. Preparation of a Molybdenum Carbene Complex by Nucleophilic Abstraction: (2,6-Diisopropylphenylimido)bis[1,l-bis(trifluoromethyl)ethoxy](2-methyl-2-phenyl- 1-propylidene)molybdenum
(NH&MoA
four steps
4%
(FaC),MeC-O-?$oAph / (F,C),MeC-0
80
Experimental Procedures
XIX
Experimental Procedure 3.1.2. Preparation of an Iron Vinylidene Complex by Electrophilic Abstraction: Carbonyl(~5-cyclopentadienyl)(dimethylvinylidene)(tripheny1phosphine)iron tetrafluoroborate HBF,, TfzO
Of?Fe-$/ Ph,P
86
Experimental Procedure 3.1.3. Preparation of a Ruthenium Carbene Complex from a Diazoalkane: Dichloro-bis(tricyclohexy1phosphine)benzylideneruthenium 1. PhCHN,, -78 OC, 5 min 2. PCy,, 20 O C , 30 min CH,CI, (PPh,),RuCI2
.c
pcY$ CI,
I
ph
-
/
91
Experimental Procedure 3.1.4. Preparation of a Molybdenum Vinylidene Complex from a Carbyne Complex: Tetrabutylammonium { Cyano(ethoxycarbony1) vinylidene} (dicarbonyl){ hydro-tris(3,5-dimethyl-1-pyrazolyl)borato} molybdenum [NEt,]ITp'Mo(CO),]
80%
co I Tp'-Mo+CI A 0
NC-C0,Et
100%
co
C02Eq-
Tp'-ho=C+
NBU,+
CN
60
95
Experimental Procedure 3.2.1. Cyclopropanation with an Iron Carbene Complex: 1,l-Diphenylcyclopropane 100 OC, dioxane 14h c
Ph
88%
x
Ph
Ph
106
xx
Experimentul Procedures
113
124
129
133
133
Experimental Procedures
XXI
Experimental Procedure 3.2.7. RCM with a Tungsten Catalyst in Homogeneous Phase: Diethyl 3-Cyclopentene-1,l -dicarboxylate
* Et0,C
Et0,C
C0,Et
6
C0,Et
PbEt4,PhMe 91%
150
Experimental Procedure 3.2.8. RCM with a Ruthenium Catalyst in Homogeneous Phase: 2-Phenyl-3,6-dihydro-2H-pyran
-0
L
OPh +
. )
86%
156
Experimental Procedure 3.2.9. Cross Metathesis with a Molybdenum Catalyst in Homogeneous Phase: (E)- 1-Phenyl-1-octene 2,6-(iPr),C,H,,
N
“fc
(CF3),M*CO--EjlCi
Ph-
+
3
(CF,),MaCC)’
Ph
94%
161
XXII
Experimentul Procedures
Experimental Procedure 4.1.1. Preparation of an Enantio ically pUre Rhodium(I1) Complex: Dirhodium(I1) Tetrakis[methyl2-0~0-1-(3-phenylpropanoyl)4(S)-imidazolidinecarboxylate]; Rh2(4S-MPP1M),(MeCN),
175
Experimental Procedure 4.2.1. Prepara Insertion: Ethyl trans-2-0x0- 1,4-diph
01e~ularC-H
Rh,(OAch
C0,Et 6h
CHzC'z
61W
182 Experimental Procedure 4.2.2. Preparation of an Indole by Intramolecular C-H Insertion: Methyl (2S)-2-(3-Acetyl-2-hydroxy-S-nitro1H-I -indolyl)-3phenylpropanoate Ph
65%
187
Experiinetztul Procedures
XXIII
Experimental Procedure 4.2.3. Alkylation of Indole by Intermolecular C-H Insertion: 3-( lH-3-Indolyl)-2,4-pyrrolidinedione
0 1
+
I - ' 64%
#
192 Experimental Procedure 4.2.4. Etherification of a Serine Derivative by Intermolecular 0-H Insertion: Methyl (2S)-3-[(Ethoxycarbonyl)methoxy]-2-(benzyloxycarbony1amino)propanoate
197 Experimental Procedure 2.5. Ammonium YliL,: Formation and Stevens Rearrangement: Diethyl2-Benzyl-2- { [(ethoxycarbonyl)methyl](methyl)amino } malonate EtozC~cozEt NZ
+
Cu,PhMe PhnNnCO,Et
I
EtozC
CozEt
__c
92%
Ph3(NACOzEt
I
200 Experimental Procedure 4.2.6. Oxonium Ylide Formation and 2,3-Sigmatropic Rearrangement: Ethyl 2,5-Dimethoxy-4-pentenoate
205
XxTv
Experimentul Procedures
Experimental Procedure
d Intramolecular
e0 C0,Et
BF,OEt,
* 100%
0 C0,Et
92%
bMe
Me
210
Experimental Procedure 4.2.8. Enantioselective, Intramolecular Cyclopropanation: 6,6-Dimethyl-3-oxabicyclo[3.1 .O]hexan-2-one
84% 92% ee
22 1
Experimental Procedure 4.2.9. Enantioselective, Intermolecular Cyclopropanation: Ethyl (lS)-2,2-Dimethyl- 1-cyclopropanecarboxylate
A +
N2
L.
91%
x
C0,Et
224
Experimentul Procedures
xxv
Experimental Procedure 4.2.10. Cycloaddition of an Acylcarbene Complex to an Enol Ether: Ethyl 5-Ethoxy-2-trifluoromethyl-4,5-dihydro-3-fkroate
227 Experimental Procedure 4.2.11. Cycloaddition of an Acylcarbene Complex to an Alkyne: Ethyl 2-Methyl-5-phenyl-3-furoate
227 The experimental procedures in this text are intended for use only by persons skilled in organic synthesis, and are conducted at ones own risk. WILEY-VCH and the author disclaim any liability for any injuries or damages claimed to have resulted from the experimental procedures described herein. In many of the reactions presented benzene is used as solvent. The replacement of benzene by a less toxic solvent, such as, e.g., toluene, might in many instances lead to comparable results, and is strongly recommended.
This Page Intentionally Left Blank
1 The Carbon-Metal Double Bond
When the development of carbene-complex chemistry began in the mid seventies, two different patterns of reactivity emerged and led to a, maybe overemphasized, division of these compounds into (electrophilic) Fischer-type and (nucleophilic) Schrock-type carbene complexes (Figure 1.1).
Nu-
Electrophilic carbene complex:
LnM5? -
R
LnM-eN RU
Nucleophilic carbene complex:
E+
R
L"L+E
R
R
Fig. 1.1. Reactivity of carbene complexes towards electrophiles (E+) and nucleophiles (Nu-).
Today, however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called 'carbene complexes' or 'alkylidene complexes', terms not associated with any specific chemical behavior.
1.1 Reactivity of Carbene Complexes Carbenes are electron-deficient intermediates, capable of reacting with organic compounds in several ways. Typical reactions of uncomplexed carbenes include cyclopropanation, C-H insertion, and reaction with lone pairs to yield ylides (Figure 1.2).
2
I The Carbon-Metul Double Bond
R,C -H
1
$ R H ,,
Fig. 1.2. Typical reactions of free carbenes.
The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. 0, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as o-donors and Tc-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept o-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. Four different types of metallic fragment can now be considered: (a) (b) (c) (d)
good o-acceptor, good x-back-donator; poor o-acceptor, good Tc-back-donator; good o-acceptor, poor Tc-back-donator; poor o-acceptor, poor x-back-donator.
In situation (a) a strong carbon-metal bond results. To this group belong the typical Schrock-type carbenes [e.g. Np3Ta=CH(tBu)], many of which are nucleophilic at carbon. Situation (b) should also lead to nucleophilic carbene complexes, albeit with a weaker carbon-metal bond. Typical reactions of nucleophilic carbene complexes include carbonyl olefination (Section 3.2.4) and olefin metathesis (Section 3.2.5). Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)SCr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(I1) and palladium(I1) carboxylates with diazoalkanes. Also platinum ylides [ 1,2], resulting from the reaction of diazoalkanes with platinum(I1) complexes, have a strong Pt-C o bond but only a weak Pt-C ~c bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes,
1.2 Fischer-Type und Schrock-Type Curbene Complexes: Theoretical Treatment
3
electrophilic carbene complexes undergo cyclopropanation, C-H insertion, and ylide formation reactions. It can be concluded that the reactivity of a carbene complex will mainly depend on the ability of the substituents R (Figure 1.3) and the metallic group M to release electrons into the empty p orbital of the carbene carbon atom.
free carbenes
Rh, Cu, Pd, Pt carbene complexes
Fischer-type
x back-donation from metal to carbon increases ~~
nucleophilicity of the carbene C-atom increases
Schrock-type
-
Fig. 1.3. Reactivity of carbene complexes as a function of the electronic interaction between metal and carbene.
1.2 Fischer-Type and Schrock-Type Carbene Complexes: Theoretical Treatment Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7c-acceptors at the metal, are electrophilic at the carbene carbon atom (C,). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without 7caccepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic? Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [ 3 -81 and Schrock-type [9171 carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energydifference between the empty carbene 2 p orbital and a d orbital (of suitable symmetry) of the group L,M.
1 The Carbon-Metul Double Bond
4
Cr(ll), d4, 18 e
Ta(V), do, 10 e
Fischer-type carbene complex
Schrock-type carbene complex
Fig. 1.4. Typical Fischertype and Schrock-type -_ c-aibene complexes.
Complexes with a strong n interaction between the metal and the carbene fragment will have an energy-rich n* orbital, and will, therefore, not be good substrates for nucleophilic additions. Moreover, in complexes with large d(meta1)2p(carbon) overlap, electrons will be transferred to a greater extent from the metal to the electron-deficient (and more electronegative) C,. Thereby the partial negative charge and electron density at C, will increase and facilitate electrophilic attack at this atom (Figure 1.5). On the other hand, complexes with weak n interaction between the metal and the carbene will have an energetically low-lying n* orbital. In addition to this, electron-transfer from the metal to C, will be less efficient, thus leading to a more positively charged carbene fragment. Hence, carbene complexes with a large energy gap and poor orbital overlap between the metal d orbital and the carbene 2 p orbital will be prone to react with nucleophiles.
,'
carbene2p
metal nd I
'
metal nd
MO
MO
small energy-difference between metal nd and carbene 2p
large energy-difference between metal nd and carbene 2p
nucleophilic carbene complex
electrophilic carbene complex
Fig. 1.5. Orbital interaction in nucleophilic and electrophilic carbene complexes.
1.3 Olefin Metathesis and Olefin Cyckq~ropunution
5
Different calculations have shown [ 13,171 that in Schrock-type tantalum carbene complexes the d orbitals of the metal interact very efficiently with the empty carbene 2 p orbital. In addition to this, the energy of the tantalum d orbitals is only slightly lower than the energy of the 2 p orbital of methylene, so that a strongly binding, low-energy ~c orbital and a high-energy 7c* orbital result. Hence the nucleophilicity of Schrock-type carbenes is a result of (a) a strong M-C .n bond with extensive electron transfer from metal to carbon, and (b) a high-energy ~ c *orbital, hardly accessible to nucleophiles. Although Schrock-type carbenes are formally electron-deficient at the metal, backbonding to the carbene is very effective, also because there are no other Tc-acceptors with which to share the electrons. Fischer-type carbene complexes, on the other hand, have a metallic group L,M with d orbitals of lower energy than the group H,Ta'-. This leads to a lower-lying K* orbital, more susceptible to nucleophilic attack, and to a weaker M-C TC bond [3,181. The decisive difference between, e.g., [Cp(CO),Fe]+ and H,Ta3- is the smaller amount of orbital overlap of the former with the carbene 2 p orbital, resulting in less efficient transfer of electron density from the metal to C,. Although Fischer-type carbene complexes are formally low valent, backbonding to the carbene is less effective than in tantalum alkylidene complexes. The structure of rhodium(I1) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(I1) carboxylate isonitrile complexes [20,2 11. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant ~c backbonding from rhodium to carbon.
1.3 Olefin Metathesis and Olefin Cyclopropanation Two of the most characteristic reactions of carbene complexes are olefin metathesis and olefin cyclopropanation. Olefin metathesis is a reaction in which the CC double bond of an alkene is cleaved, and one of the resulting alkylidene fragments combines with the metal-bound carbene to form a new alkene. The second alkylidene fragment forms a new carbene complex with the metal. Olefin cyclopropanation is a reaction in which a 0 bond is formed between the metal-bound alkylidene and each of the two carbon atoms of the alkene, to yield a cyclopropane. Fischer-type and Schrock-type carbene complexes not only differ in their electrophilicity at C, but also have strikingly different reactivity towards alkenes (Figure 1.6). The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates
6
1 The Carbon-Metal Double Bond
Ph\D/Ph**
Ph-
ArN
Ph
RO-)kJ
35% Ro 1 [Mo(VI), do, 12 e]
PhCH,CI,
Ph\D/Ph* ZIE 90: 10
P -78 OC
39%
Ph
CH,CI,,
(CO),W--J 2
h
6 -78
oc c
17%
[W(II), d4, 18 el
PhCH,CI, Ph&ph
Z/E> 99:l
l+
-78 OC
Ph Cp(CO),Fe=/
* 88%
P
h
6
PF,-
3 [Fe(lV), d4, 18 el
Fig. 1.6. Different reactivities of Fischer-type and Schrock-type carbene complexes [22241. ROH: (CF,),(CH,)COH; ArNH,: 2,6-diisopropylaniline; complex 1 is formed in situ from the corresponding 2-methyl-2-phenyl- 1 -propylidene complex and styrene.
both metathesis and cyclopropanation and leads to the formation of a mixture of diphenylcyclopropane and stilbene. How can this different reactivity be rationalized? Experimental evidence and numerous theoretical investigations indicate that metathesis and cyclopropanation proceed by different reaction mechanisms, each requiring a characteristic electronic configuration of the group L,M. Olefin metathesis proceeds via reversible formation of metallacyclobutanes by [2 + 21 cycloaddition (Figure 1.7). The precise pathway for such a cycloaddition has been calculated for molybdenum complexes such as 1 (Figure 1.6) [9]. These calculations suggest that although Mo-C and C-C bond formation is concerted the Mo-C bond is formed more quickly than the C-C bond. It was also found, beautifully consistent with experimental results, that the activation barrier for [2 + 21 cycloaddition is lowered by increasingly electron-withdrawing alkoxy ligands. The key feature of efficient metathesis catalysts seems to be their ability to form, before the [2 + 21 cycloaddition step, a z complex with the alkene (Figure 1.7). Comparison of catalyst 1 with the iron complex 3 shows that the latter, although cationic, will not be able to bind to an olefin, because this would give rise to a complex with 20 valence electrons. A similar argument can be used
1.3 Olejn Metathesis and Olejin Cyclopropanation
Fig. 1.7. Mechanism of carbene-complex-catalyzed olefin metathesis.
-
-
L,M=CH,
L,M=CH,
7
L,M- - -
to explain the different products obtained upon thermolysis of the tungsta- and titanacyclobutanes shown in Figure 1.8. Thermolysis of the tungsten complex mainly yields propane and propene [25], whereas the titanacyclobutane, which has also been shown to catalyze the metathesis of olefins, undergoes a reversible [2 + 21 cycloreversion [26,27] to yield ethylene and a (highly reactive) carbene complex. [2 + 21 Cycloreversion of the tungstacyclobutane would require, if the mechanism sketched in Figure 1.7 is to be followed, the formation of a carbene-ethylene TC complex with 20 valence electrons. This complex would be a very energy-rich, unstable intermediate. The analogous reaction with titanium proceeds smoothly, however, because only 16- and 18-electron complexes appear as intermediates.
H,C =C H, [W(IV), d2, 20 e]
““Ti*
Fig. 1.8. Thermolysis of metallacyclobutanes [25,26].
CP’ [Ti(lV). do, 16 el
55 OC
(Cp) Ti-CH, 2,
-
-\tBu [Ti(IV), do, 18e]
Calculations [28] on the formation of cyclopropanes from electrophilic Fischertype carbene complexes and alkenes suggest that this reaction does not generally proceed via metallacyclobutane intermediates. The least-energy pathway for this process starts with electrophilic addition of the carbene carbon atom to the alkene (Figure 1.9). Ring closure occurs by electrophilic attack of the second carbon atom
8
I The Carbon-Metul Double Bond
of the olefin on the carbene carbon atom from the direction opposite to the metal. The formation of both C-C bonds is concerted, but not synchronous. Experimental results [29] also support this mechanism.
Fig. 1.9. Possible mechanism of the cyclopropanation of alkenes with electrophilic carbene complexes [28].
It should be noted that cyclopropanes can also be formed from nucleophilic carbene complexes, and olefin metathesis can also be catalyzed by Fischer-type carbene complexes. The following observations indicate that metallacyclobutanes, i.e. the intermediates of olefin metathesis, can undergo reductive elimination with simultaneous formation of cyclopropanes. Small amounts of cyclopropanes are formed [30] when alkenes are treated with Tebbe reagent (Cp2TiCH2A1C1Me,) in the presence of bases. Under these conditions Cp,Ti=CH, is generated and quickly reacts with olefins to give titanacyclobutanes [3 11. Titanacyclobutanes also undergo reductive elimination to yield cyclopropanes when treated with iodine [ 3 2 ] . Dithioacetals R,C(SR’),, moreover, react in the presence of two equivalents of Cp,Ti[P(OEt),], with unactivated alkenes to yield cyclopropanes, probably via a titanium carbene complex (see Section 3.2.2.1 [33]). Several other observations suggest that nucleophilic carbene complexes, similarly to, e.g., sulfur ylides, can cyclopropanate acceptor-substituted olefins by an addition-elimination mechanism. If, e.g., acceptor-substituted olefins are added to a mixture of a simple alkene and the metathesis catalyst PhWCI,/AlCl,, the metathesis reaction is quenched and small amounts of acceptor-substituted cyclopropanes can be isolated [34]. These observations indicate that there is no sharp borderline between cyclopropanating and metathesis-catalyzing carbene complexes. Fortunately the number of carbene complexes which mediate both cyclopropanation and alkene metathesis is rather small, and in the detailed overview given in the following sections it will become apparent that most carbene complexes are highly selective and thus valuable reagents for organic synthesis.
1.4 Characteristic NMR Data
9
1.4 Characteristic NMR Data In most known carbene complexes both the metal-bound carbon and the protons attached to it are magnetically strongly deshielded. The chemical shifts of the carbene carbon atom (C,) in these complexes range among the highest observed to date, often surpassing those observed for unstabilized carbocations. Typical chemical shifts for C, are listed in Tables 1.1 and 1.2. As can be concluded from these values, there is no direct correlation between the nucleophilicity of a carbene complex and the chemical shift of C,. Even strongly nucleophilic carbene complexes, such as, e.g., Ns,Ta=CH(SiMe,), have C, resonances at > 200 ppm. At first glance this might be a surprise, because tantalum alkylidene complexes react similarly to, e.g., phosphorus ylides. In the latter, however, the phosphorus-bound carbon atom is shielded, as would be expected for an electron-rich carbon atom [6 (Me,P=CH,) 2.3 ppm]. For theoretical treatment of chemical shifts in carbenes, see [35,36] and references therein.
10
I The Carbon-Metal Double Bond
Table 1.1. Chemical shifts for carbon atoms (C,) and protons (Ha) in representative heteroatom-substituted carbene complexes L,M=C,(R)H,. Carbene Complex
C,
Solvent Ref.
Carbene Complex
C, Solvent Ref. (Hu)
(Ha)
CHPh, \
dHPh,
Ph
Cp
156
[b]
[39]
278
[b]
[40]
270
Ibl
[42]
263
[dl
[37]
364
[b]
[44]
330
[el
[37]
333
[bl
[46]
338
[el
[37]
318
[bl
[47]
OEt
ON-)o<
oc ( C O ) , W G P h
cp, OC-/M"<
OEt
oc cp;F\e ),+tBu
oc co
PF,-
'
290
[rl
[48]
1.4 Characteristic NMR Data
11
Table 1.2. Chemical shifts for carbon atoms (C,) and protons (Ha) in representative non-heteroatom-substituted carbene complexes L,M=C,(R)H,. Carbene Complex
C,
Solvent Ref.
Carbene Complex
(HA
cp\ Cp-Ti=CH, / Me,P
286 (12.1)
[dl
C, Solvent Ref. (Ha)
287 (12.4)
[dl
241 (5.75)
[dl
224 (10.82)
[dl
tBu
CP; Cp'-Ta=CH,
H'
240 (10.2)
[dl
SiMe,
361
Lb1
(Ar: 2,6-iPr,C,H3) Ph
cco,,w=(
348
Cp [d
Ph
'w=/
Cd
342
[el
%
Ph
OC-,Fecp\
1lbTf-
E&P
(iPr),P
1
.co
Ph
366
[el
(17.71 15.7)
295 (20.0)
[el
297
[dl
( 1 9.0)
OC -Ru=CiPr),P
12.0)
This Page Intentionally Left Blank
2 Heteroatom-Substituted Carbene Complexes
In this section the preparation and uses of heteroatom-substituted carbene complexes L,M=C(X,)R(, - ), (n = 1, 2; x: NR,, OR, SR) will be discussed. In these complexes the electron deficit at the carbene carbon atom is compensated both by electron-donation from the lone pairs on the heteroatom and by d-electron backbonding from the metal (Figure 2.1).
Fig. 2.1. Resonance stabilization of carbene complexes by heteroatoms (X: NR,, OR, SR).
L"M=(x R
-
Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C, than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatomsubstituted carbenes, which are very weak 7c-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. Structural parameters and other data have been calculated for several models of heteroatom-substituted carbene complexes [3-5,8]. Particularly stable are coordinatively saturated, 1%electron carbene complexes of the type (CO),M=C(X)R (M: W, Cr; X: OR, NR,; R: H, alkyl, aryl). These complexes are often referred to as Fischer-type carbene complexes, in honor of E. 0. Fischer, who prepared these compounds for the first time in 1964 [61]. Since then these compounds have attracted broad interest, and many hundreds of heteroatom-substituted carbene complexes have been synthesized. Thereby valuable new insights were gained into the nature of the carbon-metal double bond. These complexes are also becoming increasingly important for organic synthesis, both as reagents and as catalysts.
14
2 Heteroatoin-Substituted Carbene Complexes
2.1 Generation of Heteroatom-Substituted Carbene Complexes In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy1oxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C, without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2.
-.-I
Rl+ L,M=c=( R
ROC-
or
L,,MEC-R
co I
L,,.,,M-R
I
t + L-
L,M +
1'
: QR
Fig. 2.2. Synthetic routes to alkoxycarbene complexes.
2.1.1 From Acyl Complexes Nucleophilic acyl complexes can be 0-alkylated with hard electrophiles to yield the corresponding alkoxy- or (acy1oxy)carbene complexes. The first carbene complex ever isolated [61] was prepared by this route; the intermediate, anionic acyl complex was generated by addition of phenyllithium to tungsten hexacarbonyl (Figure 2.3). Other methods for preparing acyl complexes include the acylation of metallates and the treatment of alkyl carbonyl complexes with nucleophiles.
2. I Generation of Heteroutom-Substituted Curhene Complexes
1. PhLi, Et,O
1. H+
2. Me,N+
W(c%
15
0(CO)5w=(
OMe
2.CH,N,
NMe,'
Ph orange powder mp 102.5 OC (dec.)
(CO),W=( Ph orange crystals mp 59 OC
(Fischer, Maasbol, 1964)
Fig. 2.3. The first synthesis of a stable alkoxycarbene complex [61].
2.1.1.1 From Acyl Complexes Generated from Carbonyl Complexes Metal carbonyls can react with organolithium compounds to yield nucleophilic acyl complexes. 0-Alkylation of the latter leads to the formation of alkoxycarbene complexes. This methodology, introduced by E. 0.Fischer (Figure 2.3, [61]), has proven to be very robust and is currently one of the most commonly used procedures for preparing alkoxycarbene complexes. A broad range of different organolithium compounds and other nucleophiles react with carbonyl complexes, generating (sometimes isolable) acyl complexes [62-651. 0-Alkylation of these acyl complexes is usually effected by treatment with trialkyloxonium salts [66-701 or alkyl triflates (65,71,72], but acyl halides [73-751, diazoalkanes [76], aromatic diazonium salts [77], alkyldiphenylsulfonium salts [78], dimethoxycarbenium salts [79], or silyl halides [80] have also been used successfully. Illustrative examples of the conversion of carbonyl complexes into carbene complexes by this two-step methodology are listed in Table 2.1. Experimental Procedure 2.1.1. Preparation of a Chromium Carbene Complex from Chromium Hexacarbonyl: [Methoxy(methyl)carbene]pentacarbonylchromium 137, p 1301 OMe
MeLi
220.06
250.13
Methyllithium (40 mL of a 0.5 M solution in diethyl ether, 20 mmol) is added dropwise over 2 h to a well-stirred suspension of chromium hexacarbonyl (4.4g, 20 mmol) in diethyl ether (600 mL) at room temperature. A yellow, clear solution results. The solvent is evaporated under reduced pressure without heating, and the residue is dissolved in degassed (N2) water (100 mL) and pentane (200 mL). While stirring vigorously trimethyloxonium tetrafluoroborate (3.1 g, 21 mmol) is added in small portions. The aqueous phase should have become slightly acidic
16
2 Heteroatom-Substituted Carbene Complexes
Table 2.1. Heteroatom-substituted carbene complexes prepared from carbonyl complexes and carbon nucleophiles. Starting Material
Reagents
Product
1. MeLi, Et,O 2. [Me,Ol[BF,I
Cr(C0)6
2. TMSOTf
Ref.
92%
[81]
OSiMe,
1. Me,P=CH, Cr(C0)6
Yield
(CO),Cr<
-
51%
PMe,
1801
WI
Me0
1. BuLi, Et,O
2. Cr(CO), 3. MeOTf
\
59%
1831
51%
[84] ~ 7 1
68%
[85]
56%
[86]
OC-,Mo ON
57%
[37] ~ 7 1
OC-,Mo ON
88%
[45]
90%
[71] [881
\
dOM
All
All
::3
I
MOM
I . BuLi 2. Cr(CO), 3. [Me,Ol[BF,I
RO
(R: triisopropylsilyl)
1. 2-BuLi, THF 2. Cr(CO), 3. MeS0,F Boc OMe
I
&cr(co,5 Boc
1. BuLi, THF 2. W(CO), 3. SiO,, H,O
1. MeLi, Et,O, 2. lEt~Ol[BF41, CH,CI,, 1 h 1. MeLi 2. MeOTf, CH,CI,
1. MeLi 2. Me4NBr 3. 3-pentyn-I-yl triflate. C H X L
OMe
2. I Generation of Heteroutom-Substituted Curbene Complexes
17
Table 2.1. continued. Starting Material
Reagents
Product
10
Cp-Mn oc h' OC' P
12
13
CP, OCI
BB
OCNRU
14
1. BuLi 2. Ph,SnCo(CO),
Ref.
88%
[37]
80%
[37]
71%
[37] ~ 9 1
44%
[90]
Me0
oc' OMe
Bu3Snv OMe
1-
15
'c0'5"m""'
1. fBuLi 2. W(CO),, Et,O 3. [Me,Ol[BF41
11
Yield
I
oc-p. CO oc
I . tBuLi 2. [Me,OI[BF,l,
oc
I
45%
(911
1. 48% 2. 72%
[92]
oc-'co-
acetone
when addition is complete, otherwise more oxonium salt must be added. The phases are separated and the aqueous phase is extracted with pentane (50-mL portions; until the extract remains colorless). The combined organic extracts are dried over sodium sulfate and concentrated to 75 mL. Crystallization at -15 OC yields 4.6g (92%) of the title compound as yellow crystals, sufficiently pure for most purposes. Analytically pure material (mp 59°C) can be obtained by column chromatography (degassed silica gel/pentane). 'H NMR ([D6]acetone) 6 3.00/3.17 (3H), 4.39/4.89 (3H). Carbonyl complexes also react with non-carbon nucleophiles. The resulting carbonic acid derivatives can serve as starting material for the preparation of bisheteroatom-substituted carbene complexes [93]. Heterocyclic carbene complexes can be obtained from nucleophiles with a leaving group in P-position (Table 2.2).
18
2 Heteroatom-Substituted Carbene Complexes
Table 2.2. Heteroatom-disubstituted carbene complexes prepared from carbonyl complexes and non-carbon nucleophiles. Starting Material
Yield
Ref.
50%
[941
60%
[95]
98%
[96]
15%
[97]
95% Meolfo(l+ Cp-feBF,-
[98]
Reagents, Conditions
Product
0 (C0)5Mn-f
AgPF,, 20 "C, acetone. 1 h
JC'
0
D
Ih
ON
2-brornoethanol, NaH, 5 min; then KPF,
11+
OC\ CpF;e<
0
oc
0
PF,-
aziridine, MeCN, Br(CH,),NH,Br, 25 "C, 10 min OEt
(CO),Ni==(
Ni(CO),
NHMe
OH
Qd
OC NH
(PhC=CPh),W(CO)
k
P
h
,
53%
[99]
25 "C, CH,CI,, 16 h
2.1.1.2 From Acyl Complexes Generated from Metallates In addition to the direct nucleophilic alkylation of carbonyl complexes, the acylation of metallates with, e.g., carboxylic acid chlorides [73,100,102] or anhydrides [79] is a practical way of generating acyl complexes (Fighe 2.4). Illustrative examples are given in Table 2.3.
2. I Generation of Heteroutom-Substituted Carbene Complexes
19
(Hegedus, 1998)
Fig. 2.4. Generation and 0-alkylation of nucleophilic acyl complexes from metallates and acyl halides [loll.
Table 2.3. Heteroatom-substituted carbene complexes prepared from carboxylic acid derivatives and metallates. Starting Material
Reagents, Conditions
Product
Yield
Ref.
82%
[IOO]
11%
[lo31
OMe
2
p
h
v
OAc
4
c
C
then K2[W(CO)5]; [Me,O][BF,]
c
K,[Cr(COM; then~[Me,O][BF,] ~
OAc
,
o OAc
5
l
OAc
F C O C I
OMe
~ (CO),Cr
Na[Cp(CO),FeI; then [Me,O][BF,]
~
~ OAc
OAc
woMe -
[I041
Fe-L
O d c' ,
2.1.1.3 From Acyl Complexes Generated by Other Methods A further method for preparing acyl complexes consists in the treatment of alkyl complexes containing at least one carbonyl ligand with a strong ligand [44,105,106]. Thereby 1,l-insertion of the carbonyl group into the metal-carbon bond can
20
2 Heteroutom-Substituted Carbene Complexes
be induced to yield an acyl complex (Figure 2.5). This reaction can also be conducted in the presence of trialkyloxonium salts, the alkoxycarbene complexes being obtained directly [ 1071 (Figure 2.5). LiBr. 20 OC.THF. 1 h then H,PO, (CO),Mn-CH, 66%
-
OH
(Moss, 1973)
Br(CO),Mn< orange-yellow needles mp 33-34 OC
1. SnCI, 2.NaBPh4
c p \ ~ ~ ~ ~ H B P h 4 -
___)
Ph,P
CO
51% (all steps)
o.P )P h,’
0
(Marson, Randall, Winter, 1994)
I
&
oc, Cp?’
oc
[Et,O][BF,], CO (1 atm) 25 “C, CH,CI,, 24 h 76%
OEt OC>Fe* cp I
oc
‘
l+ BF,-
(Hossain, 1993)
Fig. 2.5. Preparation of hydroxy- and alkoxycarbene complexes from alkyl complexes by 1,l-insertion of carbon monoxide [106- 1081.
Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate; this then reacts with water and tautomerizes to the acyl complex.
2. I Generation of Heterontom-Substituted Curbene Complexes
21
2.1.2 From Isonitrile Complexes Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [ 109- 1 121. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electronpoor olefins [ I 131, isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6).
-
+
L,M-C=N-R
-
NuH
L,M=C=N-R
/IJ-R
L,M=C
\
Nu
Fig. 2.6. Preparation of aminocarbene complexes from isonitrile complexes (Z: electron withdrawing group; X=Y: dipolarophile).
Multicomponent condensations have also been described; in these an isonitrile, a carbonyl compound and a suitable transition metal complex are combined in one step to afford heterocycloalkylidene complexes. Examples of the use of isolated or intermediate isonitrile complexes for the preparation of aminocarbene complexes are given in Table 2.4.
2.1.3 From a-Haloiminium Salts and Metallates Haloiminium salts can react with metallates or similarly nucleophilic transition metal complexes to yield heteroatom-substituted carbene complexes (Figure 2.7) [ 1201. This reaction is closely related to the acylation of metallates with derivatives of carboxylic acids (Section 2.1.1.2). Examples are given in Table 2.5.
yellow solid rnp 91 -94 OC (Stone, 1975)
Fig. 2.7. Conversion of a-haloiminium salts into aminocarbene complexes [ 1201
22
2 Heteroutom-Substituted Curbene Complexes
Table 2.4. Heteroatom-substituted carbene complexes prepared from isonitrile complexes. Starting Material
Yield
Ref.
KOrBu, Me1
89%
[Ill]
KF, 20 "C, MeOH, 2 d
-
[116]
PhCHO, NaOMe
-
[117]
-
[118]
71%
[115]
68%
[I191
Reagents, Conditions
Product
HO
1
Ph3P(CO),W-CEN&
Me,SiO,
(CO),Fe-CZN
4
4=J JPh,
(CO),W-CEN
@CN NC
5
Jh, (CO),Cr-CzN
6 Cp-re-CO
MeNCO, PhMe, 20 "C, 1 h
rBuNC, PhCHO, PhNH,CI, 20 "C, MeOH
NC
2.1.4 From Carboxamides and Metallates Mechanistically similar to the reaction of nucleophilic metallates with a-haloimines is their reaction with amides. In this case formation of the carbene complex requires treatment with a silyl chloride (Figure 2.8) [42,125,126].
Fig. 2.8. Generation of aminocarbene complexes from metallates and carboxamides.
2.1 Generution of Heteroatom-Substituted Curbene Complexes
23
Table 2.5. Heteroatom-substituted carbene complexes prepared from chloroiminium salts. Starting Material
Reagents, Conditions
Product
Yield
Ref.
As illustrated by the examples given in Table 2.6, this methodology is well suited for converting elaborate organic carboxamides into the corresponding carbene complexes.
Experimental Procedure 2.1.2. Preparation of a Chromium Carbene Complex from a Carboxamide: { [(4)-2,2-Dimethyl-4-phenyloxazolidin-3-yl]methylene} pentacarbonylchromium [1261 1. Na,[Cr(CO),]
Ph 206.26
Ph 381.31
Disodium pentacarbonylchromate is prepared by adding a solution of sodium naphthalenide (88 mmol) in THF (160 mL) to chromium hexacarbonyl (40 mmol) in THF (240A). The resulting solution of disodium pentacarbonylchromate is transferred to a 500-mL graduated cylinder (to estimate the concentration). This solution can be stored at -20 "C under argon for at least two months.
24
2 Heteroatom-Substituted Carbene Complexes
Table 2.6. Heteroatom-substituted carbene complexes prepared from carboxamides. Starting Material
Reagents, Conditions
Product
Yield
Ref.
59%
[40]
fPh
(co)5crYN'F 2
3
4
PhArro
&: H
.(.ZN0 H :
PhANYCr(C0'5 I 35%
U
[I271
2. I Generation of Heteroatom-Substituted Curbene Comp1exc.s
25
2.1.5 From Vinylidene Complexes Electrophilic vinylidene complexes, which can be easily generated by a number of different methods [ 1281, can react with non-carbon nucleophiles to yield carbene complexes (Figure 2.9; for reactions with carbon nucleophiles, see Section 3.1).
Fig. 2.9. Generation of carbene complexes from vinylidene complexes.
L,M=C H
2.1.5.1 From Vinylidene Complexes Generated from Alkynes Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the acarbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [ 129 -1351. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [ 136- 1411. Vinylidene complexes can further undergo [2 + 21 cycloadditions with imines, forming azetidin-2-ylidene complexes [ 142,1431. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [ 1431 (Table 2.7).
2.1.5.2 From Vinylidene Complexes Generated from Alkynyl Complexes Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.1 I , Table 2.7, Entries 11, 12) [ 134,1441. If a given vinylidene complex is not sufficiently electrophilic, protonation at Cp can promote nucleophilic addition at C, by intermediate formation of an electrophilic carbyne complex [89] (Figure 2.9, Section 2.1.8). A further method for the preparation of electrophilic vinylidene complexes consists in the dehydratization of acyl complexes [145] (Section 3.1.2).
26
2 Heteroatom-Substituted Carbene Complexes
Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes. Starting Material
Reagents, Conditions
Product
1
Yield
Ref.
36%
[146]
61%
[I341
60%
[I341
50%
[47]
H
Me C
2
HSiMe3
\6
CI-Ru-CI I
NaPF,, MeOH
Me3P Me C
3
4
\6
CI-Ru-CI I Me3p
OC-Fe-OH, cp;
Od
5
NaPF,, MeOH
1'
BF,-
OC F -,e
CH,CI,, MeOH; then MeOTf
Ph, /Ph P, ,NCMe Phl 'Ph E
oc
100% [147] /
P
O
CH,CI,
H
Ph/ 'Ph E
E: C0,Et
6
85%
[I481
7
51%
[I491
2. I Generation
of
Heteroatom-Substituted Carbene Complexes
27
Table 2.7. continued. Starting Material cp\ OC--Mn=C=CH,
8
I
oc
Reagents, Conditions
Ph*KPh
Product
OC3Mn&Ph
Ref.
75%
[I431
72%
[l50]
53%
[I431
68%
[89]
96%
[I511
y
od
-78 "C, 30 min
Yield
I
Ph NfiS
9
L-J
Et0,C'
82 "C, DCE, 2 h
Et0,C
(two diastereomers 8: 1) Ph .
10
'
cp\ OC -Re=C =CH2 I
oc
11
12
CP', OC-MnI Ph,P
' Ph
-50 OC, 1 h
xo
BF,Et,O; then MeOH
OC-Mn Ph,P
I
PhCHO, Et,O, BF,Et,O, -40 "C Cp(CO),W
2.1.6 From Carbenes and Carbenoids Diaminocarbene complexes were reported as early as 1968 [ 1.521. Preparation and applications of such complexes have been reviewed [ 1531. Because of n-electron donation by both nitrogen atoms, diaminocarbenes are very weak n-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [ 18,1531. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3).
28
2 Heteroatom-Substituted Carbene Complexes
ROH
-
/--
LnM
OR
L"M
e
R
L,M=C R
I
LnM*R
Fig. 2.10. Generation of heteroatom-substituted carbene complexes from alkynes.
Fig. 2.11. Conversion of alkynyl complexes into heteroatom-substituted carbene complexes (E+: electrophile).
Fig. 2.12. Strategies for the preparation of cyclic diaminocarbene complexes.
2. I Geizerution of' Heteroatom-Substituted Curbene Complexes
29
Besides the addition of non-carbon nucleophiles to carbonyl and isonitrile complexes (Tables 2.2, 2.4), heteroatom-disubstituted carbene complexes can be prepared by direct addition of stable or latent carbenes to suitable complexes. The synthetic routes sketched in Figure 2.12 are those commonly used for preparing imidazoline-2-ylidene or imidazolidine-2-ylidene complexes. The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154- 1591. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [ 1601. Stable, uncomplexed imidazoline2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [ 162- 166]), are also convenient starting materials for the preparation of carbene complexes [ 167,1681. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169- 1711. Examples of such preparations are given in Table 2.8.
2.1.7 From Alkyl Complexes by a-Abstraction A further general route to heteroatom-substituted carbene complexes is based on the a-abstraction of nucleophiles from alkyl Complexes (electrophilic abstraction; Figure 2.13). Fig. 2.13. Generation of
heteroatom-substituted carbene complexes by a-abstraction.
- x-
LnM<
L,M-
X = H, OR, halogen
J
Y = NR,, OR, halogen
This methodology has proven particularly useful for preparing non-heteroatomstabilized carbene complexes (Section 3.1.2), but is also suitable for certain types of heteroatom-substituted carbene complex [ 1771 (Table 2.9). Experimental Procedure 2.1.3. Preparation of an Iron Carbene Complex by a-Abstraction of Hydride: Dicarbonyl(q5-cyclopentadienyl)(phenylthiocarbene) iron hexafluorophosphate [ 1791
1 NalHg 2. PhSCH,CI
[Cp(CO),Fe],
363.93
oc CP-Le-/ Od 300.16
SPh
[Ph,CJ[PF,J +
OC CP--\Fe 444.12
30
2 Heteroatom-Substituted Curbene Complexes
Table 2.8. Preparation of heterocycloalkylidene complexes. Starting Material
+[$
Reagents, Conditions
Product
Yield
Ref.
42%
[172]
82%
[I751
65%
[I761
f
I HFe(CO),-
I 10 "C, 3 h
[N)=FeIcOl,
N
N
\
\
3
r:x:> O 'M'
Me0
3
[Rh(COD)CI],, 1 10 "C, PhMe, 2h
L O M e
C O M e
Ph
I
6
\OMe )=Rh(COD)Cl
[
p'>c10,-
N"
N LPh
Sodium amalgam is p sodium (2.0g, 87 mm inert gas (caution! ex0 temperature a solution THF (150 mL) is added an
[(CP')fiCl,lZ> NEf, 20 "C, THF, 3 h
9 N/NYRhc'(cp'' LN-Ph
lgam has cooled to room in dry, degassed room temperature
31
2. I Generation of Heteroatom-Sub.Ftituted Carbene Complexes
Table 2.9. Preparation of heteroatom-substituted carbene complexes by a-abstraction. Starting Material
1
COC\ PlFe--\ Ph,P
2
3
04
Yield
>80% [I781
1'
78%
[I791
81%
[I801
IPh,CI[PF,I
OC\ Cp-,FeT OC
BF,, 20 "C, C6H6
Ref.
1'
OC\ Cp-FeT PF,Ph,P OMe
S ' Ph
OC\ Cp- Fe-CF,
Product
[Ph3CI[PF61 OMe
OC\ Cp-Fe
Od
Reagents, Conditions
PF,-
SPh
OC, F1' Cp-Feq BF,Od F
for 3 h. The ferrate solution is decanted from the amalgam into a round-bottom flask, and freshly distilled chloromethyl phenyl sulfide (9.1 mL, 10.8g, 68.1 mmol) is added dropwise at 0 "C by means of a syringe over 10 min. The mixture is stirred at room temperature for 1 h and then concentrated to dryness under reduced pressure. Degassed dichloromethane (50 mL) and degassed hexane (100 mL) are added and the mixture is left to settle for 1 h. Filtration through neutral alumina (100 g), concentration, and column chromatography (silica gel, dichloromethanehexane 2: 1) yields 8.7 g (5 1%) Cp(CO),FeCH,SPh as bright yellow crystals, mp 66 "C. Triphenylmethyl hexafluorophosphate (1 1.3 g, 29.1 mmol) is added in one portion to a solution of Cp(CO),FeCH,SPh (8.7g, 29.0 mmol) in degassed dichloromethane (75 mL). The resulting dark green mixture is stirred for 5 min, and anhydrous diethyl ether (degassed) is added while stirring. Filtration, rapid washing with cold diethyl ether (3x 25 mL) and recrystallization from dichloromethane and diethyl ether gives 10.1 g (78%) of the title complex as golden-yellow crystals, mp 126 "C (dec.). The compound slowly abstracts hydride from diethyl ether, and is slightly air- and moisture-sensitive, but can be stored for a few months at 25 "C under nitrogen. 'H NMR (200 MHz, CD,Cl,) 6 5.64 (s, 5H), 7.62 (s, 5H), 15.52 (s, 1H). 13C NMR (75 MHz, CD,NO,) 6 90.40, 128.45, 131.52, 133.29, 142.31, 208.72, 317.35.
32
2 Heteroatom-Substituted Carbene Complexes
2.1.8 From Carbyne Complexes Similar to vinylidene complexes, carbyne complexes can also react with nucleophiles to yield heteroatom-substituted carbene complexes (Figure 2.14) [ 122,1811841, When imines are the nucleophiles used, the initially formed iminium intermediates can undergo intramolecular electrophilic alkylation of the other ligands (e.g. Entry 2, Table 2.10; see also [143]). In addition to this, carbyne complexes can also react with azides to give metallatriazoles [ 185,1861 (Entry 6, Table 2.10). Table 2.10. Heteroatom-substituted carbene complexes prepared from carbyne complexes. Starting Material
Reagents, Conditions
Product
NaOiPr
Yield
Ref.
69%
[187]
45%
[I881
63%
[189]
91%
[I901
bh
oc
Ph PheN, NEt3, 20 "C, CH,CI,, 1 h
oc
BudNCl, -50 "C, CH2C1,, 1 h
oc
4
oc
-20 "C, CH,CI,
oc
Cp-ReLToJ'BC1,\ I
>co
-20 "C, CHZCI,
oc
oc \
cp-wI
oc
N,-CO,Me 20 "C, pentane,
48 h
Cp-ke=(I
NF
OC
Cp T R e
oc
<Sc, oc
Meo2C7
\ /N--N
cp-w
Od
P
40%
[I901
85%
[I861
2. I Genercition of Heternatom-Substituted Curhene Complexes
33
Fig. 2.14. Conversion of electrophilic carbyne complexes into heteroatom-substituted carbene complexes.
2.1.9 Other Methods As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a nonheteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.1 1).
X = OR, NR,
Fig. 2.15. Preparation of heteroatom-substituted carbene complexes by olefin metathesis.
Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [ 191,1921. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.1 1). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. Photolysis of cationic alkoxycarbene iron complexes [ 1931 or alkoxycarbene manganese complexes [ 1941 has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [ 1951. Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [ 155,1961. The reaction of alkoxy(ary1)carbene iron complexes with two equivalents of an isonitrile leads to the formation of azetidin-2-ylidene complexes [ 1971, For other reactions of Fischer-type carbene complexes with isonitriles see [ 1981.
34
2 Heteroatom-Substituted Curbene Complexes
Table 2.11. Preparation of heteroatom-substituted carbene complexes by photolysis and olefin metathesis. Starting Material
Reagents, Conditions
Product CP,
,co
R
o
hv, C,H,, CO
O
CP,
hv, C6D6
M
Yield
Ref.
30%
[199]
48%
[200]
e
co
&,Me
dYzCP 45%
hv, 20 "C, CbD6, 3d
[201]
H
(cisltrans 78:22)
iNpoMe Cr(CO),
O-l
75%
[202]
65%
[125]
PhKOMe I 10 "C, PhMe,
6h
Ph (CO),Cr=( Ph
0x0
20 "C, heptane
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. Because many of the reactions involving heteroatom-substituted carbene complexes are cascade processes involving several different reaction types, they are
2.2 Synthetic Applicutions of Heteroatom-Substituted Curbene Complexes
35
difficult to organize according to reaction mechanism. In the following sections, therefore, the different synthetic applications of these complexes will be classified in terms of the type of product obtained. The focus will be on reactions which lead to metal-free products. Several reviews have appeared in which different aspects of the applications of heteroatom-substituted carbene complexes in organic synthesis are discussed [203 -2101.
2.2.1 General Considerations The heteroatom-substituted carbene complexes most frequently used in organic synthesis are carbonyl complexes of the type (CO),M=C(X)R (M: Cr, Mo, W; X: OR, NR,; R: H, alkyl, aryl, vinyl, alkynyl, etc.). To some extent such complexes behave as carboxylic esters or amides, the (CO),M=C group having electronic properties similar to those of a carbonyl group (Figure 2.16).
Fig. 2.16. Typical reactions of Fischer-type alkylcarbene complexes.
The carbene-bound alkyl groups are acidic { pK, [(CO),Cr=C(OMe)Me in H,O] 12.3) and can be easily deprotonated and alkylated [45,211,212] or acylated [213] (Figure 2.16). Stereoselective aldol-type additions can be realized with the aid of Fischer-type alkylcarbene complexes [214-2161. In these reactions the metallic fragment can either play the role of a ‘bulky carbonyl group’ or stabilize a given conformation of the substrate by chelate formation [2 16,2171. As in carboxylic esters it is possible to substitute alkoxy groups of Fischertype carbene complexes by non-carbon nucleophiles, such as other alcohols [73,214,218], enols [219], aliphatic amines [43,64,66,220-2241, aniline [79], imines [225], or pyrroles [226]. Strong nucleophiles can also lead to a dealkylation of methoxy-substituted carbene complexes (S,2 at the methyl group, [227]), in the same way as methyl esters can be cleaved by nucleophiles such as iodide. Carbon
36
2 Heteroatom-Substituted Curhene Complexes
nucleophiles can also attack the carbene carbon atom of Fischer-type carbene complexes to give a non-heteroatom-substituted carbene complex by substitution of the heteroatom-bearing group (Figure 2.19; see also Section 3.1). Heteroatom-substituted vinylcarbene complexes are easily prepared by aldol condensation of aldehydes with alkylcarbene complexes [228]. The latter also react readily with imidates to yield either (2-aminoviny1)- or (2-a1koxyvinyl)carbene complexes [229]. The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a$-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [ 15I], additions of nucleophilic radicals [2311, 1,3-dipolar cycloadditions [232,233], inter[234-24 1] or intramolecular 1220,2421Diels-Alder reactions, as well as SimmonsSmith- [243], sulfur ylide- [244] or diazomethane-mediated [ 1511 cyclopropanations of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents can lead via conjugate addition to substituted 1,4cyclohexadien-6-ylidene complexes 12451. Also alkynylcarbene complexes can react as Michael acceptors with nucleophiles, forming 1,3-dien-l-ylcarbene complexes (Figure 2.17). Both carbon nucleophiles, such as, e.g., enamines [246-2491, and non-carbon nucleophiles, such as imidates [250], amines [64,131,2511, aliphatic alcohols [48,79,252], phenols 12521, and thiols [252] can add to the C-C triple bond of alkynylcarbene complexes. Further reactions of the C-C triple bond of alkynylcarbene complexes include 1,3-dipolar [253,254], Diels-Alder [64,234,238,255-2581 and 12 + 21 cycloadditions [259 -2611, intramolecular Pauson-Khand reactions [43,262], and C-metallation of ethynylcarbene complexes [263]. Because of the strong electron-withdrawing effect of the M(CO), fragment, reactions of nucleophiles, dienes or 1,3-dipoles with vinylcarbene or alkynylcarbene complexes are usually faster than with the corresponding a$-unsaturated esters [242,253,264]. In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis 1206,265-2671 (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electronrich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis 1268-2721 and modification [ 103,140,148,2731of sensitive natural products. Heteroatom-disubstituted carbenes form stable complexes with transition metals; these have not yet found broad application in organic synthesis. The use of these complexes as catalysts for various transformations is being investigated [ 153,175,274,2751.
2.2 Synthetic Applications of Heteroatom-Substitirterl Carbene Complexes OMe
OMe Nu
37
OMe
OMe R
OMe R
Fig. 2.17. Typical reactions of Fischer-type vinyl- and alkynylcarbene carbene complexes [e.g. L,: (CO),Cr].
2.2.2 Demetallation and Formation of Acyclic Products Heteroatom-substituted carbene complexes are normally rather stable compounds. Some reaction conditions, however, lead to selective demetallation of these complexes. The organic fragment can thereby be converted into different types of product (Figure 2.18). Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(pheny1)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 OC, decompose at room temperature to yield acyl radicals [277]. Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [ 150,2531. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [ 1411. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatomsubstituted carbene complexes to non-heteroatom-substituted carbene complexes
38
2 Heternatom-Substituted Curbene Complexes
k
OMe LnMH
0
LnM
+
MeOH Me0
Fig. 2.18. Typical products resulting from heteroatom-substituted carbene complexes upon thermolysis, oxidation, or reduction.
\ - MeO-
Fig. 2.19. Typical reactions of Fischer-type carbene complexes with carbanions and ylides.
is discussed in Section 3.1. Carbon nucleophiles generally react with Fischertype carbene complexes as sketched in Figure 2.19. Organolithium compounds or enolates can replace the heteroatom-bearing group, forming non-heteroatomsubstituted carbene complexes [279]. Alternatively the metallic fragment can be displaced to give enol ethers. The anionic intermediates formed from Fischer-type carbene complexes and alkynyllithium compounds are stable at -78 "C and react with aldehydes and imines to yield five-membered heterocycles [280]. Ylides, such as diazoalkanes or phosphorus ylides, can also substitute the metallic fragment of heteroatom-substituted carbene complexes. Of great synthetic potential are demetallations with simultaneous formation of CSn bonds (Figure 2.20) [ 137,146,230,281,2821. These reactions presumably proceed via a heterobimetallic intermediate containing an Sn-M-C group. Reductive elimination of the metal M from this intermediate leads to formation of the carbontin bond. The resulting alkyl- or vinylstannanes are valuable synthetic intermediates,
39
2.2 Synthetic Applications of Hetermtom-Substituted Curbene Complexes
Table 2.12. Demetallation of heteroatom-substituted carbene complexes with simultaneous C-H, C-N, and C-0 bond formation. Starting Material
Reagents, Conditions
OMe 1
(CO),Cr+ CH3
w(c0)3cqt BF30H-
Ph*
H,O/MeCN 1 : I , KOH (pH 12), t, 21 10 s (25 "C)
Product MeOH, MeCHO, (CO),Cr(OH)-
-40 2 eq. "C,NaBH,, MeOH, CH,C12,2h
OR
Yield
Ref.
-
(2831 [284]
76%
[I511
76%
[243]
80%
(1511
67%
(2151
QR
pyridine N-oxide, 25 "C, THF, 14 h
5
(CO),Cr
(R: 8-phenylmenthyl)
'
8
BF30H-
Ph*
(CO),Cr Q A P h
H,O, air, 23 "C, CH2CI,,24h
DMSO, 25 "C, 16h
Ph%
Qa Ph
0
40
2 Heteroatom-Substituted Curbene Complexes
Table 2.12. continued, Starting Material
Reagents, Conditions
OMe
Product
Yield
Ref.
96%
[253]
OMe
Ph
Ph CGO),
10
y"
DMSO, 25 "C, Id
O *'M.
Ph
% ' Me
yN
Ph
E10,C
Et0,C
hv, MeCN, 2.5 h; then 0,
Fig. 2.20. Reaction of heteroatom-substituted carbene complexes with nucleophilic and electrophilic tin derivatives.
2.2 Synthetic Applications
of
Heteroatom-Substituted Cnrbene Comple.res
41
Fig. 2.21. Photolytic generation and synthetic applications of chromium ketene complexes.
e.g. as precursors for C-radicals or organolithium compounds. Illustrative examples of some of the demetallation reactions discussed above are given in Tables 2.12 and 2.13.
2.2.3 Photochemical Transformations A remarkable reaction, discovered by McGuire and Hegedus in 1982 [292], is the photochemical conversion of heteroatom-substituted chromium and molybdenum carbene complexes into intermediates with ketene-like character (Figure 2.2 1). This reaction has been reviewed by Hegedus [203]. Photolytically generated chromium ketene complexes can acylate nucleophiles, such as alcohols [293], amines [294,295], phosphorus and sulfur ylides [296], and even arenes [73]. In addition, similar to ketenes, these intermediates undergo [2 + 21 cycloadditions with alkenes [ l o l l , imines [297,298] or aldehydes under mild conditions to yield the same type of four-membered cyclic products as would be obtained from alkoxy- or aminoketenes. Many of these reactions have also been performed intramolecularly, leading to polycyclic structures. A valuable aspect of these ketene complexes is the high diastereoselectivity with which acylations or cycloadditions sometimes occur. With the aid of chiral auxiliaries high asymmetric induction can be achieved at the newly formed stereogenic centers. By utilizing this methodology enantiomerically enriched a-amino acid derivatives [205] and other interesting molecules have been prepared, both in solution [74,299,300] and on solid support [301] (Tables 2.14, 2.15).
42
2 Heteroatom-Substituted Carbene Complexes
Table 2.13. Demetallation of heteroatom-substituted carbene complexes with simultaneous C-C and C-Sn bond formation. Starting Material
Reagents, Conditions
Product
CH2N2,5 "C, Et,O, < 1 min
H2C=(OM,
Yield
Ref.
93%
[287]
71%
[243]
90%
[276]
CH3
CH,Br,, LDA, -78 "C, THF
Br&
Ph
80 "C, C,H,, 2 h Ph
H
& -fpco') H
Bu,SnOTf, NEt,, Et20, 25 "C
/Y0-YSnBu3 83%
[146]
81%
[288]
75%
[289]
62%
[290]
77%
[291]
H
Bu,SnH, pyridine, 60 "C, hexane, 8 h (86% de)
CICHJ, MeLi; then H,O
(1.2 PS eq.), n B80u "C, ,
PhMe, 1.5 h
PhLSnBu3
(only Z isomer) OTBS
100 "C, C,H,
I
OMe
2.2 Synthetic Applications of Heteroatom-Substituted Curbene Complexes
43
Table 2.14. Reactions of photochemically activated chromium carbene complexes with nucleophiles. Starting Material
Reagents, Conditions
Product
Ph,P+CO,Bn
2 ( c 0 ) 5 c r 5
hv, C,H,
Yield
Ref.
60%
[302]
82%
12991
71%
[294]
70%
[303]
CO
MeO,C-CO,tBu (> 93% de)
(64% de) Ph
Ph
H,NXCO,Me hv, 0 "C, THF,
5
co
0 Ph
Ph
+uxCO2Me
phyy (> 95% de)
6
( C O Y r 3 (OMe
&J~~ p
hv, CO, ZnC1,
i
Ph
e
0
THF
hv, 25 "C, MeOH, 5 d
Meo&
I
I
C02Me
44
2 Heteroutom-Substituted Curhene Complexes
Table 2.15. Cycloaddition reactions of photochemically activated chromium carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
71%
[304]
OMe BnO
CH,
hv, 70 "C, CO, CH,CI,; then H,, Pd OMe
C0,Me
(single diastereomer) Me0 R
Ph
(co),c~+
2 0
R (R:octyl),
hv, 25 "C, CO, CH,CI,
84%
0
[I011
W I 0 (> 97% de) Me0
OH
2.2 Synthetic Applications of Heteroatom-Substituted Curbene Complexes
45
Experimental Procedure 2.2. I. Photolysis of a Chromium Carbene Complex: 2-Benzyl-4-benzyloxy-4-methyl-2,3,4,4a,7,7a-hexahy~o1H-cyclopenta[c]pyridin-3-one [294]
(CO),Cr<
O 7P h
326.23
+
YJ/$ 185.27
hv, CO
GTph OBn
347.46
A dried Pyrex pressure tube is charged under argon with [methyl(benzyloxy)carbene]pentacarbonylchromium (209 mg, 0.64 mmol) and a solution of 2-benzyl2-azabicyclo[2.2.1]hept-5-ene(59 mg, 0.32 mmol) in dry THF (5 I&). The tube is purged three times with carbon monoxide, placed under carbon monoxide (approx. 5 atm), and photolyzed with a 450-W medium-pressure mercury lamp (Conrad Hanovia 7825) for 15 h. Concentration and radial chromatography (hexane/ethyl acetate 6: 1) yields 56 mg (51%) of the title compound as a clear oil. I3C NMR (75 MHz, CDC1,) 6 18.4, 33.2, 39.3, 50.4, 51.5, 55.6, 66.1, 80.1, 127.5, 127.6, 127.7, 128.3, 128.4, 128.8, 128.9, 133.7, 137.4, 138.2, 170.7.
2.2.4 Cyclopropanation Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptorsubstituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. Several reaction sequences have been reported in which Fischer-type carbene complexes are converted in situ into non-heteroatom-substituted carbene complexes, which then cyclopropanate simple olefins [306,307] (Figure 2.22). This can, for instance, be achieved by treating the carbene complexes with dihydropyridines, forming (isolable) pyridinium ylides. These decompose thermally to yield pyridine and highly electrophilic, non-heteroatom-substituted carbene complexes (Figure 2.22) [46].
46
2 Heteroutom-Substituted Curbene Complexes
1
Fig. 2.22. Intramolecular cyclopropanation reactions with in-situ-generated alkylidene complexes [46].
Fig. 2.23. Cyclization of enynes with electrophilic and nucleophilic carbene complexes.
Closely related to the ring-closing metathesis of enynes (Section 3.2.5.6), catalyzed by non-heteroatom-substituted carbene complexes, is the reaction of stoichiometric amounts of Fischer-type carbene complexes with enynes [266,308 -3151 (for catalytic reactions, see [316]). In this reaction [2 + 21 cycloaddition of the carbene complex and the alkyne followed by [2 + 21 cycloreversion leads to the intermediate formation of a non-heteroatom-substituted, electrophilic carbene complex. This intermediate, unlike the corresponding nucleophilic carbene
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
47
Table 2.16. Cyclopropanations with stoichiometric amounts of heteroatom-substituted carbene complexes. Starting Material
Fc
Reagents, Conditions
Product
I-hexene, BHT, 152 "C, DMF, 0.5 h
Yield
Ref.
88%
[319]
90%
[320]
57%
[226]
76%
[321]
74%
[322]
74%
(3231
91%
[266]
89%
[266]
Fc
(97% de)
(Fc: ferrocenyl) 100 "C, PhMe,
2h
@CO,Me
3
100 "C, BHT, THF. 5 h
Me0,C
Ph
Et 100 "C, THF, 1 h
" 'BuO f l E t
&CO,Et
65 "C, THF, 1 h
Me0 (cisltrans 26:74)
Bu
(cO),Mo% OMe
Ar
100 "C, THF, air; then H,O, HCI
H
OEt
(Co),Cr=(
70 "C, MeCN, 4 h; then hydrolysis /
-
OEt
-
(CO)&r=(
70 "C, MeCN, 4 h; then hydrolysis
Ts-N
4 (4852)
48
2 Heteroatom-Substituted Carbene Complexes
Table 2.16. continued. Starting Material
9
r
Reagents, Conditions
(co),Cr=(
Ts-N$
then hydrolysis OEt
///
Ref.
71%
[266]
40%
[266)
51%
[324]
51%
[71]
.O
70 "C, MeCN, 4 h;
Ts-
Yield
OEt
Ts- s C O z M e
10
Product
(co),Cr<
+C0,Me
Ts-N
70 OC, MeCN, 4 h; then hydrolysis
11
w
complexes, can undergo an intramolecular cyclopropanation reaction (Figure 2.23). Intermolecular variants of this cyclopropanation reaction have also been reported [88,317,318]. The reaction of enynes with Fischer-type carbene complexes can also lead to the formation of cyclobutanones (Figure 2.23) [ 3 151. The mechanism for this reaction is likely to be rearrangement of the intermediate, non-heteroatom-substituted vinylcarbene complex to a vinylketene, which undergoes intramolecular [ 2 + 21 cycloaddition to form the observed cyclobutanones.
51
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
Fig. 2.25. Alternative mechanism for the Dotz benzannulation reaction [329,330].
OTBS
I
1.5 eq. Ac,O 50 OC,heptane. 47 h
*
+
35%
EtO
I
(2.9 eq.)
MeO-0
OMe OTBS Me0
fredericamycin A (Boger, 1995)
Fig. 2.26. Synthesis of fredericamycin A utilizing a Dotz benzannulation reaction [268].
52
2 Heteroutom-Substituted Curbene Complexes
Aryl(dialky1amino)carbene chromium complexes do not yield aminonaphthols upon treatment with alkynes, but form indene derivatives. Vinyl(dialky1amino)carbene complexes, however, react with alkynes to yield aminophenols as the main products if solvents of low nucleophilicity are used [335]. (2-Amino-lviny1)carbene complexes do not undergo benzannulation when treated with alkynes, but form cyclopentadienes or heterocycles instead [2511. The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(a1koxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alky1)carbene complexes with alkynes [339]. Despite the many limitations, the Dotz benzannulation remains a powerful tool for the preparation of substituted phenols. One example of the use of a Dotz benzannulation as the key step in a synthesis of the potent natural antibiotic fredericamycin A (as racemate) is sketched in Figure 2.26. Additional examples of Dotz benzannulation reactions are given in Table 2.17. Further examples have been reported (see, e.g., [269,270,340-3441).
2.2 Synthetic Applications of Heteroatom-Substituted Curbene Complexes
53
Table 2.17. Examples of the preparation of substituted phenols by use of the Dotz benzannulation reaction. Starting Material
&
Reagents, Conditions
MeCN,82"C, 40 min
Product
Yield
Ref.
63%
[345]
30%
[84]
60%
[85]
93%
[331]
69%
[346]
66%
[335]
(R: 2,4,6-Me,C6H,) OTlPS
TIPSO
""y
3-hexyne, 50 "C, THF, 6 h /
TIPSO
OH
TIPSO
TIPSO
(+ 40% of Cr(CO), complex)
l-pentyne,6O0C, THF, 42 h boc
Boc
6Me
Me
DMAD, THF
C0,Me C0,Me
Me0 OH
T S i
M e ,
90 "C, PhMe,
5 min; then CAN, HNO, SiMe,
0
23
1-pentyne, 80 "C, heptane, 41 h
(c0'5c
NMe,
54
2 Heteroatom-Substituted Carbene Complexes
Table 2.17. continued. Starting Material
Reagents, Conditions
Product
1 -pentyne, 50 "C,
THF, 24 h OMe
1-pentyne,
TBSOTf, 2,6-lutidine, 50 "C, CH2C12
302.16
1.9 Hz, 1H).
Yield
Ref.
80%
[336]
2.2 Synthetic Applicutions of Heteroutom-Substituted Curhene Complexes
55
2.2.5.2 Other Thermal Benzannulations In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dotz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocyclizations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 + 21 cycloreversion) and pyrrolidine to yield a substituted benzene. Table 2.18. Preparation of substituted benzenes with heteroatom-substituted carbene complexes. Starting Material
Reagents, Conditions
Product
OEt
TS-
&
Yield
Ref.
FSiMe3 (CO),Cr=(
b
Ts-N
67 "C, THF; then hydrolysis
TBS
TBS
90 OC,C,D,, 18 h;
then SiO,
[348]
63%
[349]
/
3
?
84%
56
2 Heteroatom-Substituted Curbene Complexes
OMe benzannulation
-
Fig. 2.27. Generation and possible transformations of ( 1,3-butadien- 1 -yl)carbene complexes.
2.2.6 Formation of Five-Membered Rings Five-membered carbo- or heterocycles can be prepared with the aid of heteroatomsubstituted carbene complexes in several different ways. In the following sections the focus will be on cyclization reactions in which the carbon-metal double bond plays a decisive role.
2.2.6.1 Cyclization of (1,3-Butadien-l-yl)carbeneComplexes In most of the reactions of heteroatom-substituted carbene complexes with alkynes the first event is insertion of the alkyne into the carbon-metal double bond. If vinylcarbene complexes undergo insertion reactions with alkynes, ( 1,3-butadien-1y1)carbene complexes result (Figure 2.27).
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
57
Depending on the types of substituents and the precise reaction conditions (1,3-butadien- 1-yl)carbene complexes can undergo direct cyclization to yield cyclopentadienes [337,350]. As mentioned in Section 2.2.5.1, cyclopentadiene formation occurs particularly easily with aminocarbene complexes [35 I]. Alternatively, in particular at higher reaction temperatures, CO-insertion can lead to the formation of a vinylketene complex, which, again depending on the electronic properties of the substituents and the reaction conditions, can cyclize to yield cyclobutenones, furans [91,352], cyclopentenones, furanones 1911, or phenols (Dotz benzannulation) (207,25 1,3531. For the formation of cyclopentadienes from ( 1,3-butadien- 1-yl)carbene complexes several mechanisms can be considered (Figure 2.28). The precise substitution pattern of the carbene complex, which has a decisive influence on the stability of the different potential intermediates, will determine the mechanism in each case.
R
reductive elimination
Fig. 2.28. Possible mechanisms for the formation of cyclopentadienes from ( 1,3-butadien1 -yl)carbene complexes [333,354].
One possible mechanism is electrophilic attack of the complexed carbene carbon atom at the terminal carbon of the diene. The resulting zwitterionic intermediate can now eliminate the metallic group (CO),M directly, or, alternatively, the metallic group can migrate to yield a new, more stable zwitterion (stabilization of the ally1 cation by the heteroatom X). A further possible mechanism is the concerted electrocyclization of the metalla1,3,5-triene to yield an intermediate metallacyclohexadiene 13331. Reductive elimination of (CO),M would also lead to the formation of cyclopentadienes. Two examples of cyclopentannulation reactions, which most likely proceed via an ionic mechanism, are sketched in Figure 2.29. In the first example both the
58
2 Heteroutom-Substituted Curbene Complexes Ph
I
(+ other stereoisomers, total yield of dimes: 77%)
+
W(CO),
co,
- W(CO), *
Et2N*oEt
(Aumann, 1991,1996)
(Barluenga, 1995)
Fig. 2.29. Examples of diastereoselective cyclopentannulations of ( 1,3-butadien- l-y1)carbene complexes [191,353-3551.
metallatriene and a zwitterionic intermediate were isolated and characterized by X-ray crystallography. In the second example the stereochemical outcome of the reaction can also be rationalized by an ionic mechanism. Although the authors proposed that this [355] and similar reactions [258] to proceed via a tungstacyclohexadiene, such an intermediate would not account for the diastereoselectivity observed. An ionic mechanism however, as shown in Figure 2.29, could explain the stereochemical outcome of this reaction: steric repulsion between the methyl group at the 1,3-cyclohexadiene and the (CO),W fragment is likely to favor a conformation in which the cyclohexadiene-bound methyl group and the (CO),W fragment are located on opposite sides of the cyclohexadiene ring. Steric repulsion between the phenyl group and the (CO),W group will tilt the phenyl group in such a way that the distance between the 'upper' edge (see Figure 2.29) of the phenyl
2.2 Synthetic Applications
Heteroatom-Substituted Curbene Complexes
of
59
group and the carbene carbon atom becomes shorter than the distance to the ‘lower’ edge of the phenyl group (which is located on the same side of the cyclohexadiene ring as the (CO),W fragment). If electrocyclization now occurred the tungsten atom would attack the phenyl group at the carbon which is closest to it, i.e. at the ‘lower’ edge. This would lead to the formation of the not-observed diastereomer (if the reductive elimination of (CO),W from the putative tungstacyclohexadiene takes place with retention of configuration). If, instead of electrocyclization, electrophilic attack of the closer ‘upper’ edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (CO),W would yield a l-methoxy1,3-~yclopentadiene.Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. Generally, ionic mechanisms will be favored for reactions of heteroatomsubstituted dienylcarbene complexes, because of the stabiblization of charges by lone pairs. Further illustrative examples of the synthesis of five-membered carbo- or heterocycles by intermediate formation of vinylketene- or butadienylcarbene complexes are listed in Table 2.19. Cyclopentannulation with a Tungsten Alkynyl-
y-3-phenyl-5,9b-dihydro-4H-cyclopenta[a]naphine [353]
/
482.10
199.30
\
I\ OEt
357.50
A solution of 1-(3,4-dihydronaphthalen-1-yl)pyrrolidine (99 mg, 0.50 mmol) in dry dichloromethane (0.5 mL) is added to a solution of [ethoxy(2phenylethynyl)carbene]pentacarbonyltungsten (24 1 mg, 0.50 mmol) in dry dichloromethane (0.5 mL), and the resulting mixture is shaken at 25 “C for 5 min. Column chromatography (silica gel, gradien tane/dichloromethane and then with pentanekthyl acetate) of the title compound as a pale yellow oil. lH NMR (300 MHz, CDCl,) 6 1.48 (t, 6.9 Hz,3H), 1.67 (m, 4H), 2.41 (m, 2H), 2.96 (m, 2H), 2.74 (m, lH), 2.83 (m, lH), 2.92 (m, lH),3.52 (m, lH), 4.00 (m, 2H), 5.46 (s, lH), 7.16 (m, 4H), 7.33 (m, 4H), 7.80 (m, 1H).
60
2 Heteroutom-Substituted Curbene Complexes
Table 2.19. Synthesis of five-membered carbo- or heterocycles from heteroatom-substituted carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
75%
(3561
60%
(3561
62%
[258]
2 equivalents
-%/ 125 OC,C,H,
0
2 equivalents
% 90 "C, C,H,, 4 h
Me,SiO
Y 3
60 "C, THF, 18 h
- C0,Me THF
0
AoEt 80%
[331]
1Bt1
55 "C, THF, 16 h
97% [359]
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
61
Table 2.19. continued. Starting Material
Reagents, Conditions
Product
Yield
Ref.
84%
[258]
92%
[258]
97%
[360]
94%
[353]
Ar: 4-MeOC6H,; 25 "C, THF, 15 h OSiMe,
AN/-f 25 "C, THF, 15 h
Ph-
10
65 "C, THF, MeCN, 14 h
w
EtO N ''
"D 25 "C, CH2C12, - 5 min
Experimental Procedure 2.2.6. Cyclopentannulation with a Molybdenum Arylcarbene Complex: 3-Hexyl-5-methyl- 1 -indanone [346]
370.17
110.20
230.35
A mixture of [4-tolyl(methoxy)carbene]pentacarbonylmolybdenum (300 mg, 0.81 mmol), toluene (12 mL) and 1-octyne (116 mg, 1.05 mmol, 1.3 eq.) is heated at 90°C for 5 min. Diethyl ether and ceric ammonium nitrate (20 mL of a 0.5 M solution in 0.1 M nitric acid) are added and the resulting mixture
62
2 Heteroatom-Substituted Curbene Complexes OMe
0
X = OH, OSiR,, NHZ, CHZ,
2 = electron-withdrawing
X
group
(CO),M
Fig. 2.30. Intramolecular acylation reactions with carbene-derived vinylketene complexes.
Table 2.20. Preparation of five-membered rings by cyclization of vinylketene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
77%
[362]
96%
[363]
74%
[364]
OEt
1
(CQ,C~=(
4/Jo
CozEt
Et0,C
NEt,; then HC1 0
2
70 "C, MeCN; then hydrolysis (cis/bans98:2)
3 BnO
)I,// w
OEt (co),cr+,
70 "C, MeCN; then hydrolysis
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
Ph-Ph 80 OC, C,H,
63
12 h D
70%
yellow solid, rnp 150 OC
Fig. 2.31. Stevens rearrangement of (Rud'er' 1992) chromium-carbenederived ammonium ylides
110 O C , PhMe, 12 h 81%
[3651.
I
M=Cr
R
M=Mo,W
i
R eMJ - fR 0
- Cr(CO), 0
OMe
0
Fig. 2.32. Possible mechanism of the formation of cyclopentenones and cycloheptadienones from alkoxy(cyc1opropyl)carbene complexes 13731.
2.2.6.2 Cyclization of Functionalized Carbene Complexes Vinylketene complexes, generated by treatment of heteroatom-substituted carbene complexes with alkynes, can react intramolecularly with different nucleophiles to yield cyclic compounds (Figure 2.30, Table 2.20). Four- to ten-membered rings have
64
2 Heteroatom-Substituted Carbene Complexes
Table 2.21. Preparation of five-membered, nitrogen-containing heterocycles by rearrangement of chromium-carbene-derived ammonium ylides. Starting Material
Reagents, Conditions
Product
Yield
Ref.
85%
(3671
Ph
0
+
PMP
3
hv, CO, Et,O
0
been prepared using this strategy [361]. Further examples for such cyclizations are given in Tables 2.23 and 2.24.
2.2.6.3 Rearrangement of Ammonium Ylides Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21.
2.2 Synthetic Applications of Heteroutom-Substituted Curbene Complexes
Hex OMe
65
flLi r
-
-78 OC, THF
Ph
OMe ( C O ) , W - - H e x Ph
BF,OEt, Li+
D
(Iwasawa, 1998)
Fig. 2.33. Generation of anionic propargyl tungsten complexes and conversion to pyrroles [280].
2.2.6.4 Other Methods The reaction of alkoxy(alky1)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). Five-membered carbocycles can also result from the reaction of alkoxy(cyc1opropy1)carbene complexes with alkynes [37 1,3721 (Entries 4-5, Table 2.22). The mechanism of this remarkable reaction has not yet been thoroughly investigated. One possible mechanism is sketched in Figure 2.32. Cycloheptenones are formed from cyclopropylcarbene complexes when the reaction with alkynes proceeds by a mechanism analogous to the Dotz benzannulation reaction. The formation of sevenmembered rings is in fact observed in the case of molybdenum (Entry 5 , Table 2.24) and tungsten [374] carbene complexes. The corresponding chromium complexes, however, undergo fragmentation with formation of ethylene and cyclopentenones. The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33; see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or N-sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33; Entry 6, Table 2.22).
66
2 Heteroatom-Substituted Curbene Complexes
Table 2.22. Preparation of five-membered carbo- and heterocycles with the aid of heteroatom-substituted carbene complexes. Starting Material
Reagents, Conditions
Product
OMe
OTBS
Yield C0,Me
(co),c~=(
Ref.
99% [376]
Ph
r+ 80 "C, DCE, 8 h
OTBS
OMe
2
(CO),Cr+
0
73% [370]
Ph
7 0 "C, THF, CO, 24 h
4
68% [373] OMe 100 "C, dioxane, 1% H,O, 6 h
110 "C, PhMe, 1% H20, 24 h
5
57% [377]
(CO)&r (cisltrans 6:94)
I. PhCCLi 2. PhCHO 3. NEt,, I, 4. MeOH
51%
n
[280]
Ph
2.2.7 Formation of Six-Membered, Non-Aromatic Carbocycles and Six-Membered Heterocycles In addition to the benzannulation reactions discussed in Section 2.2.5, other reactions of heteroatom-substituted carbene complexes are known which lead to the formation of six-membered rings. Alkoxycarbene complexes have a reactivity
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
67
similar to that of carboxylic esters. Several examples have been reported in which intramolecular nucleophilic attack at the carbene carbon atom of these complexes leads to six-membered rings. Carbon nucleophiles can lead to the formation of non-heteroatom-substituted carbene complexes, which can eventually undergo an elimination reaction to yield alkenes (Entry 1, Table 2.23). Intramolecular nucleophilic attack of the metal-bound carbon atom by an alcohol can yield heterocyclic carbene complexes, which can eliminate the metallic fragment and yield enol ethers (Entry 2, Table 2.23). Entry 3 in Table 2.23 is an example of the addition of a nucleophilic, carbon-centered radical to a vinylcarbene complex, forming a 2-pyranylidene complex. If the Dotz benzannulation reaction is conducted with ortho-disubstituted arylcarbene complexes, the final aromatization step is blocked and cyclohexadienones can be isolated (Figure 2.34) [356,378,379].
OMe
Fig. 2.34. Formation of cyclohexadienones from 2,2-disubstituted vinylcarbene complexes.
As illustrated by the examples in Table 2.23, this reaction enables the stereoselective generation of quaternary carbon atoms. The resulting cyclohexadienones can be chemically modified in several different ways, and hence are valuable synthetic intermediates. Intramolecular acylation reactions with ketene complexes, generated, for instance, by thermolysis or photolysis of carbene complexes, can also be used for the preparation of six-membered rings. Illustrative examples are shown in Table 2.23. Few examples of the preparation of six-membered heteroaromatic compounds using Fischer-type carbene complexes have been reported [224,25 1,38I]. One intriguing pyridine synthesis, reported by de Meijere, is sketched in Figure 2.35. In this sequence a (2-aminoviny1)carbene complex first rearranges to yield a complexed 1-azadiene, which undergoes intermolecular Diels-Alder reaction with phenylacetylene. Elimination of ethanol from the initially formed adduct leads to the final pyridine. A further useful reaction sequence, reported by Aumann [219], is based on the Diels-Alder reaction of 2-pyranylidene complexes with enamines (Figure 2.36). Retro-Diels-Alder reaction of the initially formed 3-oxabicyclo[2.2.2]octan2-ylidene complex leads to elimination of metal hexacarbonyl and formation of a substituted cyclohexadiene. Although this sequence can also be performed with the corresponding carbonyl compounds (2H-2-pyranones), these normally
68
2 Heteroatom-Substituted Cnrhene Complexes
Table 2.23. Preparation of six-membered, non-aromatic carbo- and heterocycles by use of heteroatom-substituted carbene complexes. Starting Material Me-Cp OCoc +<
Reagents, Conditions
Product
Yield
Ref.
79%
[380]
50%
[206]
62%
[231]
73%
[379]
78%
[363]
1. BuLi; then
2
Ph
2. PPh,
Q0 Ph
(cisltrans 9 1 :9)
q l rNaObll,$eOH, Ph.'''
(CO),Cr
(R: 8-phenylmenthyl)
9 #u..Ar
O P P h
(CO),W
1.5 eq. [CpzTiC1lz, -20 "C; then HCI
(co),w d
P
h
(cisltrans 58:42)
/
0
OTr
4 (CO),Cr
90 "C, heptane, 18h
OEt
70 "C, MeCN; then hydrolysis
OMe (84% de)
H
H
2.2 Synthetic Applications of Heteroatom-Substituted Carbene Complexes
OEt
(CO),Cr.N
Ph
5 u
69
fhm
h ]*[
I
H (mixture of stereoisomers) (de Meijere, 1993)
Fig. 2.35. Synthesis of substituted pyridines with the aid of Fischer-type alkynylcarbene complexes [382].
+
7=cr(c0)5
WCO),
CH,CI,, NEt,, 2OoC 4h
94% Ph'
/
n 20 OC, C6D6,5 h
-
(Aumann, 1996)
- Cr(CO),
Fig. 2.36. Preparation of cyclohexadienes by Diels-Alder complexes with enamines [219].
reaction of 2-pyranylidene
require high temperatures to undergo Diels-Alder reaction [383]. The cycloadditionkycloreversion with carbene complexes, however, takes place at room temperature. Heteroatom-substituted vinylcarbene complexes are also excellent dienophiles, which usually undergo Diels-Alder reactions under very mild reaction conditions (see Experimental Procedure 2.2.7). Unfortunately the outcome of reactions between Fischer-type vinylcarbene complexes and dienes is difficult to predict ([256]; compare, e.g., Experimental Procedures 2.2.7 and 2.2.9). The course of
70
2 Heteroatom-Substituted Carbene Complexes
these reactions depends strongly on the precise substitution pattern of both reactants and on the solvent. It is not yet possible to formulate reliable guidelines for these interesting reactions.
2.2.8 Formation of Seven-Membered Rings If 1,3-butadienes are cyclopropanated by use of vinylcarbene complexes, the divinylcyclopropanes which result can rearrange to cycloheptadienes [7 1,24 1,384 -3871 (Figure 2.37). Heteroatom-substituted vinylcarbene complexes react particularly well with donor-substituted 1,3-butadienes to yield the corresponding cycloheptadienes [264]. Some of these reactions proceed at room temperature. The intermediate divinylcyclopropanes have occasionally been isolated [264]. As indicated in Figure 2.37, for donor-substituted dienes in particular the formation of zwitterionic intermediates
2.2 Synthetic Applications of Heteroatom-Substituted Curbene Complexes
71
Fig. 2.37. Formation of cycloheptadienes by reaction of vinylcarbene complexes with 1,3-dienes.
should also be considered as viable alternative to transient divinylcyclopropane formation. 1- or 2-Aza- 1,3-butadienes also react with Fischer-type vinylcarbene complexes generating substituted dihydroazepines. Divinylaziridines have been proposed as intermediates in these interesting transformations [206,258]. In the reaction of vinyloximes with heteroatom-substituted carbene complexes one equivalent of carbene complex is required for reductive cleavage of the N-0 bond (Experimental Procedure 2.2.8). Non-heteroatom-substituted vinylcarbene complexes are readily available from alkynes and Fischer-type carbene complexes. These intermediates can undergo the inter- or intramolecular cyclopropanation reactions of non-activated alkenes. Cyclopropanation of 1,3-butadienes with these intermediates also leads to the formation of cycloheptadienes (Entry 4, Table 2.24). The formation of cycloheptadienones from alkoxy(cyclopropy1)carbene complexes and alkynes (Entry 5, Table 2.24) [388,389] proceeds essentially by the same mechanism as the Dotz benzannulation reaction (see Figure 2.32). The cyclopropyl group participates in the electrocyclic rearrangement as the equivalent of a vinyl group. Further methods for preparing seven-membered rings with the aid of heteroatomsubstituted carbene complexes include the intramolecular acylation reactions with ketene complexes discussed in Section 2.2.6.2. Experimental Proce re 2.2.8. [4 + 31 Cycloaddition of a Chromium Vinylcarbene Complex 1-Azadiene: truns-4-(2-Furyl)-2-methoxy-5-methyl-4,5dihydro-3H-azepine [3901
72
2 Heteroatom-Substituted Carbene Complexes
Table 2.24. Preparation of seven-membered rings by use of heteroatom-substituted carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
71%
[264]
0 1
Ph
(CO),Cr
20 "C, MeCN, 24 h; then HCI
then SiO,
tBuHN 91%
[390]
94%
[258]
65%
[388]
63%
[363]
84%
[364]
OMe
Ph
0
OSiMe,
OMe (co),Mo=( C0,Et
BU
70 "C, C,H,, 40 min
r> OMe
f
6
Ph Ph-Ph
Ph,P, 65 "C, THF. 3 h
OH
Me
\
70 "C, MeCN; then hydrolysis OEt
7
0
(co),Cr=( TsHN
70 "C, MeCN: then hydrolysis
m Ts I
2.2 Synthetic Applicutions of Heteroutom-Substituted Curbene Complexes
73
A solution of [2-(2-furyl)vinyl(methoxy)carbene]pentacarbonylchromium (656 mg, 2.0 mmol) and E-Zbutenal oxime (85 mg, 1.0 mmol) in THF (50 mL) is heated under reflux for 20 h. The mixture is left to cool to room temperature and silica gel (3 g) is added. After stirring for 3 h the mixture is filtered, concentrated, and the residue purified by column chromatography (silica gel, hexane/ethyl acetate/triethylamine 1O:l:l). 174 mg (85%) of the title compound is obtained as a yellowish oil. 'H NMR (300 MHz, CDCl,) 6 1.0 (d, 6.9 Hz, 3H), 2.4 (dd, 4.0, 13.2 Hz, lH), 2.55 (m, lH), 2.8 (dd, 6.4, 13.2 Hz, lH), 3.2 (m, lH), 3.7 (s, 3H), 5.2 (dd, 5.0, 8.2 Hz, lH), 6.1 (d, 3.0 Hz, lH), 6.3 (m, lH), 6.4 (dd, 2.2, 8.2 Hz, lH), 7.3 (m, 1H).
Experimental Procedure 2.2.9. [4 + 31 Cycloaddition of a Chromium Carbene Complex to a 2-Aminodiene: 6-(2-Furyl)bicyclo[5.4.0]undecan-2,4-dione[264]
u 213.33
328.20
248.31
The 2-aminodiene (0.21 g, 1.0 mmol) is added to a solution of the chromium carbene complex (0.33 g, 1.0 mmol) in acetonitrile (8 mL). The resulting mixture is stirred at room temperature for 3 h and concentrated under reduced pressure. The residue is dissolved in hexane and the solution is filtered and kept overnight at -20 "C, whereupon Cr(CO), and Cr(CO),(MeCN) crystallize. Decantation gives a clear solution which is concentrated under reduced pressure. THF (10 mL) and aqueous hydrochloric acid (3 N, 10 mL) are added to the residue and the resulting mixture is stirred at room temperature for 3 h. The mixture is extracted with diethyl ether (3 x 20 mL) and the combined extracts are washed with a saturated aqueous solution of NaHCO, (2 x 20 mL) and with brine (20 mL). Drying (sodium sulfate), concentration and purification of the crude product by column chromatography (silica gel, hexane/ethyl acetate 3: 1) yields 0.20g (81%) of the title compound. Colorless needles, mp 126-127 OC (hexane). 13C NMR (CDCl,) 6 21.6, 23.5, 26.1, 27.5, 38.5, 41.9, 42.7, 54.2, 61.9, 105.2, 110.1, 141.5, 156.0, 200.7, 201.1.
This Page Intentionally Left Blank
3 Non-Heteroatom-Substituted Carbene Complexes
This section deals with alkylidene complexes L,M=CR, and vinylidene complexes L,M=(C),=CR, in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. Because hydrogen, alkyl, or aryl groups can compensate only to a limited extent the electron deficit of the carbene carbon atom, it is mainly the metal and its ligands which provide stabilization in this type of carbene complex. For this reason the reactivity of these compounds depends mainly on the nature and oxidation state of the metal and on the electronic properties of the remaining ligands. Non-heteroatom-substituted carbene complexes of almost all transition metals are known. Depending on the oxidation state of the metal, the overall charge of the complex, and the properties of the additional ligands, the reactivity of alkyl or aryl carbene complexes can vary greatly. Some examples of compounds with strikingly different chemical properties are shown in Figure 3.1.
tx at -1 1
70 min electrophilic carbene cyclopropanates alkenes OC:
no decomposition at 80 OC
no decomposition at 145 O C
nucleophilic carbene olefinates carbonyl compounds
inert towards water or air catalyzes olefin metathesis in the presence of AICI,
Fig. 3.1. Non-heteroatom-substituted carbene complexes covering a broad range of different reactivities [391-3931.
76
3 Non-Heteroatom-SubstitutedCarbene Complexes
3.1 Generation of Non-Heteroatom-Substituted Carbene Complexes The impressive number of different reactivities of non-heteroatom-substituted carbene complexes parallels the many possibilities for their preparation. The most important synthetic approaches are sketched in Figure 3.2.
From ylides
L,M
+
R
a-Abstraction
AR 1
From carbyne complexes
1
Electrophilicaddition to alkenyl or alkynyl complexes
R+
From alkynes or cyclopropenes
[2 + 21 Cycloreversion
Fig. 3.2. Synthetic approaches for the generation of non-heteroatom-substituted carbene complexes.
Many non-heteroatom-substituted carbene complexes have been prepared from alkyl complexes by a-abstraction, for which two mechanistically different pathways must be considered (Figure 3.3): (a) abstraction of an electrophilic group X+ (nucleophilic abstraction), with the electron pair which formed the C-X bond remaining in the newly formed carbene complex (e.g. a-deprotonation, a-desilylation, etc.), or
3.1 Generution of Non-Heteroutom-SubstitutedCurbene Complexes
- x+
- x-
abstraction of an electrophile
abstraction of a nucleophile
77
D
t
X+ = H', SiR,'
1-
X- = H-, RO-, SR,, halide, N, L"y+R X R
-
Lxj
reductive elimination
Fig. 3.3. Possible mechanisms of carbon-metal double-bond formation by a-abstraction.
(b) abstraction of a nucleophilic group X- (electrophilic abstraction), with the electron pair which formed the C-X bond remaining on X- (e.g. a-abstraction of hydride, halides, alkoxides, thioethers, N,, etc.). In complexes in which the metal has free coordination sites a-abstractions can also proceed via 1,l-insertion of the metal M into the C-X bond (intramolecular oxidative addition) [394,395]. Such insertion can be followed by reductive elimination of LX (Figure 3.3). Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc.; Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1 S ) , and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). A further way of generating carbon-metal double bonds is based on the [2 + 21 cycloreversion of metallacyclobutanes. This method has proven particularly useful for the generation of synthetically valuable titanium and zirconium carbene complexes (Section 3.1.7).
78
3 Non-Heteroatom-SubstitutedCurbene Complexes
3.1.1 a-Abstraction of Electrophiles (Nucleophilic Abstraction) The first non-heteroatom-substituted carbene complex was prepared by Schrock in 1974 [392] (Figure 3.4). Treatment of tris(neopenty1)tantalum dichloride with neopentyllithium led to the formation of neopentane and (2,2-dimethyl- 1propylidene)tris(neopentyl)tantalum. This carbene complex reacts violently with water or oxygen, but can be sublimed (80°C) and stored indefinitely at room temperature under argon. 2 Me,CCH,Li 20 OC, pentane, 1 h
- CMe,, - 2 LiCl D
100%
orange solid, rnp 71 'C (Schrock, 1974)
Fig. 3.4. Preparation of the first tantalum carbene complex [392]
The mechanism of this remarkable a-elimination reaction has been scrutinized by several research groups [ 17,4931,396-4041. From the experimental data obtained this process is best described as an intramolecular deprotonation of one neopentyl ligand by another, the latter being released as neopentane (Figure 3.4). The driving force for the (exothermic, [397]) elimination of neopentane from penta(neopenty1)tantalum results from the formation of a very strong carbontantalum double bond (1 26 kcal mol-', C-Ta single bond: 44 kcal mol-'), although the release of steric strain also contributes significantly to the energetics of this reaction. Steric crowding probably also plays a role in other alkane eliminations from alkyl complexes induced by the substitution of small ligands by bulkier ones [401,405-4111 (Figure 3.5).
3. I Generation of Non-Heteroatom-Substituted Curbene Complexes
79
Me P O P M e , - CMe,, PPh,
-
(Teuben, 1989)
5
-
green-brown solid
F5C60H,-35 OC (Gibson, 1994)
R = C,F,
3 LiCH,Ph, TMEDA -40 'C, THF, 1 h CrCI,(THF),
(Gambarotta, 1994)
D
60%
+ l+ I
MeCN, 20 'C Et,O, 1 h
+,1' BF,-
(Schrock, 1995)
74%
pale yellow solid
orange powder
Fig. 3.5. Examples of a-elimination of alkanes from alkyl complexes induced by ligand exchange 150,412-41 51.
80
3 Non-Heteroatom-Substituted Carhene Complexes
Experimental Procedure 3.1.1. Preparation of a Molyb um Cartme Complex by Nucleophilic Abstraction: (2,6-Diisopropylphenylimido)bisf 1,1-bis(trifluoromethyl)ethoxy](2-methyl-2-phenyl-1-propylidene)molybdenum [37,4 16,4171 TMS-CI, NEt,, ArNH2, 70 "C DME, 6 h (NH102M024 98%
339.95
A r N d P h
\rt 712.92
Ph
TfOH, 20 % DME, 3 he 75%
ArN\,
Mo(CI),(DME) ArNI/
Ph(Me),CCH,MgCI 20 OC, EGO, 3 h m
84%
607.52
NA TfO,do-
r
LiOCMe(CF& y Ph 20 OC, Et,O, 1 h
;y)JoMe
C
86%
N A r yPh
RO./,!,,-RO' 765.54
791.68
yellow crystals
ArNH, = 2,6-diisopropylanikne; ROH 1,I-bis(trifluorornethy1)ethanol
For related tungsten complexes, see [418]. For an alternative synthesis, see [413,419]. All operations are conducted under inert atmosphere using Schlenk techniques. Ammonium dimolybdate [(NH,),Mo,O,, 10.0g, 29.4 mmol] is suspended in DME (1 50 mL) and, while stirring at room temperature, triethylamine (23.8 g, 235 mmol) in DME (10 mL), tri in DME (20 mL), and 2,6-diisopropyl ven order. The r mL) are added in th re. Filtration through for 6 h at 70°C and d concentration of Celite, exhaustive washing of the solid the combined filtrates under reduced pres Mo[N(2,6-iPr2C,H,)],C1,(DME),which is used in the next synthetic step without further purification. 2.0 M solution Neophylmagnesium chloride (PhMe,CCH, in ether, 50 mmol) is added dropwise to Celite and con -40°C and 14. , 4.42g, 29.5 mmol) olution of Mo[N(2,6-
3.I Generution o f Non-Het~r~~utom-Suhstituted Curbene Complexes
81
Solid LiOCMe(CF3)2(459 mg, 2.44 mmol) is added in one portion to a stirred solution of Mo[N(~,~-~P~~C,H~)](CHCM~~P~)(O,SCF,)~(DME) (966 mg, 1.22 mmol) in diethyl ether (50 mL) at -3OOC. The resulting mixture is stirred at room temperature for 1 h, concentrated under vacuum, and the remaining solid is extracted with pentane. Filtration of the mixture through Celite and evaporation of the solvent under reduced pressure yields a dark yellow solid, which is recrystallized from pentane at -40°C. 800 mg (86%) of Mo[N(2,6iPr2C6H,)](CHCMe2Ph)[OCMe(CF,)2]2 is obtained as yellow crystals. 'H NMR (300 MHz, C&) 6 1.17 (s, 6H), 1.18 (d, 12H), 1.53 (s, 6H), 3.56 (sept, 2H), 6.92-7.19 (m, 8H), 12.12 (s, 1H); l3C NMR (75 MHz, C6D6) 6 18.8, 23.8, 28.7, 30.4, 55.4, 81.3 (sept, 29 Hz), 123.5, 124.1 (4,289 Hz), 124.2 (4,289 Hz), 125.9, 126.7, 128.6, 129.6, 147.6, 148.0, 154.0, 284.9.
[Ph,C][BF,], 20 OC CH,CI,, 10 min
Me,P=CH,
- Ph,CMe
CP\
/CH3 Ta-CH, Cp/ \CH,
cP\
or LiN(TMS),
/CH3
BF,-
97%
CP/T"\
CH,
82%
cP/
\CH,
pale green needles (Schrock, 1975)
Ar-N (Schrock, 1988) bI
red crystals
ArNH, = 2,6-diisopropylaniline
Me,CCH,Li, 25 'C C,H, 10 min (Osborn, 1987)
75% brown oil
tBuLi, - 4 O T Et,O (iPr),P-0s
/
CI
90%
-
_JPh
PSZC(W,P
(Werner, 1985)
yellow crystals mp 118 'C (dec.)
Fig. 3.6. Generation of carbon-metal double bonds by intermolecular a-deprotonations and a-eliminations [403,421-4231.
82
3 Non-Heteroatom-Substituted Carbene Coniplexes
OAr H3C>+a-CH, H3C b A r
hv, CeH, - CH,
ArO
(Rothwell, 1986)
D
100%
A,.o’
C ‘ H,
ArOH = 2,6-di(tert-butyl)phenol
hv, pyridine
- CMe, (Hoffman, 1988)
yellow, air-stable solid
Fig. 3.7. Photolytically induced a-elimination of alkanes from alkyl complexes [424,426].
a-Deprotonation of alkyl ligands in transition metal alkyl complexes can also be accomplished intermolecularly [398,409,420]. Figure 3.6 shows some examples of CH-acidic alkyl complexes, which can readily be deprotonated and/or dehydrohalogenated with different types of base. As a general guideline it can be stated that a-deprotonation of transition metal alkyl complexes will be easy when the metal is in a high oxidation state, the complex is positively charged, and the additional ligands are poor 7c-donors. A further possibility of inducing the elimination of alkanes from transition metal alkyl complexes is photolysis [398,424-4271. Two examples of photolytic a-eliminations leading to non-heteroatom-substituted alkylidene complexes are shown in Figure 3.7. In many of the reported preparations of stable carbene complexes from alkyl complexes, alkyl groups without P-hydrogen (e.g. neopentyl, 2,2,2-trifluoroethyl, trimethylsilylmethyl, methyl, benzyl) were chosen in order to avoid p-elimination. There are, however, also examples of moderately stable, non-heteroatom-substituted alkylidene complexes with hydrogen in the P-position to the metal (see, e.g., Figure 3.8).
3.1.2
a-Abstraction of Nucleophiles (Electrophilic Abstraction)
The abstraction of nucleophilic groups from the a-position of transition metal alkyl complexes is a versatile method for the preparation of carbene complexes. This type of abstraction can occur under a variety of reaction conditions, depending on the leaving groups chosen, the metal, and the additional ligands. The most common leaving groups are hydride, alkoxides, carboxylates, thioethers, and halides. Few examples of the generation of non-heteroatom-substituted carbenes by a-abstraction of pyridine have been reported [46,428].
3.1 Generation of Non-Heteroatom-SubstitutedCarbene Complexes
LiD
THF, -78 OC Cp(CO),Fe Cp(CO),Fe-l *
\o
[Ph,C"F,I CH,CI, -78TiqJ,0min
83
Cp(CO),Fe D
orange plates mp 180-181 OC (dec.) (Jones, 1980)
ON\ Cp-Re-CH, / Ph,P
-
[Ph,CI[PF,I CH,CI, -78OC, 10min
1. lPh,ClIPFsl 2. BuLi
70%
Cp-Re Ph,P
D
56%
ON\ Cp-Re/ Ph,P
1+ 9 PF,-
(Gladysz, 1983)
25 OC, Et,O, 3 h
R = SiMe,
yellow crystals (Schrock, 1997) [Ph,CI[AsF,I
oc, PPh,
yp~phlt
0cwAPh,
100%
AsF,-
(Brookhart, 1982)
green, air-stable solid (txat 50 OC: 15 h)
[Ph,CI[PF,I -78OC, CH,CI,
*
(Templeton, 1997)
100%
Fig. 3.8. Generation of carbon-metal double bonds by a-hydride abstraction [54,394,4324341.
3.1.2.1 a-Abstraction of Hydride Numerous examples have been reported of transition metal alkyl complexes which can be converted into carbene complexes by a-hydride abstraction [429-4311. This process can also proceed intramolecularly by oxidative insertion of the metal into the a-C-H bond. Figure 3.8 shows some illustrative examples of iron, rhenium, and
84
3 Non-Heteroatom-SubstitutedCurbene Complexes
tungsten alkyl complexes which undergo oxidation to the corresponding carbene complexes by inter- or intramolecular a-hydride abstraction. Conjugate hydride abstractions have also been used for the generation of carbonmetal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. EtCHO, BF,OEt, -40 OC, CH,CI, 2 h 94%
Fig. 3.9. Preparation of a cationic, non-heteroatom-substituted tungsten carbene complex by conjugate hydride abstraction [435].
Normally, hydride can be readily a-abstracted from alkyl complexes when the metal is in a low oxidation state and bound to good n-accepting ligands. In complexes with P-hydrogen, p-elimination might become the main reaction, specially when highly substituted alkenes are formed (Figure 3.10). [Ph,CI[PF,I -78 "C, CDZCI,
Ph,P
70%
-
ON y l ' p ~ ~ - (Gladysr, 1983) cp-he/ Ph,P
Fig. 3.10. P-Elimination of alkyl ligands can compete with carbene complex formation [433].
3.1.2.2 a-Abstraction of Oxygen-Bound Leaving Groups A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes 1436 -4391 (Figure 3.11). These carbene complex precursors can be prepared from alkoxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-4451 or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) 1391,446-4521.
3. I Generation of Non-Heteroatom-Substituted Curbene Complexes
0
[Me,O][BF,] or MeOTf 20 OC, CH,CI,, 12-24 h D
85
OMel' CWO),Fe<
X-
up to 95% X- = BF,-: yellow solid, mp 141-143 OC
Me,CuLi, -78 OC CH,CI,, Et,O, 0.5 h
49%
NaBH,, NaOMe -78 OC, MeOH
I
(0-methylation + reduction)
OMe
OMe CWO),F~+
HBF,, -23 OC, Et,O
I
TMS-OTf, -78 'C, CH,CI,
Cp(CO),Fe(Casey, 1985)
I
OTf71+
(Brookhart, 1983)
Fig. 3.11. Generation of cationic iron alkylidene complexes by acid-catalyzed a-abstraction of alkoxide [44 1,4471.
a-Alkoxyalkyl and a-siloxyalkyl complexes have also been prepared by alkylation at the metal of anionic, nucleophilic complexes [e.g. metallates L,M(CO),-Na+] with a-haloethers [453], aldehydes [454,455], or ketone derivatives [456]. Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(1V) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction; these include tetrafluoroboric acid [457-4591, trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460]. In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [ 178,4621 (see Section 2.1.7), Also non-heteroatom-substituted tungsten [440,443,444,45 1,452,4611, molybdenum [437], and chromium carbene complexes [440] have been prepared by a abstraction of alkoxide. Some a-alkoxy- or a-(acy1oxy)alkyl complexes undergo thermal a-abstraction without the need for additional catalysts (Figure 3.12).
86
3 Non-Heteroutom-Substituted Curbene Complexes
Na,Mo(CO),
Mo(CO),
(Rees, 1972)
Ph yellow crystals, rnp 180 OC (dec.) Ph
- -x
C;pp 'Ph
o
CpzWHz
70%
70 OC
H Cp,W\
/
F
Ph
,Zr(Ph)(Cp),
C6H6, 45%
h,
CP,w=.\ Ph
(Caulton, 1981)
green solid
Fig. 3.12. Generation of alkylidene complexes by thermal a-abstraction of oxygen-bound leaving groups [53,463].
Examples of conjugate a-eliminations of alkoxide and hydroxide have also been reported (Figure 3.13). Few cationic iron carbene complexes have been characterized spectroscopically [55,104,464], because most such compounds quickly degrade at room temperature [466]. Besides elimination of the alkylidene ligand, one major decomposition pathway can be disproportionation 14591 (Figure 3.14).
3.1 Generation of Non-Heteroatom-Substituted Curbene Complexes
87
(Gladysz, 1993)
oc' OC\MnAOMe I I
CP
-4 je-YoH
4
1'
Et20, -15OC HBF4
(Case)', 1985)
BF,-
O oc C 7 ' " Y
OCoc
orange solid dec. at room temp.
Fig. 3.13. Generation of alkylidene complexes by conjugate a-abstraction of oxygen-bound leaving groups 1447,4651.
Cp-Fe OC\
41+ BF,-
OC
20 OC, CH2CI,, 1 h
l+
2 Cp-Fe=CH,' OC\ Oc'
(Casey,1985)
BF,78%
Od
BF,-
-60%
OC\ ,Cp
/Fe.
oc
CH3
l+
oc +
\
Cp-Fe-11
Od
I
BF,-
(Jolly, Pettit, 1966)
Fig. 3.14. Decomposition reactions of cationic iron(1V) carbene complexes 1447,4591.
3.1.2.3 a-Abstraction of Thioethers Because a-alkoxyalkyl iron complexes are thermally unstable [467] they cannot be stored for long periods of time. More suitable carbene precursors are the corresponding a-(dimethylsu1fonium)alkyl complexes, which can be stored indefinitely under ambient conditions [468-4731. These complexes are prepared by S-alkylation of a-(methy1thio)alkyl complexes, which can be prepared by alkylation of metallates with a-halothioethers, by addition of C-nucleophiles to (alky1thio)carbene complexes, or by addition of thiols to carbene complexes.
88
3 Non-Heteroatom-Substituted Carbene Complexes
Alternatively, a-(dialkylsu1fonium)alkyl iron complexes can be prepared from ahaloalkyl iron complexes by silver- or thallium-promoted nucleophilic substitution with thioethers [474]. Dimethylsulfide can be eliminated from a-(dimethylsu1fonium)alkyl complexes simply by heating (Figure 3.15). The resulting, very reactive, electrophilic iron carbene complexes cannot usually be isolated but are generated directly in the presence of a suitable reactant, e.g. an olefin. Cationic nickel [475] and tungsten [476] carbene complexes have been prepared by similar routes. 1. Na, THF 2. CICH,SMe 3. Me1 4. NaBF,, H,O
[CP(CO),F~I, 70%
-
100 o c MeNO, or dioxane
Cp(CO),Fe\+
BF,-
-
one-pot procedure (Helquist, 1992)
Fig. 3.15. Preparation and thermolysis of (dimethylsulfonium)methyl iron complexes [468].
a-(Pheny1thio)alkyl iron complexes can be prepared either by treatment of complexes as Cp(L),Fe-Na+ with a-chloroalkyl phenyl sulfides [477] or, alternatively, by reaction of carbanions with (pheny1thio)carbene complexes [ 179,478 -48 11. S-Methylation of a-(pheny1thio)alkyl iron complexes leads to spontaneous elimination of methyl phenyl sulfide and generation of the cationic iron carbene complex (Figure 3.16). Suitable alkylating agents for this purpose are fluorosulfonic acid methyl ester or trialkyloxonium salts. RMgX or RLi, THF
I+BF,-
-
-
R
Cp(CO),Fe--( SPh
[Me,O][SbCI,]
(Helquist, 1987, 1988)
Fig. 3.16. Generation of reactive, cationic iron(1V) carbene complexes from a-(phenylthio)alkyl complexes [470,482].
3.I Generation of Non-Heteroutom-Substituted Curbene Complexes
89
3.1.2.4 a-Abstraction of Halides Halides are, not surprisingly, also suitable leaving groups for the generation of carbon-metal double bonds by a-abstraction. a-Haloalkyl complexes can be converted into carbene complexes either thermally [459,483] or by treatment with Lewis acids [lSO]. The vinylogous variant of this reaction has also been reported (Figure 3.17). Direct treatment of organic geminal dihalides or trihalides with strongly nucleophilic transition metal complexes can also lead to the formation of carbene complexes, presumably via intermediate a-haloalkyl complexes [484-4891. Examples of such reactions are sketched in Figure 3.17. -
50 OC
& 1’
OC-‘Fe=CH,
(Jolly, Pettit, 1966)
BF,-
/
oc -
L,Ru
A
95%
purple solid (Grubbs, 1997)
Ph
Na,W(CO),, 20 ‘C HMPA, THF, 1 h
+
)pW(C,,,
(Kawada, Jones, 1980)
10%
Ph’ bright yellow crystals rnp 215 OC (dec.)
Ph
CH,CI,,
20 O C
PCY3 (Caulton, 1997) I
PCY3 burgundy solid
Fig. 3.17. Generation of carbon-metal double bonds by a-abstraction of halides [459,490 -4921.
90
3 Non-Heteroatom-SubstitutedCarbene Complexes
3.1.3 From Ylides Electrophilic transition metal complexes can react with organic ylides to yield alkylidene complexes. A possible mechanism would be the initial formation of alkyl complexes, which are converted into the final carbene complexes by electrophilic a-abstraction (Figure 3.18). This process is particularly important for the generation of acceptor-substituted carbene complexes (Section 4.1).
Fig. 3.18. Formation of carbene complexes from ylides. X: N,, PR,, SR,, ArI, ArSO,, TaL,.
The mechanism sketched in Figure 3.18 implies that the starting complex L,M has a free coordination site (or a readily replaceable ligand) and can act as an electrophile. Therefore reactions of this type will occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex L,M.
3.1.3.1 From Diazoalkanes It has been known for a long time that the decomposition of diazoalkanes can be catalyzed by transition metal complexes [493-4961. Carbene complexes were proposed as possible intermediates by Yates in 1952 [497]. However, because reactions of diazoalkanes with metal complexes tend to be difficult to control, it was not until 1975 [498] that stable carbene complexes could be directly obtained from diazoalkanes (Figure 3.19).
PhPh' Cu powder, THF 20 O C , 20 h, N,
-
OC-Mn-0
oc/
(Herrrnann, 1975)
D
82%
oc/
Ph
mp 91-92 OC
Fig. 3.19. First preparation of a stable carbene complex from a diazoalkane [498].
3.1 Generution of Non-Heteroutom-SubstitutedCarbene Complexes
91
Numerous carbene complexes have since been prepared by this method [ 1,52,60,499-5031, even utilizing highly reactive diazoalkanes such as diazome-
thane [504]. Because of their high nucleophilicity and reactivity, non-acceptorsubstituted diazoalkanes can displace even strongly bound ligands, such as phosphines. Examples of such reactions are shown in Figure 3.20. mental Procedure 3.1.3. Preparation of a Ruth Diazoalkane: Dichloro-bis(tricyclohexy1phosph
Carbene Complex zylideneruthenium
[571 1. PhCHN,, -78 OC, 5 rnin 2. PCy,, 20 OC, 30 rnin CH (PPh,),RuCI,
d
99%
pcY3
Ph
cI-a"=/ CI' LCY,
961.88
824.99
For an alternative procedure, starting from PhCHC1, and Ru(COD)(COT), see [484]. A cold solution of phenyldiazomethane [507] (0.99 g, 8.38 mmol; -50 "6) (10 mL) is added to a solution of RuCI,(PPh,), [508] (4.00g, 4.16 dichloromethane (40 mL) at -78 "C. The color of the mixture changes ge-brown to green-brown. After 5-10 min at -70 "C a cold solution of ylphosphine (2,57g, 9.16 mmol) in dichloromethane is added with a syringe. The resulting mixture is stirred at room temperature for 30 min, filtered, concentrated to half of its original volume, and filtered again. Addition of methanol (100 mL) leads to precipitation of a solid, which is filtered off, washed with acetone and methanol, and dried under reduced pressure. 3.40g (99%) of the title compound is obtained as a purple d i d . 'H NMR (400 MHz, 6 1.16-1.25, 1.39-1.46, 1.67, 1.77, 2.58-2.62 (all m, 33H), 7.33 (t, 7.6 , 8.44 (d, 7.6 Hz, 2H), 20.02 (s, 1H); I3C NMR (100 MHz, CD,Cl,) 6 8.24, 30.04, 32.49, 129.27, 129.49, 131.21, 153.17, 294.72. Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophilicity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO),W=C(OMe)Ph [509], [Cp(CO),Fe(THF)][BF4] [5 10,51 11, and (CO),Cr(COD) [52,5121. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes.
92
3 Non-Heteroatom-SubstitutedCurbene Complexes
Ph,CN,, 20 OC pentane, 1.5 h
.
(iPr),Sb
(iPr),Sb
C i - i h q hPh
Cl-Lh-11
I
88%
(iPr),Sb
(Werner, 1997)
(iPr), b mp 61 OC (dec.)
CH,N,
20 OC
I . EtzO, C6H6
(PPhJ,Os
82%
N 'O
PPh, CI\ ,Os=CH, ON 6Ph,
(Roper, 1983)
orange crystals mp 209-210 OC CHZN,, -50 "C TH F
PPh, OC-L-1
P
I
93%
PPh,
PPh, (Roper, 1984)
OC>/r=CH,
' I
PPh,
orange solid rnp 110 OC
-
-
N,CHSiMe, SiMe,
C6H6 ___.)
70%
C6H6,25OC
tK 1
(Andersen, Bergman, 1996)
2
/ y
styrene 20 'C, 2 d
1
4
\
Fig. 3.20. Examples of the preparation of non-heteroatom-substituted carbene complexes from diazoalkanes 160,504-5061.
Complexes of the type [Cp(CO),Fe=CR,]+X-, which are probably the cyclopropanating intermediates when using [CP(CO)~F~(THF)] [BF,] as catalyst for diazodecomposition, have been isolated, characterized spectroscopically, and shown to cyclopropanate olefins.
3.1 Generation c~ Non-Heteroutom-Substituted Carbene Complexes
93
The most effective catalysts for diazodecomposition known today are palladium(II), copper(I), and rhodium(I1) complexes, although stable alkylidene complexes have not yet been isolated from the reaction of these catalysts with diazoalkanes.
3.1.3.2 From Other Ylides Alkylidene complexes can be prepared by ligand displacement with phosphorus ylides [S1 3 3 141 or nucleophilic tantalum carbene complexes [409,S IS]. This methodology has, however, not found widespread use. Representative examples are given in Figure 3.21.
Cp,Ta,
PMe, /
Et,P=CHMe, 60 OC C,H, 24 h, - PEt,
-PMe,
CH,
+
*
Cp,Ta-CH,
--.-
PMe3
50%
NPh
Cp,Ta,
K-
NPh 90°C, C,H,
21 h (Grubbs, 1993)
D
82%
CI
L = P(Ph),Me
CI golden-yellow powder
1. 2-(MeO)C,H,CH=PPh, Na/Hg, 20 OC NPh
Ro W ‘’ RO’
II
(Schrock, 1979)
CH3 colorless needles
I‘
I C‘ I
THF ROH = Me(CF,),COH
NPh
C,H, THF, 4.5 h 2. CuCI, C,H, 13 h (Grubbs, 1993)
64% burnt-orange crystals
Fig. 3.21. Preparations of non-heteroatom-substituted alkylidene complexes from phosphorus ylides [516,517].
3.1.4 From Carbyne Complexes Carbyne complexes, i.e. complexes with a carbon-metal triple bond, have been known since 1973 [5 181. Since then several ingenious synthetic approaches have made this class of compounds readily accessible [ S 19,5201. Carbyne complexes
94
3 Non-Heteroutom-Substituted Curbene Complexes
ior
R
R
R
R
R
R
R
R
Fig. 3.22. Conversion of carbyne complexes into carbene complexes.
can be suitable starting materials for the preparation of carbene complexes (Figure 3.22) [181]. The conversion of carbyne complexes into carbene complexes can be achieved either by electrophilic or by nucleophilic addition to the carbon-metal triple bond. In addition to this, carbyne complexes can be converted into carbene complexes by [2 + 21 cycloaddition to alkynes or alkenes. The capacity of carbyne complexes to undergo electrophilic or nucleophilic addition reactions depends, as for a-abstractions from alkyl complexes, on the electronic properties of the remaining ligands and on the nature and oxidation state of the metal. Cationic carbyne complexes with strong n-accepting ligands will normally tend to react with nucleophiles, whereas anionic carbyne complexes with n-donating ligands are more likely to add electrophiles at the carbon-metal triple bond.
3.1.4.1 Nucleophilic Additions to Carbyne Complexes Some carbyne complexes, in particular cationic ones with good n-accepting ligands, can react with nucleophiles to give carbene complexes [ 187,5211. Several reductions of carbyne complexes to carbene complexes by treatment with metal hydrides have been reported. Similarly, organolithium or other carbanionic reagents can react with electrophilic carbyne complexes to yield carbene complexes. Illustrative examples of both reactions are sketched in Figure 3.23. Experimental Procedure 3.1.4. Preparat' plex from a Carbyne ene}(dicarbonyl)( [526] [37] pp 15
olybdenum Vinylidene Comyano(ethoxycarbony1) y1)borato)molyb-
3.1 Generation of Non-Heteroatom-Substituted Carbene Complexes
NC-Co,Et,
co 496.62
NaH
co
CO,Eq
~pl-Ao=c==( 113.12
I
234.99
607.42
95
24.00
k0
NBu,+
CN ’102.52
Further reading: [527-5291,Solid 4-(dimethylamino)phenyldiazonium tetrafluoroborate (30.0g, 128 mmol) is added in portions to a vigorously stirred suspension of [NEt,][Tp’Mo(CO),] (77.4g, 127 mmol) in dichloromethane (3.0 L). The resulting mixture is stirred for 2 h, concentrated under reduced pressure to 900 mL, and washed with 18% hydrochloric acid, water, 10% aqueous sodium carbonate, and water, in that order. Drying over magnesium sulfate, concentration, and batchwise chromatographic purification (silica gel, gradient elution with dichloromethanehexanes) yields 50.6g (80%) of Tp’(CO),Mo=CCl as a yellow, air- and moisture-stable crystalline solid. Sodium hydride (0.48g of a 60% dispersion in paraffin, 12.0 mmol) is added to a stirred solution of ethyl cyanoacetate (1.36g,12.0 mmol) in THF (100 mL) at O’C, followed by addition of solid Tp’(CO),MwCCl (2.00g, 4.03 mmol). The resulting mixture is heated under reflux for 24 h, left to cool to room temperature, and treated with solid tetrabutylammonium bromide (1.29g, 6.14 mmol). Water is added and the mixture extracted with dichloromethane. The combined extracts are washed with water, dried over magnesium sulfate, and concentrated to small volume under vacuum. Treatment of the residue with diethyl ether leads to precipitation of 2.94g (quant.) of the title compound as a yellow, air-stable solid. 13C NMR (CDCl,) 6 12.57, 13.64, 14.78, 14.94, 86.89,105.00,105.58,124.00,143.40,143.88, 16.00,19.65,23.96,58.64,58.72, 151.04, 151.75,171.60,229.31,300.00. Closely related to the a-addition of nucleophiles is the P-deprotonation of electrophilic carbyne complexes. In many of the examples reported [ 143,53033I ] the resulting vinylidene complexes could not be isolated but were generated in situ and either oxidized to yield stable carbene complexes [532]or used as intermediates for the preparation of other carbyne complexes [527].Cationic carbyne complexes can be rather strong acids and undergo quick deprotonation to vinylidene complexes with weak bases [143].An interesting example of the use of anionic vinylidene complexes as synthetic intermediates is sketched in Figure 3.24.
96
3 Non-Heter[~utom-SuhstitutedCurbene Complexes
-
Et,AIH, -78 OC CH,CI,, 15 min
oc
74%
oc cp-keTh
(Fischer, 1978)
Od red powder, mp 149 OC
yellow crystals (Roper, 1980)
Cp-he-Ph OC
Od
1+
-
MeLi, -40 OC Et,O, 16 h
BCI,-
46%
Cp-he OC oc
(Fischer, 1976)
orange crystals, mp 74-75 OC
oc Cp-hnGCH,
Od
l+
MeLi, -50 OC pentane, 3 h
BcI,-
D
bn< C ::
(Fischer, 1976)
oc
19%
dark red crystals, mp c 20 OC
- 1) MeMgBr
Ph,P-Mn Qh
+
Ph 56%
Ph
Ph
20%
(Geoffroy, 1993)
Fig. 3.23. Examples of the conversion of carbyne complexes into carbene complexes [521 -5251.
3.1.4.2 Electrophilic Additions to Carbyne Complexes Electron-rich carbyne complexes can react at the carbyne carbon atom with electrophiles to yield carbene complexes. Numerous examples of such reactions, mostly protonations, have been reported [5 191. Depending on the nucleophilicity of the carbyne complex, such reactions will occur more or less readily. The protonation of weakly nucleophilic carbyne complexes requires the use of strong acids, such as triflic [533], tetrafluoroboric [534] or hydrochloric acid [535,536]. More electronrich carbyne complexes can, however, even react with phenols [537,538], water [393,539], amines [4 18,540,5411, alkyl halides, or intramolecularly with arenes (cyclometallation, [542]) to yield the corresponding carbene complexes. A selection of illustrative examples is shown in Figure 3.25.
3.1 Generation of Non-Heteroatom-Substituted Carbene Complexes
-
97
BuLi, -78 OC
YP
% 92%
~ ( M e O ) , PI - ~ ~ = C ~ ’ - L i ’ ]
( M ~ o ) , P - M ~ ~THF, 45 min
co
LO
HCI, 20 OC THF, 4 h (MeO),P-MoLO
46% dark red crystals (McElwee-White, 1992)
Fig. 3.24. Generation of nucleophilic vinylidene complexes by deprotonation of carbyne complexes [530].
ph3p
CI-0s
1
CI To1 / d
(Roper, 1980)
OC’6Ph, air-stable solid mp 255 OC
PhOH, 20 OC CH,CI, 5 min
Me,CO Me3co--J+, I
(Schrock, 1985)
D
73%
Me,CO
orange crystals
NEt,, DME. 20 OC Et,O, 20 rnin
I
CI (Schrock, 1987)
D
Br
Pic B ,r
I c,o
40 OC, CH,CI,,
2h
Pic
, (Mayr, 1987)
46% Pic = 4-methylpyridine
dark brown crystals
Fig. 3.25. Generation of carbene complexes by addition of electrophiles to carbyne complexes [533,538,540,5431.
98
3 Non-Heteroatom-SubstitutedCurbene Complexes
3.1.5 From Alkynyl and Alkenyl Complexes Non-heteroatom-substituted carbene complexes are in principle accessible either by electrophilic or by nucleophilic addition to alkynyl or alkenyl complexes (Figure 3.26).
L,,M=C-
-/
ElNu-
L , M e
E+
L,,M=C-
El+
--/
Fig. 3.26. Conversion of alkenyl and alkynyl complexes into carbene and vinylidene complexes.
Of these potential approaches addition of electrophiles only has attained some relevance in the preparation of non-heteroatom-substituted carbene complexes. Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-5511. This approach is one of the most appropriate for the preparation of vinylidene complexes [ 1281. Figure 3.27 shows illustrative examples of such reactions.
3.1.6 From Alkyne and Cyclopropene Complexes Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding x-complexes. With terminal alkynes the corresponding x-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [ 128,130,132,133,547,556-5691, With mononuclear q2-alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-H bond to yield a hydrido alkynyl complex, followed by 1,3-hydrogen shift from the metal to C [570,571], or (b) concerted formation of the M-C, bond and 1,2-shift of H, to [572]. Vinylidene complexes are valuable intermediates for the preparation of heteroatom-substituted carbene complexes (Section 2.1.5) and other organometallic compounds, including non-heteroatom-substituted carbene complexes [573,574]. Examples of further transformations of vinylidene complexes include addition reac-
EP
99
3.I Generation of Non-Heteroatom-SubstitutedCurbene Complexes
--(-
-
HBF,, -23 OC Et,O
Cp-Fe OC\
Od
-11 OC, Et,O t x 70 rnin
Cp-Fe OC\ + -('BF-,
Od
(78% for both steps)
yellow solid
OC\ Cp-Fe-1
(1'
BF,-
oc' ~.
(Casey, 1982)
HBF,, 0 OC CH,CI,, Et,O
Tp'-W
co
85%
Ph
-
Ph,P \ Cp-Ru-Ph
T p I - 4 4 '+BF4-
co Ph red-orange crystals
Ph,P (Bruce, 1985)
100%
dark green crystals
ICH,CN, NH,PF, 20 OC, CH,CI,, 18 h
Ph,P
\ Cp-Ru Ph,P
I
(Templeton, 1992)
Br,. THF
I
Ph,P
Ph-
D
Ph
72%
Ph,P \ Cp-RU=C Ph,P
I
Br
< 1'
pF,-
(Lin, 1996)
CN
pale red solid
Ph,P
-Mn cp\ I oc
<
1. X
2 BuLi
cp\
ph3p-Mn-
ocI
O
, BF,OEt,
1
-Li+
2. NH, orange solid (Geoffroy, 1992)
Fig. 3.27. Preparation of carbene complexes by addition of electrophiles to alkynyl and alkenyl complexes [89,391,552-5551.
tions [573,575], cycloadditions [ 150,546,574,576-5781, and elimination reactions [130,132,579]. The examples sketched in Figure 3.28 illustrate the broad scope of this preparative procedure. Similar to alkynes, cyclopropenes also readily form transition metal n-complexes which can isomerize to carbene complexes thermally, photochemically, or che-
100
3 Non-Heteroatom-SubstitutedCarbene Complexes Ar-Li -78 O C , THF, 1.5h
ON \ Cp-Mo-CO I
-
(Ar = tolyl)
oc
ON
\ I
Cp-Mo-Ar
1Li'
ON
+
oc Ar
[Me,O][ BF,], 20 OC
CH,CI,,
20 h
*
Cp-Mo=C
I
oc
Ar (Dotz, 1995)
64%
orange crystals
orange crystals
Fig. 3.27. continued.
mically (Figure 3.29) [581-5901. However, this strategy for preparing carbene complexes suffers from the difficult access to cyclopropenes. The use of propargyl halides [491] (cf. Section 3.1.2.4) seems to be a more practical route for the synthesis of vinylcarbene complexes.
3.1.7 By [2 + 21 Cycloreversion Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 21 Cycloreversion can be induced thermally, chemically, or photochemically [49,591-5951. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 21 cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. Examples of sequential [2 + 21 cycloaddition and [ 2 + 21 cycloreversion reactions leading to the formation of stable carbene complexes are sketched in Figure 3.30. [2 + 21 Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-6061 and zirconium [599,6011 carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 + 21 cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 21 cycloreversion.
3. I Generation of Non-Heteroatom-SubstitutedCarbene Complexes
20 OC, CH,CI, Et,O, 3 d 80%
1 atm CO, 20 OC CH,CI, pypy-of PhMe,P
‘B F,-
w
CP-p=Ci
1’
P
(
‘FezCP
/
co
BF,-
hv, (> 280 nrn) CH,CI,, 12 h
90%
PF,-
(Schumann, 1992)
(MeO),P Ph orange-red crystals
PhMe,P
,
\ Cp-MozC-
1’ h
I
PhMe,P
Ph,Th
1’
101
BF,-
(Selegue, 1988)
1’
cp\ Ph, ,F~=cPhyPp/,ph X O p h
BF4-
‘Ph
Ph/P Ph
(Nakanishi, 1992)
mp 140 OC (dec.)
red crystals Ph Ph
1’
MeOH, NaPF,
PF,CI
(Dixneuf, 1992)
Ph
Fig. 3.28. Conversion of alkynes and alkyne complexes into vinylidene complexes [37,58,59,133,559,560,562.580].
3.1.8 Other Methods In addition to the methods described, non-heteroatom-substituted carbene complexes have also been prepared by reactions which do not belong mechanistically to any of those described in previous sections. Carbene complexes can be prepared by reaction of stabilized carbenes or carbenoids (e.g. a-haloorganolithium compounds) with transition metal complexes [6 101. This method is particularly useful for the preparation of donor-substituted
102
3 Non-Heteroutom-SubstitutedCurbene Complexes
P(iPr),
Ph-, 20 OC PhMe, 1.5 h 75%
*
90%
10%
brown solid mp 97 OC (dec.)
violet crystals rnp 106 OC (dec.)
P(iPr), CI,&s%Ph CI’ ‘CO P(iPr),
(Esteruelas, 1995)
I
yellow solid
Fig. 3.28. continued.
Ph-Ph
20 ‘C, Et,l 81%
( M e O ) , CI Py~~--ph I
Ph
NPh HgCI,, 20 ‘C CD,CI, ___c
(Grubbs, 1993)
Fig. 3.29. Isomerization of cyclopropene complexes into vinvlcarbene comdexes
carbene complexes (Section 2.1.6). Similarly, carbene complexes can result from the reaction of geminal organodilithium compounds with transition metal dihalides [61 I]. I-Metallacyclopent- 1-enes have been obtained by formal [2 + 31 cycloaddition of ethylene to Re(CtBu)(CHtBu)(OR), (a carbenekarbyne complex) [612]. Nucleophilic tungsten(I1) complexes can react directly with cyclopentanone to yield cyclopentylidene complexes [6 131. Ligand exchange [409,614-6 161 and ligand transformation of carbene complexes have been widely used for the fine-tuning of catalytically active carbene complexes.
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
CH
hv, C (Pyrex), GH,, 4 25 h OC
(
c
0
)
5
w
103
b
Ph (Cooper, 1984)
m (c0)5w=(
Ph
+
phAiMP
37% purple solid
Ar = 2,6-/Pr,C,H,
'OMe
/
(Schrock, 1989)
Fig. 3.30. Preparation of carbene complexes by sequential [2 + 21 cycloadditions and [2 + 21 cycloreversions of carbene and carbyne complexes to alkenes and alkynes [596-5981.
3.2 Synthetic Applications of Non-HeteroatomSubstituted Carbene Complexes Non-heteroatom-substituted carbene complexes play a key role both as reagents and catalysts in organic synthesis and as intermediates in the preparation of other organometallic compounds. However, discussion of applications in inorganic synthesis would surpass the scope of this book. Here the focus will be on those reactions which lead to metal-free compounds and hence are particularly relevant to the organic chemist.
3.2.1 General Considerations Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply
104
3 Non-Heteroatom-SubstitutedCurbene Complexes
CP Si=CH,
(Grubbs, 1981,1982,1987)
Cd
SiMe,
-
-20 OC,CH,CI,
HSiMe3
SiMe,
(Petasis, 1995)
Me,Si
20 min D
33%
h
(Grubbs, 1986)
PMe,Ph, HMPA -40 OC, PhMe D
70%
A
CP\ / / cp
/qPMe,Ph
(Schwartz, 1983)
yellow-orange oil
Fig. 3.31. Generation of carbene complexes by [2 metallacycles [26,27,605,607-6091.
+ 21 cycloreversion
of four-membered
consistently to all known carbene complexes, can be helpful for estimating the reactivity of the compounds. Low-valent, 18-electron (Fischer-type) carbene complexes with strong nacceptors usually are electrophilic at the carbene carbon atom (C,). These complexes can undergo reactions similar to those of free carbenes, e.g. cyclopropanation or C-H insertion reactions. The carbene-like character of these complexes becomes more pronounced when electron-accepting groups are directly bound to C, (Chapter 4), whereas electron-donating groups strongly attenuate the reactivity (Chapter 2).
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
105
Schrock-type carbene complexes are usually high-valent, electron-deficient complexes without n-accepting ligands. These complexes often behave as C nucleophiles and typical reactions include carbonyl olefination and olefin metathesis.
3.2.2 Cyclopropanation The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [6 171 their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(1) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(I1) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(I1) or palladium(I1) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically.
R - R
-X
X -
X
catalytic cyclopropanation
stoichiometric cyclopropanation
< 1 eq. L,M
Y
carbene complex precursor
L,M
RKR ML"
isolated carbene complex
Fig. 3.32. Catalytic and stoichiometric cyclopropanation of alkenes with carbene complexes.
In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling.
106
3 Non-Heteroutom-Substituted Curbene Complexes
3.2.2.1 Stoichiometric Cyclopropanations Because electrophilic carbene complexes can cyclopropanate alkenes under mild reaction conditions (Table 3.1) [438,618-6201, these complexes can serve as stoichiometric reagents for the cyclopropanation of organic compounds. Thoroughly investigated carbene complexes for this purpose are neutral complexes of the type (CO),M=CR, (M: Cr, Mo, W) and cationic iron(1V) carbene complexes. The mechanism of cyclopropanation by electrophilic carbene complexes has been discussed in Section 1.3.
Scope and Limitations In the stoichiometric cyclopropanation reaction of electrophilic carbene complexes electron-rich alkenes, such as enol ethers, usually tend to give higher yields than non-donor-substituted alkenes. This reflects the electrophilic character of these complexes. The suitability of neutral, non-heteroatom-substituted carbene complexes of chromium, molybdenum, and tungsten for cyclopropanation has been investigated by several research groups. However, these reagents have not been used extensively as cyclopropanating agents in organic synthesis. This is probably because of the difficulty of their preparation, their low stability, and their tendency to promote olefin metathesis [621,622]. For the more reactive iron complexes, however, some interesting applications have been found (Table 3.2). With in situ-generated [Cp(CO),Fe=CHMe] [BF,] it is, for instance, possible to cyclopropanate alkenes efficiently with ethylidene [442,47 1,4721. This reaction is otherwise not easily accomplished, because potential synthetic equivalents of ethylidene or higher alkylcarbenes R,CH-CH with hydrogen in the P-position quickly rearrange to the corresponding alkenes R,C=CH,. Cyclopropanations with [Cp(CO),Fe=CMe,][BF,] [391,545] and higher alkylidenes [457,544] have also been realized (Table 3.2), but yields are often low. A very useful precursor for carbene complexes is the complex [Cp(CO),FeCH, SMe,][BF,] [468,469]. This compound, which can be stored indefinitely under ambient conditions, can be converted thermally, under essentially neutral conditions, into the carbene complex [Cp(CO),Fe=CH,][BF,], which cyclopropanates a wide range of olefins in good yields. Compared with other methylenating agents (diazomethane, Simmons-Smith reagent), this iron carbene precursor has the advantages of (a) safe storage and handling, and (b) carbene-generation under neutral reaction conditions. This second quality is particularly valuable when sensitive natural products or elaborate intermediates [623] need to be cyclopropanated selectively.
3.2 Synthetic Applications of Non-Heteroutom-Substituted Curbene Complexes
MeLi Cp(CO),Fe=CHSPh+ PF,-
50%
[Me,O][ BF,], 0-25 OC 3h CH,CI,
51%
-
-
mH& +
H
107
(60:40)
(Helquist, 1989)
H
Fig. 3.33. Stoichiometric, intramolecular cyclopropanations with iron(1V) carbene complexes [477,624].
(s, 4H), 7.12-7.40 (m, 1OH); 13C NMR (75 .9, 128.2, 128.4, 145.8,
108
3 Non-Heteroatom-Substituted Carbene Complexes
Table 3.1. Cyclopropanation with stoichiometric amounts of chromium, molybdenum and tungsten carbene complexes. Carbene Complex
I
(CO),Cr-T Ph
Substrate
Conditions (Precursor)
-30 "C (a-ether) -80
Product
'ha
-18 "C ='\Ph
[625]
11%
[437]
50%
[434]
82%
[443]
69%
[443]
36%
[443]
Meo%
-78 "C (a-pivalate)
w
80%
oc;
Me0
(CO)
Ref.
(CO),Cr
then 0, (a-ether)
4
Yield
(a-ether) cisltrans 64:36
-78 "C (c0)5w7Ph
(a-ether) ph%
cisltrans 1 :99
-78 "C (a-ether)
(c0)5w7Ph
-78 "C (a-ether)
A
phqph 89%
[626]
85%
16261
OMe
cisltrans 36:64
-78 "C (c0)5W=\ph
(a-ether) cisltrans 91 :9
(c0)5w=7\Ph
3.2 Synthetic Applications
of Non-Heteroatom-Substituted
Carbene Complexes
109
Table 3.1. continued. Carbene Complex
Substrate
Conditions (Precursor)
Product
100 "C, 2.5 h,
lo (==w.& Oc (
Yield
Ref.
10%
[452]
65%
[452]
27%
[621]
neat
Ph
Ph E t O d (cO),w=(
Ph
37 "C, 3 h, neat
ph)&oEt Ph
To1
l2
(CO),W=(
To1
'3 oc, IP P h 3 1 +
oc' 14
w=CH
TP', OC-W=CH, Ph-
1
-78 "C (L,W-Me; hydride abstraction)
52%
[438] [4341
P
-78 "C (L,W-Me; hydride abstraction)
I
73%
[54]
+
PF,-
To1
P h G
AsF,-
\
CP
70 "C, 3 h, neat
h
g
A further interesting reaction sequence is based on the conversion of carbanion equivalents (e.g. Grignard reagents, organolithium compounds, cuprates, lithium enolates) into stable a-(pheny1thio)alkyl iron complexes [470]. Abstraction of thiophenol can be achieved by S-alkylation with Meerwein salt; thioanisole is released and a highly electrophilic iron carbene complex is generated. The latter can undergo different types of inter- or intramolecular reaction [479,482]. Although in most of the examples reported so far intramolecular carbene C-H insertions were the predominant reaction pathway [478], some intramolecular cyclopropanations have also been reported (Figure 3.33). Stereoselectivity
Cyclopropanations with carbene complexes, such as those listed in Tables 3.1 and 3.2, are usually stereospecific (i.e. Z-alkenes yield cis-cyclopropanes) [29]. This suggests, in agreement with theoretical investigations [28], that this reaction is a concerted process. One remarkable feature of these cyclopropanation reactions is the high cis-diastereoselectivity frequently observed [619,629]. Enantioselective cyclopropanations using enantiomerically pure tungsten [54], iron [458,483,630], and ruthenium [581] carbene complexes have also been at-
110
3 Non-Heteroatom-SubstitutedCurbene Complexes
Table 3.2. Cyclopropanation with stoichiometric amounts of cationic iron and nickel carbene complexes. Carbene Cornplexl'l
Substrate
Conditions (Precursor) 0 OC (a-ether)
oc \
Cp-Fe-
od
P 'h
oc \
Cp-Fe-
od
P 'h
\
02
P 'h
[24]
ph&
57%
[24]
cisltrans > 99: I
pha/ph 88% [24]
3
78%
[24]
75%
[24]
48%
[441]
75%
[442]
80%
[459]
cisitram > 99: 1
Ph
\
Cp-Fe-
od
54%
-78 "C (a-ether)
oc
Ref.
phw
cisltrans > 99: 1
-78 "C (a-ether)
Yield
cisltrans 89: 1 1
-78 "C (a-ether)
oc Cp-Fe-
Product
P 'h
-78 OC (a-ether)
A
oc \
-18 O C (a-ether)
oc
cisltrans 5O:SO
Ph
\
-78 "C (a-ether) cisltrans > 98:2
*
oc \
Cp-Fe-
od
M 'e
-18 O C (a-ether)
oc \ I
Cp-Fe=CH,
oc
50 "C (a-bromide)
cisltrans 82: 18
0
[a] All carbene complexes listed are monocationic. Treatment with HBF, (wethers) or thermolysis (a-SMe,) was used for carbene generation.
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
1 11
Table 3.2. continued. Carbene Complexla]
Substrate
oc
Conditions (Precursor)
Product
Cp- Fe=CH,
I
96%
[473]
62%
[468]
C0,Me
oc
Ref.
phXph 100 "C (&Me,)
\
Yield
C0,Me
5 yo -CO,Me
oc
l2
S0,Ph
\ Cp-Fe=CH, I
100°C
S0,Ph
(&Me,)
68%
[628]
oc
l3
Cp-Fe
I
=(
Ph/,
-65 "C (a-ether)
phq
oc oc
y
~~
l4 Cp-Fe\
od
45%
[447]
45%
[447]
66%
[I041
49%
[475]
-15 "C Ph/,
(y-allylalcohol)
% h p+
cisltrans 33:67
oc ph%
cisltrans 23 177 Ph,P
16
>i=CHZ
80 "C (a-SMe,)
CP
tempted. No asymmetric induction could be obtained with tungsten complexes and only low enantioselectivities were obtained in the cyclopropanation of E- 1phenylpropene with enantiomerically pure [Cp(CO)(Ph,P)Fe=CH,] [BF,] (generated from the corresponding chloride [483] (9% ee) or menthyl ether [458] (39% ee)). With the enantiomerically pure ethylidene complex [Cp(CO)(Ph,P)Fe=CHMe] [OTfl, however, vinyl acetate could by cyclopropanated with up to 95% ee [55,631]. Unfortunately the preparation of these resolved iron complexes is very tedious, and,
112
3 Non-Hetermtom-Substituted Carbene Complexes
also in view of the low stability of the iron carbene precursors, these reagents will probably not gain in popularity. Recent results 16321 indicate, though, that the iron complex [Cp(CO),Fe(THF)] [BF,] can be used as a catalyst for the cyclopropanation of olefins with phenyldiazomethane. The complex [Cp(CO),Fe=CHPh] [BF,] has been proposed as reactive intermediate. Thus, if enantiomerically pure, configurationally stable complexes of the type [Cp(CO)(R,P)Fe(THF)] [BF,] could be prepared, these might be useful catalysts for enantioselective cyclopropanations. Titanium Carbene Complexes A mechanistically interesting synthetic procedure for the synthesis of vinylcyclopropanes was reported in 1997 by Takeda [33] (Figure 3.34, Table 3.3). Mg, 2 P(OEt),, 20 OC MS 4 A, THF, 2 h CI CP,d CP'
'c,
&R'
R2?-R D
or SR *
RZ&R'
R2Y-sR
42-93%
I
SR
Fig. 3.34. Cyclopropanation with titanium carbene complexes generated in situ [ 3 3 ] .
The mechanism proposed for this intriguing reaction consists in desulfurization of the dithioacetal by one equivalent of the intermediate titanium(I1) complex and formation of a titanium vinylcarbene complex. [2 + 21 Cycloaddition to the alkene can lead to a titanacyclobutane intermediate which undergoes reductive elimination to yield the final cyclopropanes. The inference of a Tebbe reagent-like organotitanium intermediate is further substantiated by the report that the same reagent formed from dithioacetals and Cp,Ti[P(OEt),], olefinates ketones and esters [633] and undergoes olefin metathesis with ally1 silanes 16341 (see also [635]). This cyclopropanation reaction is indeed surprising, because titanacyclobutanes do not normally undergo reductive elimination with simultaneous formation of cyclopropanes upon thermolysis, but rather [2 + 21 cycloreversion [31].
3.2 Synthetic Applicutions of Non-Heteroatom-Substituted Curbene Complexes
1 13
Table 3.3. Cyclopropanation of alkenes with titanium carbene complexes generated in situ 1331. Dithioacetal
Alkene
Product
Yield
cishrans 2 2 7 8
ZIE 4:96
ZIE 18:82
Experimental Procedure 3.2.2. Cyclopropanation with a Titanium Carbene Complex: (E)-1-Hexyl-2-(2-phenylethenyl)cyclopropane1331
A>
Ph
+
222.37
Cp,TiCI,, Mg, P(OEt),, MS 4 A, 20 OC. THF, 6 h &Hex 112.22
228.38
A flask is charged with finely powdered molecular sieves (4 A, 50 mg), magnesium turnings (97 mg, 4.0 mmol), titanocene dichloride (249 mg, 1.0 mmol), THF (5 mL), triethyl phosphite (0.34 mL, 326 mg, 2.0 mmol) and 1octene (224 mg, 2.0 mmol), in the order given. The resulting mixture is stirred for 2 h at room temperature and a solution of 2-[(E)-2-phenylethenyl]-1,3-ditbiane (111 mg, 0.5 mmol) in THF (2 mL) is added. The mixture is stirred for 4 h and then diluted with hexane (30 mL). Filtration through Celite, concentration of the filtrate, and purification of the product by preparative TLC gives 82 mg (72%) of the title compound as a mixture of cis and trans cyclopropanes (cisltruns 60:40).
1 14
3 Non-Heteroatom-Substituted Curbene Complexes
3.2.2.2 Catalytic Cyclopropanations with Diazoalkanes Some transition metal complexes readily react with ylides to yield electrophilic carbene complexes. If these complexes can transfer the carbene to a given substrate in such a way that the original transition metal complex is regenerated then this complex can be used as a catalyst for the transformation of the ylide (carbene precursor) into carbene-derived products (Figure 3.35).
p
R
Fig. 3.35. Mechanism for the catalyzed cyclopropanation with ylides as carbene complex precursors.
Diazoalkanes are the carbene complex precursors most commonly used for the catalytic cyclopropanation of alkenes. Reactions involving this type of ylide will be discussed in this section.
Scope and Limitations: Catalysts From the mechanism sketched in Figure 3.35 it can be concluded that catalytically active complexes need a free coordination site (or a readily displacable ligand) and must act as a Lewis acid towards the diazoalkane. Some of these catalysts can, therefore, also coordinate strongly to alkenes [636]; this might lead to slight deactivation of a given catalyst if large amounts of an electron-rich olefin are present [626,637]. Strong nucleophiles (iodide, pyridine, amines, thiols) can also attenuate or completely destroy the activity of some catalysts [636]. Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brprnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(I1) acetate [6406421, rhodium(I1) carboxylates [626,643] and copper(1) triflate [636]. Copper(I1)
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
115
salts can also be used [644], because reduction of Cu(I1) by diazoalkanes to catalytically active Cu(1) is generally fast [636]. The complexes (CO),Cr(COD) [52,512] and [Cp(CO),Fe(THF)] [BF,] [632] have also been successfully used to catalyze cyclopropanations with aryldiazomethanes. The high cis-selectivity observed with [Cp(CO),Fe(THF)][BF,] parallels the stereoselectivity seen when stoichiometric amounts of the complex Cp(CO),Fe=CHPh+X- are used as cyclopropanating agent. The chemoselectivity of a given catalyst-carbene complex usually increases if the additional ligands become more electronegative [636]. Scope and Limitations: Substrates
The transition metal-catalyzed cyclopropanation of alkenes with diazomethane is a valuable alternative to Simmons-Smith methodology [645]. Because of the mild reaction conditions under which this reaction takes place, diazomethane is the reagent of choice if sensitive olefins are to be cyclopropanated [646-6481. Since the discovery of this reaction by Dull and Abend [617] several improved procedures have been reported. Besides copper(1) complexes, palladium(I1) acetate has become the most widely used catalyst for this reaction. For cyclopropanation of alkenes devoid of base-sensitive functional groups a ‘one-pot’ procedure has been developed [649]. In this procedure diazomethane is generated in a biphasic system from N-methyl-N-nitrosourea and potassium hydroxide in the presence of a palladium complex (e.g. Pd(acac),, (PhCN),PdCl,, or Pd[P(OPh),],) and the alkene. In this way the handling of diazomethane is elegantly avoided. In cyclopropanations with electrophilic carbene complexes, yields of cyclopropanes tend to improve with increasing electron density of the alkene. As illustrated by the examples in Table 3.5, cyclopropanations of enol ethers with aryldiazomethanes often proceed in high yields. Simple alkyl-substituted olefins are, however, more difficult to cyclopropanate with diazoalkanes. A few examples of the cyclopropanation of enamines with diazoalkanes have been reported [650]. The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptorsubstituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3dipolar cycloaddition to the olefin [651-6541. The resulting 3H-pyrazolines can either be stable or can isomerize to lH-pyrazolines. 3H-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions.
1 16
3 Non-Heteroatom-SubstitutedCarbene Complexes
Scope and Limitations: Diazoalkanes The limitations of cyclopropanation with diazoalkanes are mainly determined by the nature of the latter, which are hazardous to handle. The use of isolated diazoalkanes is confined to small-scale laboratory applications. Most electrophilic carbene complexes with hydrogen at Cp will undergo fast 1,2-proton migration with subsequent elimination of the metal and formation of an alkene. For this reason, transition metal-catalyzed cyclopropanations with non-acceptor-substituted diazoalkanes have mainly been limited to the use of diazomethane, aryl-, and diaryldiazomethanes (Tables 3.4 and 3.5). Some diazoalkanes cyclopropanate olefins in the absence of any catalyst [6586601. Thus, for instance, upon generation from N-cyclopropyl-N-nitrosoureaat 0 "C diazocyclopropane spontaneously cyclopropanates methylenecyclopropanes [658]. Thermal, uncatalyzed cyclopropanations of unactivated olefines with aryldiazomethanes can already occur at only slightly elevated temperatures (e.g. at 80 "C with I-naphthyldiazomethane [661]). Hence, for enantioselective cyclopropanations with a chiral catalyst, low reaction temperatures should be chosen to minimize product formation via the uncatalyzed pathway. The most common byproducts encountered in cyclopropanations with diazoalkanes as carbene precursors are azines and 'carbene dimers', i.e. symmetric olefins resulting from the reaction of the intermediate carbene complex with the diazoalkane. The formation of these byproducts can be supressed by keeping the concentration of diazoalkane in the reaction mixture as low as possible. For this purpose, the automated, slow addition of the diazoalkane to a mixture of catalyst and substrate (e.g. by means of a pump or a syringe motor) has proven to be a very valuable technique.
Stereoselectivity Cyclopropanations with diazomethane can proceed with surprisingly high diastereoselectivities (Table 3.4) [643,662-6641. However, enantioselective cyclopropanations with diazomethane and enantiomerically pure, catalytically active transition metal complexes have so far furnished only low enantiomeric excesses [650,665] or racemic products [666]. These disappointing results are consistent with the results obtained in stoichiometric cyclopropanations with enantiomerically pure Cp(CO)(Ph,P)Fe=CH,+X-, which also does not lead to high asymmetric induction (see Section 3.2.2.1).
3.2.2.3
Catalytic Cyclopropanations with Other Carbene Precursors
Sulfur Ylides Sulfonium ylides R,S=CR', [672,673] and metallated sulfones [674-6761 can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
1 17
Table 3.4. Palladium- and copper-catalyzed cyclopropanations with diazomethane. Catalyst
Substrate
Conditions
Product
0
1
Pd(OAc),
dPh Et20,00C Ph
Ph
2
4
Pd(OAc),
Pd(PhCN),Cl,
Yield
Ref.
98%
[642]
97%
[641]
63%
16671
88%
[649]
100%
16641
A
COPh
A
/
O
P
h
Et,O, 0 "C
MoPh
CH2C12,Et20, KOH;(insitu CH2N2)
/d.'ph
CH2C12,Et,O,
0tPh
71%
R,N
0
II
0
0
(IR,2R)/( 1S,2s) 93:7
[663] see also [6681 ~ 9 1
R: 2-thienyl Ph
7
Ph
Ph
Ph
Et20, -15 "C
Pd(OAc), Ph
Ph
Ph
98%
[670]
68%
16551
Ph
(lS,2S)/(lR,2R) 69:31
NHBoc
OAc
NHBoc
two diastereomers 1 : I
such as Michael additions or even uncatalyzed cyclopropanations of alkenes 16771. It is probably for this reason that the reaction has not become more widely accepted in organic synthesis. From a theoretical point of view metal-catalyzed cyclopropanation of alkenes with sulfur ylides is, however, very interesting, because it nicely illustrates the chemical similarity of diazocompounds and sulfur ylides.
1 18
3 Non-Heteroutom-Substituted Curbene Complexes
Table 3.5. Transition-metal-catalyzed cyclopropanations with diazoalkanes. Diazoalkane
Substrate
Catalyst, Conditions
Product
Rh2(0Ac)4, Et20, 25 "C
Yield
Ref.
92%
[626]
92%
(6261
38%
[626]
6%
(6261
80%
[632]
56%
[671]
ph*
'OMe
cisltrans 13~87
Rh2(0Ac)4, Et20, 25 "C
Ph\njOBu cisltrans 7 1 :29
Rh2(0Ac)4, Et20, 25 "C
Ph\n/Ph cisltrans 77~23
'"\4
Rh,(OAc),,
Et20, 25 "C
Bu
cisltrans 47:53
[Cp(CO),Fe(THF)I[BF417 -78 "C, CH2CI2
ph&ph
cisltrans 96:4
Cu(OTf),, 25 "C, neat alkene O2N
Rh,(OAc),, THF
&
6%
38%
[657]
55%
[6571
CI
cisltrans 60:40 CI
Rh2(0Ac)4, THF
CI
cisltrans 84:16 CI.
CI
cisltrans 96:4
lo
N \2\
3.2 Synthetic Applicutions qf Non-Heteroutom-SubstitutedCurbene Complexes
119
Cyclopropenes As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone dimethylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD),, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2 + I ] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[ 1.1 .O]butane also reacts with Ni(COD), to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). Interestingly, copper(1) salts also catalyze the cyclopropene-vinylcarbene isomerization [68 11. In this case the transient carbene complexes again show electrophilic behavior, behavior similar to that of the complexes formed from copper(1) salts and diazoalkanes or sulfonium ylides.
3.2.3 C-H Insertions One of the most fascinating transformations of free carbenes, generated for instance by photolysis of diazoalkanes or by a-elimination, is their insertion into aliphatic C-H bonds. This ability of carbenes is not only of theoretical interest, but also a unique tool for the synthesis of highly strained compounds such as, e.g., bicyclo[ 1.1 .O]butanes.
3.2.3.1 C-H Insertions of Nucleophilic Carbene Complexes Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). R
C-H insertion D
Fig. 3.36. Possible mechanism of the insertion of nucleophilic carbene complexes into aliphatic C-H bonds.
I,
L,M+
alkane elimination
Y
H
As already discussed (Section 3.1.1) the elimination of, for instance, neopentane from penta(neopenty1)tantalum corresponds to an a-deprotonation of one alkyl ligand by another, the latter being eliminated as neopentane. Hence in the reverse reaction the carbene carbon atom of the (nucleophilic) carbene complex must formally deprotonate the incoming alkane with simultaneous electrophilic attack of the metal at the newly formed, carbanionic alkyl group (Figure 3.36).
120
3 Non-Heteroatorn-SubstitutedCarbene Complexes
Table 3.6. Transition-metal-catalyzed cyclopropanations with carbene precursors other than diazoalkanes. Carbene Precursor
1
ph\ S=CH, Ph/
2
ph\ ,S=CH, Ph
Substrate
Catalyst, Conditions Cu(acac),, 20"C,THF
-BU
@ O / Y
C0,Me
..(.)'
Cu(acac),, 20 "C, THF
2
Yield
Ref.
48%
[672]
45%
(6721
50%
[679]
C0,Me
C0,Me
9
Product
Ni(COD),, 40 "C, C6H6, exothermic
..C0,Me
C0,Me
cop
MeoxoMe LCO,Me
Ni(COD),, 0 "C, Et,O Me0
MeoxoMe &CO,Me
Jt*, 80%
(6781
74%
[678]
"C0,Me
Ni(COD),, 0 "C, Et,O
f
"C0,Me
Me0
cisltrans 8192
pCO,Me
,
39%
D
[680]
t
23%
il, CuCI, P(OPh),, -30 "C, 10 min
C0,Me
72%
[681] [6821
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
-
80 OC. C6Dl, - CMe,
-
Ti Nd
Nd
[Ti(IV), do,8 el-
AD
(Girolami, 1997)
N d C6D1f (structural proposal based on the products from aqueous hydrolysis)
- p-20 OC. C,D6 - CMe,
NP\ T',
121
- c p \ Ti -
YD
cp\T',
cd
Cd
(Hessen, 1995)
c6D5
iTi(IV), do, 16 el
RHN \ RHN-Zr-Cy RHN'
c - C ~ H 100 ~ , OC C6H12, 8 h 35%
-
-
-
NR
(Wolczanski, 1996)
RHN'
-
R = tBu,Si
RHN R,N-\,rd
-
[Zr(lV), do, 8 el NR
NR
II
RHN-V-CH3 RHN' R = tBu,Si
80 OC, 16 h
NR
II
RHtV+NR
*-,
C6H6
II
RHN-V-Ph
RHN' ,
C H (Me) c RHN-V RHN'
(Horton, 1993)
Fig. 3.37. Insertion of nucleophilic carbene and imido complexes into aliphatic and aromatic C-H bonds [408,683,689,691].
Inter- and intramolecular (cyclometallation) reactions of this type have been observed, for instance, with titanium [408,505,683-6851, hafnium 141 11, tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688]. Not only carbene complexes but also imido complexes L,M=NR of, e.g., zirconium [689,690], vanadium [69 11, tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. It should be mentioned here that nucleophilic titanium carbene complexes, generated in situ from dithioacetals and c ~ , T i [ P ( o E t ) ~(see ] ~ e.g. Sections 3.2.2.1 and 3.2.4.2), can undergo M-H insertions with silanes, germanes and stannanes [694]. This reaction represents an interesting alternative procedure for the che-
122
3 Non-Heteroatom-Substituted Carbene Complexes
mica1 modification of these important synthetic intermediates. Similarly, isolated ruthenium carbene complexes have been used to alkylate silanes by Si-H insertion [581].
3.2.3.2
C-H Insertions of Electrophilic Carbene Complexes
Carbene C-H (and Si-H, [695]) insertion is characteristic of electrophilic carbene complexes. In particular the insertion reactions of acceptor-substituted carbene complexes (Section 4.2) have become a valuable tool for organic synthesis. Few examples of C-H insertions have been reported for carbene complexes without electron-withdrawing groups attached to the carbene carbon atom [696]. Most of these are C-H insertions of cationic iron(1V) carbene complexes. Calculations performed for cyclopropanation with Fischer-type carbene complexes [28] indicate that the electrophilic attack of the carbene complex at the alkene and the final ring closure are concerted. Extrapolation from this result to the C-H insertion reaction (in which a o-bond instead of a n-bond is cleaved) suggests that C-H bond cleavage and the formation of the new C-C and C-H bonds might also be concerted (Figure 3.38).
[Fe(ll), d6, 18 e]
Fig. 3.38. Possible mechanism for the insertion of electrophilic iron(1V) carbene complexes into aliphatic C-H bonds.
However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. This unique C-C bond-forming reaction has been applied to the synthesis of natural products [480,4811. In the examples reported, intramolecular C-H insertion into R,C-H groups was used for the construction of more elaborate, polycyclic carbon frameworks. Representative examples are listed in Table 3.7.
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
123
Table 3.7. Intramolecular C-H insertion reaction of cationic iron carbene complexes generated in situ by S-alkylation of 1 -(phenylthio)alkyl complexes (see Experimental Procedure 3.2.3). Carbene Precursor
A
Product
Yield
Reference
124
3 Non-Heteroatonz-SubstitutedCurbene Complexes
ene
The results listed in Table 3.7 might seem surprising, in that p-elimination does not seem to be a problem. In one example only (Entry 1, Table 3.7) was a small amount of the elimination product obtained. For other alkyl-substituted iron carbene complexes this elimination quickly becomes the main reaction pathway (Figure 3.14). This behavior is consistent with the generally observed trend, that olefin formation from electrophilic carbene complexes is inhibited by electronwithdrawing groups at the P-carbon, and assisted by electron donors at this position. This is because the cationic intermediate which undergoes the elimination reaction will be destabilized if the group R (Figure 3.39) is electron-withdrawing, as is the case in Helquist’s carbene complexes (Table 3.7).
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
1 25
-
+
H+
R
-----+Hi
Cp(Co)zFexR
I
Fig. 3.39. One possible decomposition pathway for cationic iron(1V) carbene complexes.
Hence, cationic iron carbene complexes such as Cp(CO),Fe+=CHCHZR, in which Z is an electron-withdrawing group, might also be suitable for intermolecular cyclopropanation or C-H insertion reactions. The use of such carbene complexes in organic synthesis has not yet been thoroughly investigated, but could fruitfully supplement the chemistry of acceptor-substituted carbenes.
3.2.4 Carbonyl Olefination The most useful methods for the formation of C-C bonds are based on the addition of C-nucleophiles to carbonyl compounds. Among the many variations of this basic scheme phosphorus ylides, capable of olefinating aldehydes or ketones in a single step, have proven to be exceedingly valuable reagents in organic synthesis. As discussed in previous sections, high-valent carbene complexes of early transition metals have ylide-like, nucleophilic character. Some Schrock-type carbene complexes react with carbonyl compounds in the same manner as do phosphorus ylides, namely by converting the carbonyl group into an alkene. It is particularly interesting, that some titanium and tantalum carbene complexes olefinate derivatives of carboxylic acids. These reagents are, moreover, much less basic than phosphorus ylides, and thus enable the olefination of strongly C-H acidic carbonyl compounds.
3.2.4.1 Carbonyl Methylenation In 1978 [30] Tebbe reported the preparation and characterization of the product formed from titanocene dichloride and two equivalents of trimethylaluminum (Figure 3.40). This product (‘Tebbe reagent’) is a four-membered methylene complex containing titanium and aluminum. The mechanism of its formation has been studied by Grubbs [697] (among others). Unlike phosphoranes, which normally react with ketones and aldehydes only, the Tebbe reagent methylenates a broader range of carbonyl compounds, including esters, amides, and carbonates (to yield enol ethers, enamines, and ketene acetals, respectively) [698]. Elaborate organic substrates and dicarbonyl compounds can also be methylenated in high yields with the Tebbe reagent (Table 3.8) [699-7021. Acid chlorides or
126
3 Non-Heteroatom-SubstitutedCurbene Complexes 2 AIMe,, 20 OC PhMe, 60 h 80-90%
*
CP'
'c(
Tebbe reagent
0
Fig. 3.40. Preparation and use of the Tebbe reagent for carbonyl methylenation.
2.5 eq. Tebbe reagent -40 OC to 25 "C THF/CH,CI, 2.1
1. 5 eq. rBu,Al 2. Swern oxidation
z
74%
69%
\ (Paquette, 1992)
Fig. 3.41. Ring expansion by tandem double Tebbe-Claisen methodology [699].
anhydrides are converted into titanium enolates of the corresponding methyl ketones [703]. Some nitriles can be coupled to P-ketoenamines by treatment with titanium methylene complexes [704]. An elegant ring expansion methodology has been developed by Paquette [699] (Figure 3.41). This synthetic sequence is based on the double methylenation of an a-(acy1oxy)aldehyde to give an ally1 vinyl ether, which undergoes (reductive) Claisen rearrangement when treated with trialkylalanes. Most experimental data suggest that the actual methylenating agent derived from the Tebbe reagent upon treatment with a weak base, is the highly reactive carbene complex Cp,Ti=CH, [709]. This complex is a typical Schrock-type carbene, because it is high-valent [Ti(IV)], electron-deficient (16 valence electrons) and nucleophilic at carbon. The carbonyl olefination is mechanistically closely related to olefin metathesis (Figure 3.40). [2 + 21 Cycloaddition of the carbene complex to the carbonyl
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCurbene Complexes
127
Table 3.8. Examples of methylenations with the Tebbe reagent and with dimethyl titanocene. Substrate
Reagent (Conditions)
Product
excess Tebbe reagent, 20 "C, THF, 9 h (intermediateClaisen rearrangement)
cp\ /Ti
/%
Yield
Ref.
52%
[706]
74%
[707]
68%
[708]
72%
[708]
'CH, 65 "C. THF 0 Phd0,TBS
PhdO,TBS
compound leads to a metallaoxetane which upon ring fission yields the new alkene and a metal-oxo complex. The force driving this (exothermic [710]) process is the formation of a strong M-0 double bond [397]; this renders the reaction irreversible. Hence, in contrast to olefin metathesis, which can be catalyzed by a carbene complex, in carbonyl olefination stoichiometric amounts of the organometallic reagent are required. Besides the Tebbe reagent, other types of transition-metal-based methylenating reagents have been developed. Some of these are prepared in situ from several components and the actual methylenating agent and the mechanism of its formation remain unknown. A selection of such methylenating reagents is given in Table 3.9. Some of these reagents are much easier to prepare and to handle than the Tebbe reagent, and are increasingly being used in organic synthesis. In particular the methylenation of ketones with dibromomethanehitanium tetrachloride/zinc ('Lombard0 reagent') proceeds very smoothly even with sensitive substrates, and has almost completely replaced the Tebbe reagent.
128
3 Non-Heteroutom-SubstituteclCurbene Complexes
Table 3.9. Scope and limitations of different types of methylenating reagent. Methylenating Reagent CP, A / Cp'T? /A', CI
Scope and Limitations
Reference
methylenation of aldehydes, ketones, esters, thioesters, amides, carbonates; can also induce olefin metathesis
in situ preparation: [711-7131
methylenation of aldehydes, ketones, esters, thioesters, amides, carbonates; less air-sensitive than Tebbe reagent
16981
methylenation of aldehydes, ketones; no reaction with esters
[7141
methylenation of ketones
[7 151
'Tebbe reagent' CP,Ti CP'
x
Cp,ZrCl,, Zn, CH,Br2, THF
Cp,TiCI,, Zn, CHJ,, THF Zn, TiCI,, CH,Br,, THF, CH,CI,
methylenation of aldehydes, ketones; no reaction with esters, THP-ethers, TBS-ethers, carboxylic acids, alcohols
[7 16,7171
CrCI,, CHI,, THF
conversion of aldehydes and ketones R,C=O into R,C=CHI
[7 181
conversion of aldehydes, ketones, and esters RXC=O into RXC=CHSPh
[7191
methylenation of aldehydes, ketones, esters, amides, imides, thioesters, acylsilanes, anhydrides, carbonates
[708,709,720]
Cp,TiMe,, THF or PhMe, 60-75 "C r Z n -\ B r Zn THF LZ&Br
methylenation of aldhydes and ketones
[7211
(Nysted reagent) BF,OEt,, THF, 0 "C MoOCI,(THF),, MeLi or AIMe,, THF
methylenation of aromatic aldehydes, ketones
[722-7241
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carhene Complexes
129
Experimental Procedure 3.2.4. Methylenation of an Ester with Tebbe Reagent: 1-Phenoxy-1-phenylethene[712,7131 AIMe.. PhMe
0
II
198.22
Ph'
249.00
196.25
Trimethylaluminum (20 mL of a 2 M solution in toluene, 40 mmol) is added to titanocene dichloride (5.0 g, 20 mmol) under inert gas. The resulting mixture is stirred at room temperature for 3 d with evolution of methane. The mixture is then cooled to 0 "C and a solution of phenyl benzoate (4.0 g, 20 mmol) in THF (20 mL,) is added over 5 min. The resulting mixture is left to warm to room temperature within 45 min and is then diluted with anhydrous diethyl ether (50 mL). Approximately 50 drops of a 1 M aqueous sodium hydroxide solution are carefully added over 10-20 min. When gas evolution has ceased anhydrous sodium sulfate is added and the slurry is filtered through a pad of Celite. After rinsing with diethyl ether the combined filtrates are concentrated and the product is purified by column chromatography (150 g basic alumina, pentane/diethyl ether 9:l). 2.8 g (70%) of the title compound is obtained. 'H NMR (250 MHz, CDC1,) S 4.45 (d, 2.3 Hz, lH), 5.05 (d, 2.3 Hz, lH), 7.06-7.1 1 (m, 3H), 7.29-7.38 (m, 5H), 7.66-7.70 (m, 2H).
3.2.4.2 Carbonyl Olefination with Higher Alkylidenes There are unfortunately few Tebbe-type reagents capable of transferring alkylidenes other than methylene. If in the preparation of the Tebbe reagent triethylaluminum is used instead of trimethylaluminum, other types of product result [725]. Higher homologs of the Tebbe reagent can be prepared from Cp,TiCl(CH=CHMe) and diisobutylaluminum hydride [726] and the resulting reagent can be used for the olefination of ketones (Entry 3, Table 3.10). These (rather difficult-to-make) reagents have, however, not been further used in organic synthesis. Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-7291, probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereoselectivity (Table 3.1 1). A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes.
130
3 Non-Heteroatom-Substituted Carbene Complexes
Table 3.10. Carbonyl olefination with Tebbe-type titanium reagents. Reagent, Conditions
&
Substrate
Product
Do0""'-
Yield
Ref.
55%
[731]
83%
[732]
50%
[726]
0 OC, CH,CI,, 0.5 h
Do 0"" vo
98% [733] (NMR)
\ C,H6 or PhMe, reflux
&
5
Cp-Ti CP T
/ P
h
98% [733] (NMR)
Ph
C6H6or PhMe, reflux SiMe, I
6
CP' "\Ti+SiMe, I
SiMe,
25 "C, PhMe, 1-12 h
Ph
e0 ..~ .
0x0 Ph'
23 "C, PhMe, 10 h 8
P Cp'TB CP,
50 "C, PhMe, 10-15 h
72%
[608]
83%
[605]
65%
[734]
0x0
fi I,, Ph
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes Table 3.11. Carbonyl olefinations with nucleophilic carbene complexes. Reagent, Conditions
Substrate
Product
Yield
Ref.
[7 101 R' A
X
NP
R': Me; X: OEt; 2: 1 R': H; X: NMe,; 5:95
25 "C, Et,O or pentane
60% 77%
50%
B'zo
NP
25 "C, Et,O or pentane 3
'',pBu \PMe,Ph
"'
75 "C, PhMe, 3 h 4
cp,FBu \PMe,Ph
80%
[735]
85%
[735]
ZIE 49:s 1
Ph-BU
N 'P h n '
"'
[710]
ZIE 4753
75 "C, PhMe, 3 h N I1
OR' A
P
ROyWRO
R: CMe(CF,), Ar: 2,6-iPr,C6H3; PhMe or C,H,, 0.5-6 h
0
ukn
17361 R': Ac, 20 "C; ZIE 33:66 R': H, 20 "C; ZIE = 1 1 :89 R': H, -78 "C; ZIE < 1 :99
70% -
70%
V361 RO'
R: CMefCF,L Ar: 2,6-iPr,C,H3;
ChHhr0 . 5 4 h
R': Ac; 20 "C R': H, 20 "C; ZIE < 1 :99
0% 88%
131
132
3 Non-Heteroutom-Substituted Curbene Complexes
Table 3.12. Carbonyl olefinations with carbene-complex-like reagents generated in situ. Reagent, Conditions
Substrate
Product
Yield
Ref.
88%
17371 [7 171
87%
17381
79%
[738]
64%
16331
79%
[633]
Br
ZIE 92:8
Zn, TiCI,, TMEDA, 25 "C, THF, 2 h
Zn, TiCI,, TMEDA, 25 "C, THF, 0.5 h
0". & SPh
Br
Zn, TiCI,, TMEDA, 25 OC, THF, 3 h 4
Cp,TiCl,, Mg, P(OEt13, 20 "C, THF, 3 h; then Me,C(SPh),, 5 min; then add lactone, 2 h
ZlE91.9
%Ph O
e
P
h
Cp,TiCI,, Mg, P(OEt),, 20 OC, THF, 3 h; then O PhY Ph
5 min; then add ketone, 0.5 h
6
Cp,TiCI,, Mg, P(OEt),, 20 "C, THF, 3 h; then PhCH(SPh),, 5 min; then add ester, 2 h
ZIE 53:47
o T H e x
OEt
,,THex 61%
[633]
79%
[739]
ZIE 77:23
Cp,TiCI,, Mg, P(OEt),, 20 "C, THF, 3 h; then 7
SPh M e , S i A S P h
10 min; then add lactone, 0.5 h
ZIE 56:44
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
133
Experimental Procedure 3.2.5. Alkylidenation of an Ester: 5-Methoxy-5undecene [737]
Arne + AB1 L," TiCI,,TMEDA,Zn
Bu
117.17
243.97
184.32
To THF (10 mL) at 0 "C under inert gas are added, in this order, titanium tetrachloride (4 mL of a 1 M solution in dichloromethane, 4 mmol), TMEDA (1.2 mL, 0.93 g, 8.0 mmol), and zinc dust (0.59 g, 9.0 mmol). The resulting mixture is stirred at 25 "C for 30 min and a solution of methyl pentanoate (0.12 g, 1.0 mmol) and 1,l-dibromohexane (0.54 g, 2.2 mmol) in THF (2 mL) is added. The dark mixture is stirred at 25 "C for 2 h, cooled to 0 "C, and treated with potassium carbonate (1.3 mL of a saturated aqueous solution). After stirring for a further 15 min at 0°C ether (20 mL) is added and the resulting mixture is filtered through a short column of basic alumina (activity 111) using etherhiethylamine (200: 1, 100 mL). Concentration of the filtrate and column chromatography (basic alumina, pentane) gives 0.18 g (96%; Y E 91:9) of the title compound. I3C NMR (CDCI,, 2 isomer) 6 13.9, 14.1, 22.3, 22.6, 24.7, 29.4, 29.8, 31.1, 31.6, 56.4, 110.1, 155.0.
Experimental Procedure 3.2.6. Alkylidenation of a Ketone: 4-Methyl-1-phenylocta-1,3-diene [633] MS 4 A, Mg, Cp,TiCI,, P(OEt),L
100.16
222.37
phL 200.33
A, 150 mg), magnesium turnings (43 mg, 1.8 mmol), titanocene dichloride (374 mg, 1.5 mmol), THF (4 mL), and triethyl phosphite (0.51 mL, 2.9 mmol) is stirred under argon at room temperature for 3 h. A solution of (E)-2-(2-phenylethenyl)-1,3-dithiane (122 mg, 0.55 mmol) in THF (1 mL) is added, followed after 5 min by dropwise addition of a solution of 2-hexanone (50 mg, 0.50 mmol) in THF (2.5 mL). The resulting mixture is stirred at room temperature for 30 min, diluted with hexane (30 mL), and filtered through Celite. Concentration of the filtrate and purification of the crude product by preparative TLC (hexane/ethyl acetate 98:2) yields 74 mg (74%; W E 60:40) of the title compound. A mixture of finely powdered molecular sieves (4
134
3 Non-Heteroatom-SubstitutedCarbene Complexes
"
1 'TiCp,
* H
(Nicolaou, 1996)
rn0Jh+ a0Jh - 2
0
RO-Mo,
Rd
3Ph
ROH = (CF,),MeCOH
(Grubbs, 1993)
Fig. 3.42. Tandem alkene metatheses-carbonyl olefinations [740-7421.
One remarkable application of carbene complexes is the combination of olefin metathesis with carbonyl olefination. If a given substrate has both C-C and C-0 double bonds, it might be possible to realize with a given carbene complex olefin metathesis to yield a new carbene complex, followed by an intramolecular carbonyl olefination step. As emphasized above, because of the irreversibility of the carbonyl olefination, stoichiometric amounts of carbene complex will be required. Figure 3.42 shows some examples of such tandem-metathesis-olefination reactions.
3.2.5
Olefin Metathesis
Olefin metathesis refers to a process in which two alkenes exchange their alkylidene fragments. This reaction has recently been reviewed [743 -7471. An exciting account on the discovery of olefin metathesis has been written by Eleuterio [748].
3.2 Synthetic Applications of Non-Heteroutom-SubstitutedCarbene Complexes
135
4 eq. Tebbe reagent reflux, THF D
0 H
OBn
61%
H OBn
(Nicolaou, 1996)
OBn
Fig. 3.42. continued.
Streck has discussed economic and ecological aspects of the industrial application of this reaction [749]. A thorough discussion of the subject is to be found in a monograph by Ivin and Mol [750].
3.2.5.1 General Considerations Olefin metathesis reactions have been grouped into the different categories shown in Figure 3.43. The metallacyclobutane mechanism of olefin metathesis has been discussed in Sections 1.3 and 3.1.7. For metathesis of acetylenes carbyne complexes are generally required (Figure 3.44), and both heterogeneous and homogeneous catalytic systems have been developed for this purpose. Homogeneous, single-component catalysts such as, e.g., W(=CCMe,)(OCMe,), or W(-CMe)(OCMe2CF3),, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,o-diynes, and RCM of a,odiynes [75 I]. When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-7541. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal tert-butyl groups have been prepared with the aid of a tungsten carbene complex [755].
136
3 Non-Heteroatom-SubstitutedCurbene Complexes Exchange metathesis
Self metathesis (degenerate)
Self metathesis (productive)
-
2
2
A
./ +
m+./
-
Bu Cross metathesis
2
-
Ph-ph
Bu P
P
h
Ring-openingmetathesis polymerization (ROMP) r
1
H
n
Fig. 3.43. Types of olefin metathesis reaction.
Industrial Applications
In contrast with other types of carbene complex reaction discussed in this book, which mainly are tools for the synthesis of fine chemicals, the development of olefin metathesis was essentially driven by the petrochemical industry. The first examples of this reaction were the polymerization of norbornene and the metathesis of propylene to 2-butene and ethylene, described by Eleuterio [756] and Peters [757]. The catalysts initially used for exchange metathesis were supported metal oxides, such as MoO,/Al,O, or WO,/SiO,, and temperatures as high as 500 "C were necessary. The first catalysts for ring-opening polymerization were Ziegler-Natta-type catalysts (TiCl,/AIR,).
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
131
Ring-closing metathesis (RCW
Acyclic diene metathesis (ADMET)
Fig. 3.43. continued.
/
r
1
Fig. 3.44. Reactions of carbyne and carbene complexes with alkynes.
As early as 1966 Phillips Petroleum Co. began to operate a 50 000 t/y metathesis plant in Canada, producing 2-butene and ethylene from propene (Phillips Triolefin Process). Currently, the most important industrial application of olefin metathesis is probably the 'Shell Higher-Olefin Process' (SHOP, Figure 3.45). In this process ethylene is first oligomerized under nickel catalysis to a mixture of C, to C,, terminal alkenes. The desired C,-C,* fraction is separated by distillation and further used as copolymer for polyethylene production, or is hydroformylated and reduced to detergent alcohols. The undesired oligoethylene fractions (< C, and > C,8) are mixed and isomerized with an alkali metal-based catalyst to internal alkenes. These are subjected to repeated exchange metathesis (100-125 "C, supported oxide catalyst) 10-1 5 % of the desired detergent-range alkenes being obtained per pass. Alternatively, cross metathesis with ethylene can be used to produce terminal alkenes of lower molecular weight. The world-wide capacity for the Shell higher-olefin process reached lo6 tly in 1990.
138
3 Non-Heteroatom-Substituted Curbene Complexes
// NilPR,
I
oligomerization
metathesis detergents polymers
-
1
distillation c6-c18
*
Fig. 3.45. Schematic representation of the Shell higher-olefin process (SHOP).
Further important industrial applications of olefin metathesis include the synthesis of 3,3-dimethyl-1-butene (‘neohexene’, intermediate for the production of musk perfume) from ethene and 2,4,4-trimethyl-2-pentene, the manufacture of a,o-dienes from ethene and cycloalkenes (reversed RCM), and the ROMP of cyclooctene and norbornene to VestenamerB and NorsorexB, respectively.
3.2.5.2 Heterogeneous Catalysts Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl, [758], Me,Sn/A1,03 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. The known catalyst systems for olefin metathesis can be grouped into heterogeneous catalysts, homogeneous multi-component catalysts, and homogeneous single-component catalysts. Heterogeneous catalysts are mainly used for gas-phase olefin metathesis in industrial production plants. These catalysts consist of a support (A1,0,, TiO,, Nb,O,, SiO,, etc.) which provides mechanical stability and a high surface area (> 200 m2 g-I), but can also have a decisive influence on the activity of the final catalyst. The support is mixed or impregnated with 1-20% of either a transition metal oxide or another metal derivative (see Table 3.13) [761,762]. Activation by treatment with H, [763,764], 0, [765-7671, CO [768], or HC1 at high temperatures [769] is usually necessary to obtain efficient catalysts. Activation can also be achieved by photolysis [764,766] or by treatment of the supported metal derivative with lower alkyl cyclopropanes [766,768]. It is assumed that the activation step generates free coordination sites or carbene complexes. Cyclopropanes are
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCurbene Complexes
139
believed to undergo oxidative insertion with the support-bound metal to yield metallacyclobutanes, which decompose into ethylene and a carbene complex by [2 + 21 cycloreversion [768]. Many heterogeneous and homogeneous metathesis catalysts require a so-called cocatalyst, such as BuLi, EtAlCl,, R,Sn [770], or R,Pb [771], for which several possible functions are being discussed. Organolithium compounds can react directly with the transition metal derivative to yield catalytically active carbene complexes (see Section 3.1.1). Alternatively, metal oxides can be reduced by the cocatalyst, generating free coordination sites. One further possibility is the formation of a metal-cocatalyst complex, which becomes the actual catalyst. This last option is probably the case for AlX,R,, "), as the activity of the resulting catalysts usually increases with increasing Lewis-acidity of the aluminum derivative used. Generally the activity of heterogeneous catalysts for olefin metathesis increases with its Lewis acidity [772]. Some heterogeneous and homogeneous metathesis catalysts also require promoters or additives, such as water, alcohols, or alumosilicates. Table 3.13. Heterogeneous catalysts for alkene metathesis.
6
7
Catalyst
Temperature
Substrate
MOO,, PTiO,, Me,Sn
25 "C
propen e
MOO,, SiO,
20 "C
propene
Mo(CO),, S O ,
27 "C
propene
Mo,(OAc),, SiO,
25 "C
propen e
WC1,(2,6-Br,C6H@),, NbO,, SiO,, iBuAIC1,
20 "C
2-pentene
WCI(CO),Cp, SiO,, iBuAICI,
20 "C
2-pentene
140 "C
1-butene
Re2%
8
Re,O,, AI,O,, PbEt,
35 "C
I-hexene
9
MeReO,, SO,, AI,O,
20 "C
2-pentene
10
polystyrene-bound (RCy,P),CI,Ru=CHCH=CPh,
20 "C
cis-2-pentene
Reference
140
3 Non-Heteroatom-SubstitutedCarbene Complexes
Table 3.14. Examples of multi-component catalysts for homogeneous-phase alkene metathesis. Catalyst
Temperature
Substrate
Reference
(Ar: 2,6-iPr,C,H3) 2
WCI,, Bu,Sn
30 "C
norbornene
3
WCI,, Et,AI,CI,, PhNH,
20 "C
propene
4
WOC1,(2,6-Br,C6H,O),, PbEt,
120 "C
2-alkenes
5
W(2-MeC6H,O),Cl,, Et,AI,CI,
20 "C
I-octene
6
WOCI,, EtAICI,
20 "C
2-pentene
V911
25 "C
cyclooctene
13931
20 "C
dicyclopentadiene
[792,793]
8
+ N,CHCO,EI
3.2.5.3 Homogeneous Catalysts Small-scale preparations and the chemical modification of fine chemicals and elaborate intermediates are usually conducted in solution. For this purpose soluble metathesis catalysts of predictable and reproducible activity are generally preferred. The catalytic systems presently known can be grouped into multi-component and single-component catalysts (Tables 3.14-3.16). Although the applications of Fischer-type carbene complexes are discussed in Chapter 2, their use as catalysts for olefin metathesis (a minor aspect of the chemistry of these compounds) will be discussed in this section. Low-valent, 18-electron carbene complexes (Fischer-type) are electronically and coordinatively saturated and must usually be activated to become efficient metathesis catalysts. This activation can be effected by thermal or photolytic [781,782] removal of one ligand, e.g. carbon monoxide, or by treatment with a Lewis acid (Table 3.15) [783]. High-valent, electron-deficient carbene complexes can either require activation (e.g. by a Lewis acid) to catalyze metathesis (Table 3.14) or not require any
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
14 1
Table 3.15. Fischer-type carbene complexes as catalysts for homogeneous-phase alkene metathesis. Catalyst
Temperature, Activator
Substrate
Reference
1
[(CO),Mo=C(O)Ph][NBu,], MeAICI,
-
styrene
WI
2
(CO),W=C(Ph),
40 OC
cycloalkenes, internal alkenes
[795,796]
3
(CO),W=C(OMe)Ph
50 "C
norbornene, cyclobutene, alkynes
[797,798]
4
(CO)5W=C(OMe)Ph
PhCgH
cyclopentene
[7991
5
(CO),W=C(OMe)Ph
hv
COD
[78 11
25 "C
norbornene
[goo1
6
+ AICI,
further activation (Table 3.16) [606]. Some of the known (Schrock-type) carbene complexes, which catalyze alkene metathesis without any further additive, have proven to be exceedingly efficient catalysts. These complexes can initiate ROMP at high rates, thus generating growing polymer chains of similar lengths. As the chaincarrying alkylidene complexes undergo practically no terminating side-reactions [22,396,604,784,785] some of these catalysts are able to induce living polymerization, and thus enable the production of polymers with very narrow molecular-weight distributions. The length of the polymer chains can be controlled by the amount of catalyst used and also by deliberate termination of the polymerization after a certain time. This can, for instance, be achieved by addition of an aldehyde, which reacts irreversibly with the carbene complex. Unlike in radical or anionic polymerizations, in ROMP with single-component metathesis catalysts the growing polymer chain remains able to further grow even after consumption of the monomer. This enables the manufacture of block copolymers with interesting physicochemical properties by sequential addition of different monomers to such 'living' systems. Most known single-component catalysts can be easily modified by simple ligand exchange; this enables precise fine-tuning of the activity and the stereochemical output of a given catalyst.
142
3 Non-Heteroatom-SubstitutedCarbene Complexes
Table 3.16. Alkylidene and carbyne complexes as single-component catalysts for homogeneous-phase alkene metathesis. Catalyst
Temperature
Substrate
Reference
15 "C
cis-2-pentene
[6 161
-
norbornene
W1
20 "C
cyclopentene
[8021
20 "C
norbornene, dicyclopentadiene
[427]
20 "C
2-pentene
40 "C
norbornene
[go31
80 "C
dicyclopentadiene
[804,805]
(Ar:2,6-iPr2C,H,) 2
Me,Si7 /-TaMe,Si
S ,M i e,
'SiMe,
4
R0.d
Ro'
NPh LSiMe,
ROH: (CF,),MeCOH generated by hv-induced elimination of TMS from the dialkyl complex 5
4, ROH: (CF,),MeCOH
6
The optimum catalyst for a given reaction depends primarily on (a) the energetics of the reaction and (b) the functional groups present in the substrate. If, for instance, a strained cycloalkene such as norbornene or cyclobutene is to be polymerized, a catalyst of low activity will be sufficient to attain acceptable reaction rates. RCM
143
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
can often also be accomplished with catalysts of low activity, in particular when five- or six-membered rings are formed and the equilibrium is continuously shifted by removal of one of the products (e.g. ethylene). Highly active catalysts will, on the other hand, be required when acyclic or highly substituted olefins have to be metathesized effectively. If olefin metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH,Cl, > toluene > THF). As discussed above, metathesis is initiated by the formation of a n-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize ally1 thioethers have, however, been described [787].
3.2.5.4 Scope and Limitations of Molybdenum- and Ruthenium-Based Homogeneous Catalysts Because of the enormous synthetic potential of molybdenum- [22] and rutheniumbased [57,806] single-component catalysts, a closer look at the scope and limitations of the most promising compounds known to date is appropriate. The systematic exploration of the synthetic possibilities offered by these new catalysts has just begun, and many new developments are to be expected in the near future 1744,746,747,8071. As quick reference for the organic chemist, the most relevant chemical properties of two types of frequently used catalyst (Figure 3.46) are listed below. These carbene complexes are quite robust and well-suited to the metathesis of elaborate organic intermediates.
I
k'
N
.
(CF,),MeCO -M , o& ( C(CF,),MeCO F , ) , M e C O/- l ? \lox
Fig. 3.46. Suitable catalysts for homogeneous-phase olefin metathesis.
lI
\ / R
R = Me, Ph
R' = Ph, CH=CPh,
A
B
Metathesis Activity Molybdenum complexes A (Figure 3.46) react efficiently with terminal and internal alkenes in toluene (e.g. 500 eq. Z-2-pentene are metathesized in 2 min at 25 "C; 20 eq. of styrene in 2 h at 25 "C). These catalysts also oligomerize 2,4-hexadiene [808] and 1,5-hexadiene [809] and promote RCM of enol ethers. Isomerization of alkenes by catalysts A is a potential catalytic side-reaction [810-8121.
144
3 Non-Heteroatorn-Substituted Curbene Complexes
Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl- 1-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of ally1 ethers to enol ethers has been observed with complexes B [582].
Stability and Reactivity towards Functional Groups Although solutions of molybdenum complexes A in methylcyclohexane or benzene are stable for several weeks at room temperature [416], these complexes are sensitive towards oxygen and protic solvents [8 13,8141. Aldehydes are quickly olefinated by complexes A, whereas variable reactivity is observed towards ketones [416,736]. With carboxylic esters usually no reaction occurs. Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60°C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [8151, or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. The increased stability of ruthenium carbene complexes towards oxygencontaining compounds might be because later transition metals, having more d-electrons, are softer and hence react better with soft bases, e.g. olefins. The early transition metals, on the other hand, having few d-electrons, are generally harder and react preferentially with hard bases, such as water or carbonyl compounds. 3.2.5.5
Ring-Opening Metathesis Polymerization (ROMP)
The mechanism of ROMP is sketched in Figure 3.47. Because of the high demand for new polymers with improved properties, ROMP has received much attention since the discovery of olefin metathesis [396,816,817]. One of the first applications envisioned of ROMP was the preparation of polyenes with rubber-like properties by ROMP of cyclopentene. Although this polymer did not have the optimum product profile, the polymer obtained by ROMP of cyclooctene can indeed be used in blends with other rubbers, conferring on the final product improved mechanical properties and simplifying its processing. This polymer, called Vestenamer 8012, is being produced by Chemische Werke Huls, Marl, Germany, and the annual capacity in 1990 reached 12 000 t. Comparison of the energetics of ROMP for the series of homologous lower cyclic alkenes (Table 3.17) suggests that three-, four-, and eight-membered and larger rings, should be polymerizable, because AG for ROMP of the parent compounds is negative. Because the free energy of the polymerization of five- to seven-membered
3.2 Synthetic Applications of Non-Heteroutom-Substituted Carbene Complexes
145
Initiation
Propagation
R'
Termination
Fig. 3.47. Mechanism of ring-opening metathesis polymerization catalyzed by carbene complexes.
Table 3.17. Thermodynamic parameters for the conversion of liquid monomer into solid amorphous polymer [750]. Monomer
Cyclobutene Cyclopentene Cyclopentene Cyclohexene Cyclohexene Cycloheptene Cyclooctene Cycloocta-I ,5-diene Cycloocta-l,5-diene Norbomene
Polymer
Z Z E Z E
70% E 48% E Z E 45% E
AHo (kJ mol-') -121 -15.4 -18 2 -2 -18
-13 -25 -3 3 -62.2
As0 (kJ mol-') -52 -51.8 -52 -3 1 -28 -31 -9 -5 -5 -50
AGO (25 O C ;
(kJ mol-') -105 -0.3 -2.6 6.2
1.3 -1 -13
-19 -24 -47
rings is, however, close to zero or positive, such compounds might not polymerize upon treatment with a metathesis catalyst. Additional substituents, and the reaction conditions can, however, have a decisive impact on ROMP. Subtle effects can operate, and it is often not possible to predict whether or not a system will undergo ROMP (Figure 3.48). As for other types of polymerization, ROMP can only be initiated if the concentration of monomer reaches a certain value and the temperature does not exceed a given limiting value. Hence, polymerization occurs more readily at high
146
3 Non-Heteroatom-SubstitutedCarbene Complexes Cycloalkenes which undergo ROMP SiMe,
0
Q
0%
Q
R
Cycloalkenes which do not undergo ROMP
,,XPh
p p ro EtO
Fig. 3.48. Selection of cycloalkenes and their suitability for ROMP [750,8181.
monomer concentrations and at low temperatures. Interesting reaction sequences have been realized by adjustment of these two parameters. It is, for instance, possible to prepare cyclopentenes by ring-closing metathesis of a 1,6-heptadiene at high temperature in solution, and then polymerize the cyclopentene by ROMP at a lower temperature and at high concentration [8 121. Further interesting applications of ROMP include the preparation of conjugated polyenes by oligomerization of synthetic equivalents of cyclobutadiene [819] or 1,6methanobenzene [8 161. With the discovery of ruthenium carbene complexes, which can catalyze ROMP in the presence of air and water [584,820], ROMP could be extended to hydrophilic
3.2 Synthetic Applications
of Non-Heteroatom-Substituted Carbene
Complexes
147
cycloalkenes. Carbohydrate-substituted polymers have been prepared by ROMP of norbornene or 7-oxanorbornene derivatives in water [821-8241. Several norbornenes substituted with biologically relevant molecules (e.g. nucleotides [825], amino acid derivatives [826], p-lactams [827]) or other functionalities, such as, e.g., N,N-dipyridylcarbamoyl groups as chelating ligands [828], have also been successfully polymerized [8 16,829,8301. Polymers produced by ROMP are generally equilibrium mixtures of a highmolecular-weight fraction of linear polymers (mwt > lo5 g mol-') and a lowmolecular-weight fraction of cyclic oligomers. In addition to these two components, the polymer can also contain different amounts of other polymers, produced by a different polymerization mechanism (Figure 3.49). This can become a serious problem with very reactive monomers. For instance, ROMP of norbornene with ReCl(CO)S/EtAICl, at 100 "C yields 98.6% of ring-opened product. At llO"C, however, only 5.1% of the ROMP-product but 94.9% of the ZieglerNatta-polymerization-product (poly-2,3-norbornylene) are obtained [83 11. Similar behavior has been observed with cyclobutene [750, p 2601.
ROMP t
Q
w
Fig. 3.49. Types of product arising from olefin polymerization with metathesis catalysts.
Stereoselectivity
The stereochemistry of polyenes formed by ROMP depends on the monomer used, on the catalyst, and also on the solvent [812]. ROMP tends to yield predominantly Z-polyenes when catalysts with several sterically demanding or chelating ligands are employed (see e.g. [832]). If, on the other hand, the transition metal bears few or small ligands, the stereochemistry of the polyene will depend mainly on the 1,2and 1,3-interactions of the C-substituents on the intermediate metallacyclobutanes. Depending on the monomer used, either 2- or E-polyenes may be obtained with steriplly nondemanding catalysts. An exhaustive review of the stereochemical outcome of the ROMP of different mono- and polycyclic alkenes is given by Ivin and Mol [750].
148
-
3 Non-Heteroatom-Substituted Carbene Complexes
\
RCM and ADMET of dienes
RCM of enynes
Fig. 3.50. The mechanism of ring-closing metathesis of dienes and enynes.
3.2.5.6 Ring-Closing Metathesis (RCM) When dienes are subjected to olefin metathesis, either oligomerization (ADMET) [808,809,833-8371 or cyclization (RCM) can occur [743,747,807] (Figure 3.50). Scope and Limitations
As in other types of metathesis it is mainly thermodynamic factors which determine whether ADMET or RCM becomes the preferred reaction pathway. Because of the strain of three-, four-, and eight- to eleven-membered rings, these will be difficult to prepare by metathesis. Five- and six-membered rings, on the other hand, are readily formed [838,839], and it might not be possible to polymerize 1,6- or 1,7-dienes, even in the absence of a solvent. Generally low substrate concentrations and high temperatures will increase the ratio RCM/ADMET. Also additional substituents at the diene backbone, which force the diene into an appropriate conformation, can lead to a negative AGO for RCM. Some common 'tricks' for achieving this are the
3.2 Synthetic Applications of Non-Heternatom-SubstitutedCarbene Complexes
149
Fig. 3.51. Common catalysts for homogeneous-phase olefin metathesis.
introduction of gem-dimethyl groups or rings (e.g. 1,2-phenylene) between the two C-C double bonds to be ligated. Macrocyclic lactones [840-8501, lactams [842,85 11, carbamates [852], pyridinium salts [853], ethers [811,854-8581, peptoids [859-8641, and calixarenes [865] can also be prepared by RCM. As in other macrocyclizations, yields can usually be improved by lowering the concentration of the reactants. Macrocyclization of esters of allylglycine with diols has been successfully used to prepare derivatives of 2,7-diaminosuberic acid [86 1,8641. The latter are surrogates of cystine, and therefore of interest for the preparation of peptide mimetics. For unknown reasons protected allylglycine derivatives can not be directly ‘dimerized’ by self metathesis [864]. However, catechol [864], ethylene glycol [861J, and 1,2- or 1,3-di(hydroxymethyl)benzenederivatives [860] of allylglycine are suitable templates for the formal self metathesis of this amino acid via RCM. One drawback of RCM-based macrocyclizations is the formation of EIZ-isomeric mixtures. Because high yields are often obtained, however, and the experimental set-up is simple, RCM is an attractive route to macrocyclic compounds. Also in macrocyclizations the conformation of the starting diene has an influence on the RCMIADMET ratio. Substituents on the diene which form hydrogen bonds between each other or with the intermediate carbene complex, or external Lewis acids can have an important effect on the course of the reaction. Such effects, often subtle, might explain the variable results sometimes obtained in macrocyclizations [842], but make synthesis-planning and the formulation of general guidelines difficult. Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 21 cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization.
150
3 Non-Heter~)atom-SubstitutedCurbene Complexes
The most commonly used catalysts for RCM are the three carbene complexes sketched in Figure 3.5 1. The order of reactivity of these three catalysts towards alkenes (but also towards oxygen) is 1 > 3 > 2. As illustrated by the examples in Table 3.18, these catalysts tolerate a broad spectrum of functional groups. Highly substituted and donor- or acceptor-substituted olefins can also be suitable substrates for RCM. It is indeed surprising that acceptor-substituted alkenes can be metathesized. As discussed in Section 3.2.2.3 such electron-poor alkenes can also be cyclopropanated by nucleophilic carbene complexes [34,678] or even quench metathesis reactions [34]. This seems, however, not to be true for catalysts 1 or 2. Molybdenum catalysts such as 1 can also lead to the isomerization of alkenes [810-8121. Care is due in particular if enantiomerically pure olefins with the stereogenic center near the C-C double bond are to be metathesized, or when strained rings are to be formed [81 I]. The most critical cyclization reactions are those in which tetrasubstituted alkenes or eight- to eleven-membered rings are formed [845,869]. Particularly interesting is Entry 38 in Table 3.18, a tandem ring-openinghing-closing metathesis. In this reaction ring-opening metathesis of the norbornyl group leads to the formation of a carbene complex, nicely disposed to undergo RCM with a vinyl group to yield a cyclooctene. A selection of preparations of carbocyclic compounds by RCM is given in Table 3.18.
3.2 Synthetic Applications of Non-Hetert,utom-Substituted Carbene Complexes
15 1
Table 3.18. Formation of cycloalkenes by ring-closing metathesis. Substrate'"
Conditions
2% W(O)C1,(2,6Br2C,H,O),, 4%PbEt,, 9 0 ° C C6H3C13' OTBS
A E E
5
9
m
Yield
9
5% 1,65 "C, C,H,, 24 h; (5%yield with3)
4% 1, 20 "c, C,H,, 15 min
oG
CH2C12, 6%3,20°C, 12 h
[a] E: C0,Et
70% [771] I8 121
(undergoes ROMP; 5 M in DME, -30 "C, 0.3% 1 )
89% [814]
EEpOH OH
6% 1,20 "C, pentane, 3.5 h
E
Ref.
2% W(O)C1,(2,6Br2C,H30)2, 4%PbEtd,90"C, PhMe., 1 h
5% 3,20 "C, CH2C12,24 h; (no reaction with 1)
H O &
6
*
COzMe
Product
97%
[814]
84%
[741]
88% [871]
flAo To1
55% [872]
152
3 Non-Heteroutom-SubstitutedCurbene Complexes
Table 3.18. continued. Substrate"'
4% [RuCI,(CO),],, 80 "C, CO, PhMe, 4 h
10
11
Conditions
:q SiMe,
c /Jp
79% [873]
80% [873]
TBSO
93% [867]
OSiEt,
15% 2, 100 "c, PIiMe, 1.5 h
14
2% 1,20 "C, CH,CI,, 15 rnin
h-$,
Ref.
WPh
4% PtCI,, 80 "C, PhMe, 1 h
OSiEt,
13
Yield
4% [RuCI,(CO),],, 80 "C, CO, PhMe, 1 h
TBSO
12
Product
(9
78% [874]
85% (7411
DCH0 DCHO 82% [813]
15
4
5% 3 , 2 0 "C, CH,CI,, 24 h
97% [814]
17
5% I, 65 O C , C6H6,24 h; (no reaction with 3)
61% [814]
18
5% 3,20 "c, CH,CI,, 24 h
16
/
96% [814]
3.2 Synthetic Applications cf Non-Heteroatom-SubstitutedCarbene Complexes Table 3.18. continued. Substrate'"
Conditions
Product
Yield Ref.
91%
C0,Me
22
23
F
O
B
[876]
C0,Me
,
@
13% 1, 20 "C, pentane, 3 h
80% [871]
(CO),W=C(OMe)Ph, 1%
75 "C, PhMe, 18 h /
/
0.25% 1, 25 "C, neat, 1 h
95%
[880]
153
154
3 Non-Heteroatom-Substituted Curbene Complexes
Table 3.18. continued. Substrate'"'
Conditions
Product
Yield Ref. 62%
5% 1 , 4 0 "C,
[844]
CO,, 72 h OBn
10% 2, KO "C,
C6H6,35 h; (only ADMET with 1)
29
E
BnoDoB" 40% [881]
E
96% [814]
5% 3 , 2 0 "C, CH,CI,, 4 d
5% 1 , 6 5 "C,
31
96% (8141
C6H6924h; (25% yield with 3)
8% MTO on Si02/AI,0,, 40 "C, FCI,CCF,CI, CH,CI,, 7 d
4% 2,20 "C, C6H,, 24 h
33
Me0 HO
36
[882]
99% [882]
2%3,8OoC,17h, I ,2-dichloroethane
34
35
58%
63%
[877]
HO
5% 2,25 "C,
C6H6,4 h
3% 1, 55 "c, hexane, 3 h; then TBAF
75% [883]
92%
[869]
3.2 Synthetic Applications of Non-Heteroatom-Substituted Curbene Complexes
155
Table 3.18. continued. Substratel'l
38
Conditions
Product
Yield
Ref.
ethylene, 8% 1, CH2CI2,25 "C
for 1 h. The mixture is left to cool to room temperature and filtered through a ca. 3 mm bed of silica gel. After rinsing with toluene (50 mL) the combined filtrates eous sodium hydroxide solution (1%, 15 mL) and then with are washed with water (15 mL). Concentration and distillation (62-66 'C, 15 Tom) yields 12.0g f the title compound as a colorless liquid. 'H NMR (300 MHz, CDCI,) , 3.02 (s, 4H), 4.19 (9,7 Hz, 4H), 5.62 (s, 2H); 13C NMR
14.06, 40.88, 58.86, 61.50, 127.79, 172.12,
Stereoselectivity When large rings are constructed by RCM, UE-isomeric mixtures can result. Hence RCM-based macrocyclizations are best suited for the preparation of intermediates in which the C-C double bond formed by metathesis can be removed (by, e.g., hydrogenation or oxidation) at a later stage of the synthesis. When trienes with two diastereotopic C-C double bonds are subjected to RCM, two diastereomeric dienes can be formed. Few examples of such reactions have been reported [886,887]. Interestingly, the stereochemical outcome of such cyclizations can be controlled by the choice of the catalyst (Entries 8 and 9, Table 3.19). Kinetic resolution of chiral dienes has been realized by RCM with enantiomerically pure molybdenum carbene complexes [888- 8901. High enantiomeric excesses of recovered diene (> 99% ee) and cycloalkene (93% ee) could be obtained.
156
3 Non-Heteroatom-Substituted Carbene Complexes
Synthesis of Heterocycles
Table 3.19 lists examples of the preparation of nitrogen-containing heterocycles by RCM. As mentioned in Section 3.2.5.3, free amines can partly deactivate metathesis catalysts. With the highly reactive molybdenum catalyst 1 it is, however, possible to cyclize dienes containing a basic amino group. If the less reactive catalysts 2 or 3 are to be used, protonation or acylation of the amine can be used to reduce their nucleophilicity. This will generally lead to higher yields with smaller amounts of cata1y st. The examples in Table 3.19 show that RCM is a versatile tool for the preparation of lactams. This methodology is increasingly being used for the synthesis of natural products or analogs thereof [891-8931. In most of the reported examples N,N-disubstituted amides were used as starting materials, because the most stable conformer ( E ) of amides such as RCONHR can not undergo a cyclization reaction. As amides tend to have high C(0)-N bond rotation barriers, cyclization reactions generally proceed more readily with N,N-disubstituted amides (see e.g. [8931). However, as Entries 5 and 13 in Table 3.19 demonstrate, N-monosubstituted amides can also cyclize upon treatment with a metathesis catalyst. Particularly noteworthy examples are Entries 8 and 9 in Table 3.19; these represent a diastereoselective RCM in which the stereoselectivity is controlled by the catalyst [886]. Entries 17, 23 and 24 (Table 3.19) illustrate the use of RCM for the solid-phase synthesis of lactams [894]. RCM induces both ring closure to the lactam and cleavage from the support. Although elegant at first glance, the usefulness of this methodology will be limited if the products must be used without further purification (as is usually the case for compound libraries prepared by parallel synthesis). Because relatively large amounts of catalyst are required, the crude products will only be acceptable for assays in which transition metal complexes do not interfere. Table 3.20 lists examples of the preparation of oxygen-containing heterocycles by RCM. Further examples, including lactones [895], pyrans [896,897], chromenes [839l, tetrahydrofurans [838], phosphonates [898], and oxepines [856,899-9021, have been reported. For references to macrocyclizations see ‘Scope and Limitations’ in this section.
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
Table 3.19. Formation of nitrogen-containing heterocycles by ring-closing metathesis. Substrate
Conditions
Product
4% 1, 20 "c, C6H6,3 h
1
4% 2,20 "c, CH,Cl,, 36 h; then NaOH
p h L N a
5% 3, 20 "c, CH,CI,, 2 d
BOC-N
4% 3, 20 "c, C,H,, 32 h
Yield
Ref.
85%
[903]
79%
[813]
97%
[814]
Boc9 95%
[904]
76%
[905]
77%
[903]
66%
[906]
HO
HO
4% 1,20 OC, C6H6, 1.5 h
Ph
50% 2, 50 "C, C,H,, 24 h
F3kP 61%
[887]
97%
[887]
TBSO 0
TBSO
cisitrans 8:92
10% 1,80 "C, Cf,H6,3 d 0 TBSO
TBSO cisltrans
86: 14
157
158
3 Non-Heteroatom-SubstitutedCurbene Complexes
Table 3.19. continued. Substrate
Conditions
Product
1% 2,80 "C, C,H,, 40 min
Yield
Ref.
86%
[907]
70%
[908]
TsN+OAC
3% 2, 110 "c, PhMe, 48 h
BnO
ph-Ns
12
13
10% 2,20 "C, C,H,, 60 h TrO
14
/ J
5% 2,20 "C, C,H,, 72 h
86%
[903]
95%
[905]
93%
[909]
49%
[905]
91%
[910]
0
15
10% 2, C,H,, 20 "C, 1 d, 75 "C, 13 d
o
e
16
C0,Me
17
5% 3.20 "C. CH,CI,, 16 h; Ar: 2,4-dinitrophenyl
I
Oso"'" 62%
[911]
88%
[907]
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes Table 3.19. continued.
w>
Yield
Ref.
10% 3,40OC, CH,CI,, 3 h
100%
(9121
4% 1,20"C, CdL 1h
73%
[903]
5%C&,2,20 92"C, h
73%
[909]
84%
[910]
16%
[911)
77%
[907]
87%
[914] ~921
Substrate
Conditions
TBSO
19
Product TBSO
0
20
Phv
21
c & 0
0
0
BocHN
POI
24
d' I
COzMe
5%3,80"C, 16h, 1,2-dichloroethane Ph
100% 3,50"C, PhMe, 16 h
Dog
0 Pol
1% 2,80 "C, C6H6,2.5h
C0,Me
4% 3,20 "C, CH,CI,, 5 h 0
159
I60
3 Nnn-Heteroutom-Substituted Curbene Complexes
Table 3.19. continued. Substrate
Conditions
Product
Yield
Ref.
Ts
10% 3 , 4 0 "C, CH,CI,, 3 h 0
28
70% [912] 0
Fx
m
9% C6H6,4h 1,20 "C,
0
50%
[910]
0
MeOC , -,
MeozCl 10% 2,60"C, 29
x2
24 h
51%
[883]
78%
[910]
74%
[844]
I
g; BOG
30
15% 1,50 "C,
C&, 4 h
I
F,COd
COCF,
1% 1,40 "C,
CO,, 72 h; [Conc. (diene) 3 mM]
\
ZiE 29171
3.2 Synthetic Applicutions ( f Non-Heteroutom-SubstitutedCurbelie Complexes
161
Ally1 1-phenyl-3-butenyl ether (94 mg, 0.50 mmol) is added to a homogeneous, orange-red solution of the ruthenium complex 2 (9.3 mg, 0.01 mmol, 2%) in dry benzene (CAUTION!, 15 mL) under argon. The progress of the reaction is followed by TLC. After stirring for 5 h at 20 "C the reaction is quenched by exposure to air (until greenish-black; 6 h) and concentrated. The residue is purified by column chromatography (gradient elution with etherkexane 0: 1 to 6:94). 69 mg (86%) of the title compound is obtained as a colorless oil. (Note: A similar result will probably be obtained if the lengthy quenching procedure is omitted.)
3.2.5.7 Cross Metathesis The treatment of equivalent amounts of two different alkenes with a metathesis catalyst generally leads to the formation of complex product mixtures [925,926]. There are, however, several ways in which cross metathesis can be rendered synthetically useful. One example of an industrial application of cross metathesis is the 'ethenolysis' of internal alkenes. In this process cyclic or linear olefins are treated with ethylene at 50 bar/20480 "C in the presence of a heterogeneous metathesis catalyst. The reverse reaction of ADMET/RCM occurs, and terminal alkenes are obtained. Much more challenging is the targetted introduction of carbon substituents at terminal olefins by means of cross metathesis. Because of the mild reaction conditions under which alkene metathesis proceeds, cross metathesis could become an extremely valuable tool for the synthetic chemist if the critical parameters for productive cross metathesis between different, functionalized olefins were understood. The examples listed in Table 3.21 illustrate the synthetic possibilities of cross metathesis. In many of the procedures reported, advantage is taken of the fact that some alkenes (e.g. acrylonitrile, styrenes) undergo slow self metathesis only. Interestingly, it is also possible to realize cross metathesis between alkenes and alkynes (Table 3.21, Entries 11-13), both in solution and on solid supports [927,928]. Experimental Procedure 3.2.9. Cross Metathesis with a Molybdenum Catalyst in Homogeneous Phase: (E)-1-Phenyl-1-octene [929]
A \y +B 1
(CF,),MeCO-t!!to (CF,),MeCd
Ph
+ /
J - hP
Ph104.16
765.54
112.22
188.32
28.05
162
3 Non-Heteroatom-SubstitutedCarbene Complexes
Table 3.20. Formation of oxygen-containing heterocycles by ring-closing metathesis. Substrate
Conditions
Yield Ref.
Product
RPh A
A+
93% [915]
Ph
13% 1,60 "C, hexane, 2 h
87% [871]
92% [916]
1% 2,40 "C, CO,, 12 h
1 %
\
8 (.,. SiMe,
e3
68% [917] 0
O 7CCI,
3% 3,20 'C, PhMe BnO
62% [844]
0
CCI,
93% [918]
.
OBn
13% I, 20 "C, pentane, 3.5 h I
Ph
A0
aPh 84% [871]
Ph
I
/o""
5% 3, 20 OC, CH,CI,, 16 h
43% [911] QPh
Pol
5% 2, ethylene, 22 OC, CH,CI,, 24 h (> 99% ee)
81% (> 99% ee)
[919] [9201
3.2 Synthetic Applications of Non-Heteroutom-Substituted Curbene Complexes
Table 3.20. continued. Substrate
10
Conditions
-0
y0YPh 10% 3 , 2 5 "C, CH,CI,, 4 h
II
Product
Yield Ref.
coyph &
87% [813]
0
95% [921]
H
8% 2 , 5 5 "C, C& 2 h; (only 20% yield for cis-diastereomer)
12
13
10% 3 , 2 5 "C, CH,CI,, 4 h
60% [883]
e(J
95% [%I]
ti
Ph
14
3-6% 3 , 4 0 "C, CH,CI,, 3 h
I
I
-Si
30% 3,25-60 "C, C6H6, 2 d; [conc. (diene) 5 mM]
15
16 A
c
o
~
~
85% [922] W.31
][I! 45% %
5-7% 3 , 4 0 "C, , CH,CI,, Ih
89% [924] AcO
AcO
(f 10% dimer)
10% 2 , 7 0 "C, PhMe, 60 h, high dilution
86% [845]
163
164
3 Non-Heteroatom-Substituted Carbene Complexes
Table 3.21. Preparation of alkenes and dienes by cross metathesis. Starting Components (Ratio)
H e x 4
Conditions
Product (Yield)
HexT TNo2 1% 1,20 "C, CH2CI2, 1 h
/
P
[929]
/
(48%;
(33:67) Hex
Ref.
+ 39% 7-tetradecene) C
(33:67)
N
5% 1,20 "C, Et,O, 10 h
(75%; Z/E 90:10) 5% 1,20 "C, fco2Bn @CN
v(33:67)
CN
CH,CI,, 3 h (44%; ZlE85:15;
+ 6% hornodirner of ester) 2% 1,20 "C, DME, 4 h
p h L s i M e 3
P311
(80%; ZIE 19:81)
(33:67) 5% I , 40 "C, CH,CI,, 12 h
P
oqT (74%; ZIE 3 1 :69)
(33 :67)
P
O
3% I , 40 "C, CH,CI,, 4 h (4456)
[933]
O
C0,Me
C0,Me
(66%; ZIE 33:67)
5% 1,40 "C, CH,CI,, 2 h (33:67) 5% I, 40 "C, CH,CI,, 16 h
(40:60) (98%)
165
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes
Table 3.21. continued. Starting Components (Ratio)
Conditions
Product (Yield)
Ref.
10% 1,40 OC, CH,CI,, 8 h Me,Si (77%; 92% ee)
(97% ee; 40:60)
C0,Me
CO,Me
10% 3,20 "C, DCE, 30 h
P341 BocHN -ph
(33:67)
(52%; + 40% homodimer of ester) 7% 3, 20 "c, CH,CI,, 24 h
(25:75)
l2
(90"/; ZIE 25:75)
p\7
1% 3,20 "C, CH,CI,, 24 h
OBn
(25:75)
(82%; ZiE 5050)
0
3% 3,CH,CI,, ethylene, 45 20 h "C, l3
~
O
A
~
o
A
P361 c
C
(74%)
The molybdenum complex 1 (342 mg, 0.45 mmol, 1%) is added to a mixture of dichloromethane (90 mL), 1-octene (5.0 g, 44.6 mmol) and styrene (9.29 g, 89.2 mmol). The resulting mixture is stirred at room temperature for 1 h and then filtered through a pad of silica gel. After rinsing with dichloromethane the combined filtrates are concentrated and the crude product is purified by column chromatography (silica gel). 7.9 g (94%) of the title compound is obtained as a colorless oil. All the metathesis reactions discussed in this section require only catalytic amounts of a carbene complex. The use of stoichiometric quantities of carbene complexes in organic synthesis is limited to cheap metals such as, e.g., titanium.
166
3 Non-Heteroutom-Substituted Curbene Complexes
Table 3.22. Examples of the preparation of dienes by ring-opening cross metathesis, Starting Components (Ratio)
Conditions
Product (Yield)
Ref.
5% 1 , 2 0 "C, CH,CI,, 3 h
(40:60)
R;b HB:
0
0
( 1 7:83)
5% 3 , 2 0 "C, CH2C12,24 h
:go "."'LAr
Wl
OMe
[61%, Ar: 4-MeOC6H,]
8%3 , 2 0 "C, CH2C12,3 h
0
(l0:90)
5% 3 , 2 0 "C, CH,CI,, 3.5 h
I.! ..,,(y
SiMe3 I...
&-NBoc 0
(50:SO)
(83%; ZIE 33:67)
7% 3, 20 "c, CH,CI,, 16 h HO
OH
'OH
(40:60)
(58%; ZIE 67:33)
10% 3 , 2 0 "C, CH,C12, 18 h Pol
(+ regioisomer; 5050; Ar: 3-chlorophenyl)
[938]
167
3.2 Synthetic Applications of Non-Heteroatom-SubstitutedCarbene Complexes Table 3.22. continued. Starting Components (Ratio)
Conditions
Product (Yield)
5% 3,20 "C, CH,CI,, 20 h
<
Ref.
SiMe,
C0,Pr
Pr0,C
(40:60)
[9381
(92%; ZIE 60:40)
1% 2,20 "C, CH,CI,
eHeX [9421
O=CHex
(33:67)
(63%; ZIE 70:30)
10% 2,20 "C, CH,CI,
[8 181 [942]
H
(33:67)
(53%; ZIE 10:90)
+
& H
Hex
(1 3%; ZIE 69:3 1 )
(33:67)
2% WCI,, C,H,, 80 "C, 10 h
CCN (68% yield at
[9431
40% conversion)
Apart from the tandem metathesis/carbonyl olefination reaction mediated by the Tebbe reagent (Section 3.2.4.2),few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp,TiCl,, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored.
168
3 Non-Heteroutorn-Substituted Curbene Complexes
Table 3.23. Cyclizations presumed to involve vinylidene complexes. Substrate
Conditions
Product
Yield
Ref.
79%
[945]
68% BocHNT\ eq. Mo(CO),(NEt,), 20 "C, NEt,, Et,O, 20 h BocND 1
OH
Me0,C
%\
-
[I491
20%
[945]
HO
HOi
C0,Me
[945]
0.5 eq. NaH; then 0.5 eq. Mo(CO),(NEt,), 20 "C, Et,O, 24 h
Meo2Co 60%
[946]
Me0,C
Meo2ck 0.5 eq. NaH; then 0.5 eq. Mo(CO),(NEt,), 35 "C, Et20, 12 h
62%
[946]
3.2.5.8 Ring-Opening Cross Metathesis One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [8 181) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,9371. The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology.
3.2 Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes
169
3.2.6 Other Applications of Non-Heteroatom-Substituted Carbene Complexes Although terminal alkynes can easily be converted into vinylidene complexes, vinylidene complexes have not yet been extensively used as intermediates in organic synthesis [ 150,546,576-578,9441. Some cyclization reactions, which might proceed by transient formation of vinylidenes, are listed in Table 3.23 (see also Sections 2.1.5.1 and 2.2.2). Electrophilic carbene complexes generated from diazoalkanes and rhodium or copper salts can undergo 0-H insertion reactions and S-alkylations. These highly electrophilic carbene complexes can, moreover, also undergo intramolecular rearrangements. These reactions are characteristic of acceptor-substituted carbene complexes and will be treated in Section 4.2.
This Page Intentionally Left Blank
4 Acceptor-Substituted Carbene Complexes
In contrast with non-acceptor-substituted carbene complexes, most of which are rather stable compounds, only few acceptor-substituted carbene complexes have been isolated [500,502,947,948]. In particular, acceptor-substituted carbene complexes relevant to organic synthesis (e.g. copper or rhodium acylcarbene complexes) are normally highly reactive and have remained elusive to spectroscopic characterization (for theoretical treatments, see Section 1.2). The inference that these intermediates are indeed carbene complexes is in part based on the observation that the modes of generation and the reactivity of these reactive species correlate well with those of less reactive carbene complexes.
4.1 Generation of Acceptor-Substituted Carbene Complexes The most important synthetic access to acceptor-substituted carbene complexes is the reaction of ylides with electrophilic, coordinatively unsaturated transition metal complexes (Figure 4.1; see also Section 3.1.3).
Fig. 4.1. Generation of acceptor-substituted carbene complexes from ylides. X: N,, SR,, S(O)Me,, ArI; Z: COR, CO,R, CONR,, SO,R, CN, P(O)(OR),.
Because acceptor-substituted carbene complexes can normally not be isolated, generation must occur in the presence of a suitable substrate. If during carbenetransfer from the intermediate carbene complex to the substrate the complex L,M (Figure 4.1) is regenerated, then catalytic amounts of this complex only will be
172
4 Acceptor-Substituted Curhene Complexes
sufficient to achieve complete conversion of the ylide. This catalytic variant is the most common way in which acceptor-substituted carbene complexes are generated and used in organic synthesis.
4.1.1 From Acceptor-Substituted Diazomethanes The most frequently used ylides for carbene-complex generation are acceptorsubstituted diazomethanes. As already mentioned in Section 3. I .3. I , non-acceptorsubstituted diazoalkanes are strong C-nucleophiles, easy to convert into carbene complexes with a broad variety of transition metal complexes. Acceptor-substituted diazomethanes are, however, less nucleophilic (and more stable) than non-acceptorsubstituted diazoalkanes, and require catalysts of higher electrophilicity to be efficiently decomposed. Not surprisingly, the very stable bis-acceptor-substituted diazomethanes can be converted into carbene complexes only with strongly electrophilic catalysts. This order of reactivity towards electrophilic transition metal complexes correlates with the reactivity of diazoalkanes towards other electrophiles, such as Brginsted acids or acyl halides.
4.1.1.1 Preparation of Acceptor-Substituted Diazomethanes The great popularity of diazocompounds as carbene complex precursors is mainly because of the ease of their preparation and handling. In addition to this, catalytic diazodecompositions often proceed very cleanly and in high yields. Acceptorsubstituted diazomethanes can be prepared in several different ways (Figure 4.2). Bis-acceptor-substituted diazomethanes are most conveniently prepared by diazo group transfer to CH acidic compounds either with sulfonyl azides under basic conditions [949,950] or with I-alkyl-2-azidopyridinium salts [95 I ] under neutral or acidic conditions [952-9541. Diazo group transfer with both types of reagents usually proceeds in high yield with malonic acid derivatives, 3-keto esters and amides, 1,3-diketones, or p, y-unsaturated carbonyl compounds [955,956]. Cyano-, sulfonyl, or nitrodiazomethanes, which can be unstable or sensitive to bases, can often only be prepared with 2-azidopyridinium salts, which accomplish diazo group transfer under neutral or slightly acidic reaction conditions. Other problematic substrates include amides of the type Z-CH,-CONHR and p-imino esters or the tautomeric 3-amino-2-propenoic esters, which upon diazo group transfer cyclize to I ,2,3-triazoles [957 -9591. When 2-alkyl-3-keto esters or 2-aryl-3-keto esters are treated with sulfonyl azides under basic conditions, nucleophilic deacylation occurs to yield 2-alkyl/aryl-2-diazo esters [960-9631. Nucleophilic deacylation can also be used to convert acceptorsubstituted diazoketones into the corresponding acceptor-substituted diazomethanes [964,965]. In all these deacylation reactions it is the most electrophilic carbonyl group which is attacked by the nucleophile and cleaved off. Diazo esters can also be prepared from glycine esters by treatment with nitrous acid [966] or with alkyl nitrites. Further methods include the oxidation of hydrazones, oximes (Forster reaction), and semicarbazones, the base-induced
4.1 Generation of Acceptor-Substituted Carbene Complexes
173
R ,H N, , ' z Zl L
RS02N3
or
base
($,
BF,-
NaNO,
O
R
RSOzN3
N3
pyrrolidine N,
NZ
zlYR NZ
Fig. 4.2. Synthetic routes to acceptor-substituted diazomethanes. Z', Z2: electronwithdrawing groups.
fragmentation of N-sulfonylhydrazones (Bamford-Stevens reaction) [967], and the ring fission of 45dihydro- 1,2,3-triazoles [968]. Acceptor-monosubstituted diazomethanes can be further converted into other types of diazo compound. C-Acylation of diazoacetic esters generally requires very reactive acylating agents, such as acid chlorides [969,970] or bromides [971]. C-Alkylations of acyldiazomethanes are best accomplished by metallation followed by treatment with a carbon electrophile [972-9771. C-alkylation can also occur without any base if sufficiently electrophilic aldehydes or ketones are used [973,978 -9821 or if the alkylation proceeds intramolecularly [983]. In addition to the methods sketched in Figure 4.2, diazoketones are frequently prepared from acyl halides and diazomethane. Because this methodology requires the use of distilled diazomethane, it is hazardous and not well suited to large-scale preparations. Acceptor-substituted diazomethanes can be explosive, and low-molecular-weight diazo compounds, in particular, should be handled with care. Ethyl diazoacetate has a half-life of 109 h at 100 "C in inert solvents [984, p 4251, but traces of acid or catalytically active salts can dramatically accelerate the thermal decomposition. Monoacyldiazomethanes are thermally less stable than diazoacetates [985], whereas bis-acceptor-substituted diazomethanes generally have high thermal and chemical stability.
174
4 Acceptor-Substituted Carbene Complexes
Table 4.1. Transition metal complexes suitable for the conversion of acceptor-substituted diazomethanes into carbene complexes. Catalyst
References
rhodium(I1) carboxylates Rh2(02CR)4
[987-9951
enantiomerically pure rhodium( 11) complexes
[996-10081
other rhodium complexes
[1009-loll]
palladium complexes
[655,1012]
copper(1) and (11) complexes
[636,1013-10231
enantiomerically pure copper complexes
[1024-10321
cobalt(I1) and (111) complexes
[ 1033-10351
platinum(O), (11) and (IV) complexes ruthenium(1) and (11) complexes enantiomerically pure ruthenium porphyrins
[I0361 [ 175,1037-10401 [ 1041-10431
other enantiomerically pure ruthenium complexes
[948,1044,1045]
osmium complexes
[1039,1046,1047]
methylrheniumtrioxide (MTO)
[ 10481
[510,511] (CO),W=C(OMe)Ph (CO),Cr(COD)
W I [52,512]
4.1 Generution of Acceptor-Substituted Curbene Complexes
175
4.1.1.2 Catalysts for Diazodecomposition As shown in Figure 4.1, the initial step of the conversion of an ylide into a carbene complex is an electrophilic attack at the ylide. Reactions of this type will, therefore, occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex L,M. Complexes which efficiently catalyze the decomposition of even weakly nucleophilic ylides can easily react with other nucleophiles also, such as amines or thiols. This has to be taken into account if reactions with substrates containing such strongly nucleophilic functional groups are to be performed. Different catalysts suitable for the conversion of acceptor-substituted diazomethanes into carbene complexes are listed in Table 4.1. Of these, the very efficient rhodium(I1) and copper(1) complexes are by far the most commonly used catalysts. For a comparative study of several different catalysts, see [986]. Enantiomerically pure copper and rhodium complexes enable enantioselective catalysis of carbene-mediated reactions. Such reactions will be discussed more thoroughly in Section 4.2. Experimental Procedure 4.1.1 describes the preparation of an enantiomerically pure rhodium(I1) complex which has proven efficient for catalysis of different types of carbene complex-mediated C-C-bond-forming reactions with high asymmetric induction. Experimental Procedure 4.1.1. Preparation of an Enantiomerically Pure Rhodium(I1) Complex: Dirhodium(I1) Tetrakis[methyl2-0~0-1-(3-phenylpropanoyl)4(S)-imidazolidinecarboxylate]; Rh2(4S-MPPIM),(MeCN), [ 10001
1
0 Rh ~h~~~0 Rh@W,
+
IWSCrJ)~l
441.99 4 276.29
1389.06
Methyl 2-0x0- 1-(3-pheny lpropano y l)-4(S)-imidazolidinecarboxylate is prepared from N-Cbz-L-asparagine by Hofmann degradation (Br2, NaOH; 8 l%), esterification (MeOH, SOCl,; 92%), acylation (pyridine, Ph(CH,),COCl; 97%), and hydrogenation (PdIC, H,; 100%) [1000,1049]. A mixture of rhodium(I1) acetate (228 mg, 0.5 16 mmol), the imidazolidinone (1.70 g, 6.15 mmol), and dry chlorobenzene (20 mL) is heated under reflux for 18 h in a flask fitted with a Soxhlet extraction apparatus into which a thimble is placed containing an oven-dried mixture of sodium carbonate and sand (2: 1, 5 g). The progress of the ligand-exchange reaction can be monitored by HPLC (p-Bondapak-CN column, methanol). The resulting blue solution is concentrated under reduced pressure, and the residue is purified by column chromatography (reversed phase silica, Bakerbond Cyan0 40 mm prep. LC packing, methanol).
176
4 Acceptor-Substituted Curbene Complexes
4.1.2 From Other Ylides Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. Iodonium ylides (PhI=CZ1Z2) [ 1017,105010561, sulfonium ylides [673], sulfoxonium ylides [ 10571 and thiophenium ylides [ 1058,10591 react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations.
4.1.3 From Other Carbene Complexes The intermolecular reaction of alkynes with acylcarbene complexes normally yields cyclopropenes [587,1022,1060- 10621. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result. In particular intramolecular cyclopropanations of alkynes, which would lead to highly strained bicyclic cyclopropenes, often yield rearrangement products of the latter. In many instances these products result from a transient vinylcarbene complex, which can be formed by two different mechanisms (Figure 4.3). As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [ 10641. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. Alternatively, [2 + 21 cycloaddition of carbene complexes to alkynes, followed by [2 + 21 cycloreversion can also lead to the formation of vinylcarbene complexes (Sections 3.2.5.6 and 2.2.4). In the example shown in Figure 4.4 either of these mechanisms leads to insertion of the alkyne into the C-Rh double bond of the initially formed acylcarbene rhodium complex. The resulting vinylcarbene complex undergoes intramolecular cyclopropanation of the 1-cyclohexenyl group to yield a highly reactive cyclopropene, which is trapped by diphenylisobenzofuran.
I 77
4.2 Synthetic Applicutions of Acceprt~r-S~bstitiiteu' Curbene Comple.xes
xez+
7 R
R
yZ
z%R
+
ML"
/
L,M
R
R
Fig. 4.3. Possible mechanisms for the formation of vinylcarbene complexes from alkynes and electrophilic carbene complexes.
0 Rh,(OAc),, CH,CI,
25 OC. 2h
0
(Padwa, 1993)
Fig. 4.4. Formation of vinylcarbene rhodium complexes by intramolecular addition of acylcarbene rhodium complexes to alkynes [ 106.51.
The intramolecular addition of acylcarbene complexes to alkynes is a general method for the generation of electrophilic vinylcarbene complexes. These reactive intermediates can undergo inter- or intramolecular cyclopropanation reactions [ 1066 - 10681, C-H bond insertions [1061,1068- 10701, sulfonium and oxonium ylide formation [ 10711, carbonyl ylide formation [ 1067,1069,1071], carbene dimerization [ 10661, and other reactions characteristic of electrophilic carbene complexes.
I78
4 Acceptor-Substituted Carbene Complexes
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes Acceptor-substituted carbene complexes are highly reactive intermediates, capable of transforming organic compounds in many different ways. Typical reactions include insertion into o-bonds, cyclopropanation, and ylide formation. Generally, acceptor-substituted carbene complexes are not isolated and used in stoichiometric amounts, but generated in situ from a carbene precursor and transition metal derivative. Usually only catalytic quantities of a transition metal complex are required for complete conversion of a carbene precursor via an intermediate carbene complex into the final product. In the following sections the synthetically most useful reactions will be presented, ordered according to the type of reaction. Recent reviews covering transformations with acceptor-substituted carbene complexes include [38,995,1072- 10791.
4.2.1 General Considerations Carbene complexes generated from e.g. rhodium(I1) or palladium(I1) carboxylates and unsubstituted or acceptor-substituted diazoalkanes are similar in reactivity to free carbenes. These complexes readily undergo, for instance, C-H insertion, cyclopropanation, ylide formation or intramolecular rearrangements. Carbene complexmediated transformations, however, usually proceed with greater selectivity and yield than the corresponding reactions of the uncoordinated carbenes. In addition to this the type of metal complex used can have a profound effect on the chemoand stereoselectivity of the corresponding carbene complexes [21,990,1080- 10821. The same metallic fragments characteristic of Fischer-type carbene complexes, e.g. (CO),Cr, also are capable of forming complexes with acceptor-substituted carbenes. The resulting carbene complexes are highly reactive and can cyclopropanate simple olefins [52,509]. Spectroscopic characterization of such complexes has, however, not yet succeeded. The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(I1) acetate. The successful design of new synthetic sequences using electrophilic carbene complexes thus requires careful assessment of potential side-reactions.
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
179
Ph I
6
P
6
Ph
12%
Ac 0
Ph
C0,Me Ph
22%
Ph
18%
AC
16% (Zaragoza, 1995)
Fig. 4.5. Rhodium(I1) acetate-catalyzed decomposition of amino acid-derived diazoamides [1083,1084].
4.2.2
C-H Insertions
Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptorsubstituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,10721074,1076,1085,1086].
4.2.2.1
Scope and Limitations
The possible mechanisms of C-H insertion of electrophilic carbene complexes have already been discussed in Section 3.2.3. The regioselectivity of C-H insertions with acceptor-substituted carbene complexes parallels the regioselectivity of oxidants. Generally, C-H insertions take place more easily into electron-rich C-H bonds. In other words, the new C-C bond will preferentially be formed at the carbon atom at which hydride abstraction (and carbocation formation) by an oxidant would have been most favorable. Hence, ethers or other compounds containing heteroatoms capable of stabilizing carbocations will be preferentially alkylated by electrophilic carbene complexes at the heteroatom-bound carbon atom. In line with this, electron-withdrawing groups generally deactivate C-H bonds towards electrophilic attack by acceptor-substituted carbene complexes [ 1087,10881.
180
4 Acceptor-Substituted Curbene Complexes
Z
Fig. 4.6. Possible mechanism of the C-H insertion of electrophilic carbene complexes into aromatic C-H bonds (Z: electron-withdrawing group).
C-H Insertions into vinylic C-H bonds are also a common reaction of electrophilic carbene complexes. Insertions into aromatic or heteroaromatic C-H bonds can proceed via cyclopropanation and rearrangement (Figure 4.6).
4.2.2.2 Stereoselectivity As early as in 1973 it was shown [lo891 that the C-H insertion of acceptorsubstituted carbene complexes can take place with retention of configuration (e.g. Table 4.5, Entry 3) [953,1090,1091]. In the case of intramolecular C-H insertions into methylene groups high diastereoselectivities are often observed when 4-6membered rings are formed (see examples in Tables 4.4-4.8). Enantiomerically pure rhodium(I1) complexes have been shown to catalyze enantioselective, intramolecular C-H insertions. Because high enantioselectivities (> 97% ee [1092]) can be achieved, it can be inferred that in the transition state of these C-H insertions the catalyst is in close proximity to the reacting groups. With the same type of catalyst highly enantioselective (up to 93% ee) intermolecular C-H insertions can also be realized [1093].
4.2.2.3 Intramolecular 1,2-C-H Insertions In acceptor-substituted carbene complexes with hydrogen at Cp. fast hydride migration to the carbene will usually occur [1094,1095]. The resulting olefins are often formed with high stereoselectivity. 1,2-Hydride migration will also occur in P-hydroxy carbene complexes, ketones being formed in high yields (Table 4.2). Intramolecular 1,5-C-H insertion can sometimes compete efficiently with 1,2-insertion [ 10961.
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
18 1
Table 4.2. Intramolecular 1,2-C-H insertion of acceptor-substituted carbene complexes. Starting Material
Reagents, Conditions -78 "C, 1% Rh2(02CCF3)4, CH,Cl2, 1 h
Product
-78 "C, 1% Rh2(02CCF3)4, CH2C12, 1 h
OMe
5% Rh,(OAc),, 20 "C, C,H,,
16h
NZ
Ref.
94% [lo971
C02Me
-78 O C , 1% Rh2(02CCF3)4, CH,C12, 1 h
N2
m+
7
Ph
Yield
80% [lo971
Hi5c72 92% [I0971
R
78% [I0981
0.5% Rh,(OAc).+,
20 "C, CH,CI,, lh
Ph&Ph BocNH
57% [974]
N,
4.2.2.4 Intramolecular 1,3-C-H Insertions Few examples of the formation of cyclopropanes by intramolecular C-H insertion of electrophilic carbene complexes have been reported. This methodology for cyclopropane preparation seems only to be suitable for polycyclic compounds with little conformational flexibility. Illustrative examples are listed in Table 4.3.
4.2.2.5 Intramolecular 1,4-C-H Insertions Intramolecular carbene C-H insertion is a suitable method for the preparation of four-membered carbo- or heterocycles. In particular p-lactams [ 1088,1101110.51 are formed easily, and often in high yields, upon transition metal-mediated diazodecomposition of N-alkyldiazoacetamides. 1,2-Azaphosphetidines [I 1061 also can be prepared by intramolecular C-H insertion into nitrogen-bound methylene groups. Few examples of the preparation of p-lactones by intramolecular C-H insertion have been reported [ 1107,1108]. This might be because of the ease with which p-lactones undergo ring-opening reactions or decarboxylation [ 1 1091. Examples of the formation of four-membered rings by intramolecular C-H insertion of electrophilic carbene complexes are listed in Table 4.4.
182
4 Acceptor-Substituted Carbene Complexes
Table 4.3. Intramolecular 1,3-C-H insertion reactions of electrophilic carbene complexes. Starting Material
1
Reagents, Conditions
Product
copper bronze, cyclohexene
Yield
Ref.
94% [lo991
N2
cuso,, 1 10 "C, 2
68% [IIOO]
PhMe, high dilution 0
4.2.2.6 Intramolecular 1,5-C-H Insertions Intramolecular carbene C-H insertion frequently leads to the formation of fivemembered rings [967,990,1021,1113- 11281. In particular 1-diazo-2-alkanones tend to yield cyclopentanones exclusively when treated with rhodium(I1) carboxylates. The use of enantiomerically pure catalysts for diazodecomposition enables the preparation of non-racemic cyclopentane derivatives [ 1005,1052,1074,1092,1129]. Intramolecular 1,5-C-H insertion can efficiently compete with 1,2-C-H insertion
4.2 Synthetic Applications o j Acceptor-Substituted Carbene Complexes
183
Table 4.4. Intramolecular 1,4-C-H insertions of electrophilic carbene complexes generated from diazocarbonyl compounds. Starting Material
Reagents, Conditions
Product
Yield
Ref.
53%
[IIIO]
0
I Yo Rh,(OAc),, 80 "C, C6H6,4 h PMP C0,tBu
(mixture of diastereomers)
1% Rh,(OAc),, 40 "C, CH,CI,, 20 h
Ph/YCOZEt
C0,Me 0
1% Rh,(OAc),, 40 "C, CH2CI2, 4h
O+N2
40 "C, CH2CI,, 2?h
CNXN2 Go
67% [ 1 1 1 1 ]
(97% ee)
(E: C0,Me)
K
Eto2C
N2
Co2Et
0.5% Rh,(OAc),, 20 "C, CH,CI,, 1.5 h
[1096,1116,1124], as illustrated, e.g., by the examples in Table 4.5 (Entries 13 and 14). Wolff rearrangement of the transient acylcarbene complex can, however, sometimes compete with intramolecular C-H insertion [ 1 1301. Although diazoacetamides are usually converted into p-lactams upon diazodecomposition, with some substrates pyrrolidinones can be the main product (Table 4.6) [ 1101,1102,1140,1141]. For instance, 1-diazoacetyl-2-ethylpyrrolidineyields, upon treatment with rhodium(I1) complexes only 1-methyl-3-pyrrolizidinone (i.e.
184
4 Acceptor-Substituted Curhene Complexes
Table 4.5. Preparation of cyclopentanones by intramolecular carbene 13-C-H insertion. Starting Material
G2
Reagents, Conditions
Product
I%Rh,(OAc),,
0 (80% de)
1% Rh,(OAc),, 40 "C, C,H,
-20 "C, CHZCI,,
2% PhtN
+N2Ph
Ph
x:
0 I/
I/
0
(98% ee) 0
NHPh "Ph 0
20 "C, CH,CI,,
3h AcO
N3"',~&N2
AcO
1% Rh,(cap),, 20 "C, CH,CI,,
5h OMe
OMe
Yield
Ref.
185
4.2 Synthetic Applications of Acceptor-Substituted Curbene Complexes Table 4.5. continued. Starting Material
Reagents, Conditions
Product
Yield
Ref.
I 3% Rh,(OAc),, 20 "C, CH,CI,
9
eN2
+
PO(OEt),
lo
61% [I1351 Ph
11
5% Rh,(OAc),, CH,CI,, 12 h
Rh2(0Ac)4
-$
PO(OEt),
85%
[I1361
62%
[I911
2% Rh,(OAc),, CH,CI,, 6 h
70% [I1371
Rh,(OAc),,
53% [I1381
0
6"
C0,Me
12
rn
C0,Me
l3
14
40 "C, CH,CI,
20 O C , 0.05% rhodium(l1) octanoate, CH,CI,, 10 min 20 "C, 0.01% rhodium(I1) octanoate, CH,CI,, 5 min
91% [I1391 p h (>~ 90% " cde)o ~ M e
-CO,Me
89% [I1391
186
4 Acceptor-Substituted Carbene Complexes
Table 4.6. Preparation of pyrrolidines and pyrroles by intramolecular carbene 13-C-H insertion. Starting Material
Reagents, Conditions
Product
Yield
Ref.
CH,CI,, 2%
htNx'Bu Me0,C Ph$N-Ar .+'
(Ar: 4-nitrophenyl)
qPh 2
P/'P/
80% [I 1451
(74% ee)
,Th7yh 2% Rh,(OAc),,
98% [I1461
BryNxo N, C0,Et 20 "C, CHZCI,,
30 min Br
C0,Et
3
I Ph
N,
20 QC,2% Rh2(02CC3F7)4, CH,CI,, 2 h; then TIPSOTf, NEt,
C0,Et
53% [I1431
&OTIPS \
Ph
PhAN&
4
C0,Et
I
N,
Ph
I
20 "C, 2% fi,(HNCOCF3),, CH,CI,, 10 min; then TIPSOTf, NEt3
C0,Et
94% [I0801
p J $ s o T (l L P h
moTBS HN2 0
S0,Ph
5% rhodium(I1) octanoate, 80 "C, 0 C&,3 h 1
53% [I1471
S0,Ph TBS
0
20 "C, 2% Rh,('LS-MEPY), (see Fig. 4.20), CH,CI,, 20 h
C0,Me
a5
72% [I1161
(racemic)
1% Rh,(OAc),, 40 "C, CH,CI,, 4h
88%
[IIlO]
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
1 87
the product of 1,5-C-H insertion) [ 11421. Non-racemic pyrrolidinones can be obtained with enantiomerically pure rhodium(I1) complexes as catalysts [ 1088,11011. Further carbene precursors, which readily lead to the formation of five-membered rings upon diazodecomposition, are diazoacetanilides (Table 4.6) [ 1080,1110,1143]. With these substrates insertion into an aromatic C-H bond to yield 2-indolinones is often the predominant reaction pathway. The regioselectivity can however strongly depend on the type of catalyst and on the structure of the substrate (compare, e.g., Entry 4 in Table 4.6 with Experimental Procedure 4.2.1) [1143]. Diazoacetanilides also cyclize to 2-indolinones upon catalysis by acids [ 1 1441. ecu1)-3-
NO2
382.38
ide is prepared by N-arylation of L-phenylalanine 4-fluoronitrobenzene, 110OC, 10 h, ioxin-4-one, toluene, 110 "C, ethylamine, acetonitrile, 20 "C, 1.5 h, 90%).
phenyl)-N-(2-diazo-3-oxobutyryl)-L-pheny1alanine 01) in 1,2-dichloroethane (45 mL) is added over f rhodium(I1) acetate (48 mg, 0.11 mmol, 2.6%) in f,2-dichloroethane (30 mL) by means of a syringe motor. Concentration yields with a small amount of hot methanol. Recrystallization eptane yields 1.05 g (65%) of the title compound as mp 200-202 OC, [a],20-278 " (c 1.0, chloroform). on of a sample from heptane/e etate yields colorless plates, .53 (s, 3H), 3.41 (dd, 10.7, C. 'H NMR (200 MHz, CDC ,3.63 (dd, 5.1, 14.2 Hz, lH), 3. 3H), 5.34 (dd, 5.1, 10.7 Hz, lH), 6.88 (d, 8.8 Hz, lH), 6.99-7.19 (m, 5H), 8.08 (dd, 2.2, 8.8 Hz, IH), 8.20 (d, 2.2 Hz, 1H). Several examples have been reported for furanone formation by intramolecular C-H insertion of electrophilic carbene complexes [ 1006,11481 (Table 4.7). Yields can, however, be low with some substrates, possibly as a result of several potential side-reactions. Oxonium ylide formation and hydride abstraction, in particular, [ 1090,1149- 11521 (see Section 4.2.9) seem to compete efficiently with the formation of some types of furanones.
I 88
4 Acceptor-S~b,~titlrted Curbene Complexes
Table 4.7. Preparation of five-membered, oxygen-containing heterocycles by intramolecular carbene 1,5-C-H insertion. Starting Material
Reagents, Conditions
Qf
1 BnO ""
Product
80 1%"C, Rh,(OAc),, C,H,, 4 h
Yield
Ref.
62% [1107]
$0
BnO""
OTBS
OTBS 0
3
"3-
,,gc02Me
R
0.06% rhodium(I1) octanoate, 20 "C, CH,CI,,
wYo2Me 0
20 min
BnOi (R: geranyl)
58%
[I1201
BnOi (+ 32% C-3 diastereomer)
CHZCIZ, 0.5%
Bno/\Yoyo 04Q-E I/ I/
BnO
N2
/Yh7Yh
[ 11541
BnO' (86% de, 94% ee)
0L > E
a0
I/
(98% de, 97% ee)
(E: C0,Me) CH2CIz,0.5%
70% [I1551
Ac,
0
B ~ o " * ' ' ' ~65%~
I/
H
CHZCI,, 0.5% 6
0
A>E
0
I/
m
o
I/
(98% ee)
82% [lo071
I 89
4.2 Synthetic Applications of Acceptor-Substituted Carbenr Complexes Table 4.7. continued. Starting Material
Reagents, Conditions
Product
Yield
0
Ref.
67% [1156]
(94% ee)
0
1% rhodium(I1)
50%
[I0691
C&,, 4 h n
4.2.2.7 Intramolecular 1,6- and 1,7-C-H Insertions The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [ 1 157,11581. Electron-rich arenes or heteroarenes [ 1159- 11621 and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8.
4.2.2.8 Intermolecular C-H Insertions Few examples of preparatively useful intermolecular C-H insertions of electrophilic carbene complexes have been reported. Because of the high reactivity of complexes capable of inserting into C-H bonds, the intermolecular reaction is limited to simple substrates (Table 4.9). From the results reported to date it seems that cycloalkanes and electron-rich heteroaromatics are suitable substrates for intermolecular alkylation by carbene complexes [ 11651. The examples in Table 4.9 show that intermolecular C-H insertion enables highly convergent syntheses. Elaborate structures can be constructed in a single step from readily available starting materials. Enantioselective, intermolecular C-H insertions with simple cycloalkenes can be realized with up to 93% ee by use of enantiomerically pure rhodium(I1) carboxylates [ 10931.
190
4 Acceptor-Substituted Curbene Complexes
Table 4.8. Preparation of six- and seven-membered carbo- and heterocycles by intramolecular carbene C-H insertion. Starting Material
Reagents, Conditions
Product
Yield
Ref.
(78% ee) OMe
IPh
1 eq. CuCI, -45 "C, CH,CI,, 3h
90% [I0171 Me0
83% [ I 1631
3% Rh,(OAc),, 80 "C, C6H,, 2.5 h CF3
& AN2
82% [I1641
0
1.1 eq. DMAD,
60% [I1331
Rh,(OAc),, 25 "C, C,H,, 1 h
0 Ph
Ph
co+!N2 1% Rh2(cap),, 40 "C, CH,CI,, 10h
O
y
2
L O T , ,
67% [I1531
H
1% Rh,(OAc),, 20 "C, CH,CI,, lOh
80% [lo901 (two diastereomers50:50)
4.2 Synthetic Applicutions of Acceptor-Substituted Carbene Complexes
Table 4.9. Intermolecular C-H insertion reactions of electrophilic carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
62%
[993]
R
I
1 H
H
N2
10% Rhz(OAc),, 120 oc, 6 h, pinacolone
1% Rh2(OAc),, 4 eq. thiophene,
20 OC, PhF, 15 h
H
H
(R:3,4-dirnethoxybenzyl)
b?
94% [I1661
OH
% qPh 0
3
\\
1% Rh2(OAc),, 25 "C, THF, 0.5 h
73% [lo711
Ph
C0,Me
Ph
4
"K
0
co2Me
N2
83% [lo931
(81% ee)
//
5
0
N P C O , ~ B U
//
5% Rh,(OAc),,
20 "C, CH2C12, 18h
85% C0,tBu
I11671
191
192
4 Acceptor-Substituted Carbene Complexes
Experimental Procedure 4. Insertion: 3-(lli-3-1ndolyl)-
U ~ X C-H
12b.09
4.2.3 Si-H Insertions Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-11711. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4dimethyltetrahydro-2-furanone) in the presence of dimethyl(pheny1)silane leads to the a-silylester with 80% de (67% yield; [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(I1) acetate to yield a-silylesters with up to 70% de [956].
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
193
4.2.4 C-C Insertions Carbenes do not usually undergo intermolecular C-C bond insertions [1172]. Intramolecular 1,2-insertions into C-C bonds are, however, frequently observed [976], and have, e.g., been used for the synthesis of strained bridgehead olefins [ 1 1731. Further examples are listed in Table 4.10. Table 4.10. 1,2-C-C Insertion of electrophilic carbene complexes. Starting Material
1 OH
Reagents, Conditions
Product
oy&
0.5% 20 "C, Rh,(OAc),, CH,Cl,, lh
Yield
Ref.
57%
[974]
NZ
0.2% Rh,(OAc),, 60 "C, hexane,
2
80% [ I 1741
1.5 h; then SiO,
C0,k
3
OH
73% [1175]
20 "C, CHCI,, 5 rnin
NZ
H
0.5% Rh,(OAc),,
C0,Et
4% U d O A c h , 20 "C, pentane, 30 rnin
0,
tBu0
4.2.5
C0,Et
90%
[977]
tBU0
X-H Insertions (X: N, 0, S)
Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X: N, 0, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7).
194
4 Acceptor-Substituted Carbene Complexes
yZ ML,
+
RNX'H (X = N, 0,S )
ML"
2 - 2
Y -
RXX'H
RNX
Fig. 4.7. Possible mechanism of the X-H bond insertion of electrophilic carbene complexes.
Ylide formation, and hence X-H bond insertion, generally proceeds faster than C-H bond insertion or cyclopropanation [ 11761. 1,2-C-H insertion can, however, compete efficiently with X-H bond insertion 111771. One problem occasionally encountered in transition metal-catalyzed X-H bond insertion is the deactivation of the (electrophilic) catalyst L,M by the substrate RXH. The formation of the intermediate carbene complex requires nucleophilic addition of a carbene precursor (e.g. a diazocarbonyl compound) to the complex L,M. Other nucleophiles present in the reaction mixture can compete efficiently with the carbene precursor, or even lead to stable, catalytically inactive adducts L,M-XR. For this reason carbene X-H bond insertion with substrates which might form a stable complex with the catalyst (e.g. amines, imidazole derivatives, thiols) often require larger amounts of catalyst and high reaction temperatures. 4.2.5.1
N-H Insertions
The first reports of N-H insertion reactions of electrophilic carbene complexes date back to 1952 14971, when it was found that aniline can be N-alkylated by diazoacetophenone upon treatment of both reactants with copper. A further early report is the attempt of Nicoud and Kagan [ 11781 to prepare enantiomerically pure a-amino acids by copper(1) cyanide-catalyzed decomposition of a-diazoesters in the presence of chiral benzylamines. Low enantiomeric excesses (< 26% ee) were obtained, however. Because of partial deactivation of many catalysts by aliphatic amines, less nucleophilic derivatives such as carboxamides or carbamates are usually used as substrates for carbene N-H insertion. The usefulness of this reaction for the preparation of heterocycles under mild conditions became apparent in 1978, when chemists from Merck, Sharp & Dohme reported the synthesis of bicyclic p-lactams by intramolecular carbene N-H insertion 11 1791. Intramolecular N-alkylation of p-lactams by carbene complexes is one of the best methods for preparation of this important class of antibiotic and many p-lactam derivatives have been prepared using this methodology [ 1180 -11861 (Table 4.11). Intramolecular N-H insertion can also be used to alkylate amines [1187-11891, y-lactams [1190], and carbamates [1191-11931 (Table 4.11). Intermolecular N-H insertion of electrophilic carbene complexes has occasionally been used for the preparation of amino acid derivatives and other types of intermediates (Table 4.12) 1956,1043,1194- 12011.
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
195
Table 4.11. Preparation of nitrogen-containing heterocycles by intramolecular N-H insertion of electrophilic carbene complexes. Starting Material
Reagents, Conditions 0.5% Rh,(OAc),, 1% NEt,, 0 "C, CH,CI,, 14 h
Ph+Nz
1
Product
Ref.
63% (12021
"--)-'-fO ,NJ
WN2
Boc
BocNH
2
Yield
C0,PNB
0. I%Rh,(OAc),,
100% 112031
0
80 OC, PhMe 0 -
C0,PNB
0
OMe
O h C02Bn
GyNZ
0
5% Rh,(OAc),, 80 "C, C6H6,
25 rnin
X
O
57% [1204]
0
C02Bn
%&
0
1% Rh,(OAc),, 20 "C, C,H,, 1 h
50% [1205] 0
C0,PNB
OPNB
0.3% Rh,(OAc),, 20 "C, CHCI,, 2 h
0
94% [1188] Et0,C
R
=
TBSO
TBSO
4
~
F,C
80 "C, 5% Rh2(02CCF3)4, DCE, 10 h
o a o c
C02Bn
0.3% 20 OC, Rh,(OAc),, CH,CI,
"
77% 110571
n
C02R2 0 I\
V
(R': PhOCH,CONH; R2: 4-nitrobenzyl)
O
H
87% [I0511
CO,RZ
Ph
0
C0,Me
1% Rh,(OAc),, 80 "C, C6H6,
C0,Bn
30 rnin
8 N2
HN,
Rh,(OAch, 80 'C, C,H6
9
75% [1206] 0
"'(F$,H
50% [1207]
tBu0,C 0
'0
196
4 Acceptc~r-Substituteu'Curbene Complexes
Table 4.12. Intermolecular N-H insertions of electrophilic carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
I10 "C, 5 eq.
1
EtOzCyPO(OEt), NZ
VNm= H 2% Rh,(OAc),,
46% [1198] I
Boc
PhMe, 15 h 2
EtOzCKPO(OEt),
4-nitroaniline, 110 "C, 5 eq. N2
3
K cozMe
Ph
NZ
o z N ~ N ~ ( o E t 81% ) z
2%Rh2(OAc),,
PhMe, 15 h
H
[1198]
C0,Et
PhYozMe
110 "C, 1 eq.
67% [I1971
diethy lamine,
vNo
2% Rh2(OAc),, PhMe, 12 h
OEt
PhMe, 14 h
OEt
4.2.5.2 0 - H Insertions The reaction of acceptor-substituted carbene complexes with alcohols to yield ethers is a valuable alternative to other etherification reactions [ 1152,1209- 121 11. This reaction generally proceeds faster than cyclopropanation [ 11761. As in other transformations with electrophilic carbene complexes, the reaction conditions are mild and well-suited to base- or acid-sensitive substrates [1212]. As an illustrative example, Experimental Procedure 4.2.4 describes the carbene-mediated etherification of a serine derivative. This type of substrate is very difficult to etherify under basic conditions (e.g. NaH, alkyl halide [ 1213]), because of an intramolecular hydrogen-bond between the nitrogen-bound hydrogen and the hydroxy group. Further, upon treatment with bases serine ethers readily eliminate alkoxide to give acrylates. With the aid of electrophilic carbene complexes, however, acceptable yields of 0-alkylated serine derivatives can be obtained.
4.2 Synthetic Applications of A~cept(~r-Substitute~ Carbene Complexes
197
Experimental Procedure 4.2.4. Etherification of a Serine Derivative by Intermolecular 0-H Insertion: Methyl (2S)-3-[(Ethoxycarbonyl)methoxy]-2-(benzyloxycarbony1amino)propanoate Zaragoza, F., Ullmann, A., unpublished results
Ho/yCo*Me (
“ZEt
+
HN\CO,Bn
N*
2S3.26
114.10
441.99
-“X
C0,Me
Rh,(OAc),
Eto2C
C0,Bn
339.3s
A solution of ethyl diazoacetate (1.33 g, 11.7 mmol, 2.9 eq.) in dichloromethane (20 mL) is added over 18 h to a refluxing mixture of N-Cbz-serine methyl ester (1.02 g, 4.03 mmol), dichloromethane (10 mL), and rhodium(I1) acetate (42 mg, 0.095 mmol, 2.4%). The resulting mixture is concentrated under reduced pressure and the residue is purified by column chromatography (60 g silica gel, gradient elution with heptanelethyl acetate 9:l to 6:4). 0.85 g (62%) of the title compound is obtained as a colorless oil. ‘H NMR (200 MHz, CDCl,) 6 1.22 (t, 7 Hz, 3H), 3.72 (m, 4H), 4.01 (m, 3H), 4.16 (q, 7 Hz, 2H), 4.45 (m, lH), 5.09 (s, 2H), 6.01 (d, br, 7 Hz, lH), 7.23 (m, 5H); I3C NMR (50 MHz, CDCl,) 6 14.01, 52.51, 54.25, 60.97, 66.82, 68.18, 71.17, 127.83, 127.96, 128.33, 136.24, 156.00, 169.95, 170.38. A broad range of compounds can be 0-alkylated with carbene complexes, including primary, secondary, and tertiary alcohols, phenols, enols, hemiaminals, hydroxylamines, carboxylic acids, dialkyl phosphates, etc. When either strongly acidic substrates [ 12141 and/or sensitive carbene precursors are used (e.g. aliphatic diazoalkanes [ 12151 or diazoketones) etherification can occur spontaneously without the need for any catalyst, or upon catalysis by Lewis acids [1216]. Enantiomericaliy pure phenyldiazoacetic esters [ 12171 and vinyldiazoacetic esters [956] react with alcohols upon transition metal catalysis to yield aalkoxyesters with low diastereoselectivity (< 53% de). Intramolecular 0-H insertion enables the preparation of 3-%membered cyclic ethers in high yield [979,1193,1218- 12201. Examples of 0-H insertion reactions are given in Tables 4.13 and 4.14. One side-reaction occasionally observed in carbene complex-mediated etherifications is the oxidation of the substrate by hydride abstraction (see Section 4.2.9).
4.2.5.3 S-H Insertions S-Alkylation of thiols by carbene complexes can be a useful approach to a-(alky1thio)- or a-(arylthio)ketones, although few examples of intramolecular [975,1193] or intermolecular [497,1043,1230- 12331 S-H bond insertion reactions of electrophilic carbene complexes have been reported. Yields are sometimes low, probably because of the poisoning of the catalyst by the thiol. Examples are given in Table 4.15.
198
4 Acceptor-Substituted Carbene Complexes
Table 4.13. Preparation of cyclic ethers by intramolecular 0-H insertion of electrophilic carbene complexes. Starting Material
1
P
h N3
Reagents, Conditions
Yield
Ref.
&(OAc)4
90% [1221]
1Yo Rh,(OAc),, 80 "C, C6H6
92% [I2221
q C02Et N2
C02tBu
2
Product
*
(mixture of diastereomers)
4
H 0 a c 0 2 k
13% Rh,(OAc),, 20 "C, CH,CI,, 48 h
C0,Et
3% Rh,(OAc),, 80 "C, C,H6, 40 min
C0,Me
C?Tco4.2.6
1% Rh,(OAc),, 1 10 "C, PhMe,
90%
[lo801
53% [I2231
77% [I2241
16h
Ylide Formation
Acceptor-substituted carbene complexes are electrophilic intermediates which react readily with lone pairs, giving the corresponding ylides. These can be valuable intermediates, capable of undergoing a broad range of synthetically useful transformations. This subject has been treated in several reviews [38,995,1077- 1079,10861.
4.2.6.1 Ammonium Ylides Tertiary amines can react with electrophilic carbene complexes to yield ammonium ylides which usually undergo Stevens rearrangement (Figure 4.8) leading to products of a formal carbene C-N bond insertion.
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
199
Table 4.14. Preparation of ethers by intermolecular 0-H insertion of electrophilic carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
66%
[993]
80 "C, C,H,, 1 eq. 1
J
C0,Me NZ
1Yo Rh,(OAc),, 20 min
0
72% [I2251
2
/ \
tBu
CH,CI,, 6 h; (E: C0,Et)
/ \
tBu
tBu
tBu
40 "C, 2 eq.
1% Rh,(OAc),, CH,CI2, 1.5 h tevt- butanol,
4
Rh2(0Ac)4, 130 "C, 1.5 h 2 eq. catechol, 1% Rh2(OAc),, 80 "C, C,H,, 15h
(EtO),OPKCOZMe
5 NZ
(mixture of diastereomers)
"6' 42% [1227]
(E1O),OPyCO,Me
83% [1201] HO
6
7
Rh2(0Ac)4, EtozC-fCozEt 84% [I2281 EtozC-TfCo2Et ethanol 0NZ
r.4
) ,N
Nz
8% 2-propanol, Rh,(OAC),, cNLo'( 20"C, CH,Cl,, ) ,N H 20 h; then TFA
40% [1229]
200
4 Acceptor-Substituted Carbene Complexes
Table 4.15. Preparation of thioethers by S-alkylation of thiols with electrophilic carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
41%
[975]
2%Rh,(OAc),,
80 "c, C&, 5 min
2
2% Rh2(OAc),,
3
C0,Et
80 " c , C6H6, 1.5 h
C0,Et
73% [1193]
N2
Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptorsubstituted carbene complexes.
Allylammonium ylides can undergo 2,3-sigmatropic rearrangement [ 12341. With weakly nucleophilic amines, C-H bond insertion or hydride abstraction can compete efficiently with ammonium ylide formation. Intramolecular N-alkylation of tertiary amines, followed by Stevens rearrangement, enables rapid preparation of elaborate polycyclic structures [ 1235- 12381. Illustrative examples are given in Table 4.16.
186.17
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
20 1
Table 4.16. Generation and rearrangement of ammonium ylides from diazocarbonyl compounds and tertiary amines. Starting Material
Reagents, Conditions
Product
Yield
Ref.
84%
[I0181
Bn02C
5% Cu(acac),, 1 10 "C, PhMe
& (900/, de)
/N
fxN2 3% Rh,(OAc),, 20 "C, CH,Cl,
99% [1135]
ph*
5% Cu(acac),, 110 "C, PhMe, 1.5 h
86% [I2391 Ph
0
62% [I2401
2% Cu(acac),, 80 "C, C,H,
3% Rh,(OAC),+, 80 "C, DCE, 10 h
& j
52%
[954]
C02Me Ph CNJPh
K
F3c
N2
\ 65% [I1991
202
4 Acceptor-Substituted Carbene Complexes
4.2.6.2 Azomethine Ylides The intermolecular reaction of imines with acceptor-substituted carbene complexes generally leads to the formation of azomethine ylides. These can undergo several types of transformation, such as ring closure to aziridines [ 1242- 12451, 1,3-dipoIar cycloadditions [ 1 133,1243,1246- 12481, or different types of rearrangement (Figure 4.9).
Fig. 4.9. Formation and transformations of azomethine ylides from imines and electrophilic carbene complexes.
The scope of these reactions has not yet been thoroughly investigated. The examples listed in Table 4.17 suggest that azomethine ylides generated by intramolecular, carbene-mediated N-alkylation of imines enable convergent and fast
4.2 Synthetic Applications o j Acceptor-Substituted Carhene Complexes
203
preparations of polycyclic heterocycles. Entry 2 is particularly interesting because the azomethine ylide is only formed from the E-oxime. With the corresponding Z-oxime intramolecular C-H insertion into the vinylic C-H bond of the oxime occurs [ 11331. Table 4.17. Generation and rearrangement of azomethine ylides from acceptor-substituted carbene complexes and imines. Starting Material
1
Reagents, Conditions
Meo2CK co2Me N,
Product
llO°C, 1.2eq. PhCH=NMe, 1.2 eq. PhCHO, Rh2(0Ac)4, PhMe. 3 h
Ref.
68% [1133]
Ph
Ph
2.7 eq. DMAD, Rh2(0Ac)4, 40 "C, CH2C12, 2.5 h N,
Yield
C0,Me
OMe
(R: OMe)
3 eq.DMAD, 0.1% Rh2(OAc),, 80 "c,C6H6, 12 h
Meo&
1.2 eq. DMAD, 0.3% Rh2(OAc),, 20 "C, CHCI,, 1 h
0
65%
[1249]
4.2.6.3 Nitrile Ylides The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [ 11941, presumably formed by the mechanism sketched in Figure 4.10.
204
4 Acceptor-Substituted Curbene Complexes
Fig. 4.10. Possible mechanism for the formation of 1,3-oxazoles from acylcarbene complexes and nitriles.
Table 4.18. Preparation of 1,3-oxazoles from acceptor-substituted carbene complexes and nitriles. Starting Material
Reagents, Conditions
Product
Yield
Ref.
5% Rh,(OAc),,
98% [I2511
N,N-diethy Icyanamide (neat), 60 "C, 3 h O,N
1Yo Rh,(OAc),,
propionitrile, 25 "C, 3 h
&N2
0
2% Rh,(OAc),, chloroacetonitrile, 80 "C, 10 h OMe
K
Meo2c Co2Me N2
I % Rh,(OAc),, 0.1 eq. PhCN, 68 "C, CHCI,, 29 h
Ph
Trapping of the intermediate acyl nitrile ylide with dimethyl acetylenedicarboxylate leads to pyrroles in low yields (< 18%) [1250]. Representative examples of the preparation of oxazoles with carbene complexes are listed in Table 4.18.
4.2 Synthetic Applications qf Acceptor-Substituted Curbene Complexes
1
205
(R = CH,CHR,)
Fig. 4.11. Generation and transformations of oxonium ylides from electrophilic carbene complexes.
4.2.6.4 Oxonium Ylides Ethers can react with electrophilic carbene complexes to yield oxonium ylides, which usually undergo either elimination reactions or 1,2-alkyl shifts to yield products of a formal carbene C-0 bond insertion (Figure 4.1 1) [ 1020,1255- 12591. With allyl ethers highly diastereoselective, concerted 2,3-sigmatropic rearrangements are often observed [ 1260- 12641. An elegant application of such stereoselective rearrangements was reported by Pirrung in 1991 (Figure 4.12). Intramolecular 0-alkylation of an allyl ether followed by 2,3-sigmatropic rearrangement was the key step in the first total synthesis of (+)-griseofulvin, a natural fungicide. In this example the 2,3-sigmatropic rearrangement proceeded both with high diastereoand enantioselectivity (Figure 4.12). If chiral catalysts are used to generate the intermediate oxonium ylides, nonracemic C-0 bond insertion products can be obtained [ 1265,12661. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [ 1 135,12591, an observation consistent with a homolysisrecombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free carbenes [ 1267,12681 acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [ 107 1,10931. Examples of reactions which presumably involve oxonium ylides are listed in Table 4.19. Experimental Procedure 4.2.6. Oxonium Ylide Formation and 2,3-Sigmatropic Rearrangement: Ethyl 2,5-Dimethoxy-4-pentenoate [ 12641
102.13
114.10
189.23
206
4 Acceptor-Substituted Curbene Complexes
-
C0,Me
6 steps
32%
Me0
D
MeO+O
Nz
62%
OMe
I
5% Rh,(piv), 80 'C, C,H,, 1 h
3. TFA 4. (PhO),P(O)N,
OMe
5. HCI, H,O D
MeO
56%
-
1, NaOMe, MeOH 2. CH,N,, THF
II
47%
'
0
0 Me0
P (+)-griseofulvin
(Pirrung, 1991)
Fig. 4.12. Synthesis of non-racemic griseofulvin based on intramolecular oxonium ylide formation [ 12621.
4.2.6.5
Carbonyl Ylides
When acceptor-substituted carbene complexes are generated in the presence of carbonyl compounds, carbonyl ylides can be formed. These intermediates can undergo a wide variety of further transformations [38,1079] (Figure 4.13). Treatment of aldehydes or ketones with acceptor-substituted carbene complexes leads to formation of enol ethers [1271- 12741, oxiranes [1048], or 1,3-dioxolanes [989,1275] by 0-alkylation of the carbonyl compound. Carboxylic acid derivatives
207
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
Table 4.19. C-0 Bond insertion reactions of acceptor-substituted carbene complexes generated from diazocarbonyl compounds. Starting Material
Reagents, Conditions N2CHC02Et,
MeO-SiMe,
Product
Yield
Sib,
68% [I2631
1Yo Rh2(OAc),,
25 "C, CH2CI2
pN* 0 -
Ref.
-YCOZO E tW (94% de)
CH,C12, 2% Cu(MeCN),PF,, 3% A[''
62% [I0141 (57% ee)
CH,CI,, 1% Cu(MeCN),PF,
QqL?
46% [I2601 OW
0 0
C0,Me
20 OC, hexane, 96% [I2691
\\ (60% ee)
95% [I1351
15% Cu(hfacac),, 40 "C, CH2C1,
86%
40 "C, CH,Cl,, 0
1%
[988]
cLa' (8lY0 ee)
can react with electrophilic carbene complexes to yield ketene acetals [ 1 102,1276 or oxiranes [ 12771. With diazocarbonyl compounds as carbene precursors cyclization of the initially formed carbonyl ylide to 1,3-dioxoles can ensue [ 1275,12791. Dihydrofurans can result either from a$-unsaturated carbonyl compounds and electrophilic carbene complexes [ 12801 or from carbonyl compounds and vinylcarbene complexes.
- 12781
208
4 Acceptor-Substituted Curbene Complexes
Table 4.19. continued. Starting Material
Reagents, Conditions
Product
P
h
4
2% CuOTf, 20 "C, D["I
Ref.
31%
[I0301
Ph .
N ~ C O , ~ B ~
8
Yield
tBu0,C
0 0
(cisltrans 24:76\
A
B
C
D
Of great synthetic potential is the ability of carbonyl ylides to undergo interor intramolecular 1,3-dipolar cycloadditions to alkenes [ 1075,1281- 12971, alkynes [1287,1292,1293,1296- 12981, reactive ketones [1299], or acylnitriles [1297]. Asymmetric induction can be achieved with enantiomerically pure rhodium(I1) carboxylates as catalysts [ 13001. The synthetic possibilities of this process have been thoroughly investigated, in particular by the research group of A. Padwa. Elaborate polycylic structures can be prepared by intramolecular generation and cycloaddition of carbonyl ylides from simple starting materials (Figure 4.14). Some of these products have been used as intermediates for the synthesis of natural products. Some examples of transformations involving carbonyl ylides are listed in Table 4.20. Entry 1 illustrates the conversion of P-acyloxy-a-diazoesters into a-acyloxyacrylates by ring fission of a cyclic carbonyl ylide [978]. This reaction has been used for the synthesis of the natural aldonic acid KDO (3-deoxy-Dmanno-2-octulosonic acid), which is an essential component of the cell wall lipopolysaccharide of gram-negative bacteria (Figure 4.15).
209
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
f 0
(R = CHR,)
Z
?
R
W
h
4
R
/
1
HC=CR',)
R
Fig. 4.13. Formation and reactions of carbonyl ylides from carbonyl compounds and electrophilic carbene complexes.
cat. Rh,(OAc), 50 'C, C,H,
*
I
95% C0,Me
(Padwa, 1995)
Fig. 4.14. Preparation of polycyclic compounds by intramolecular 1,3-dipolar cycloaddition of carbonyl ylides to alkenes (13011.
2 10
4 Acceptor-Substituted Curbene Complexes 1. N,CHCO,Et (neat), 7 d (74%) 2. Ac,O, pyridine (100%)
1% Rh,(OAc), CHCI,
D
TBSO
HO
P
TBSO AcO
N,
100%
C0,Et
4.3. NH, AcOH , H,O 92%
OH
HO 4Hbc i , .
(Lbpez Herrera, 1997)
OH KDO
Fig. 4.15. Synthesis of KDO using ethyl diazoacetate as synthetic equivalent of the anion of glyoxylic acid ethyl ester [980].
2 11
4.2 Synthetic Applications of Acceptor-Substituted Curbene Complexes
Table 4.20. Generation and transformations of carbonyl ylides from electrophilic carbene complexes. Starting Material
Reagents, Conditions
bOSiMe, Co,Me
2
Rh,(OAc),, 70 "C, C,H,
Product
0
Yield
Ref.
66%
[1302]
70%
[992]
85%
[992]
COPe
@ ..OSiMe,
NZ
0
Ph I
3 Me0,C
Ph
80 "C, 0.2% Rh2(NHCOC3F,), Ph
I
'0'
4
Me0
Me0,C
Ph
80 "C, 0.2% Rh,(OAc),
Ph
(pol: polymeric support) PhCHO, dimethyl maleate, 4.4%
6 NZ
N
I
Rh2(02CC3F7)4, CH,CI,, 8 h
'N Z U
'I..
Me0,C
COzMe
C02Me
bU2CI
7 /
p,,Qsirn3
DMAD, 0.4 % Rh,(OAc),, 20 "C, C,H,, 0.5 h 0
49% [ 13041
2 12
4 Acceptor-Substituted Carbene Complexes
Table 4.20. continued. Starting Material
Reagents, Conditions
Product
6 %Rhz(OAc),, 20 "C, CH,CI,, lh
Yield
Ref.
75% [I2711
H
45%
9
10
w~~ CH,CI,, 5 eq. 2-cyclohexenone, 0.1% rhodium(I1) octanoate
[I3051
62% [1271] (mixture of diastereomers)
The outcome of reactions involving carbonyl ylides is not always as easy to predict, as the examples in Table 4.20. Depending on the basicity of the intermediates, proton migrations might occur and unexpected results can be obtained. Two examples of such peculiar conversions and the proposed mechanisms are sketched in Figure 4.16.
4.2 Synthetic Applications of Acceptor-Substituted Curbene Complexes
\ N / ~ ~
2 13
0.2% Rh,(OAc), DMAD, 20 “C C,H, 10 min c
Meozca Me0,C
\AyPh
PkN/
60%
0
* o
Me0,C
C0,Me
(Padwa, 1996)
Fig. 4.16. Atypical reactions of carbonyl ylides generated from carbonyl compounds and acceptor-substituted carbene complexes [ 1276,13061.
4.2.6.6 Sulfonium Ylides If acceptor-substituted carbene complexes are generated in the presence of thioethers, ylide formation is generally the mostly favored process. The resulting sulfonium ylides are often sufficiently stable to be isolated [975,1307- 13091. Typical reactions of sulfonium ylides include 1,2-alkyl migration, leading to products of
2 14
4 Acceptor-Substituted Carbene Complexes
a formal carbene C-S insertion reaction (Figure 4.17). This rearrangement proceeds particularly smoothly with ally1 thioethers through a concerted 2,3-sigmatropic rearrangement [975,1231,1234,1266,1310- 13131. If enantiomerically pure catalysts are used to generate the sulfonium ylide, the 2,3-sigmatropic rearrangement can take place with significant asymmetric induction [ 1314,13151. Thioacetals are also suitable substrates for carbene C-S insertion [ 1264,1309,13161. In reactions involving sulfonium ylides fewer side-reactions than with oxonium ylides are usually observed. This is probably because of the stabilization of the former by dx-pn interaction; this is not possible with oxonium ylides.
z R2
'R
R'
R'
Fig. 4.17. Generation and typical transformations of sulfonium ylides.
In addition to 1,2-alkyl shifts, sulfonium ylides with P-hydrogen can also undergo fragmentation into an olefin and a thioether [ 1317,13181. When allylic thioethers are treated with two equivalents of ethyl diazoacetate in the presence of a catalyst for diazodecomposition, S-alkylation and elimination of the thioalkyl group from the initially formed a-alkylthio-4-alkenoic esters occurs to yield 2,4-dienoic esters [1319]. Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [ 1320- 132.51, aziridines [ 1321,1326,13271 or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the
4.2 Synthetic Applications
of
2 15
Acceptor-Substituted Carbene Complexes
Table 4.21. Reactions of thioethers with electrophilic carbene complexes. Starting Material
K
Meo2c co2Me
p
Reagents, Conditions
Product
Yield
Ref.
psph W0zk C0,Me
89% [I3091
0
N2
Rh2(0Ac)4, 40 "C, CH,CI,
SPh U
C02Me 0.5% Rh,(OAc),,
78% [I3111
80 "C, C6H6, 30 min
1% Rh,(OAc),,
77% [I3321
80 "C, C6H6, 30 min '
C0,Me
Rh6(c0),2
29% [lo111
(+ 6% other diastereomers)
1% Rh,(OAc),, 80 "C, C,H6
56% [I3161 H
0x0
0 x 0
2% Rh,(OAc),, 25 "C, CH,Cl,, 20 min
83% [I0711
2 16
4 Acceptor-Substituted Curbene Complexes
Fig. 4.18. Possible mechanism of the olefination of thiocarbonyl compounds with electrophilic carbene complexes [ 13361.
corresponding three-membered, cyclic product. Enantiomerically pure thioethers have been used, in combination with a catalytically active copper or rhodium complex, to convert diazoalkanes into the corresponding, more nucleophilic, chiral sulfonium ylides. These can undergo the reactions sketched in Figure 4.17 in an enantioselective fashion [ 132I , 1323,1324,1326- 13281. The conversion of acceptor-substituted diazomethanes into carbene complexes often does not proceed smoothly in the presence of sulfides. This might be because of poisoning of the catalyst by the sulfide [975]. Higher yields of sulfonium ylides can, however, be obtained by HBF,-mediated reaction of acceptor-substituted diazomethanes with sulfides [lo1 1,1328,1330,1331]. Representative examples of reactions of acceptor-substituted carbene complexes with thioethers are given in Table 4.21.
4.2.6.7
Thiocarbonyl Ylides
When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [ 1333- 13361. The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [ 13371. Few examples have been reported in which stable thiiranes could be isolated [1338]. The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carbonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [ 13381. Illustrative examples of these reactions are given in Table 4.22. When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into account that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides 113381.
4.2 Synthetic Applications
of
2 17
Acceptor-Substituted Carbene Complexes
Table 4.22. Reactions of thiocarbonyl compounds with acceptor-substituted carbene complexes. Starting Material
Reagents, Conditions
Product
Yield
Ref.
H
U
80 "C, C6H6,4 eq. N,CHCO,Et,
1
49% [1339]
0.4% Rh,(OAc),,
16h
S
CHC0,Et
MeO,d
C0,Me
tBu
4
CozMe N,
tBu
COzMe
66% [1342]
3% Rh,(OAc),, 80 "C, C6H6 tBu
4.2.6.8 Other Ylides Phosphonium ylides can be generated by treatment of diazoacetates and triphenylphosphine or triethyl phosphite with catalytic amounts of RuC12(PPh,), [ 13431 or ReOCl,(PPh,), [1344]. If these reactions are conducted in the presence of aldehydes, carbonyl olefination takes place in high yields. Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with ally1
2 18
4 Acceptor-Substituted Curbene Complexes
bromide, for instance, leads, via 2,3-sigmatropic rearrangement, to a formal insertion of the carbene into the C-Br bond [ 1231,12341. Diacylcarbene complexes can also abstract chloride from 1,2-dichloroethane, presumably by intermediate formation of a chloronium ylide [ 1 1661. Finally, stable cyclic iodonium ylides can be prepared by intramolecular iodine alkylation with electrophilic carbene complexes [ 1345,13461,
4.2.7 Cyclopropanation One of the most efficient procedures for the synthesis of cyclopropanes is the reaction of alkenes with electrophilic carbene complexes. In this process up to three stereogenic centers can be generated in one step. Cyclopropanes are a key structural element encountered in many natural products with interesting biological activity. Further, by virtue of the ability of cyclopropanes to undergo ring-opening reactions these compounds can be valuable synthetic intermediates. 4.2.7.1
Scope and Limitations
A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [ 10151, 1,3-dienes [1002], vinyl iodides [ 1347,13481, arenes [1349], fullerenes [1350], heteroarenes, enol ethers or esters [ 1351- 13541, ketene acetals, and N-alkoxycarbonyl[ 1355,13561 or N-silyl enamines [ 13571. Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,10 151. Alkynes can be converted into cyclopropenes by inter- [587,1022,1052,106010621 or intramolecular [ 10701 cyclopropanation with electrophilic carbene complexes. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result from rearrangement or other transformations of the cyclopropenes initially formed (cf. Section 4.1.3). Alkenes with strongly electron-withdrawing groups or strained alkenes are generally not suitable substrates for cyclopropanation with electrophilic carbene complexes. These alkenes tend to undergo fast 1,3-dipolar cycloaddition with diazocarbonyl compounds (i.e. the carbene complex precursors) to yield 3Hpyrazolines. The latter can either be stable or rearrange to IH-pyrazolines. In some cases 3H-pyrazolines eliminate nitrogen upon thermolysis or photolysis to yield the corresponding cyclopropanes [ 13501. The most thoroughly investigated carbene precursors for inter- or intramolecular cyclopropanations are diazoacetic acid derivatives. Cyclopropanations with vinyldiazoacetic esters have been investigated in detail by the group of H. M. L. Davies. The products resulting from the cyclopropanation of arenes often undergo electrocyclic rearrangement to cycloheptatrienes (Figure 4.6) [ 1358- 13601. Treatment of donor-substituted alkenes with acylcarbene complexes can lead to the formation of donor-acceptor-substituted cyclopropanes which can readily undergo ring fission and/or rearrangement to dihydrofurans. Reactions of this type will be discussed in Section 4.2.8.
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
2 19
4.2.7.2 Stereoselectivity Experimental results [ 13611 and theoretical treatment [28] indicate that the cyclopropanation of alkenes by electrophilic carbene complexes is a concerted process. Z-Olefins normally lead to the formation of the corresponding cis-cyclopropanes, and E-olefins yield truns-cyclopropanes. The relative configuration of the carbenebound substituent and the substituents of the alkene in the final cyclopropane seems to be mainly determined by the steric bulk of these groups. In cyclopropanations of terminal alkenes with ethyl diazoacetate low diastereoselectivities are often observed [ 1024,135 I]. These can be improved by increasing the steric demand of the substituents at the carbene or at the alkene [ 1033,13621. High diastereoselectivities can, e.g., often be achieved with tert-butyl, neopentyl or 2,6-di(tert-butyl)phenyl diazoacetate [ 13621 as carbene complex precursors (Figure 4.19). 1% Rh,(acac), 10 eq. 1-hexene 20 “C, CH,CI,
-
N,
Bu\lrl/CozEt + (37: 63)
1% Rh,(acac), 10 eq. 1-hexene 20 OC, CH,CI, c
80%
N,
Bu&cozR
+ (7 : 93)
““‘4 C0,Et
““‘4 CO,R
(Doyle, 1990)
Fig. 4.19. Enhancement of diastereoselectivity in intermolecular cyclopropanations with sterically demanding diazoacetates [ 13621.
The catalyst can also have a significant influence on the stereoselectivity of cyclopropanation reactions [ 10231. For instance, cyclopropanation of styrene with ethyl diazoacetate and copper or rhodium catalysts normally proceeds with low diastereoselectivity. With ruthenium porphyrines as catalysts, however, up to 92% de can be achieved [1041,1042]. The inter- or intramolecular cyclopropanation of achiral alkenes with enantiomerically pure diazoacetic esters [ 1016,1363,13641 or amides [ 1365,13661 does not usually proceed with high diastereoselectivity. A chiral auxiliary which occasionally gives good results is pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) [ 1016,1367,1368]. The most elegant strategy for the preparation of enantiomerically enriched cyclopropanes is based on the use of chiral catalysts (for recent reviews, see [ 1072-
220
4 Acceptor-Substituted Curbene Complexes
1
2
Ph
r( Ph
3
Rh,(SS-MEPY),
Rh,(SS-MPPIM),
Fig. 4.20. Complexes for asymmetric cyclopropanation with acceptor-substituted diazomethanes. 1 [1372], 2 [ 13731, 3 [ 10331, Rh,(SS-MEPY),, Rh2(5S-MPPIM), [1001,1074]. For related rhodium-based catalysts, see, e.g., [997,1000,1002].
10741). Since the first experiments with chiral copper complexes reported by Nozaki [650] and Aratani [ 10271 many different catalysts have been examined, both for intermolecular and intramolecular cyclopropanations (for a review, see [ 13691). Syntheses of natural products [955,1370] and drugs [ 13711 using asymmetric cyclopropanation with chiral electrophilic carbene complexes have been reported. A selection of useful catalysts is given in Figure 4.20 (see also Experimental Procedure 4.1.1). For intermolecular cyclopropanations with unsubstituted diazoacetates the highest asymmetric inductions can be achieved with the copper(1) complexes of C,-symmetric, bidentate ligands developed by Pfaltz (e.g. 1) and Evans (2). The chiral rhodium(I1) complexes known today do not generally lead to such high enantiomeric excesses as copper complexes in intermolecular cyclopropanations. For intramolecular cyclopropanations, however, chiral rhodium(l1) complexes are usually superior to enantiomerically pure copper complexes [ 13741.
4.2.7.3 Intramolecular Cyclopropanations Intramolecular cyclopropanation with acceptor-substituted carbene complexes is a powerful method for the synthesis of bicyclo[n. l.O]alkanes [ 1359,137513781. Several different ring sizes can be prepared, including macrocycles [ 1359, 13791 (Table 4.23). Intramolecular cyclopropanation has often been used for
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
catalys_t
0
O
Ph
D
+
22 1
+
Ph 4
catalyst: Rh,(O,CC,F,), catalyst: Rh,(cap),
catalyst: Rh,(cap),
(100 : 0)
(72% yield)
catalyst
catalyst: Rh,(O,CC,F,),
(0 : 100)
(95% yield)
a
&
+
(0 : 100)
(56% yield)
(100 : 0)
(76% yield)
Fig. 4.21. Chemoselectivity of different rhodium(I1) carbene complexes [990,1082].
the synthesis [ 1016,1050,1349,1380- 13841 or modification [ 10531 of natural products or analogs thereof [ 1385,13861. Several chiral complexes have been tested for their ability to control the stereochemical outcome of intramolecular cyclopropanations. High enantioselectivities have been attained using enantiomerically pure copper complexes [ 1387,13881 and rhodium complexes [ 1000,1001,1006,1049,115I , 1348,1374,1387,1389- 13921. Intramolecular C-H bond insertion and ylide formation can compete with cyclopropanation. As shown in Figure 4.21, however, the chemoselectivity of the intermediate carbene complex can sometimes be controlled by the remaining metal-bound ligands [21,990,1075,1081,12231. Experimental Procedure 4.2.8. Enantioselective, Intramolecular Cyclopropanation: 6,6-Dirnethyl-3-oxabicyclo[3.1 .O]hexan-2-one [ 13901
~~~~1- $: C0,Me
.(
[Rh,(6R-MEPY) J
154.17
916.55
126.16
ution of 3-methyl-2-butenyl diazoacetate (14.9g, 96.7 m o l ) in dichloromethane (450 mL) is added over 30h to a refluxing solution of Rh,(SR-
222
4 Acceptor-Substituted Curbene Complexes
Table 4.23. Intramolecular cyclopropanation by transition metal-catalyzed decomposition of diazocarbonyl compounds. Starting Material
Reagents, Conditions
Product
Yield
Ref.
tgu
N2
TBSO
4
f 0
OM
100% [1393]
(Ar:3,5-tBu2-4-MeC,H2) cyclopentadiene, 1% Rh2(OAc),, 40 "C, CH2C12, 3h
S0,Ph
OTBS
a
S0,Ph
98% [1393]
OTBS
PO(OMe),
20 "C. 4% Q r & h
4%Rh2(OAc),, 20 "C, CHCIj, 10 min
C0,Me
C0,Et
91% [lo121
75% [I1751
2% Rh2(OAc),, 40 "C, CH,C12, 3h
77% [1394]
N,
N, 0
I
3% Rh,(OAc),, 0 "C, CH2C12, 3 min
87% [I3951
PMB
(+ C-2 epimer; 87:13) C0,Et
-78 "C, 1% Rh2(02CCF3)4, CH2C12,1 h
-a
EtOZC..
H
86% [lo971
4.2 Synthetic Applications of Acceptor-Substituted Carbene Complexes
223
Table 4.23. continued. ~~
Starting Material
Reagents, Conditions
Product
Yield
Ref.
h
8
2)
PhS
9
0
5% Pd(OAc),,
20 "C, C,H, [Rh,(OAc), leads to sulfonium ylide formation]
Ph
3% Rh,(OAc),, 10
80 OC, DCE, 9 h
12
63% [1310]
q;J 0
61% [I0831
C0,Me
[I3961
(74% de)
(Ar:2-tolyl)
I1
@r"h H
1% Rh,(OAc),, 20 "C, CH,CI,, 5h
Q
CH,CI,, 1 .O% Cu(MeCN),PF,, 1.2%
82% [I3971
(90% ee) tdu
teU
61% [I3541
224
4 Acceptor-Substituted Carbene Complexes
4.2.7.4 Intermolecular Cyclopropanations The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072- 1074,1076,1077,1081]and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantioselectivity include copper complexes [ 1025,1026,1028,1029,1031,1373,1398- 14001, cobalt complexes [ 1033- 10351, ruthenium porphyrin complexes [ 1041,1042,1230], C,-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002- 1004,1010,1062,1353,1401 - 14051. Particularly efficient catalysts for intermolecular cyclopropanation are C,-symmetric copper(1) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. Experimental Procedure 4.2.9. tion: Ethyl (1S)-2,2-Dimethyl-l
ermole
clopropana-
CuOTf,
212.61
A +f
cozEt
J2
56.11
114.10
A solution of the chiral ligand (67 mg, 0. is added to a suspension of copper(1) tri
c
2(
C0,Et
142.20
4.2 Synthetic Applications
of Acceptor-Substituted Carbene
Complexes
225
Table 4.24. Intermolecular cyclopropanation by transition metal-catalyzed decomposition of diazocarbonyl compounds. Starting Material
Reagents, Conditions
C0,Me
Product
Yield
Ref.
N,CHCO,Et, 2.4% Rh,(OAc),, 20 "C, CH,CI,, 12h
dN-C02Bn
H
(cislfrans38:62) Boc I
Boc
N,CHCO,Me, 0.2% Cu(OTf),, PhN,H,, CH,CI,
0
45% [1407] C0,Me H
9
OTBS
?TBS
TBSO
3 eq. N,CHCO,Et, 2% Rh,(OAc),, CH,CI,, 10 min
C0,Et
85% [1408]
TBSO OTBS
OTBS
25 "C, 10 eq. EtOCH=CH,, 1% Rh,(OAc),,
CH,CI,
q+"
93% [I0661
Ph
OEt
1-butene, 1%
Ar's05? Ph
T
Ph
o"
f N2
\\
00
' 0
I/
I/
/Yh7Yh
(> 95% ee)
(AK4-tBuC,H,)
25 OC, pentane
oeE C0,Me
20 "C, CH,CI,, 1% N2kco2Me>
+
OEt
I
E
I/
I/
10 equivalents
t
O
p
OEt
(> 98% ee)
(E: C0,Me)
42% [lo601
226
4 Acceptor-Substituted Carbene Complexes
Fig. 4.22. Formation of 1,3-dipoles by ring fission of donor-acceptor-substituted cyclopropanes.
4.2.8 Formal 1,3-Dipolar Cycloadditions of Acyl- and Vinylcarbene Complexes The reaction of heteroatom-substituted alkenes with electrophilic carbene complexes can lead to the formation of highly reactive, donor-acceptor-substituted cyclopropanes. This type of cyclopropane usually undergoes ring fission and rearrangement reactions under milder conditions than do unsubstituted cyclopropanes (Figure 4.22). For this reason unstable cyclopropanes or only rearrangement products are obtained when donor-substituted alkenes react with acceptor-substituted carbene complexes [1409- 14161. In reactions of acyl- and vinylcarbene complexes with enol ethers the most common types of rearrangement observed are those shown in Figure 4.23. In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [ 14181, and other natural products [ 14191. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal 13 + 41 cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee; [1420]) can be attained using enantiomerically pure rhodium(I1) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. Rearrangement of non-donor-substituted vinylcyclopropanes can be induced by transition metals [ 1421- 14231 or thermally [ 1424- 14271. The thermal isomerization, however, requires high reaction temperatures (typically 200-500 "C).
4.2 Synthetic Applications cf Acceptor-Substituted Curbene Complexes
227
Fig. 4.23. Rearrangement of donor-substituted acyl- and vinylcyclopropanes.
Experimental Procedure 4.2.10. Cycloaddition of an Acylcarbene Complex to an Enol Ether: Ethyl 5-Ethoxy-2-trifluoromethyl-4,5-dihydro-3-furoate [14171
210.11
72.11
254.21
A mixture of ethyl 2-diazo-4,4,4-trifluoro-3-oxobutyrate(58.5 g, 0.28 mol), hydroquinone (100 mg), rhodium(I1) acetate (100 mg, 0.23 mmol, 0.08%), and ethyl vinyl ether (500 mL) is heated to 100°C for 8 h in a closed steel vessel (CAUTION!). The mixture is concentrated under reduced pressure and the residue purified by column chromatography (silica gel, ethyl acetateheptane 4:l). 65g (91%) of the title compound is obtained as an oil. lH NMR (100 MHz, CDC1,) 6 1.25 (t, 3H), 1.29 (t, 3H), 3.10 (m, 2H), 3.75 (m, 2H), 4.20 (9, 2H), 5.68 (dd, 3.0, 7.5 Hz, 1H).
Experimental Procedure 4.2.1 1. Cycloaddition of an Acylcarbene Complex to an Alkyne: Ethyl 2-Methyl-5-phenyl-3-furoate [ 143I]
156.14
102.14
230.27
228
4 Acceptor-Substituted Carhene Complexes
Table 4.25. Formal 1,3-dipolar cycloaddition of acyl- and vinylcarbene complexes to alkenes, alkynes, and dienes. Starting Material
Reagents, Conditions
p yo & 0
Product
Yield
Ref.
rhodium(I1) octanoate, CH,CI, 0
1Yo Rh,(OAc),, furan, PhF, 20 "C, 15 h
92% [I1661
1% Rh,(OAc),, N-acetylindole, PhF, 20 "C, 15 h
62% [1166]
2% Rh,(OAc),, 2 eq. triacetyl glucal, 20 "C, PhF, 10 h
34% [1166]
Ho"d~co
HO
65% [I4281
Rh,(OAc),, PhF
O
L-
0
3% rhodium(I1) octanoate, 80 "C, C,H,, 30 min
78% [I0671 \
Ph
229
4.2 Synthetic Applications of Acceptor-Substituted Curbene Complexes
Table 4.25. continued. Starting Material
Product
3%rhodium(II) mandelate, 80 "C, C&,3 h
I
cyclopentadiene; Rh2(0Ac)4,
9
6
Reagents, Conditions
Yield
Ref.
72% [I0691
A
E t o 2 c ~ C O z E t 80% [I2281
EtozCYKCo2Et then 110 "C, PhMe, 12 h
Tom:;{;2:, OTBS 0
10
&lTBS
63% [I4291
(R: Boc)
Boc'
OTBS 0
l 1 +o>
N*
NI Boc
1% rhodium(I1) octanoate, 69 "C, hexane, 1 h
53% [I4291
230
4 Acceptor-Substituted Carbene Complexes
1
Z
L
(X = 0, NR)
0-
1% rhodium(l1) octanoate 69 OC,hexane, 1 h C
Boc'
ooT
-
Boc'
0
-------c
O
49%0
\
O
S
/S
BOG'"
1
(Davies, 1996)
Fig. 4.24. Reaction of electrophilic carbene complexes with furans and pyrroles leading to ring-opening of the heterocycle [ 14291.
4.2 Synthetic Applicutions of Acceptor-Substituted Carbene Complexes
23 1
4.2.9 Ring Fission of Pyrroles and Furans The reaction of acceptor-substituted carbene complexes with pyrroles and furans can lead to surprising results. Derivatives of these heterocycles are often attacked by electrophilic carbene complexes at C-2. The resulting intermediates can either tautomerize to give the products of carbene C-H insertion, or undergo ring-opening of the heterocycle to yield a$-unsaturated carbonyl compounds [ 1162,1367,14321 or imines (Figure 4.24). These can either be isolable, or undergo further transformations. Examples of such reactions are given in Figure 4.24 and in Table 4.26. Table 4.26. Ring fission of furans and pyrroles by acceptor-substituted carbene complexes. Starting Material
Reagents, Conditions
_c-i;
Product
Yield
Ref.
20 "C, 1.75 eq. 2-hexy lfuran,
58% [I4331
1% Rh,(OAc),,
Et02C
C02Et
rhodium(I1) octanoate, CH,CI,
E
t
O
q
P
O
rhodium(I1) octanoate, CH,C1,
1% rhodium(I1) octanoate, 69 O C , hexane, 1 h Boc
60%
[987]
75%
[987]
77% [I4291
232
4 Acceptor-Substituted Carbene Complexes
4.2.10 Other Synthetic Applications of Acceptor-Substituted Carbene Complexes The normal byproducts formed during the transition metal-catalyzed decomposition of diazoalkanes are 'carbene dimers' and azines [496,1023,1329]. These products result from the reaction of carbene complexes with the carbene precursor. Their formation can be suppressed by slow addition (e.g. with a syringe motor) of a dilute solution of the diazo compound to the mixture of substrate and catalyst. Carbene dimerization can, however, also be a synthetically useful process. If, e.g., diazoacetone is treated with 0.1% RuCICp(PPh,), at 65 "C in toluene, cis-3-hexene2,5-dione is obtained in 81% yield with high stereoselectivity [1038]. A further, often observed, side-reaction is the oxidation of a given substrate by the carbene complex via hydride abstraction [1083,1084,1090,1118,1149-11521. Examples of such oxidations are shown in Figures 4.5 and 4.25. No catalysts or carbene precursors have, however, yet been reported, which lead to preparatively useful amounts of oxidized product.
1% Rh,(OAc), 20 OC, CH,CI,
0
oL +
I
OBn
58%
37% (Lee, 1995)
s I
Fig. 4.25. Hydride abstraction by acceptor-substituted carbene complexes [ 1090,11491.
5 Bibliography
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
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41 42 43 44 45 46 47 48 49 50 51
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5 Bibliogruphy 79 80 81 82 83 84 85
86 81
88 89 90 91 92 93 94 95 96 91
98 99 100 101
I02 103 104 105 106 107
108 109 110 111 112 1 I3 I I4 1 I5 116 1 I7
I I8 119 120 121 122 123 124 125
235
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236 I26 I27 128 129 130 131 132 133 I34 I35
136 137 138 139 140 141 142 143 144 145 I46 147 148 I49 150 151
I52 1.53 154 155
156 157
I58 159 I60 161 162 163 164
16.5
166 167
5 Bibliogruphy
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250 738 739 740 741 742 743 744 745 746 747 748 749 750
75 I 752 753 754 755 756 757 758 759 760 76 I 762 763 764 765 766 767 768 769 770 77 1 772 773 774 775 776 777 778 779
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252 822 823 824 825 826 827 828 829 830 83 1 832 833 834 835 836 837 838 839 840 84 I 842 843 844 845 846 847 848 849 850 85 1
852 853 854 855 856 857 858 859 860 86 I 862 863 864
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5 Bihliogruphy 865 866 867 868 869 870 87 1 872 873 874 875 876 877 878 879 880 88 I 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 91 1 912
913
253
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Doyle, M. P.; Peterson, C. S.; Parker, D. L. Angew. Chern. Int. Ed. Erzgl. 1996, 35, 1334-1336. Meyers, A. 1.; Price, A. J. Org. Chern. 1998, 63, 4 1 2 4 1 3 . Bedekar, A. V.; Koroleva, E. B.; Andersson, P. G. J. Org. Chem. 1997, 62, 2518-2526. Tanner, D.; Andersson, P. G.; Harden, A,; Somfai, P. Tetrahedron Lett. 1994, 35, 46314634. Starmans, W. A. J.; Thijs, L.; Zwanenburg, B. TetrLihedron 1998, 54, 629-636. Miiller, P.; Baud, C.; Ene, D.; Motallebi, S.; Doyle, M. P.; Brandes, B. D.; Dyatkin, A. B.; See, M. M. Helv. Chim. Acta 1995, 78, 4 5 9 4 7 0 . Yoshikawa, K.; Achiwa, K. Chern. Pharm. Bull. 1995, 43, 2048-2053. Martin, S.F.; Austin, R. E.; Oalmann, C. J.; Baker, W. R.; Condon, S.L.; de Lara, E.; Rosenberg, S. H.; Spina, K. P.; Stein, H. H.; Cohen, J.; Kleinert, H. D. J. Med. Chern. 1992, 35, 1710-1721. Davies, H. M. L.; Cantrell, W. R. Tetruherlron Lett. 1991, 32, 6509-6512. Marinozzi, M.; Natalini, B.: Ni, M. H.: Costantino, G.; Pellicciari, R.; Thomsen, C. Farmuco 1995, 50, 327-331. Bubert, C.; Cabrele, C.; Reiser, 0. Synlett 1997, 827-829. Timmers, C. M.; Leeuwenburgh, M. A,; Verheijen, J. C.; van der Marel, G. A,; van Boom, J. H. Tetrahedron: Asymmetry 1996, 7, 49-52. Muller, A,; Maier, A,: Neumann, R.; Maas, G. Eur. J. Org. Chern. 1998, 1177-1 187. Davies, H. M. L.; Ahmed, G.; Calvo, R. L.; Churchill, M. R.; Churchill, D. G . J . Org, Chern. 1998, 63, 2641-2645. Ishitani, H.; Achiwa, K. Heterocycles 1997, 46, 153-156. Davies, H. M. L.; Hu, B.; Saikali, E.; Bruzinski, P. R. J. Org. Chem. 1994, 5Y, 45354541. Davies, H. M. L.; Hu, B. Tetrahedron Lett. 1992, 33, 4 5 3 4 5 6 . Pirrung, M. C.; Zhang, J. Tetrahedron Lett. 1992, 33, 5987-5990. Davies, H. M. L.; Clark, T. J.; Church, L. A. Tetrahedron Lett. 1989, 30, 5057-5060. Alonso, M. E.; Morales, A. J. Org. Chem. 1980, 45, 45304532. Hoffmann, M. G.; Wenkert, E. Tetruhedron 1993, 49, 1057-1062. Pirrung, M. C.; Lee, Y. R. Tetrahedron Lett. 1996, 37, 2391-2394. Pirrung, M. C.; Lee, Y. R. J. Am. Chem. SOC. 1995, 117, 48144821. Davies, H. M. L.; Stafford, D. G.; Doan, B. D.; Houser, J. H. J. Am. Chern. Soc. 1998, 120, 3326-3331. Wender, P. A.; Sperandio, D. J. Org. Chem. 1998, 63, 41644165. Wender, P. A,; Husfeld, C. 0.; Langkopf, E.; Love, J. A,; Pleuss, N. Tetrahedron 1998, 54, 7203-7220. Hayashi, M.; Ohmatsu, T.; Meng, Y. P.; Saigo, K. Angew. Chern. Int. Ed. Engl. 1998, 37, 837-839. Baldwin, J. E.; Bonacorsi, S. J.; Burrell, R. C. J. Org. Chem. 1998, 63, 47214725. Davidson, E. R.; Gajewski, J. J. J. Am. Chern. Soc. 1997, I l Y , 10543-10544. Houk, K. N.; Nendel, M.; Wiest, 0.;Storer, J. W. J. Am. Chem. Snc. 1997, 119, 10545-10546. Wu, P. L.; Chen, H. C.; Line, M. L. J . Org. Chern. 1997, 62, 1532-1535. Pirrung, M. C.; Lee, Y. R. J. Chern. Soc. Chem. Comrnun. 1995, 673-674. Davies, H. M. L.; Matasi, J. J.; Ahmed, G. J . Org. Chern. 1996, 61, 2305-2313. Davies, H. M. L.; Hodges, L. M.; Thornley, C. T. Tetruhedron Lett. 1998, 39, 2707-2710. Davies, H. M. L.; Cantrell, W. R.; Romines, K. R.; Baum, J. S. Org. Synth. 1992, 70, 93-100. Wenkert, E.; Decorzant, R.; Naf, F. Helv. Chim. Acta 1989, 72, 756-766. Sheu, J.-H.; Yen, C.-F.; Huang, H.-C.; Hong, Y.-L. V. J. Org. Chem. 1989, 54, 5126-5128.
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Index
76,77 84, 87, 89, 101 - electrophilic 29-3 1, 77, 82-90 - of halides 31, 89, 110 - of hydride 29-31, 83, 109, 122, 179, 197, 232 - nucleophilic 76-82 - of oxygen-bound leaving groups 84-87, 108, 110, 111 - of pyridine 45 - of thioethers 87, 106, 109, 111, 123, 124 Acceptor-substituted alkenes - cyclopropanation 45, 47-49, 11 1, 115, 117, 120, 218 - 1,3-dipolar cycloadditions to 115, 118, 211, 212, 218 - as Michael acceptors 68 - olefin metathesis 8, 150, 151, 157-159, 164 - preparation 180, 181, 232 Acceptor-substituted diazomethanes - C-acylation/alkylation 173, 210 - as carbene complexe precursors 171232 - as cocatalysts for olefin metathesis 140 - preparation 172 - stability 173 Acetals, reaction with carbene complexes 205-208 Acetylenes - see Alkynes Acidity, of heteroatom-substituted carbene complexes 35 Acrolein dimethylacetal 206 Acrylates - see Acceptor-substituted alkenes Abstraction
Acry lonitrile
- conjugate
- cross metathesis
161, 164, 166 cycloaddition to isonitrile complexes 22 C-H Activation - see C-H Insertion Acyclic diene metathesis (ADMET) 137, 148, 149, 161 Acyl complexes - 0-alkylation/O-acylation 14-20, 85, 86 - from carbonyl complexes 14, 19, 20 - conversion to vinylidene complexes 86 - from metallates 14, 18, 19 - photolysis 33, 34 Acyl halides 15 - reaction - with acyl complexes 14, 15 - with diazo compounds 173 - with metallates 18, 19 - with titanium carbene complexes 125 Acylcarbene complexes - see Acceptor-substituted carbene complexes ADMET - see Acyclic diene metathesis Aflatoxin 226 Alcohols - as cocatalysts for olefin metathesis 139 - etherification 196-199 - reaction with alkoxycarbene complexes 35, 68 - reaction with ketene complexes 41, 43, 68, 72 Aldehydes - 0-alkylation 206, 211 - condensation with heteroatom-substituted carbene complexes 35, 36 -
268
Index
condensation with isonitrile complexes 21, 22 - conversion to oxiranes 214 - [2 + 21 cycloaddition of 41 - in olefin metathesis 141, 144, 152 125, 126, 128, 130 - olefination - reaction with diazoalkanes 173, 210 - reaction with metallates 85 Aldol additions 35 Alkanes - alkylation of 179-192 - elimination of 78-82 - metallation 119-121 Alkene metathesis - see Olefin metathesis Alkenes - from carbonyl compounds 125-134 - from diazo compounds 116, 180, I8 1 , 232 - isomerization 137, 143, 144, 150 see also [2 + 21 Cycloadditions, Cyclopropanation, etc. Alkenyl complexes, reaction with electrophiles 98 Alkenylcarbene complexes - see Vinylcarbene complexes a-Alkoxyalkyl complexes 84-86 Alkoxycarbene complexes - see Heteroatom-substituted carbene complexes Alkoxynaphthols 50 Alkoxyphenols 49 Alkyl complexes - from carbene complexes I 19, 121 p-elimination 37, 38, 57, 63, 84 Alkyl halides - reaction with nucleophilic complexes 20, 29, 35, 87-89, 91, 95, 97, 99 - reaction with acylcarbene complexes 217 Alkyl nitrites 172 Alkyl triflates 15, 16 Alkylaluminum halides - see Organoaluminium compounds Alkyne complexes - intermediates in benzannulations 49 - isomerization to vinylidene complexes 25-27, 98, 101, 102 Alkynes - benzannulations with 49-55 -
- conversion to carbene complexes
2527, 98, 101, 102, 177 - cross metathesis with alkenes 161, 165 - cyclopropanation 110, 176, 177, 218, 225 46, 135-137, 148, 165 - metathesis - metathesis polymerization 135, 137, 141 - reaction with acylcarbene complexes 227-229 - reaction with alkoxycarbene complexes 42, 49-73 Alkynyl complexes, reaction with electrophiles 25, 29, 98 Alkynylcarbene complexes - cycloadditions to 36, 37, 54 - as dienophiles 36, 37, 54, 58, 60 - as Michael acceptors 36, 37, 56, 59, 61, 69 - preparation 19, 100 Allenes - cyclopropanation 108, 118 - preparation 131 Allyl amines, cyclopropanation 117, 225 Allyl ethers - 0-alkylation 205 - isomerization by carbene complexes 144 - RCM 153, 154, 156, 162, 163 Allyl halides 21 7, 2 18 Allyl silanes - cross metathesis with alkenes 164-167 - cross metathesis with alkynes 165 - preparation 152, 162, 164-167, 207 - ROMP 146 Allyl stannanes - cross metathesis 164 - preparation 164 Allyl thioethers - S-alkylation 214, 215 - cyclopropanation 223 - metathesis 143 Aluminium trichloride, as cocatalyst for olefin metathesis 75, 140 Amalgam, sodium 29, 30 Amides - see Carboxamides Amines 43, 45, - acylation by ketene complexes 63, 64
Index addition to alkynylcarbene complexes 36, 37, 69 - N-alkylation 194-1 96, 198, 200-202 - hydride abstraction 178, 179 - quaternization 198, 200-202 - RCM 157-160 - reaction with carbene complexes 35, 156 Amino acids - from chromium carbene complexes 41, 43 - cyclopropanation 117 - olefin metathesis 149, 158-160, 165 - norbornene derivatives, ROMP 147 Aminocarbene complexes - see Heteroatom-substituted carbene complexes 2-Amino- 1,3-dienes 58, 72, 73 Aminoindenes 49, 50 Aminophenols, by benzannulation 52, 53 Ammonium ylides - from acylcarbene complexes 198, 200-202 - from chromium ketene complexes 43, 45, 63, 64 Anhydrides - reaction with metallates 19 - reaction with titanium carbene complexes 126, 128 Aniline - as additive for olefin metathesis 140 - N-alkylation by acceptor-substituted carbene complexes 194, 196 - reaction with alkoxycarbene complexes 35 Arenes - acylation by ketene complexes 41 - alkylation by C-H insertion 180, 186, 187, 189-191 - cyclopropanation 180, 21 8, 223 - see also Benzannulation Aryl iodides, I-alkylation 21 8 Arylcarbene complexes - Experimental Procedure 91 - see also Non-heteroatom-substituted carbene complexes Asymmetric cyclopropanation - see Cyclopropanation, enantioselective 1-Aza- 1,3-dienes 72 -
269
Azepines C-alkylation 183 - by RCM 159 - from vinylcarbene complexes 71, 72 - Experimental Procedure 71 Azetidines - N-alkylation 64, 201 - preparation 230 Azetidinones - conversion to carbene complexes 24 - by N-H insertion 195 - see also P-Lactams Azides - 1,3-dipolar cycloaddition 32 - as reagents for diazogroup transfer 172 - transformation 184, 198 2-Azidopyridinium salts, diazogroup transfer with 172 Azines 25, 27, 232 Aziridines - from imines and carbene complexes 71, 202 - from imines and sulfonium ylides 214 - reaction with carbonyl complexes 18 - reaction with carbyne complexes 32 Azomethine ylides 64, 202, 203 -
Back-donation 2-5 Bamford-Stevens reaction 173 Benzannulation 41, 44, 49-55 - Experimental Procedures 52, 54 Benzofurans, preparation by RCM 162 Benzoquinones, from alkoxycarbene complexes 52-54 Bicyclo[n. 1 .O]alkanes 46-48, 107, 220223 Bicyclo[ 1.1.O]butanes - as carbene complex precursors 119 - preparation 119, 222 Block copolymers 141 Bond energies 3-5 - in tantalum carbene complexes 78 Boranes 35 Boron trichloride, as cocatalyst for olefin metathesis 141 Bromination, of carbyne complexes 99 1,3-Butadiene - see Dienes
270
Index
Calixarenes 149 Carbalkoxycarbenes - see Acceptor-substituted carbene complexes Carbamates - N-alkylation 194-196 - macrocyclic 149 Carbanions - see Organolithium compounds, Grignard reagents Carbazoles 44, 53, 55 - by benzannulation - by intermolecular C-H insertion 191 Carbene complexes, acceptor-substituted 171-232 - N-alkylation of amines 194-196 - 0-alkylation of alcohols 196-1 99 - S-alkylation of thiols 197, 200 - cyclopropanations with 2 18-226 - dimerization 116, 232 - generation 171-177 - C-C insertion 193 - C-H insertion 179-1 92 - Si-H insertion 192 - reaction with lone pairs 198-218 Carbene complexes, heteroatom-substituted - by a-abstraction 29-31 - acidity 35 - from acyl complexes 14-20 - from alkynes 25, 26 - analogy to carboxylic acid derivatives 35, 36 - from carbenoids or carbenes 27-29 - from carbonyl complexes 15-17 - from carboxamides 22-24 - from carbyne complexes 32, 33 - from 2-chloropyridinum salts 21 - demetallation 37-41 - from isonitrile complexes 21 - from metallates 18, 19 - as metathesis catalysts 140, 153 - by olefin metathesis 33, 34 - from organolithium compounds 15-17 - oxidation 37, 39, 40 - photolysis 33, 64 - by photolysis 33, 34 - preparation 14-34 - reaction - with alcohols 35, 36, 68 - with alkynes 42, 49-55, 67
- with carbon nucleophiles
38, 42, 68, 124 - with tin derivatives 38, 40, 42 - with ylides 38, 42, 91 - reduction 84 - reduction by dihydropyridines 45 - theoretical treatments 3-5, 13 - from thioureas 29 - use in organic synthesis 34-73 - from vinylidene complexes 25-27 Carbene complexes, non-heteroatomsubstituted - from alkenyl- and alkynyl complexes 98 - from alkynes and cyclopropenes 98-100 - carbonyl olefination with 125-135 - from carbyne complexes 93-98 - cyclopropanation with 6, 45, 71, 105-119 - by [2 + 21 cycloreversion 100 - by electrophilic abstraction 82-89 - by nucleophilic abstraction 78-82 - in olefin metathesis 6, 7, 134, 140-144 - preparation 45, 75-103 - Experimental Procedures 80, 86, 91, 95 - stability 75, 144 - theoretical treatments 3-5 - from ylides 90-93 Carbenes 1-3 - precursors for carbene complexes 27-29 - reaction with ethers 205 - stable 27-29 Carbocations 2, 122 Carbohydrates - metal-containing derivatives 16, 19, 26, 34 - metal-free derivatives 53, 210, 21.5, 225, 228 - norbornene derivatives, ROMP 147 - preparation of derivatives by RCM 158, 160, 162 Carbon dioxide, olefination 131 Carbon monoxide 49 - conversion to acyl complexes 20 - conversion to ketene complexes 49-55 - thermal extrusion 49-51 Carbonates, olefination 125, 127, 128, 132 Carbonyl complexes - reaction - with diazoalkanes 91
Index - with non-carbon nucleophiles
19, 20 with C-nucleophiles 15-17 - photolysis 41-45 Carbonyl methylenation 125-1 29 Carbonyl olefination 125-1 35 - mechanism 125-127, 134 Carbonyl ylides 206-2 13 - Experimental Procedure 2 10 Carboxamides - N-alkylation 194-196 - 0-alkylation 206, 207, 209-213 - carbonyl olefination 126, 128, 131 - conversion to aminocarbene complexes 22-24 - cyclization of 156-1 60 Carboxylic esters, carbonyl olefination 125-135 Carbyne complexes - as catalysts for alkyne metathesis 135, 137, 142 - conversion to carbene complexes 93-97 - cycloadditions 32, 102 - preparation 94, 95 - Experimental Procedure 95 - reactions with electrophiles 96, 97 - reactions with nucleophiles 32, 33, 94-96 Catalysts - for alkyne metathesis 13.5, 137, 142 - for cyclopropanation - with acceptor-substituted diazomethanes 174, 175, 220, 224 1 14-1 18 - with diazoalkanes - for enantioselective cyclopropanations 116, 174, 220, 224 - for enyne cyclization 149, 152, 153 - for olefin metathesis 5-8, 138-144 - enantiomerically pure 155 - heterogeneous 136, 138 - homogeneous multi-component 140 - homogeneous single-component 140144 - preparation, Experimental Procedures 80, 91 - polymer-bound 139 - poisoning 8, 114, 143, 144, 194, 197, 216 - for ring-fission of cyclopropanes 119, -
336
_I"
271
for ring-fission of cyclopropenes 119, 176 - see also Cobalt complexes, Copper complexes, etc. Catechol - 0-alkylation 199 - template for macrocyclizations 149 Ceric ammonium nitrate 39, 54, 55, 70 Chemical shifts 9-1 1 Chiral catalysts - see Catalysts Chiral ligands 175, 208, 220, 224 - see ulso Ligand Chloroiminium salts 21, 23 Chloromethyl phenyl sulfide 29-3 1 Chloronium ylides 21 8 Chromenes 156, 162, 222 Chromium arene tricarbonyl complexes 60 - from benzannulations 50, 52, 54 - Experimental Procedure 52 Chromium carbene complexes - benzannulation 49-55 - Experimental Procedures 52, 54 - cyclopentannulation 60-63 - cyclopropanation with 46-49, 70, 71, 106, 108, 115 - Experimental Procedure 48 - demetallation 39, 40, 42, 69 - photolysis 41, 4 3 4 5 , 64 - Experimental Procedure 45 - preparation 15-19, 22-24, 26, 34, 79 - Experimental Procedures 15, 23 Chromium(I1) chloride 128 Chromium complexes, as catalysts for diazodecomposition 115, 174 Chromium hexacarbonyl 15, 23 Claisen rearrangement 126, 127, I99 Cobalt carbene complexes 17 Cobalt complexes, as catalysts for diazodecomposition 174, 220, 224 Cocatalysts, for metathesis catalysts 139 Combinatorial chemistry - see Solid-phase synthesis Cope rearrangement 47, 70-73, 180 Copolymers 137, 141 Copper complexes - as catalysts for cyclopropanations 114116, 118-120 - as catalysts for diazodecomposition li4-116, 118, 174, 194 -
174, 220, 221, 224 Cross metathesis 16I , 164-1 67 - Experimental Procedure I61 - stoichiometric 167 Crowded complexes 78-82 Cumulenes 87, 101 Cuprates 36, 85 Cyanamides, reaction with carbene complexes 204 Cyanides - see Nitriles [2 + 21 Cycloadditions 7 - of alkenes to carbene complexes 103, 1 34- 1 68 - of alkynes to carbene complexes 46, 50, 51, 103, 135, 137 - of alkynylcarbene complexes 36 - of carbonyl compounds to carbene complexes 126, 134 - of ketene complexes 4 1, 44 Cycloalkenes - conversion to a,o-dienes 138 - heats of formation 145 - polymerizability by ROMP 145, 146 - by RCM 148-161 Cycloalkynes 135 Cyclobutadiene, ROMP of synthetic equivalents 146 Cyclobutanones - from enynes 46-48 - from ketene complexes 4 1, 44 Cyclobutenes - metathesis 103, 141, 146, 147, 167 - ROMP 146, 147 Cyclobutenones, from vinylcarbene complexes 52, 56, 57 Cycloheptadienes 70-72, 226, 229 Cycloheptadienones 63, 72 Cycloheptanones - by C-C insertion 193 - from vinylcarbene complexes, Experimental Procedure 73 Cycloheptatrienes 179, 180, 223 Cycloheptatrienylidenes 83 Cycloheptenes 148, 152-154 Cycloheptenones 63, 65 Cyclohexadienes 67, 69, 230 Cyclohexadienones 49, 67 - enantiomerically pure
Cyclohexanones by C-H insertion 190 - from sulfonium ylides 215 - from vinylcarbene complexes, Experimental Procedure 70 Cyclohexenes 148, 152, 153, 155 Cyclohexenones 68 Cyclometallation 92, 96 Cyclooctenes - by RCM 154, 155 - ROMP 136, 138, 140, 144, 145 Cyclopentadienes - from alkynylcarbene complexes 56, 59, 61 - cyclopropanation 108, 120 - ROMP 146 - from vinylcarbene complexes 56-60 Cyclopentannulation 56-66, 122-124, 150-152, 182-189 - Experimental Procedures 59, 6 I , 124, -
150
Cyclopentanones - from alkynes 62 - by C-H insertion 182-1 85 Cyclopentenes - from alkynes 168 - from heteroatom-substituted carbene complexes 66 - by RCM 146, 148, 150-152 - ROMP 141, 142, 144-146 Cyclopentenones - from cyclopropylcarbene complexes 65 - by C-H insertion 184, 185 - from vinylcarbene complexes 56, 6 I , 63 Cyclopropanation - with acceptor-substituted carbene complexes 2 18-226 - catalytic 105, 114-1 19, 218-226 - diastereoselective 6, 109, 115, 116, 219 - enantioselective 109, 111, 112, 116, 219-221, 223-226 - Experimental Procedures 221, 224 - with heteroatom-substituted carbene complexes 4 5 4 9 , 70-73 - intermolecular 105-1 19, 224-226 - intramolecular 46-48, 107, 220-224 - mechanism 7, 8, 114 - with non-heteroatom substituted carbene complexes 5-8, 105-1 19
Index vs. olefin metathesis 5-8 stoichiometric 6, 4 5 4 9 , 70-73, 105113 - Experimental Procedures 48, 106, I13 - theoretical treatments 5-8 - with titanium carbene complexes 8, 112, 113 - see ulso Alkynes, Arenes, Dienes, Enamines, etc. Cyclopropanes - as cocatalyst for olefin metathesis 138 - divinyl, Cope rearrangement 70-73, 180 - donor-acceptor-substituted, ringfission 226 - by C-H insertion 181, 182 - metal-catalyzed ring fission 119, 226 - from metallacyclobutanes 8 - vinyl, rearrangement to cyclopentenes 226 - see also Cyclopropanation Cycl opropen es - conversion to carbene complexes 99, 100, 102, 104, 119, 176, 177 - by cyclopropanation of alkynes 176, 218, 225 - cyclopropanation with 119 - Diels-Alder reaction 177, 222 - metathesis 146, 168 - preparation and rearrangement 176, 222, 227-229 Cyclopropenylidene complexes 86, 89 Cyclopropylcarbene complexes 19, 39, 42, 63, 66, 71 [2 + 21 Cycloreversion - of metallacyclobutanes 7, 100, 103, 138, 145, 148 - of metallacyclobutenes 100, 103, 137, 148 -
Dealkoxylation - see Abstraction
Degenerate olefin cross metathesis 136 Demetallation 37 Deprotonation - of alkyl complexes 78, 81, 82 - of carbyne complexes 95, 97 - of heteroatom-substituted carbene complexes 35, 68, 99
213
Desulfurization, of thiiranes 216 Dialkoxycarbene complexes 18 Diaminocarbenes 27-30 Diaminocarbene complexes - preparation 18, 22, 27-30, 33 - see also Heteroatom-substituted carbene complexes Diarylcarbene complexes - benzannulation 50 - cyclopropanations with 109 - metathesis 103 - preparation 90, 92, 103 - see ulso Non-heteroatom-substituted carbene complexes Diazoalkanes - acylation 173 - conversion to azines 116, 232 - conversion to 3H-pyrazolines 115, 218 114- cyclopropanation of alkenes with 118 - preparation of carbene complexes from 90-93 - reaction 105, 114, 172, 173, 197, - with acids 216 - with heteroatom-substituted carbene complexes 38, 42 - with hydroxy groups 15, 197 - with thiocarbonyl compounds 216 Diazoacetanilides 186, 187 Diazoacetic acid derivatives - see Acceptor-substituted diazomethanes Diazogroup transfer 172 Diazoimides, as precursors to carbonyl ylides 210-212 Diazoketones - see Acceptor-substituted diazomethanes Diazomethane 42, 9 1, 92 - 0-alkylations with 15 - cyclopropanations with 105, 106, 115-1 17 - see also Diazoalkanes Diazonium salts 15, 95 1,l-Dibromoalkanes - see I, 1 -Dihaloalkanes 1,2-Dichloroethane, abstraction of chloride from 218 Dicyclopentadiene, ROMP 136, 140, 142
214
Index
Diels-Alder reaction - of alkynylcarbene complexes
36, 54,
58, 60 - of cyclopropenes 177, 222 - of 2-pyranylidene complexes
55, 67, 69 of vinylcarbene complexes 36 - Experimental Procedure 70 Dienes - ADMET 148 - from alkynes 42 - by carbonyl olefination, Experimental Procedure 133 - by cross metathesis 161, 165 - cyclopropanation 47, 70-73, 108, 1 1 1, 120, 218, 222 - from diazoesters 181, 214 - from furans and pyrroles 230, 231 - RCM 148-161 - reaction with vinylcarbene complexes 226, 229 - see also Diels-Alder reaction 1,l-Dihaloalkanes - carbonyl olefination with 127-129, 132, I33 - Experimental Procedure 133 - conversion to carbene complexes 89, 91 - conversion to carbyne complexes 94, 95 - Experimental Procedure 95 Dihydroazepines 71, 72, 159 Dihydrofurans - from acylcarbene complexes 226-228 - Experimental Procedure 227 - from carbonyl ylides 207, 209 - by RCM 162 - reaction with acylcarbene complexes 228 2,3-Dihydro-lH-imidazole-2-ylidenes 1 8, 22, 28, 30 Dihy dropyrans - from alkoxycarbene complexes 48, 68 - by carbonyl olefination 134 - cyclopropanation 223, 225 - by RCM 153, 156, 161, 162 Dihydropyridines 16, 45, 53, 229 1,2-Dihydropyridin-2-ylidenes 2 1 Dihydropyrroles - from 4-amino-1-butynes 168 - cyclopropanation 225 - by RCM 157, 158 - from pyrroles 225 -
2,3-Dihydrothiazol-2-ylidenes23 Dimethyl acetylenedicarboxylate (DMAD) - benzannulation 53 - as dipolarophile 203, 204, 211, 213 Dimethyl sulfoxide, as oxidant 39, 40 (Dimethylsu1fonium)methyl complexes 87, 106, 111, 123, 124 Dimethyltitanocene 127, 128 Dioxolanes, from carbonyl ylides 206, 209, 212 Diphenylacetylene 48, 63 Diphenylcarbene complexes - see Diarylcarbene complexes Diphenylcyclopropane 106 Diphenylisobenzofuran 176 1,3-Dipolar cycloaddition - to alkenyl/alkynylcarbene complexes 36, 37 - of carbonyl ylides 208-213 - Experimental Procedure 210 - of diazo compounds 115, 218 - formal, of acylcarbene complexes 226229 Disodium pentacarbonylchromate 23, 24 Disproportionation of carbene complexes 86 1,3-Dithianes - see Dithioacetals Dithioacetals - carbonyl olefination 128, 129, 132, 133 - as precursors for titanium carbene complexes 1 12, 167 - Experimental Procedures 113, 133 Divinylaziridines 7 1 Divinylcyclopropanes 70, 7 1, 180, 226 Dotz benzannulation 49-55 - Experimental Procedures 52, 54 - mechanism 49-52 - natural product synthesis 52 - scope and limitations 49-52 Electrocyclization of l-metalla-l,3,5-trienes 57 - of trienes 230 Electronic structure of carbene complexes 1-5 Electrophilic carbene complexes 1 - see also Acceptor-substituted carbene complexes P-Elimination 82, 84, 86, 124 -
Index
Enamines by carbonyl olefination 126, 128, I31 - cycloaddition to ketene complexes 44 - cyclopropanation 115, 218 - as dienophiles 55, 67, 69 - olefin metathesis 34, 144 - reaction with alkynylcarbene complexes 36, 58, 61 Enol ethers - from acrolein acetal, Experimental Procedure 205 - from alkoxycarbene complexes by thermolysis 39, 48, 68 - from alkoxycarbene complexes and ylides 42, 43 - by 0-alkylation of carbonyl compounds 206 - by carbonyl olefination 125-135 - Experimental Procedures 129, 133 - cycloaddition to ketene complexes 44 - cyclopropanation 106, 108, 109, 115, 118, 120, 218, 222, 223, 225-227 - as dipolarophiles 2 11 - olefin metathesis 33, 34, 143, 144, 151, 153, 162 - reaction with acylcarbene complexes 226 Enynes - conversion to cyclobutanones 46-48 - cyclopropanation 46-48 - cyclization 46, 149, 152, 153, 155, 158-160, 162 Episulfides - see Thiiranes Epoxides - see Oxiranes Eschenmoser thiocarbonyl olefination 216 Esters - see Carboxylic esters Ethene - see Ethylene Ethers - 0-alkylation by carbene complexes 205 - by carbene 0 - H insertion 196-198 - cyclic, formation by RCM 149, 162 - hydride abstraction from 31, 232 - macrocyclic 149, 163, 207, 223 - as solvents for benzannulations 50, -
JL
275
Ethyl cyanoacetate 95 Ethylaluminium dichloride - as catalyst for olefin metathesis 138, 140 Ethylene - cross metathesis with alkenes 137, 155, 161, 162 - cross metathesis with alkynes 165 - oligomerization I37 - production 136, 137 - as volatile byproduct of RCM/ADMET 143 Exchange metathesis 135, 136 Fischer-type carbene complexes 1-7, 13 - as metathesis catalysts 140, 153 - preparation, Experimental Procedures 15, 23, 29, 86 - see also Heteroatom-substituted carbene complexes Fluorobenzene 191, 228 Formamides, conversion to aminocarbene complexes 23 Forster reaction 172 Fredericamycine 5 1 Fullerenes, cyclopropanation 2 18 Fumarates, cyclopropanation 48, 115, 120 - Experimental Procedure 48 Furanones - from alkoxycarbene complexes 39, 57, 62 - from ally1 diazoacetates 221, 228, 229-23 1 - from carbonyl ylides 209, 212 - by C-H insertion 187-189, 232 - by 0-H insertion 198 - from oxonium ylides 206, 207 Furans 42, 21 1 - from acylcarbene complexes 21 1, 227-229 - Experimental Procedure 227 - from alkoxycarbene complexes 65 - benzannulation 54 - reaction with acylcarbene complexes 228, 230, 231 - ring fission 230, 231 - from vinylcarbene complexes 52, 56, 57, 60
276
Index
Germanes 121 Grignard reagents 88, 109 Griseofulvin 205 Hafnium carbene complexes 121 Halides, abstraction 29, 31, 217 Halo alcohols, reaction with carbonyl complexes 18 Halo ethers, reaction with metallates 85 Halo thioethers 29-3 I , 87, 88, 107 Hard and soft transition metal complexes 144 Heterogeneous metathesis catalysts 138 Homogeneous metathesis catalysts 6, 140-144 Hydrazones 172 Hydride abstraction - by carbene complexes 31, 178, 197, 232 - for preparation of carbene complexes 29, 31, 83 Hydride complexes 89, 102 Hydrolysis, of alkoxycarbene complexes 39 Imidazol-2-ylidenes 18, 22, 28, 30 Imines - cycloaddition to vinylidene complexes 25, 27 - a-halo 21-23 - C-H insertion into 184 - olefination 131 - reaction with carbene complexes 72, 202, 203 - reaction with carbyne complexes 32 - reaction with ketene complexes 41, 44, 64 - reaction with propargyl complexes 65 Indanones - from alkoxycarbene complexes, Experimental Procedure 6 1 - by C-H insertion 184, 221 Indenes - from acylcarbene complexes 189 - from alkynylcarbene complexes 58, 60 - from heteroatom-substituted carbene complexes 49, 50, 52 Indenones, from acylcarbene complexes 177, 184, 191, 215, 225
Indoles - alkylation by C-H insertion
191, 192 - Experimental Procedure 192 - from 2-alkynylanilines 53, 60, 168 - benzannulation 44, 55, 68, 191 186, - by intramolecular C-H insertion 187 - Experimental Procedure 187 - reaction with acylcarbene complexes 191, 192, 228 Indolizines 203 C-H Insertion - of acceptor-substituted carbene complexes 179-192 - Experimental Procedures 182, 187, 192 - enantioselective 180, 183, 184, 186, 188-191 - of heteroatom-substituted carbene complexes 65 - of imido complexes 121 - intermolecular 189, 191, 192 - intramolecular 107, 122-124, 179 - mechanism 119, 122 - of non-heteroatom-substituted carbene complexes 107, 119, 121-125 - Experimental Procedure 124 - regioselectivity 179 - stereoselectivity 180 C-C Insertion 193 C-N Insertion 198 C-0 Insertion 205 C-S Insertion 214 N-H Insertion 193-196 0-H Insertion 193, 196-199 - Experimental Procedure 197 S-H Insertion 193, 197, 200 Si-H Insertion 192 Iodoform 128 Iodonium ylides - from acylcarbene complexes 218 - as carbene complex precursors 176, 190, 195 Iridium carbene complexes 92 Iron carbene complexes - heteroatom-substituted 107 - benzannulations 53 - cyclopentannulations 60 - preparation 18-23, 26-31, 34
Index non-heteroatom-substituted cyclopropanation 6, 106, 109-1 12, 115, 116, 118 - enantiomerically pure 109, 1 1 1, 1 12 - C-H insertion 107, 122-124 - preparation 83, 85-89, 99, 101, 106, 109 - stability 75, 87, 99 Iron complexes, as catalysts for diazodecomposition 112, 115, 174 Isobutene, cyclopropanation 224 Isonitrile complexes - conversion to carbene complexes 21, 22 lsonitriles 33 -
-
KDO (3-deoxy-D-manno-2-octulosonic acid) 208, 210 Ketene acetals - from carbonyl ylides 207 - by olefination of carbonates 125, 127, 128, 132 - reaction with alkoxycarbene complexes 66 Ketene complexes - from carbene complexes by photolysis 41, 4 3 4 5 - Experimental Procedure 45 - [2 + 21 cycloadditions 41, 43, 44 - as intermediates in benzannulations 4 1, 44, 49-52 - reactions with nucleophiles 41, 43, 4.5, 67, 71, 72 Ketenes 60 Ketocarbene complexes - see Acceptor-substituted carbene complexes Ketones - 0-alkylation 206. 21 1, 212 102 - conversion to carbene complexes - olefination 125-134 - Experimental Procedure 133 - reaction with metallates 85 Lactams, by RCM 149, 157-160 P-Lactams - N-alkylation 194-196 - conversion to carbene complexes 24 - from heteroatom-substituted carbene complexes 40, 44
211
- by intramolecular C-H insertion
179, 181-1 83 - Experimental Procedure 182 - norbornene derivatives, ROMP 147 - transformations 159, 160, 166, 199 Lactones - by C-H insertion 181, 183, 188-190 - macrocyclic 149, 160, 163, 207, 223 - by RCM 149, 160, 163 Ligands - enantiomerically pure 175, 208, 220, 224 - exchange 33, 102, 175 - C,-symmetric 208, 220, 224 Living polymerization 141 - alkynes 135, 137 - terminating agents 141, 143, 144 Lombard0 reagent 127, 128 Macrocyclization - of diazo carbonyl compounds
207, 223 of dienes by RCM 149, 160, 163 Manganese carbene complexes - heteroatom-substituted 17, 18, 20, 27, 32, 68 - non-heteroatom-substituted 87, 90, 96, 99 Meerwein salt - see Trialkyloxonium salts Metallacyclobutanes 6, 7, 104, 134, - by [2 + 21 cycloaddition 143, 148 - reaction with carbonyl compounds 128, 130 - thermolysis of 7 Metallacyclobutenes 46, SO, 103, 104, 137 Metallacyclohexadienes 57 Metallacyclopentenes 17, 34, 102 Metallates 18-24 - generation 19, 88 - reaction - with alkylhalides 20, 88 - with carbonyl compounds 85 - with carboxamides 22-24 Metathesis - see Olefin metathesis Methylenation, of carbonyl compounds 125-129 Methyllithium - see Organolithium compounds -
278
Index
Methyltrioxorhenium 139, 154, 174 Molecular orbitals of carbene complexes 2-5 Molybdenum-based metathesis catalysts 6 - enantiomerically pure 155 - heterogeneous 136, 139 - homogeneous 141, 143, 150, 156, 161, 165 Molybdenum carbene complexes 6 - heteroatom-substituted - cyclopropanation with 47, 48 - preparation 16, 20, 100 - reaction with alkynes 42, 47, 48, 53, 61, 72 - reaction with 2-aza-l,3-dienes 61 - non-heteroatom-substituted - carbonyl olefination 129, 131, 134 - cyclopropanations with 106, 108 - preparation 79-81, 86, 95, 97. 100, 101 Morpholinones - from azomethine ylides 203 - from carbonyl ylides 213 - by 0-H insertion 43, 198 - by Stevens rearrangement 201 Musk perfume 138 Naphthols - by benzannulation
44, 49-55 by C-H insertion 190 Naphthyldiazomethane 1 16 Natural product synthesis 51, 122, 160, 163, 206, 221, 226 Neohexene 108, 138 Neopentane, elimination 78, 79, 82 Neopentylidene complexes 75, 78, 79, 81, 82, 121 Nickel carbene complexes - cyclopropanations with 111 - preparation 18, 88 Nickel complexes - as catalysts for cyclopropanations 116, 119 - ethylene oligomerization with 137 Nitrile ylides 203 Nitriles - N-alkylation by carbene complexes 203 - reaction with Tebbe reagent 126 NMR, of carbene complexes 9-1 1 Non-productive olefin metathesis 136 -
Norbomenes - ring-opening cross metathesis 166 - ROMP 136, 138, 140-142, 145, 147 Norsorex 138 Nuclear magnetic resonance 9-1 1 Nucleotides, norbornene derivatives, ROMP 147 Nysted reagent 128 Olefin metathesis
134-168 138-144 - heterogeneous catalysts 138-140 - homogeneous catalysts 140-144 - industrial applications 136-138, 144147 - light-induced 103, 138, 140 - mechanism 5-8, 100, 137, 145, 148 - preparation of heteroatom-substituted carbene complexes by 33, 34 - stoichiometric 134, 165, 167 - theoretical treatment 5-8 Olefins - see Alkenes Oligoacetylenes 135 Orbitals, molecular 2-5 Organoaluminium compounds, as cocatalysts for olefin metathesis 138-141 Organolead compounds, as cocatalysts for olefin metathesis 139, 140, 150 Organolithium compounds - as cocatalysts for olefin metathesis 139 - reaction - with carbonyl complexes 15-17, 37, 100 - with carbyne complexes 94, 96 - with heteroatom-substituted carbene complexes 36, 38, 42, 65, 68, 84, 107, 109, 124 - with non-heteroatom-substituted carbene complexes 83 Organomagnesium compounds - see Grignard reagents Organotin compounds - see Stannanes Osmium carbene complexes, preparation 22, 81, 92, 96, 97, 102 Osmium complexes, as catalysts for diazodecomposition 174 7-Oxanorbornene derivatives, metathesis 146, 147, 166 - choice of the catalyst
Index Oxazoles alkylation by C-H insertion 190 - from nitriles and carbene complexes 203 Oxazolidines 203, 2 1 1 , 2 12 Oxepanes 72, 190, 198, 207 Oxepines, preparation by RCM 156, 163 Oxidation - of arene complexes 52-54 - of heteroatom-substituted carbene complexes 3 7 4 0 - of metallacyclobutanes 8 - see ulso Hydride abstraction Oxidative insertion 77 Oximes 71, 172, 203 Oxiranes 206, 209, 21 1, 213 - from carbonyl ylides - reaction with vinylcarbene complexes 68 - from sulfonium ylides 214 Oxonium ylides 205-208 Oxophilic species, early transition metals as 144 Oxygen, stability of carbene complexes towards 144 -
Palladium complexes as catalysts for diazodecomposition 114, 115, 117, 174 - as catalysts for enyne cyclization 149 Pantolactone 192, 219 Pauson-Khand reaction 36 Peptides, by acylation with ketene complexes 43 Peptoids 43, 149, 159, 160 Phenols - 0-acylation by ketene complexes 43 - addition to alkynylcarbene complexes 36 - 0-alkylation by carbene complexes 199 - preparation 41, 44 Phenylacetylene 230 Phenylalanine 178, 187, 223 Phenylcarbene complexes - Experimental Procedure 91 - see ulso Non-heteroatom-substituted carbene complexes Phenyldiazomethane 91, 1 18 Phenylthiomethyl iron complexes - as carbene complex precursors 29, 88, 107, 123 -
279
preparation, Experimental Procedures 29, 124 Phillips Triolefin Process 137 Phosphines - alkylation 217 - stabilization of carbene complexes by 104 Phosphorus ylides 9, 125 - conversion to non-heteroatom-substituted carbene complexes 93 - generation 217 - reaction with carbene complexes 38 - reaction with ketene complexes 41, 43 Photo1ysis - of alkyl complexes 33, 34, 82 - of carbene complexes 41, 4 3 4 5 , 103 - Experimental Procedure 45 Piperidines 43, 48, 64, 209 Piperidinones - from 1 -amino-4-pentynes 68 - from ammonium ylides 45, 201 - by Diels-Alder reaction 60 - by N-H insertion 195 Platinum complexes 2, 23, 26 - as catalysts for diazodecomposition 174 152 - as catalysts for enyne cyclization Polyacetylene 135, 137 Poly(cyc1ooctene) 138, 144 Poly(cyc1opentene) 144 Polyenes 146 Polymer-supported metathesis catalysts 139 Polymeric supports - see Solid-phase synthesis Polymerization - see Ring-opening polymerization, Ziegler-Natta-polymerization Polynorbornene derivatives 146, 147 Propargyl complexes 65, 84 Propargyl halides - conversion to vinylcarbene complexes 89 2-Pyranones 39, 67 2H-Pyran-2-y lidenes - preparation 20, 26, 27, 67-69 - Diels-Alder reaction 55, 67 Pyrazolines 1 15, 2 18 Pyridines - from 2-aminovinylcarbene complexes 67, 69 -
280
Index
203 Pyrroles - alkylation by C-H insertion 186, 190 - from 4-amino- I-butynes 168 - cyclopropanation 225 - from dihydrofurans 226 - from heteroatom-substituted carbene complexes 60, 63, 65 - from nitrile ylides 204 - reaction - with alkoxycarbene complexes 35 - with vinylcarbene complexes 229, 230 - ring fission by acylcarbene complexes 230, 231 Pyrrolidines - ring expansion 43 - preparation 47, 48, 55, 223, 225 Pyrrolidinones - from 1 -amino-3-butynes 62 61 - from 2-aza- 1,3-dienes - conversion to aminocarbene complexes 24 - by C-H insertion 183, 186, 187 - by N-H insertion 195 - by Stevens rearrangement 64, 201 Pyrrolizines 64, 157, 183, 186, 223, 228 - N-alkylation
Quaternary ammonium salts - see Ammonium ylides
Quinones 52-54 - Experimental Procedure
54
Radicals in oxonium ylide rearrangements 205 - addition to vinylcarbene complexes 36, 67 Reduction - of carbyne complexes 94, 96 - of heteroatom-substituted carbene complexes 37, 42, 46 Reductive elimination 38, 57, 59, 77 Retention of configuration, in C-H insertions 180, 184 Retro-Diels-Alder reactions 55, 67 Rhenium carbene complexes - heteroatom-substituted 17, 27, 32 - non-heteroatom-substituted - in olefin metathesis 142 - preparation 79, 81-83, 87, 96, 101 -
Rhenium complexes, as catalysts for - diazodecomposition 174, 217 - olefin metathesis 139, 142, 147
Rhodium carbene complexes 30, 92, 177 Rhodium(I1) carboxylates 114, - as catalysts for diazodecomposition 118, 174 - enantiomerically pure 174-176, 220, 221, 224 Ring-closing metathesis (RCM) 135, 137, 148-1 63 - asymmetric 155 - effect of monomer concentration on 148 - Experimental Procedures 150, 156 - preparation of carbocycles 150-155 - preparation of heterocycles 156-163 - on solid supports 151, 156, 159, 162, 166 - stereoselectivity 155 - thermodynamics 144-146, 148 Ring-opening cross metathesis 166-168 Ring-opening metathesis polymerization (ROMP) 136, 138, 141, 144-148 - effect of monomer concentration on 145-147 - functionalized polymers from 146, 147 - living 141 - side reactions 147 - stereoselectivity 147 - thermodynamics 142, 144-146 Ruthenium carbene complexes - as catalysts for olefin metathesis 139, 142-144, 149, 150, 156 - enantiomerically pure 109 - C-H insertion 121 - preparation 17, 26, 89, 91, 99, 101-103 Ruthenium complexes - as catalysts for diazodecomposition 174, 217, 224 - as catalysts for olefin metathesis 140, 142, 152, 153 - see also Ruthenium carbene complexes Schiff bases - see Imines
Schrock-type carbene complexes 1-7 preparation, Experimental Procedures 80, 91 - see also Non-heteroatom-substituted carbene complexes -
Index Self metathesis 136 Semicarbazones 172 Serine, etherification 196, 197 - Experimental Procedure 197 Shell Higher Olefin Process (SHOP) 137 Sigmatropic rearrangements 43, 45, 200, 205-207, 214 - Experimental Procedures 45, 205 - see also Cope-, Claisen rearrangement Sil ane s - in benzannulations 53, 54 - C-H insertion 191 - Si-H insertion 121, 122, 192 Simmons-Smith reagent 106, 115 Sodium amalgam 30 Sodium dicarbonyl(cycIopentadieny1)ferrate 19, 29-31 Sodium naphthalenide 23 Solid-phase synthesis - 1 $dipolar cycloaddition 2 1 1 - 0-H insertion 199 - olefin metathesis 1.51, 156, 159, 162, 166 - synthesis of peptides 41 Solvents - for benzannulations 49, 50 - for olefin metathesis 143, 144, 146, 147 Stable carbenes 27-30 Stannanes 138- as cocatalysts for olefin metathesis 140 - reaction with carbene complexes 38,40, 121 Stevens rearrangement 43, 45, 64, 198, 200-202 - Experimental Procedure 200 Stoichiometric cyclopropanations 6, 45-49, 106-1 I3 Stoichiometric olefin metathesis 103, 134, 165, 167 Styrenes 161, 164, 166 - cross metathesis 108-1 1 1, 113, 11 8, - cyclopropanation 218 Sulfides - see Thioethers Sulfonyl azides 172 N-Sulfonyl hydrazones 173 Sulfoxonium-ylides 176
281
Sulfur ylides
2 1 3-2 16 as carbene complex precursors 176, 195 - cyclopropanations with 116, 120, 214 - enantiomerically pure 216 - from acylcarbene complexes -
Tacticity, of polymers prepared by ROMP 147 Tandem carbonyl olefination/Cope rearrangement 126 Tandem carbonyl ylide formation/l,3dipolar cycloaddition 208-2 13 Tandem olefin metathesis/carbonyl olefination 134 Tandem ring-opening/ring-closing metathesis 150, 153, 155 Tantalum carbene complexes - carbonyl olefination 129, 13 1 - C-H insertion 121 - olefin metathesis 142 - preparation 78, 79, 81, 82, 103 - stability 75, 78 - theoretical treatments 3-5, 78 Tebbe reagent - carbonyl olefination 125-1 29, 134 - Experimental Procedure 129 - conversion to titanium carbene complexes 104 - higher homologs 129, 130 - in situ preparation, Experimental Procedure 129 Tetraethyllead - as cocatalyst for olefin metathesis 139, 150, 151 Tetrahydroazepines - see Azepines Tetrah ydrofurans - from alkoxycarbene complexes 39 - by cycloaddition 21 1 - by C-H insertion 188 - by C-0 insertion 208 - by 0-H insertion 198 156, 166 - by olefin metathesis - reaction with acylcarbene complexes 190, 191 Tetrahydroisoquinolines 201, 210
Tetrahydro-2-pyranylidenes -
conversion to vinylstannanes demetallation 39 preparation 26, 27, 68
42
282
Index
Tetrahydropyridines - from 1-amino-4-pentynes 168 - by RCM 158, 159 Theoretical treatments - cyclopropanation 7, 8 - benzannulation 49-52 - Fischer-type/Schrock-type carbene complexes 3-5 - olefin metathesis 5-8 Thermodynamics - of carbene complex formation 78 - of carbonyl olefination 127 - of ROMP 144-147 Thermolysis - of heteroatom-substituted carbene complexes 37-39, 42, 49, 50 - of metallacyclobutanes 7, 100, 104, 153 Thiiranes - S-alkylation 216 - from thiocarbonyl compounds 2 16, 217 Thioacetal s - S-alkylation 214, 215 - see also Dithioacetals Thiocarbonyl ylides 21 6 Thioester, olefination 128, 132 Thioethers - a-abstraction 87, 106, 109, 111-113 - S-alkylation 21 3-2 16 - enantiomerically pure 2 16 - olefin metathesis 143 Thioketenes 2 17 Thiols - addition to alkynylcarbene complexes 36 - S-alkylation by acylcarbene complexes 197, 200 - poisoning of catalysts by 194, 197 Thiophenes, alkylation by C-H insertion 184, 191 Thiophenium ylides 176 Thiophenols - see Thiols Ti tanacyclobutanes - preparation 7, 8, 92, 104 - reaction with carbonyl compounds 128. 130 - thermolysis 7, 104, 153 Titanacyclobutenes 104, 130
Titanium carbene complexes carbonyl olefination 125-1 35 - Experimental Procedures 129, 133 - cyclometallation 92 - cyclopropanation with 112, 113 - Experimental Procedure I 13 - generation 30, 92, 100, 104, 125-135 - C-H insertion 121 - olefin metathesis 7, 8, 134, 153 Titanium tetrachloride 127, 128 Titanocene dichloride 112, 113, 125, 128, 132, 133, 167 Tosyl azide 172 Trialkyloxonium salts 15-19, 107, 109 Triazoles - from diazoacetamides 172 - conversion to diazoalkanes 173 Trienes, RCM 152, 153, 155, 157, 162 Triflates 15, 16 Triolefin process 137 Tropylidene complexes 83 Tungstacyclobutanes 7 Tungsten carbene complexes - heteroatom-substituted - benzannulations 55 - conversion to - azepinones 72 - furans 65 - pyrroles 65 - cyclopentannulations 58-61 - oxidation 39, 40, 70 - preparation 15-19, 22, 24, 26, 27, 32, 68 - reaction with diazoalkanes 42, 91, 174 - reaction with dihydropyridines 46 - reduction 39 - thermolysis 42, 47 - non-heteroatom-substituted - carbonyl olefination 129, 131 - cyclopropanations with 6, 106, 108, 109, 111 - enantiomerically pure 109, 111 - C-H insertion 121 - olefin metathesis 6, 75, 140-142, 153 - preparation 83, 84, 86, 88, 89, 93, 97, 99, 102, 103 - stability 75 Tungsten carbyne complexes 97, 135, 142
-
Index Tungsten-based metathesis catalysts 136, 139-142, 150, 167 - see ulso Tungsten carbene complexes, Tungsten carbyne complexes Unsaturated carbonyl compounds see Acceptor-substituted alkenes
-
Vanadium carbene complexes 23, 79 Vestenamer 138, 144 Vinyl complexes, reaction with electrophiles 98 Vinyl iodides - cyclopropanation 2 18 - preparation 128 Vinylcarbene complexes, heteroatomsubstituted - from alkylcarbene complexes 35, 36 - from alkynes 26, 27 - from alkynylcarbene complexes 36, 56, 58, 62 - from carbonyl complexes 16, 17, 56 - cyclopropanation 36, 70-73 - as dienophiles 36, 37, 69, 70 - from metallates 21 - as Michael-acceptors 36, 37 - by olefin metathesis 34 Vinylcarbene complexes, non-heteroatomsubstituted - by &abstraction 83, 84, 87, 89 - from acceptor-substituted vinyldiazomethanes 199, 218, 222, 225, 229-231 - from alkynes 46, 49, 50, 103, 177 - from carbyne complexes 96, 97 - cycloadditions 222, 227, 229, 230 - from cyclopropenes 102, 177 - from ylides 93 I-Vinylcycloalkenes, from enynes 46, 149, 152, 153, 155, 158-160, 162 Vinylcyclopropanes - preparation 46-48, 108, 11 1-113, 118, 120, 2 17, 222, 225 - rearrangement to cyclopentenes 226
283
Vinyldiazoacetic esters 192, 199, 222, 225, 227, 229-231 Vinylidene complexes - from acyl complexes, Experimental Procedure 86 - from alkenyl complexes 81 - from alkynes 25, 98, 101, 169 - from alkynyl complexes 25, 98-100 - from carbyne complexes 95-97 - Experimental Procedure 95 - cycloadditions 99 - as metathesis catalysts 142 - reaction - with electrophiles 97 - with nucleophiles 25-27 Vinylketene complexes - as intermediates in benzannulations 44, 49-5 1 - intramolecular acylations by 44, 62-64 - preparation 41, 44, 48, 63 Water, as solvent for olefin metathesis 146 Wolff rearrangement 183
144,
Ylides
90-93, 172-176 - preparation 198-21 8 - reaction - with acylcarbene complexes 232 - with alkoxycarbene complexes 38, 42, 43 Ynamines 58 - as carbene complex precursors
Ziegler-Natta catalysts, for olefin metathesis 136 Ziegler-Natta polymerization 147 Zinc halides 105, 114 Zirconium carbene complexes 100, 104, 129, 131 Zirconocene dichloride 128 Zwitterions 57-59
