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A Thesis Submitted by JOHN DAVID HARLING In PREPARATIVE RADICAL REARRANGEMENT REACTIONS FOR ORGANIC SYNTHESIS A thesis submitted by JOHN DAVID HARLING in partial fulfilment of the requirements for the award of DOCTOR OF PHILOSOPHY OF THE UNIVERSITY OF LONDON BARTON LABORATORY DEPARTMENT OF CHEMISTRY IMPERIAL COLLEGE OF SCIENCE TECHNOLOGY & MEDICINE LONDON SW7 2AY OCTOBER 1989 ABSTRACT This thesis is divided into two sections. In the first section a review of free radical rearrangement reactions is presented under the headings of ring opening processes, ring closure processes and group transfer reactions. The review illustrates how free radical rearrangement reactions; once only studied by physical chemists, have during the last few years been exploited by synthetic chemists. The second section describes the development of a novel, free radical tandem cyclopropyl carbinyl rearrangement-cyclisation strategy for regio- and stereoselective carbon-carbon bond formation. Initial studies in the development of this process are directed towards the synthesis of spiro[ 5 .4 Jdecanes. The extension of this methodology to the synthesis of tricyclo[ 3. 3. 1. 0 ] nonanes and hydrindanes, the latter via two different radical triggers, is then desribed. This is followed by a brief re-examination of the regiochemistry of stereoelectronically controlled radical cyclopropyl carbinyl ring opening reactions. The section is concluded by a formal presentation of the experimental results. 2 CONTENTS ABSTRACT 2 DEDICATION 4 ACKNOWLEDGEMENTS 5 ABBREVIATIONS 6 A REVIEW OF RADICAL REARRANGEMENTREACTIONS 8 Introduction 8 Ring Opening Reactions 11 Ring Closure Reactions 30 Group Transfer Reactions 33 Review References 39 RESULTS AND DISCUSSION 43 Introduction 43 Regioselective Synthesis of Spiro[4.5 ]Decanes 47 Stereoselective Synthesis of Spiro[4.5 ]Decanes 57 Attempted Synthesis of Spiro-Ethers 67 Synthesis of Hydrinane Derivatives 73 Attempted Synthesis of Bridged Bicyclo Compounds 97 A Reappraisal of Regiochemistry 105 EXPERIMENTAL SECTION 111 Appendix 171 References 176 3 To my parents 4 ACKNOWLEDGEMENTS First and foremost, I would like to thank my supervisor Willie Motherwell for his encouragement, infectious enthusiasm and constant stream of ideas. To all my friends, past and present, of the Barton, Whiffen and Perkin labs., I would like to say a big thank you for all their help and advice and for providing a pleasant working atmosphere. Particular thanks go to my contempories, Matt and Graham for their close friendship over the last six years. Thanks are also due to Mike and Botty for their proof reading, often carried out at short notice. I am grateful to John Bilton and Geoff Tucker for mass spectra, Paul Hammerton and Dick Sheppard for assistance with high field nmr and Ken Jones for microanalyses. Finally, I would like to thank Quest International for their financial support of this case award and also everyone in the Research Section who made my three month stay there so enjoyable. 5 ABBREVIATIONS 5N a, a'- Azobisisobutyronitrile Ar Aryl b.p. boiling point Bu Butyl t-Bu tertiary-butyl DCM Dichloromethane d doublet dd double doublet ddd double double doublet DIBAL Di-iso-butylaluminium hydride DMAP Dimethylamino pyridine DME Dimethoxy ethane DMSO Dimethyl Sulphoxide Et Ethyl g.c. gas chromatography I.r. Infra-red LDA Lithium diisopropylamide m miltiplet M Methyl m.p melting point nmr nuclear magnetic resonance NOE Nuclear Overhauser Effect Ph Phenyl q quartet s singlet 6 t triplet tert tertiary THF Tetrahydrofuran TBDMS tert-butyl-dimethylsilyl TMS Trimethylsilyl t.I.c. thin layer chromatography 7 A REVIEW OF PREPARATIVE FREE RADICAL REACTIONS INTRODUCTION The comparatively recent resurgence in interest in preparative free radical chemistry can be attributed, at least in part, to the development of a wide variety of new and mild procedures for the generation of carbon centred radicals1-4 ( Scheme 1 ). R—Cl ( Br, I, HgX, CoX ) ( X = -SMe, -OPh,— N ^ N ) \ = J R-Z Y# = n-Bu3Sn Z = H or Z = H, OH, halogen for thiohydroxamates Scheme 1 V. Substitution or functional group interconversion reactions, by free radical means, such as the deoxygenation of alcohols, or the radical variant of the Hunsdfecker reaction in which carboxylic acids are transformed into bromides using acylthiohydroxamates2 are already well developed and widely used in synthesis. Similarly, intermolecular addition reactions5 have been extensively studied and subsequently applied to synthesis ( Scheme 2 ). Bu3SnH CfiHnBr + HnC6 CN ‘CN 95% Bu3SnH C6HnBr + Hn C6 C02Me C02Me 85% Intermolecular Addition Reactions S ch e m ^2 j Radical rearrangements, although well known and thoroughly investigated from a physical standpoint, have, with the obvious exception of the now ubiquitous free radical intramolecular addition or cyclisation process, not been exploited to nearly the same extent. This point may be succinctly illustrated by the excellent reviews of Wilt6 and Beckwith and Ingold7, both of which provide exhaustive coverage of radical rearrangements with the Beckwith and Ingold review containing a large amount of useful kinetic data. It is 9 immediately apparent on inspection however, that both reviews contain very few examples of radical rearrangements which have been specifically designed and applied to organic synthesis. Very recently however, and in particular during the course of this thesis, considerable interest has been aroused by the potential for the incorporation of well understood and predictable radical rearrangements into useful synthetic sequences. Accordingly, this Group Transfer • • A-B-Cn-D w B-Cn-D—A Ring Opening and Ring Closure Isomerisation H H \c—cn—c • ^ c—cn—c• / Inversion Atom Transfer Scheme 3 review sets out to update the reviews of Wilt and Beckwith and Ingold and in so doing, to illustrate and emphasise how the foundation work 10 of the Physical chemist is now being successfully exploited by his synthetic organic counterpart. Beckwith and Ingold have classified radical rearrangements into four categories ( Scheme 3 ) ring opening, ring closures, group transfers and isomerisations which include inversions, rotations and atom transfers. This entirely valid classification is also used in this present review though isomerisations have seen little development over the last decade and will not be discussed further. 11 RING OPENING PROCESSES a) The cvclopropvl carbinvl radical rearrangement P>— Me k = 108 M'1 s'1 Cyclopropyl Carbinyl Radical Rearrangement t Scheme 4 11 This rearrangement was first observed by Roberts and Mazur8. Ironically, they were interested in the cyclopropyl carbinyl-cyclobutyl carbocation rearrangement but when they tried to prepare cyclopropyl carbinyl chloride by photochlorination of methylcyclopropane, they isolated as the major product, allyl carbinyl chloride ( Scheme 4). It was some time however before the classical radical mechanism of the rearrangement was established. The kinetics of both the ring opening and ring closure processes have been extensively studied, generally by steady state e.p.r. techniques, even to the present day9'12. Scheme 5 Because of the different pathway from the cyclopropyl carbinyl- cyclobutyl carbocation rearrangement and the rapid but precisely measured rate constant for ring opening, the cyclopropyl carbinyl radical rearrangement has been widely used as a mechanistic probe13'15. Thus, by way of illustration, Baldwin and co-workers16 have used this rearrangement recently to probe for radical intermediates during the biosynthesis of Penicillin ( Scheme 5). The product obtained from this transformation suggested the intermediacy of a radical species or labile Fe-C intermediate during C-S bond formation. In principle, cyclopropyl carbinyl radical rearrangements with polysubstituted cyclopropanes may give rise to both regio- and geometrical olefin isomers through homolysis of either of the available cyclopropane bonds. Thus, considerable attention17-21 has been focussed on whether regioselectivity can be obtained in such circumstances. Generally however, unless there is an element of symmetry17 in the molecule, mixtures of compounds are obtained although somewhat surprisingly, the ratios of these compounds can vary quite considerably depending upon the exact nature of the starting material. For disubstituted cyclopropanes, Davies and Blum18-19 have found that product ratios are not only sensitive to the radical precursor but also whether the two groups are gis. or trans to each other ( Scheme 6 ). Thus, the oi§. substituted cyclopropanes give predominantly products resulting from homolysis of the cyclopropyl bond generating the lower energy secondary radical regardless of reaction conditions. 13 ,B r VV * cis 83 : 17 (kinetic trapping) trans (kinetic trapping) 34 : 66 trans 92 : 8 (thermodynamic trapping) Scheme 6 ) However, the trans cyclopropane opens predominantly via homolysis of the cyclopropyl bond that gives the higher energy primary radical under kinetic trapping conditions while under equilibrating thermodynamic conditions the major products arise from homolysis of the other cyclopropyl bond giving the secondary radical. These observations have not yet been satisfactorily explained. The radical ring opening of gem-difluorocyclopropanes does provide an example of both regioselectivity of bond homolysis and control of olefin geometry20 ( Scheme 7 ). The regioselectivity arises from the bond strengthening of the two cyclopropane bonds attached to the aem-difluoro unit so that cleavage of the other, weaker cyclopropane bond occurs exclusively
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