Table Of ContentADVANCES 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
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Copyright © 1991 JAI PRESS LTD
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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.
