Handbook of Coordination Catalysis in Organic Chemistry PDF
Preview Handbook of Coordination Catalysis in Organic Chemistry
Handbook of coordination catalysis in organic chemistry Penny A Chaloner, MA, PhD (Cambridge) School of Chemistry and Molecular Sciences, University of Sussex Butterworths London Boston Durban Singapore Sydney Toronto Wellington All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the Publishers. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be re-sold in the UK below the net price given by the Publishers in their current price list. First published 1986 © Butterworth & Co. (Publishers) Ltd, 1986 British Library Cataloguing in Publication Data Chaloner, P.A. Handbook of coordination catalysis in organic chemistry. 1. Chemistry, Organic—Synthesis 2. Catalysis I. Title 547'.2 QD262 ISBN 0-408-10776-6 Library of Congress Cataloging in Publication Data Chaloner, Penny A. Handbook of coordination catalysis in organic chemistry. Bibliography: p. Includes index. 1. Coordination compounds. 2. Catalysis. 3. Chemistry, Organic. I. Title. QD474.C47 1986 547.1'2242 85-30878 ISBN 0-408-10776-6 Printed and bound in Great Britain by Robert Hartnoll (1985) Ltd, Bodmin, Cornwall For my mother Preface I suppose that all first-time authors begin their work filled with confidence that all will proceed smoothly and in high yield, and that they will meet the publisher's target date. Some five years later I am forced to confess that while the yield may be higher than expected the reaction has been much slower! The purpose of this book is to show the increasing importance of homogeneous catalysis by metal complexes to organic chemists. In particular, I have aimed to highlight the special selectivities which can be obtained using coordination complexes as catalysts, since I believe that it is this selectivity which will encourage future developments in the field. With few exceptions I have confined my attention to homogeneous catalysts and closely related polymer supported complexes. Although it is not possible to be comprehensive (and I apologise for omitting any of readers' favourite reactions) I believe that most of the major areas have been discussed. I owe a great deal to colleagues who have provided sympathy, advice and good cheer throughout the writing of the book. I am particularly indebted to D rJohn Brown in whose laboratory I worked on homogeneous hydrogénation and whose infectious enthusiasm first led me to an interest in catalytic processes. Last, but by no means least, I must thank the secretarial staff at Oxford, Harvard, Rutgers and Sussex Universities for their impeccable typing and endless patience. PAC. 1 Introduction In recent years it will have become apparent, even to the most casual reader of the literature, that few organic syntheses do not involve at least one step in which a metal complex is involved. The advantages of using such species as catalysts are manifold. First and foremost is that catalysis improves reaction rates, and this may be translated into an ability to carry out the desired reaction under milder conditions than would otherwise be possible. For example, acetonitrile, MeCN, is hydrolysed only slowly in concentrated base at 100°C. In the metal bound species, [(NH),_Co(MeCN)] , 3 hydrolysis is instantaneous at room temperature, representing an increase in rate by a factor of 10 . The second reason to prefer catalytic to stoichiometric reactions is economic. A catalyst is used in small quantities, typically in the region of 1 mole %. To compete with this a stoichiometric use of the reagent must be at least 99% efficient, a phenomenon which is relatively rare. Many of the catalysts which will be discussed in subsequent chapters are complexes of expensive metals and their cost is an important consideration. Additionally, some of the ligands used are constructed only by long and tedious syntheses. In theory, the catalyst may be recovered at the end of the reaction and reused but this is not always possible in practice. In this respect homogeneous catalysts are less successful than their heterogeneous analogues. Most importantly, however, a coordination catalyst may be used to improve or alter the selectivity of a reaction. It is instructive to consider the types of selectivity which may be desired. Earliest came chemoselectivity, the selectivity between different functional groups, either in the same or different molecules. Figure 1 shows two classic examples, both reductions. It is frequently of value to reduce an alkyne to a cis-alkene but not to continue reduction to give the alkane. In the other example, an otß-unsaturated aldehyde might be reduced to a saturated aldehyde, an allyl alcohol or a 1 2 r\ -*- ■*· H2 CHO CHO OH OH or Figure 1 Examples of chemoselectivity in reduction or a saturated alcohol. To take an example from the field of coordination catalysis, Bu-SnH converts 1_ to 2_ with better than 99% selectivity in the 1 2 presence of (Ph„P)Pd ' . By contrast, _3 gives £ in excellent yield in a 4 3 rhodium catalysed hydrosilylation reaction . In S_, where the double bond is not conjugated with the carbonyl group, transfer hydrogénation gives up to 90% of the unsaturated alcohol, 6 . DtPl^P^Pd Bu 5nH 3 2) H 0 2 1) Ph 5iH2/(diene)Rh(PR ) 2 3 2 2)H 0 ' 2 3 (Ir(cod)Cl) 2 R p 3 Regioselectivity involves reaction at one of two or more similar sites in a molecule. The well known Markownikov addition of HBr to alkenes is regiospecific. Many additions to double bonds catalysed by metal complexes are also regioselective or regiospecific. For example, rhodium catalysed hydroformylation of styrene gives mainly 2-phenylpropanal. Similarly, hydrosilylation of 7_ gives mainly 8^ in the presence of HCo(CO) . and radical addition of RS0C1 to 9^ catalysed by (Ph P) RuCU gives only liO6. In the 2 reaction of 3^ in the presence of (Ph„P).Pd, the catalyst changes both the regiochemistry of the reaction and its outcome. In the absence of the catalyst Michael addition yields 1^, but (Ph-P).Pd reacts with an isomer of 1^, 1_3, to give the ττ-allyl complex, 14^. This allyl complex is attacked regioselectively 7 at the less hindered site to give j^ . HCo(CO) 4 HSi(0Et)3 (Ph P) Rua 1 .S0 R R ^^ + RS0 Q 3 3 2 » R ^ ^^ 2L 7 80-98 % TO A reaction is said to be diastereoselective when the product contains more of one possible diastereoisomer than the other. Two familiar examples are shown in Figure 2. A rather more complex example in the area of g hydrogénations is the reduction of 16_ to l]_ and _18_ . In the presence of [(cod)Ir(py)(PCy„)] + as catalyst, the reaction is unselective, but with 4 N02 + R2NH -N02 NR 2 12 RJ(PPh) R NH 3z> 2 NOo 13 » Pd(PPh3)2 NR2 15 -COOMe H H D ^ I /COOMe . ^COOMe D + D2 and/or COOMe D^I^COOMe H^J^COOMe ΑΘ and / or ÖH OH Figure 2 Examples of potentially diastereoselective reactions OH + H 2 16 17 18 5 [(nbd)Rh(dppb)] + the ratio of Γ7:18 (R = Me) is 25:75 at 15 psi H and 93:7 2 at 640 psi H. At low pressures isomerisation competes with reduction and 2 H addition is rate-controlling. At elevated pressure complexation of the 2 substrate becomes rate-controlling. Various diastereoselectivities may be achieved in the palladium catalysed substitution of allyl acetates (Figure 3)" With nucleophiles such as NaCH(C00Me) and lithium and tin enolates, path a, 2 giving retention, is followed. However, with Grignard reagents, PhZnCl and CH =CHAlMe , path b predominates. Heteroatom nucleophiles such as R~NH and 2 2 RCOO" give mixtures of products. COOMe COOMe COOMe PdL, / \—OAc Nu L COOMe COOMe Nu Figure 3 Palladium catalysed substitution of allyl acetates Good enantioselectivity using coordination catalysts has only been achieved in the last decade. Hydrogénation of Yà_ to 2Q_ in the presence of rhodium catalysts of chiral phosphines has been the most widely studied reaction and optical yields up to 99% may be achieved. For example, the diene rhodium complex of 21^ gives 88% enantiomer excess in the reduction of \9_ (R = Ph, Rf = Me, RM = H) . Other recent examples of enantioselective reactions are shown in Figure 4 ' 6 COOR" (diene)RhL COOR 2 + H "* R 2 NHCOR NHCOR' 20 19 2-naphthyl) 2 NiCl 2 Ph^ ^MgCl + ^ ^ Br * Ph- 5 ^ 817o5 Me N PPh 2 2 -» R' sNEt 2 93°/o R NEto PPhh 2>fcod) 2 Figure 4 Examples of enantioselective reactions References 1. E.Keinan and P.A.Gleize, Tetrahedron Lett., 2^5, 477 (1982). 2. P.Four and F.Guibe, Tetrahedron Lett., 2^5, 1825 (1982). 3. T.Kogure and I.Ojima, J. Qrganomet. Chem., 234, 249 (1982), 4. M.Visintin, R.Spogliarich, J.Kaspar and M. Graziani, J. Mol. Catal., 24 277 (1984).
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