Free Radical Chain Reactions in Organic Synthesis PDF
Preview Free Radical Chain Reactions in Organic Synthesis
BEST SYNTHETIC METHODS Series Editors A. R. Katritzky O. Meth-Cohn C. W. Rees University of Florida Sunderland Polytechnic Imperial College of Science Gainesville, Florida Sunderland and Technology USA UK London, UK Richard F. Heck, Palladium Reagents in Organic Syntheses, 1985 Alan H. Haines, Methods for the Oxidation of Organic Compounds: Alkanes, Alkenes, Alkynes, and Arenes, 1985 Paul N. Rylander, Hydrogenation Methods, 1985 Ernest W. Colvin, Silicon Reagents in Organic Synthesis, 1988 Andrew Pelter, Keith Smith and Herbert C. Brown, Borane Reagents, 1988 Basil Wakefield, Organolithium Methods, 1988 Alan H. Haines, Methods for the Oxidation of Organic Compounds: Alcohols, Alcohol Derivatives, Alkyl Halides, Nitroalkanes, Alkyl Azides, Carbonyl Compounds, Hydroxyarenes and Aminoarenes, 1988 H. G. Davies, R. H. Green, D. R. Kelly and S. M. Roberts, Biotransformations in Preparative Organic Chemistry: The Use of Isolated Enzymes and Whole Cell Systems, 1989 I. Ninomiya and T. Naito, Photochemical Synthesis, 1989 T. Shono, Electroorganic Synthesis, 1991 William B. Motherwell and David Crich, Free Radical Chain Reactions in Organic Synthesis, 1991 Free Radical Chain Reactions in Organic Synthesis William B. Motherwell and David Crich Departments of Chemistry, Imperial College of Science, Technology and Medicine, and University College, London, UK ACADEMIC PRESS Harcourt Brace Jovanovich, Publishers London San Diego New York Boston Sydney Tokyo Toronto ACADEMIC PRESS LIMITED 24-28 Oval Road London NW1 7DX US Edition published by ACADEMIC PRESS INC. San Diego, CA 92101 Copyright © 1992 by ACADEMIC PRESS LIMITED All Rights Reserved No part of this book may be reproduced in any form by photostat, microfilm, or any other means, without written permission from the publishers This book is a guide providing general information concerning its subject matter; it is not a procedural manual. Synthesis of chemicals is a rapidly changing field. The readers should consult current procedural manuals for state-of-the-art instructions and applicable government safety regulations. The publisher and the authors do not accept responsibility for any misuse of this book, including its use as a procedural manual or as a source of specific instructions. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-12-508760-8 Typeset by Thomson Press (India) Limited, New Delhi, Printed in Great Britain by St Edmundsbury Press, Bury, St. Edmonds Suffolk, UK. "And now for the Initiation Ceremony" Dedicated with admiration, gratitude and affection to Professor Sir Derek Barton FRS Foreword There is a vast and often bewildering array of synthetic methods and reagents available to organic chemists today. Many chemists have their own favoured methods, old and new, for standard transformations, and these can vary considerably from one laboratory to another. New and unfamiliar methods may well allow a particular synthetic step to be done more readily and in higher yield, but there is always some energy barrier associated with their use for the first time. Furthermore, the very wealth of possibilities creates an information-retrieval problem. How can we choose between all the alternatives, and what are their real advantages and limitations? Where can we find the precise experimental details, so often taken for granted by the experts? There is therefore a constant demand for books on synthetic methods, especially the more practical ones like Organic Syntheses, Organic Reactions, and Reagents for Organic Synthesis, which are found in most chemistry laboratories. We are convinced that there is a further need, still largely unfulfilled, for a uniform series of books, each dealing concisely with a particular topic from a practical point of view—a need, that is, for books full of preparations, practical hints and detailed examples, all critically assessed, and giving just the information needed to smooth our way painlessly into the unfamiliar territory. Such books would obviously be a great help to research students as well as to established organic chemists. We have been very fortunate with the highly experienced and expert organic chemists, who, agreeing with our objective, have written the first group of volumes in this series. Best Synthetic Methods. We shall always be pleased to receive comments from readers and suggestions for future volumes. A. R. K., O. M.-C, C. W. R. Preface In recent times the myth of free radicals as highly reactive intermediates has been exploded by the advent of rationally designed, highly efficient free radical chain sequences permitting both the high yielding interconversion of functional groups and the formation of carbon-carbon bonds under mild neutral conditions. The extent of this change in perception is such that free radical chain reactions can now be said to occupy an equal place in the armoury of the synthetic organic chemist with long standing two electron and concerted processes. Several books and review articles dealing with various facets of this general area have appeared: the intention of this particular volume, which we hope will be useful to both the experienced practitioner and to the, as yet, un- initiated, is to provide a guide to the more practical aspects of the subject. This book does not seek to be comprehensive but rather to provide a collection of tried and tested experimental parts as well as a selection of methods of very recent vintage. We are only too well aware that free radical chain processes of the S 1 type or those mediated by transition metal redox RN couples can, and in future may well, form the material for further volumes. This is a task which we leave for others. In the final analysis, the writing of a book is not only a labour but also a test of love. Given that the dedicatee once informed one of the authors that "those who can, do, those who can't, review," we hope that we may be forgiven. We are especially grateful to our friend Dr Tom Purcell, whose modesty forbade him to append his name to the cartoon which we commissioned from him. Finally, we would like to thank those of our colleagues, and, in particular, Dr Stephen Caddick, Dr Robyn S. Hay-Motherwell and Dr A. M. K. Pennell who have provided invaluable assistance in the painstaking task of checking this work. WILLIAM B. MOTHERWELL DAVID CRICH Detailed Contents 1 Some basic concepts of free radical chain reactions 1 1.1 Introduction 1 1.2 The advantages 1 1.3 The relative "character" of various radical species 2 1.3.1 Reactivity as a function of the nature of the atom containing the unpaired electron 2 1.3.2 Character variations within carbon centred radicals . . .. 4 1.4 Determinant factors in the production of a successful free radical chain reaction 7 1.5 The initiation ceremony 10 1.6 Strategy and design in synthetic sequences involving carbon centred radical reactions 12 1.6.1 Chemoselectivity in functional group interconversions . .. 12 1.6.2 Stereoselectivity in carbon-carbon bond forming reactions . 14 1.6.3 Retrosynthetic carbon-carbon bond disconnections . . .. 17 1.7 Some practical guidelines 19 1.7.1 Initiators 20 1.7.2 Solvents 20 1.7.3 The correct order of addition of reagents 20 1.7.4 Removal of organotin residues 21 1.7.5 The role of oxygen in free radical reactions 24 1.8 Overview and perspectives 24 References 25 2 Books and review articles 27 2.1 Some earlier basic texts and reviews (pre-1980) 27 2.2 General books and reviews (post-1980) 28 2.3 Recent books and articles dealing with more specific aspects of free radical reactions 28 3 Substitution reactions 29 3.1 Dehalogenation (RX -► RH, X = halogen) 29 3.1.1 Selection of the organostannane 29 3.1.2 Selection of the halogen atom 30 3.1.3 Choice of solvent and relative concentrations 30 xii DETAILED CONTENTS 3.1.4 Reactivity and selectivity as a function of substrate structure 30 General procedure for dehalogenation reactions (RX RH) without rearrangement . . . 36 Photochemically induced organostannane dehaloge- nation of 4-bromotoluene—a typical procedure . . . 36 Preparation of 2-acetoxy-l-chloroethene 36 3.2 Deoxygenation reactions (ROH RH) 37 3.2.1 Introduction. The development of the thiocarbonyl ester method (Barton-McCombie deoxygenation) 37 3.2.1.1 Practical requirements 38 (a) Xanthate esters (X = SMe) 38 (b) Thiocarbonyl imidazolide derivatives (X — l-imidazolyl) 39 (c) Thionobenzoate esters (X = Ph) 39 (d) Phenoxythiocarbonyl derivatives (X = OPh) . . 39 3.2.2 Deoxygenation of primary alcohols via thiocarbonyl derivatives 3399 n-Octadecane from the thionobenzoate derivative of n-octadecanol 40 3.2.3 Deoxygenation of secondary alcohol thionocarbonyl derivatives 40 Deoxygenation of 3ß-thiobenzyloxy-5(x-cholestane— a typical procedure 47 3.2.4 Tertiary alcohol deoxygenation 47 General procedure for the deoxygenation of alcohols via in situ formation of oxalyl thiohydroxamates . . 50 3.2.5 Regioselective deoxygenation of 1,2- and 1,3-diol cyclic thionocarbonates 50 Preparation of 5-deoxy-l,2-0-isopropylidene-3-0 methyl-D-xy\ohexofuranose 52 3.2.6 Deoxygenation of p-toluenesulphonate esters with tri-n- butylstannane via in situ iodide formation 52 General procedure for the deoxygenation of p- toluenesulphonate esters 53 3.2.7 Deoxygenation of carboxylic acid esters 53 Preparation of cyclododecane—a typical procedure. 54 3.2.7.1 Deoxygenation of carbohydrate α-acetyl tertiary benzoates 55 Preparation of 3-C-acetyl-3-deoxy-l,2; D-5,6-di-0- isopropylidene-a-D-d\\ofuranose 55 3.2.8 Selective deoxygenation of tertiary and allylic alcohols via mixed methyl oxalate esters 56 3.2.9 Deoxygenation reactions based on chloroformate and selenocarbonate derivatives 56 Deoxygenation of phenylselenocarbonates— a typical procedure 57 3.3 Deamination reactions (RNH2->RH) 58 DETAILED CONTENTS xiii 3.3.1 Deamination via isocyanides, isothiocyanates and isoselenocyanates 58 Preparation of n-octadecane from n-octadecylamine: deamination of a primary isonitrile 62 Preparation of1,3,4,6-tetra-0-acetyl-2-deoxy-oc-D- glucose: deamination of 2-amino-2-deoxy-0L-D-glucose 63 3.4 Denitration reactions (RN0 -> RH) 65 2 Method A. General procedure for the denitration of tertiary nitro compounds 66 Method B. General procedure for the denitration of secondary nitro compounds 66 3.5 Reductive homolytic cleavage of the carbon-sulphur bond 71 3.5.1 Introduction 71 3.5.2 Thiol desulphurization (RSH -> RH) 72 Preparation of n-octane from n-octyl mercaptan . . 72 Desulphurization of mercaptans 73 3.5.3 Reductive cleavage of sulphides 74 Desulphurization of 2,3-dimethyl-7-( 1-hydroxy-l- methylethyl )-7-phenylthio-8-oxo-l -azabicyclo- [4.2.0]octane 75 3.5.4 Reduction of dithioacetals 75 Desulphurization of l,4-dithiaspiro[4.5]decane . . 76 3.5.5 Reduction of thiocarbonyl groups (RCS->RCH) . . .. 77 2 2 2 3.6 Deselenation reactions 79 3.6.1 Introduction 79 3.6.2 Reductive cleavage of unsymmetrical selenides 79 General procedures for the reduction of alkyl phenyl selenides 80 3.6.3 Reduction of selenoketals 82 3.6.4 Deselenation of acyl selenides 83 General procedures for the conversion of acyl selenides to aldehydes or nor-alkanes 85 3.7 Decarboxylation reactions 85 3.7.1 Introduction 85 3.7.2 Nor-alkanes by reductive decarboxylation of O-acyl thiodydroxamates (RC0H -+ RH) 88 2 Method A. Reductive decarboxylation using tri-n-butylstannane and in situ generation of the O-acyl thiohydroxamate from the acid chloride... 90 Method B. General procedure for the reductive decarboxylation of carboxylic acids using in situ generation from the acid chloride and t-butyl mercaptan—normal addition 90 Method C. Reduction with t-butyl mercaptan— inverse addition 91 xiv DETAILED CONTENTS Method D. Reductive decarboxylation of bridgehead dicarboxylic acid monoesters 91 Method E. General procedure for the reductive decar- boxylation ofamino acid and peptide derivatives . . 92 3.7.3 Decarboxylative rearrangement of O-acyl thiohydroxamates 93 Method A. General procedure for decarboxylative rearrangement to alkylpyridyl sulphides 96 Method B. Preparation of n-pentadecyl-2-pyridyl sulphide by photolytic rearrangement 96 3.7.4 Decarboxylative halogenation (RC0H -> RY, Y = Cl, Br, I). 96 2 General method A. Decarboxylative chlorination (or bromination) of aliphatic and alicyclic carboxylic acids 97 General method B. Decarboxylative iodination of aliphatic and alicyclic carboxylic acids 99 General method C. Decarboxylative bromination of aromatic carboxylic acids 99 General method D. Decarboxylative iodination of aromatic carboxylic acids 100 3.7.5 Decarboxylative chalcogenation (RC0H -► RSR', RSeR', 2 RTeR') 100 General procedure for decarboxylative chalcoge- nation 102 Preparation of benzyl N-t-butoxycarbonylseleno- methionate 103 3.7.6 Decarboxylative phosphonylation (RC0H -+ RPO(SPh)) . 103 2 2 3.7.7 Decarboxylative sulphonation (RC0H-► RS0Spy) 104 2 2 General method for the preparation of thiosulphonates by decarboxylative sulphony- lation of O-acyl thiohydroxamates 106 3.7.8 Decarboxylative hydroxylation (RC0H-► ROH) 107 2 General methods for the formation of norhydroper oxides 109 General method for the reduction of nor-hydroperoxides to nor-alcohols 110 General method for the fragmentation of hydroperoxides with tosyl chloride and pyridine . . Ill 3.8 Functional group interchange using organometallic derivatives of boron, mercury and cobalt 113 3.8.1 Introduction 113 3.8.2 Preparation of alcohol derivatives (R^ML,,-» ROH) . . .. 115 3.8.2.1 From organoboranes and organomercury compounds 115 eis-, tmns-3-Benzyloxycarbonyl-4-hydroxymethyl- 6-methyl-tetrahydro-l ,3-oxazine 116 3.8.2.2 From organocobalt derivatives 117
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