ORGANIC REACTION MECHANISMS - 1978 ORGANIC REACTION MECHANISMS 1978 An annual survey covering the literature dated December 1977 through November 1978 Edited by A. C. KNIPE and W. E. WATTS, The New University of Ulster, Northern Ireland An Interscience@P ublication JOHN WILEY & SONS Chichester New York Brisbane Toronto a = * An Interscience@ Publication Copyright @ 1980 by John Wiley & Sons, Ltd. All rights reserved. No part of this book may be reproduced by any means, nor transmitted, nor translated into a machine language without the written permission of the publisher. Library of Congress Catalog Card Number 66-23143 ISBN 0 471 27613 8 Printed in Great Britain by John Wright & Sons Ltd., at the Stonebridge Press, Bristol Contributors M. S. BAIRD Department of Organic Chemistry, The University, Newcastle-upon-Tyne C. BROWN Chemical Laboratory, University of Kent, Canterbury A. R. BUTLER Department of Chemistry, The Purdie Building, University of St. Andrews B. CAPON Department of Chemistry, Glasgow University D. J. COWLEY School of Physical Sciences, New University of Ulster M. R. CRAMPTON Department of Chemistry, Durham University G. W. J. FLEET Dyson Perrins Laboratory, South Parks Road, Oxford University I. GOSNEY Department of Chemistry, University of Edinburgh M. C. GROSSEL Dyson Perrins Laboratory, South Parks Road, Oxford University A. F. HEGARTY Chemistry Department, University College, Cork, Ireland A. J. KIRBY University Chemical Laboratory, Cambridge A. W. MURRAY Department of Chemistry, University of Dundee D. C. NONHEBEL Department of Pure and Applied Chemistry, University of Strathclyde J. SHORTER Department of Chemistry, University of Hull Preface ‘The Road goes ever on and on Down from the door where it began. Now far ahead the Road has gone, And I must follow, if I can. Pursuing it with weary feet, Until it joins some larger way, Where many paths and errands meet. And whither then? I cannot say.’ The Fellowship of the Ring, J. R. R. TOLKIEN [Published by permission of George Allen & Unwin (Publishers) Limited] We have little doubt that the above passage will have particular significance for the dedicated team of contributors to Organic Reaction Mechanisms. As editors, we undertake the task of scanning the entire literature of organic chemistry in search of papers of mechanistic interest; we are consequently very milch aware of the manifold development of the subject and of the difficulty in kzeping up with its relentless progress. In keeping with previous volumes we have allocated references to fourteen chapter areas. It has subsequently been the task of each author to provide an expert and comprehensive review of his subject while taking care to highlight those papers of particular interest. Thus, we hope that successive volumes of Organic Reaction Mechanisms will effectively map the major developments in this diverse area while also describing the surrounding fields of activity. The present volume, the fourteenth of the series, surveys research on organic reaction mechanisms described in the literature dated December 1977 to November 1978. In order to limit the size of the volume we must necessarily exclude or restrict overlap with other publications which review related specialist areas (e.g. photochemical reactions, biosynthesis, electrochemistry, surface chemistry, organometallic chemistry, heterogeneous catalysis). Furthermore, we try to avoid duplication between chapters. Thus, while a particular reference may occasionally be allocated to more than one chapter, we do assume that readers will be aware of the alternative chapters to which a bopderline topic of interest may have been preferentially assigned. We would once again like to express our particular thanks to our very experienced contributors, to the publications staff of John Wiley and Sons, and to Dr. R S. Cahn whose expertise as technical editor has ensured maintenance of the high standard of presentation expected of this series. August 1979 A. C. K. W. E. W. Contents 1. Reactions of Aldehydes and Ketones and their Derivatives by B. CAPON . . I 2. Reactions of Acids and their Derivatives by A. J. KIRBY . . 29 3. Radical Reactions by D. J. COWLEaYn d D. C. NONHEBE.L . 89 4. Oxidation and Reduction by G. W. J. FLEET . . 199 5. Carbenes and Nitrenes by M. S. BAIRD . . 249 6. Nucleophilic Aromatic Substitution by M. R. CRAMPTON . 281 7. Electrophilic Aromatic Substitution by A. R. BUTLER . 299 8. Carbonium Ions by M. C. GROSSE.L . . 313 9. Nucleophilic Aliphatic Substitution by J. SHORTER . . 343 10. Carbanions and Electrophilic Substitution by I. GOSNEY . 383 11. Elimination Reactions by A. F. HEGARTY . 429 12. Addition Reactions: Polar Addition by C. BROWN . . 453 13. Addition Reactions: Cycloaddition by C. BROWN . . 479 14. Molecular Rearrangements by A. W. MURRAY. . 509 Author Index, 1978 . . 625 Subject Index, 1978 . . 689 Erratum for Organic Reaction Mechanisms 1977 P. 60: The three-phase test referred to should be attributed to Rebek and Gavifia who first described the test in J. Am. Chem. Soc. 7112 (1974). 96, Organic Reaction Mechanisms 1978 Edited by A. C. Knipe, W. E. Watts Copyright © 1980 by John Wiley & Sons, Ltd. CHAPTER 1 Reactions of Aldehydes and Ketones and their Derivatives B. CAPON Chemistry Department, Glasgow University Formation and Reactions of Acetals and Ketals . . 1 Hydrolysis and Formation of Glycosides . . 5 Non-enzymic Reactions . . 5 Enzymic Reactions . . 6 (a) Galactosidases . . 6 (b) Glucosidases . . 6 (c) Lysozymes . . 7 (d) Amylases and Glucamylases . . 7 (e) Cellulases . . 8 (f) Other Enzymes . . 8 . Hydration of Aldehydes and Ketones and Related Reactions . 8 Reactions with Nitrogen Bases . . 10 Schiff Bases . . 10 Enamines . . 13 Nucleosides . . 13 Hydrazones, Oximes, Semicarbazones and Related Compounds . 14 Hydrolysis of Enol Ethers . . 15 Enolization and Related Reactions . . . 16 Homoenolization . . 18 Aldol and Related Reactions . . 19 Other Reactions . . 20 References . . 21 Formation and Reactions of Acetals and Ketals There have been reviews entitled “Transition States of Hydrolysis of Acetals, Ketals, Glycosides, and Glycosylamines”,l “Some Mechanistic Studies on the Hydrolysis of Acetals and Hemiacetal~”,a~n d “Stereoelectronic Control in Hydrolytic reaction^".^ Hemiacetals have been detected at quite high concentrations in the hydrolysis of some benzaldehyde diethyl acetals by making use of the fact that the hydrolysis of the hemiacetal is base-catalysed but that of the acetal is Addition of sodium hydroxide to a hydrolysing solution of a benzaldehyde acetal, therefore, produces a sharp increase in the concentration of benzaldehyde (and hence UV absorbance) which is proportional to the concentration of hemiacetal present. The p-value for the hydrolysis of benzaldehyde acetals is -3.4 and of hemiacetals is ca. -2; hence 1 2 Organic Reaction Mechanisms 1978 this effect is most noticeable in the reactions of p-methoxybenzaldehyde diethyl acetal. Also, as the hydrolysis of the hemiacetal is base-catalysed and that of the acetal is not, the effect is most noticeable at low pH. Under certain conditions, the hemiacetal is present in a hydrolysing solution of p-methoxybenzaldehyde diethyl acetal to the extent of 40% of the starting concentration. The ethoxytropylium ion (1) has been detected as an intermediate in the hydrolysis of tropone diethyl ketal, and methoxy- and ethoxy-diphenylcyclo- propenium ions have been detected in the hydrolysis of the acetals (2). However, no intermediate could be detected in the hydrolysis of 4-ethoxyacetophenone dimethyl acetal. Similar experiments were carried out with orthoesters and the intermediate ion could be detected in the hvdrolvsis of trimethvl orthomesitoate : by taking advantage of the rate-decreasing kffect of electrolytes- on the hydrolysis (3) x = 0 Ph (4) x = s OMe Me (5) (6) (7) of carbocations, the intermediate ion could also be detected in the hydrolysis of trimethyl 4-methoxyorthobenzoate and trimethyl orth~benzoate.T~h e rate constants for the reactions of these and several other ions with water were also determined, the ions being generated in sulphuric acid solutions. Values of kHzO were obtained by extrapolation to zero acid concentration. It was shown inter alia that the methoxy-substituted cation (3) reacts over 1000 times faster than the methythio-analogue (4) and that the a-amino-substituted ion (5) reacts almost lo9 times more slowly than ion (6). The p+-values for the reactions of ions with + (7) water is +2.1.6 This was compared with the p+-value of 1.6 estimated by Young and Jencks' using a different method as described below. Young and Jencks studied trapping, by sulphite dianion, of the ions (7) which were generated by hydrolysis of the corresponding acetophenone dimethyl ketals. The fraction trapped increases with increasing sulphite concentration up to a maximum value (f,,,) which increases with increasing electron-releasing power of substituents in the ketal. The overall rate of reaction is unaffected by sulphite. Since the fraction of a-methoxysulphonic acid formed reaches a limiting value with increasing sulphite concentration, sulphite must also catalyse the reaction of water with cations (7) as well as attack them as a nucleophile. General base catalysis of attack of water by acetate was also detected as reflected by a decrease in the yield of a-methoxysulphonic acid with increasing acetate concentration. It was con- cluded from the low ratio of the rate constant for reaction with sulphite to that with water that the reaction with sulphite was diffusion-controlled. On this basis, on the assumption that the same values apply for the rate constants for reaction of all the ions with sulphite (k, = 5 x lo9 M-l s-l), a series of values for reactions of the ions with water was calculated which, at 25 "C, ranged from 7 x lo6 s-l for I Reactions of Aldehydes and Ketones and their Derivatives 3 p-methoxy to ca. 4 x lo8 s-l for m-bromo, leading to the value of p+ = 1.6 men- tioned in the last paragraph. The value obtained by McClelland and Ahmad for the p-methoxy ion, by quite a long extrapolation from solutions in concentrated sulphuric acid to water (1.4 x lo6 s-l), was in quite good agreement. A linear plot of these values against the rate constants for reactions of sulphite with the corre- sponding carbonyl compounds was obtained. When this was extrapolated to isobutyraldehyde, p-nitrobenzaldehyde, and formaldehyde, very high values of rate constants for the reaction of the corresponding ions with water were estimated, e.g. s-l for CH2=6Me. This suggests that reactions which might involve these ions as intermediates, e.g. hydrolysis of the corresponding acetals, probably proceed by SN2 processes.' The rate constant for nucleophilic attack by water on the stabilized oxonium ion (8) is 8.5 x s-l at 25 "C! OMe Oglucose A discussion of the previously determined solvent deuterium isotope effects and Brernsted a-coefficients for the general acid catalysed hydrolysis of acetals and orthoesters has been given.g In agreement with earlier discussions,1° it was con- sidered that the rate-determining step was the concerted displacement of the alkoxycarbonium ion by the acid catalyst. In addition, however, it was thought necessary to invoke several additional intermediates to explain the difference in solvent isotope effects (and fractionation factors) between these reactions and the general acid catalysed hydrolysis of vinyl ethers. The spontaneous hydrolysis of acetal (10) with an axial OAr group is 3.3 times slower than that of (9) with an equatorial OAr group.ll This result contrasts with those previously reported which indicated that orthoester (11) should react with preferential cleavage of the axial carbon-methoxyl bond. It is possible that the acetal with an axial OAr group reacts via a half-chair transition state and cation, whereas that with an equatorial OAr group reacts via a boat or twist-boat transi- tion state and cation. If these transition states were of similar energy, the rates of cleavage of axial and equatorial bonds would be similar, as found. With this structure, both the half-chair and boat (or twist-boat) conformations can accom- modate the additional fused six-membered rings without additional strain. With the orthoester (ll), however, the ring fusion is in a different position and a boat conformation would require the other six-membered ring to be fused trans across the base of the boat which is very unfavourable. In an investigation of the tri- cyclic acetals (12) and (13), evidence was obtained that, if the system is made sufficiently rigid, fusion of the axial C(2)-0 bond of a 2-aryloxytetrahydropyran occurs faster than fission of an equatorial one.12I t was found that the acid catalysed hydrolysis of (13) occurs over 3000 times faster than that of (12). The spontaneous hydrolysis of (13) is over 1000 times slower than that of (10); this was explained as