The chemistry of

enols

THE CHEMISTRY OF FUNCTIONAL GROUPS

A series of advanced treatises under the general editorship of Professor Saul Patai

The chemistry of alkenes (2 volumes) The chemistry of the carbonyl group (2 volumes)

The chemistry of the ether linkage The chemistry of the amino group

The chemistry of the nitro and nitroso groups (2 parts) The chemistry of carboxylic acids and esters

The chemistry of the carbon-nitrogen double bond The chemistry of amides

The chemistry of the cyano group The chemistry of the hydroxyl group (2 parts)

The chemistry of the azido group The chemistry of acyl halides

The chemistry of the carbon-halogen bond (2 parts) The chemistry of the quinonoid compounds (2 volumes, 4 parts)

The chemistry of the thiol group (2 parts) The chemistry of the hydrazo, azo and azoxy groups (2 parts)

The chemistry of amidines and imidates The chemistry of cyanates and their thio derivatives (2 parts)

The chemistry of diazonium and diazo groups (2 parts) The chemistry of the carbon-carbon triple bond (2 parts)

The chemistry of ketenes, allenes and related compounds (2 parts) The chemistry of the sulphonium group (2 parts)

Supplement A: The chemistry of double-bonded functional groups (2 volumes, 4 parts) Supplement B: The chemistry of acid derivatives (2 parts)

Supplement C: The chemistry of triple-bonded functional groups (2 parts) Supplement D: The chemistry of halides, pseudo-halides and azides (2 parts)

Supplement E: The chemistry of ethers, crown ethers, hydroxyl groups and their sulphur analogues (2 parts)

Supplement F: The chemistry of amino, nitroso and nitro compounds and their derivatives (2 parts)

The chemistry of the metal-carbon bond (5 volumes) The chemistry of peroxides

The chemistry of organic selenium and tellurium compounds (2 volumes) The chemistry of the cyclopropyl group

The chemistry of sulphones and sulphoxides The chemistry of organic silicon compounds (2 parts)

The chemistry of enones (2 parts) The chemistry of sulphinic acids, esters and their derivatives

The chemistry of sulphenic acids and their derivatives

UPDATES The chemistry of a-haloketones, a-haloaldehydes and a-haloimines

Nitrones, nitronates and nitroxides Crown ethers and analogs

Patai's guide to the chemistry of functional groups-Saul Patai

\ / / c-c,

OH

The chemistry of enols

Edited by

Zvr RAPPOPORT

The Hebrew Unioersity, Jerusalem

1990

JOHN WILEY & SONS CHICHESTER-NEW YORK-BRISBANE-TORONTO-SINGAPORE

An Interscience'E' Publication

Copyright 0 1990 by John Wiley & Sons Ltd Baffns Lane, Chichester, West Sussex PO19 IUD, England

All rights reserved

No part of this book may be reproduced by any means, or transmitted, or translated into a machine language without the written permission of the publisher

Other Wiley Editorial Oljices John Wiley & Sons, Inc., 605 Third Avenue, New York; NY 10158-0012, USA

Jacaranda Wiley Ltd, G.P.O. Box 859, Brisbane, Queensland 4001, Australia

John Wiley & Sons (Canada) Ltd, 22 Worcester Road, Rexdale, Ontario M 9 W ILL Canada

John Wiley & Sons (SEA) Pte Ltd, 37 Jalan Pemimpin 05-04. Block B, Union Industrial Building, Singapore 2057

British Library Cataloguing in Publication Data:

The chemistry of enoIs. 1. Enols I. Rappoport Zvi 11. Series 547l.63

ISBN 0471917206

Typeset by Thomson Press (India) Ltd, New Delhi, India Printed in Great Britain by Courier International Ltd., Tiptree, Essex

To

Iris, Ari and Naama

Contributing authors

Y. Apeloig

V. Bertolasi

S. E. Biali

B. Capon

B. Floris

G. Gilli

J. P. Guthrie

H. Hart

A. F. Hegarty

J. R. Keeffe

A. J. Kresge

D. Milstein

Department of Chemistry, Technion, Israel Institute of Technology, Haifa 32000, Israel

Dipartimento di Chimica, Universita di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy

Department of Organic Chemistry, The Hebrew Univer- sity of Jerusalem, Jerusalem 91904, Israel

Chemistry Department, University of Hong Kong, Pok- fulam Road, Hong Kong

Dipartimento di Scienze e Tecnologie Chimiche, Seconda Universita di Roma ‘Tor Vergata’, 00173 Rome, Italy

Dipartimento di Chimica, Universita di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy

Photochemistry Unit, Department of Chemistry, Univer- sity of Western Ontario, London, Ontario N6A 587, Canada

Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, USA

Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland

Department of Chemistry and Biochemistry, San Fran- cisco State University, 1600 Holloway Avenue, San Francisco, California 94132, USA

Department of Chemistry, University of Toronto, Toronto, Ontario MSS 1A1, Canada

Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel

vii

... Vlll

P. O'Neill

2. Rappoport

J. P. Richard

J. Toullec

F. TureEek

A. C. Weedon

Contributing authors

Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland

Department of Organic Chemistry, The Hebrew Univer- sity of Jerusalem, Jerusalem 9 1904, Israel

Department of Chemistry, University of Kentucky, Lex- ington, Kentucky 40506-0055, USA

lnstitut de Topologie el de Dynamique des Systemes, Universite Paris 7, 1 rue Guy de la Brosse, 75005 Paris, France

Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA

Photochemistry Unit, Department of Chemistry, Univer- sity of Western Ontario, London, Ontario N6A 5B7, Canada

Foreword

Enols and their derivatives, such as enolates or enol ethers, play an important part in many organic reactions. Likewise, the study of the kinetics and equilibrium of the keto-enol interconversion has been in the mainstream of physical organic chemistry for many decades. Enols stabilized by bulky substituents or by electron-withdrawing or hydrogen bond accepting substituents have long been known, whereas simple enols were elusive species up to the 1970s. The use of new methods and new analytical techniques resulted in impressive advancements in the chemistry of simple enols, many of which were recently detected and their properties investigated. Simultaneously, significant developments in the chemistry of stabilized enols also took place. Consequently, the last decade has seen an increased activity in the chemistry of enols, justifying the use of the term ‘renaissance’ by several authors.

This volume deals with the chemistry of enols, with emphasis on new developments including theoretical calculations, gas phase structures, photochemistry, novel syntheses, new data on the kinetics and mechanism of enolization and keto-enol equilibria, and other studies not previously reviewed.

The chemistry of important related species, especially enolates and enol ethers is so extensive that it was decided not to include their discussion in the volume in the form of special chapters. These subjects deserve books of their own and numerous reviews dealing with them are available. They are treated in this book only when their reactions are strictly relevant to the enols themselves.

Several aspects of the chemistry of enols have been previously reviewed. Two chapters appeared in the present series on ‘The Chemistry of the Functional Groups’. Forsen and Nilsson reviewed ‘Enolization’ in The Chemistry oj’the Carboizyl Group, Volume 2 (edited by J. Zabicky, 1970) and Capon reviewed ‘Dienols (enolization of enones)’ in The Chemistry qf Enones (edited by S . Patai and Z. Rappoport, 1989).

Literature coverage in most chapters is up to mid-1989. I would be grateful to readers who call my attention to mistakes or omissions in the

present volume. I also wish to take the opportunity of thanking the US-Israel Binational Science Foundation who supported our work on enols (reported in Chapter 8) in the last decade.

Jerusalem January 1990

ix

Zvr RAPPOPORT

The Chemistry of Functional Groups Preface to the series

The series ‘The Chemistry of Functional Groups’ was originally planned to cover in each volume all aspects of the chemistry of one of the important functional groups in organic chemistry. The emphasis is laid on the preparation, properties and reactions of the functional group treated and on the effects which it exerts both in the immediate vicinity of the group in question and in the whole molecule.

A voluntary restriction on the treatment of the various functional groups in these volumes is that material included in easily and generally available secondary or tertiary sources, such as Chemical Reviews, Quarterly Reviews, Organic Reactions, various ‘Advances’ and ‘Progress’ series and in textbooks (i.e. in books which are usually found in the chemical libraries of most universities and research institutes), should not, as a rule, be repeated in detail, unless it is necessary for the balanced treatment of the topic. Therefore each of the authors is asked not to give an encyclopaedic coverage of his subject, but to concentrate on the most important recent development and mainly on material that has not been adequately covered by reviews or other secondary sources by the time of writing of the chapter, and to address himself to a reader who is assumed to be at a fairly advanced postgraduate level.

It is realized that no plan can be devised for a volume that would give a complete coverage of the field with no overlap between chapters, while at the same time preserving the readability of the text. The Editor set himself the goal of attaining reasonable coverage with moderate overlap, with a minimum of cross-references between the chapters. In this manner, sufficient freedom is given to the authors to produce readable quasi-monographic chapters.

The general plan of each volume includes the following main sections:

(a) An introductory chapter deals with the general and theoretical aspects of the group. (b) Chapters discuss the characterization and characteristics of the functional groups,

i.e. qualitative and quantitative methods of determination including chemical and physical methods, MS, UV, IR, NMR, ESR and PES-as well as activating and directive effects exerted by the group, and its basicity, acidity and complex-forming ability.

(c) One or more chapters deal with the formation of the functional group in question, either from other groups already present in the molecule or by introducing the new group directly or indirectly. This is usually followed by a description of the synthetic uses of the group, including its reactions, transformations and rearrangements.

(d) Additional chapters deal with special topics such as electrochemistry, photochemis-

xi

xii Preface to the series

try, radiation chemistry, thermochemistry, syntheses and uses of isotopically labelled compounds, as well as with biochemistry, pharmacology and toxicology. Whenever applicable, unique chapters relevant only to single functional groups are also included (e.g. ‘Polyethers’. ‘Tetraaminoethylenes’ or ‘Siloxanes’).

This plan entails that the breadth, depth and thought-provoking nature of each chapter will differ with the views and inclinations of the authors and the presentation will necessarily be somewhat uneven. Moreover, a serious problem is caused by authors who deliver their manuscript late or not at all. In order to overcome this problem at least to some extent, some volumes may be published without giving consideration to the originally planned logical order of the chapters.

Since the beginning of the Series in 1964, two main developments occurred. The first of these is the publication of supplementary volumes which contain material relating to several kindred functional groups (Supplements A, B, C, D, E and F). The second ramification is the publication of a series of ‘Updates’, which contain in each volume selected and related chapters, reprinted in the original form in which they were published, together with an extensive updating of the subjects, if possible, by the authors of the original chapters. A complete list of all above mentioned volumes published to date will be found on the page opposite the inner title page of this book.

Advice or criticism regarding the plan and execution of this series will be welcomed by the Editor.

The publication of this series would never have been started, let alone continued, without the support of many persons in Israel and overseas, including colleagues, friends and family. The efficient and patient co-operation of staff members of the publisher also rendered me invaluable aid. My sincere thanks are due to all of them, especially to Professor Zvi Rappoport who, for many years, shares the work and responsibility of the editing of this Series.

The Hebrew University Jerusalem, Israel

SAUL PATAI

Contents

1. Theoretical calculations

2. Thermodynamics of enols

3. The chemistry of ionized enols in the gas phase

4. NMR, IR, conformation and hydrogen bonding

5. The generation of unstable enols

6. Keto-enol equilibrium constants

7.

8.

9. Photochemical reactions involving enols

Enols of carboxylic acids and esters

Yitzhak Apeloig

J. Peter Guthrie

FrantiSek TureEek

B. Floris

B. Capon

J. Toullec

Kinetics and mechanism of enolization and ketonization James R. Keeffe and A. Jerry Kresge

lsolable and relatively stable simple enols Harold Hart, Zvi Rappoport and Silvio E. Biali

Alan C. Weedon

10. A. F. Hegarty and P. O’Neill

11. The biochemistry of enols John P. Richard

12. Organometallic chemistry of enols David Milstein

13. Structural chemistry Gastone Gilli and Valerio Bertolasi

Author index

Subject index

1

75

95

147

307

323

399

481

59 1

639

65 1

69 1

71 3

765

81 3

.. . X l l l

List of abbreviations used

Ac acac Ad All An Ar

Bz Bu

CD CI CIDNP CNDO CP

DBU DME DMF DMSO

ee EI ESCA ESR Et eV

Fc FD FI FT Fu

Hex c-Hex HMPA

acetyl (MeCO) acetylacetone adamantyl ally1 anisyl aryl

benzoyl (C,H,CO) butyl (also t-Bu or Bur)

circular dichroism chemical ionization chemically induced dynamic nuclear polarization complete neglect of differential overlap q5-cyclopentadienyl

1,8-diazabicyclo[ 5.4.01 undec-7-ene 1 ,Zdimethoxyethane N,N-dimeth y lformamide dimethyl sulphoxide

enantiomeric excess electron impact electron spectroscopy for chemical analysis electron spin resonance ethyl electron volt

ferrocene field desorption field ionization Fourier transform furyl(OC,H 5 )

hexy1GHl 1)

cyclohexyl(C,H I 1)

hexamethy lphosphortriamide

xv

xvi

HOMO

i- IP IR ICR

LCAO LDA LUMO

M M MCPBA Me MNDO MS

n Naph NBS NMR

Pen Pip Ph PPm Pr PTC PYr

R RT

S-

SET SOMO

t- TCNE THF Thi TMEDA To1 Tos Trityl

XYl

List of abbreviations used

highest occupied molecular orbital

is0 ionization potential infrared ion cyclotron reasonance

linear combination of atomic orbitals lithium diisopropylamide lowest unoccupied molecular orbital

metal parent molecule rn-chloroperbenzoic acid methyl modified neglect of diatomic overlap mass spectrum

normal nap hth yl N-bromosuccinimide nuclear magnetic resonance

pentyl(C5H11) piperidyl(C,H ,,N) phenyl parts per million propyl (also i-Pr or Pr') phase transfer catalysis pyridyl (C5H4N)

any radical room temperature

secondary single electron transfer singly occupied molecular orbital

tertiary tetracyanoet hylene tetrahydrofuran thienyl(SC,H,) tetramethylethylene diamine tolyl(MeC,H4) tosyl(p-toluenesulphonyl) triphenylmethyl(Ph,C)

xYlYKMe*C,H,)

In addition, entries in the 'List of Radical Names' in IUPAC Nomenclature of Organic Chemistry, 1979 Edition. Pergamon Press, Oxford, 1979, p, 305-322, will also be used in their unabbreviated forms, both in the text and in formulae instead of explicitly drawn structures.