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ADVANCES IN METAL-ORGANIC CHEMISTRY A Research Annual Editor: LANNY S. LIEBESKIND Department of Chemistry Emory University VOLUME 2 • 1991 JAI PRESS LTD London, England Greenwich, Connecticut JAI PRESS LTD 118 Pentonville Road London Nl 9JN, England JAI PRESS INC. 55 Old Post Road No. 2 Greenwich, Connecticut 06836-1678 Copyright © 1991 JAI PRESS LTD All rights reserved. No part of this publication may be reproduced, stored on a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, filming, recording, or otherwise, without prior permission in writing from the publisher. ISBN: 0-89232-948-3 Printed in the United States of America LIST OF CONTRIBUTORS Steven J. Coote Dyson Perrins Laboratory University of Oxford Oxford, England William E. Crowe Central Research and Development E.I. du Pont de Nemours & Company Wilmington, Det., U.S.A. G. Doyle Daves, Jr. Dean, School of Science Rensselaer Polytechnic Institute Troy, N.Y., U.S.A. Stephen G. Davies Dyson Perrins Laboratory University of Oxford Oxford, England William A. Donaldson Department of Chemistry Marquette University Milwaukee, Wis., U.S.A. Craig L. Goodfellow Dyson Perrins Laboratory University of Oxford Oxford, England Paul Helquist Department of Chemistry University of Notre Dame Notre Dame, Ind., U.S.A. Koichiro Oshima Department of Industrial Chemistry Kyoto University Kyoto, Japan vii viii LIST OF CONTRIBUTORS Stuart L. Schreiber Department of Chemistry Harvard University Cambridge, Mass., U.S.A. Motokazu Uemura Faculty of Science Osaka City University Osaka, Japan INTRODUCTION Volume 2 of "Advances in Metal-Organic Chemistry" continues in the same spirit as Volume 1, published approximately two years ago. Authors have been encouraged to write detailed, informal accounts of their research efforts in the field of metal-oriented organic chemistry. Although authors were given guidelines in an attempt to maintain some formatting continuity between the various chapters, I have chosen to minimize editorial interference in order to allow each author to maximize the information presented according to his own style. Topics included in Volume 2 have been selected to emphasize the virtues of metal-oriented organic chemistry utilizing stoichiometric as well as cata- lytic reagents. In addition to processes of value for the synthesis of generally useful organic structures (Chapter 3, 'Transition Metal Catalyzed Silyl- metallation of Acetylenes and Et B-Induced Radical Addition of Ph SnH to 3 3 Acetylenes" by Koichiro Oshima; Chapter 4, "Development of Carbene Complexes of Iron as New Reagents for Synthetic Organic Chemistry" by Paul Helquist; and Chapter 7, "Palladium-Mediated Methylenecyclopropane Ring Opening: Applications to Organic Synthesis" by William A. Donaldson), a topic of relevance to the synthesis of the pharmaceutically interesting C-glycosides is included (Chapter 2, "Palladium-Mediated Arylation of Enol Ethers" by G. Doyle Daves, Jr.). The last few years have witnessed a re- surgence of interest in synthetic applications of arene complexes of chromiumtricarbonyl and two chapters are included within Volume 2 (Chapter 1, "Synthetic Applications of Chromium Tricarbonyl Stabilized Benzylic Carbanions" by Stephen G. Davies, Steven J. Coote and Craig L. 6 Goodfellow and Chapter 5, "Tricarbonyl (^ -Arene) Chromium Complexes in Organic Synthesis" by Motokazu Uemura). Chapter 6, "7t-Bond Hybrid- ization in Transition Metal Complexes: A Stereoelectronic Model for Conformational Analysis" by William E. Crowe and Stuart L. Schreiber, addresses the origins of the interesting conformational properties of ix X INTRODUCTION organometallic complexes. It is an important first step to the rational appli- cation of organometallic complexes to stereoselective organic synthesis. A survey of the chapter titles in both Volumes 1 and 2 will show an obvious emphasis on transition metal chemistry; however, it is my intent to begin to expand the scope of chapters published in forthcoming volumes to include metals from all regions of the periodic table. Atlanta, Georgia Lanny S. Liebeskind January 1991 Samuel Candler Dobbs Professor of Chemistry SYNTHETIC APPLICATIONS OF CHROMIUM TRICARBONYL STABILIZED BENZYLIC CARBANIONS Stephen G. Davies, Steven J. Coote and Craig L. Goodfellow OUTLINE I. Introduction 2 II. Preparation of Arene Chromium Tricarbonyl Complexes 5 III. Decomplexation of Arene Chromium Tricarbonyl Complexes 6 IV. Benzylic Carbanions Derived from (C H R)chromium 6 5 Tricarbonyl Complexes 7 V. Benzylic Carbanions Derived from the Chromium Tricarbonyl Complexes of Xylenes, Indanes and Tetralins 13 VI. Influence of Meta and Para Substituents on the Benzylic Deprotonation of (Arene)chromium Tricarbonyl Complexes 19 Advances in Metal-Organic Chemistry, Volume 2, pages 1-57 Copyright CO 1991 JAI Press Ltd All rights of reproduction in any form reserved ISBN: 0-89232-948-3 1 2 S.G. DAVIES et a/. VII. Influence of Ortho Substituents on the Benzylic Deprotonation of (Arene)chromium Tricarbonyl Complexes 27 VIII. Benzylic Carbanions Derived from (Styrene)chromium Tricarbonyl Complexes 32 IX. Benzylic Carbanions Derived from (/?-Heterosubstituted arenekhromium Tricarbonyl Complexes 33 X. Benzylic Carbanions Derived from (oc-Heterosubstituted arenekhromium Tricarbonyl Complexes 39 XI. Benzylic Carbanions Derived from (a,/?- Diheterosubstiuted arenekhromium Tricarbonyl Complexes 48 XII. Conclusions 55 References and Notes 55 I. INTRODUCTION The ease of preparation and wide range of chemical and stereochemical properties imparted to arenes on complexation to chromium tricarbonyl has 1 resulted in numerous studies of their synthetic applications. This review will deal with one aspect of the chemistry of (arene)chromium tricarbonyl com- plexes, namely the synthetic applications of chromium tricarbonyl stabilized benzylic carbanions. However, a very brief outline of all the general chemical properties of these complexes is given as an introduction. (Arene)chromium tricarbonyl complexes are bright yellow to red in colour. The complexes are generally air sensitive in solution; although as solids, whilst they should be stored under an inert atmosphere, they may be handled and weighed in air. The X-ray crystal structure of (benzene)chromium tricarbonyl 1 is shown 2 in Figure l. The 12 atoms which comprise the benzene unit are essentially coplanar with the chromium lying under one face, equidistant from all the carbon atoms. The chromium-arene carbon bond lengths are 2.23 A and the chromium to the centroid of the benzene ring distance is 1.73 A. In solution there is rapid rotation of the chromium tricarbonyl fragment about the chromium to benzene centroid axis. The carbon monoxide ligands thus provide an effective steric block to the whole face of the benzene to which the chromium tricarbonyl fragment is bound. The geometry about the chromium atom is pseudo-octahedral with the benzene occupying three of the coordina- tion sites. Chromium Tricarbonyl Stabilized Benzylic Carbanions 3 (a) (b) Figure 1. X-ray crystal structure of (benzene)chromium tricarbonyl 1: (a) side view and (b) Newman projection from the benzene centroid to the chromium. The above structural features are common to all (arene)chromium tricar- bonyl complexes although some perturbation from planarity of the arene 3 occurs when substituents possess lone pairs or are very bulky. Complexation of arenes to chromium tricarbonyl causes an upfield shift of about 2 ppm in the 'H-NMR spectrum of the aryl hydrogens. For example, the 'H-NMR spectrum of (benzene)chromium tricarbonyl is a singlet at <55.31 compared to free benzene at (57.37 in deuteriochloroform as solvent. Coordination of arenes to chromium tricarbonyl increases the acidity of the aryl protons by stabilizing, via induction, the conjugate base, an aryl anion. This may be illustrated by the ready fluoride-mediated desilylation of (phenyltrimethylsilane)chromium tricarbonyl 2 under conditions where 4 phenyltrimethylsilane itself is completely inert. Arenes bound to chromium tricarbonyl are susceptible to nucleophilic addition reactions. Thus (chlorobenzene)chromium tricarbonyl 3 is convert- 5 ed to (anisole)chromium tricarbonyl 4 on treatment with methoxide. 4 S.G. DAVIES et a/. Complexes of arenes possessing benzylic leaving groups exhibit enhanced rates of S 1 solvolysis when the leaving group can adopt an orientation N antiperiplanar to the chromium to arene centroid axis. This enhanced rate of solvolysis results from neighbouring group participation by a lone pair on the chromium assisting the ionization process to form the corresponding resonance-stabilized carbenium ion. Such neighbouring group participation also accounts for the conversion of ( + )-(S')-a-methylbenzyl alcohol)chro- mium tricarbonyl 5 under Ritter reaction conditions to ( — )-6 with complete 6 retention of configuration. Complexation of arenes to chromium tricarbonyl also enhances the kinetic acidity of benzylic protons in the conformation which places the benzylic C-H bond antiperiplanar to the chromium-arene centroid axis. The resulting benzylic carbanions are also stabilized relative to their uncomplexed an- alogues by derealization of the negative charge onto the chromium. These effects may be illustrated by the ready desilylation of (benzyltrimethyl- silane)chromium tricarbonyl 7 under conditions where benzyltrimethylsilane 7 itself is inert.
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