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Sulfur-Containing Radicals: from EPR Spectroscopic Studies to Synthetic Methodology

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Sulfur-Containing Radicals: from EPR Spectroscopic Studies to Synthetic Methodology Sulfur-containing Radicals: From EPR Spectroscopic Studies to Synthetic Methodology A Thesis Presented to the University of London in Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy Alistair John Fielding February 2002 Christopher Ingold Laboratories Department of Chemistry University College London London WCIH OAJ UCL ProQuest Number: 10015687 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10015687 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 7 believe a leaf of grass is no less than the journey-work of the stars. ” Walt Whitman This thesis is dedicated to my late Auntie, Doreen Latter. 1930-2001 ABSTRACT A variety of novel homochiral silanethiols have been prepared and investigated as polarity-reversal catalysts for the enantioselective hydrosilylation of a prochiral alkene. The enantiomeric excesses of the products were disappointingly small. A computer modelling procedure, based on molecular mechanics, was applied in order to understand the results and quantitative agreement with experiment was reasonably good. Silanethiols have been oxidised to the corresponding disulfides and these have been investigated as photochemical sources of silanethiyl radicals. Electron paramagnetic resonance (EPR) spectroscopy has been used to study the reactivity of these silanethiyl radicals and the results have been compared with the known chemistry of alkanethiyl radicals. Competitive hydrogen-atom abstraction from, and thiyl radical addition to, propene and cyclopentene, together with addition to phosphines, alkyl isocyanides and the Sh2 dealkylation of trialkylboranes have all been studied. The radical-chain isomérisation of allyl silyl ethers takes place on hearing in the presence of an initiator and an arenethiol as a protic polarity-reversal catalyst. Various thiols were tested and pentafluorothiophenol was found to be particularly effective in this role. It has been shown that thermodynamic equilibrium between isomers is established under the conditions used for these experiments. This methodology has also been applied to the isomérisation of allyl alkyl ethers to alkyl vinyl ethers. As a model for the corresponding reactions of thiyl radicals, EPR spectroscopy has been used to determine the relative molar rate constants for abstraction of allylic and benzylic hydrogens from cyclic and acyclic acetals by rgyf-butoxyl radicals using competition experiments. Absolute rate constants and Arrhenius activation parameters for p-scission of R^O(R^O)CR^ have been determined by a steady-state EPR method and the results can be understood in terms of angle-strain and stereoelectronic effects. Calculations using density functional theory have been carried out in support of this work and the agreement between computed and experimental results is remarkably good. ACKNOWLEDGEMENTS I would like to express my sincere thanks to my supervisor, Professor Brian Roberts, for his continual guidance and patience during the course of this project. I would also like to thank my co-workers past and present. Dr Y. Cai, Dr H.-S. Dang, Dr. M B. Hague, Dr K.-M. Kim and Shree. Special thanks goes to Teika for all her help and friendship. It would not have been possible without the sustained support given by various technical staff. I am grateful to John Hill, David Knapp, Alan Stones, Dick Waymark and Charles Willoughby for their skills. I am also indebted to the Engineering and Physical Sciences Research Council for provision of a maintenance grant. Finally, I would like to thank my parents for their constant support. CONTENTS ABSTRACT 4 ACKNOWLEDGEMENTS 6 CHAPTER 1 GENERAL INTRODUCTION: THE CHEMISTRY OF THIYL RADICALS 13 1.1 Generation of thiyl radicals 13 1.2 Hydrogen-atom abstraction by thiyl radicals 14 1.3 Addition of thiyl radicals to carbon-carbon multiple bonds 14 1.4 Reactions of alkyl thiyl radicals at coordinatively unsaturated centres 16 1.4.1 Phosphorus(III) compounds 16 1.4.2 Trialkylboranes 16 1.4.3 Isocyanides 17 1.5 Thiols as protic polarity-reversal catalysts 17 1.6 REFERENCES 20 CHAPTER 2 SYNTHESIS OF SILANETHIOLS AND THEIR APPLICATIONS AS POLARITY-REVERSAL CATALYSTS 2.1 INTRODUCTION 22 2.1.1 Preparation of organosilanethiols 22 2.1.1.1 Preparation of silanethiols by a heterolytic mechanism 22 2.1.1.2 Preparation of silanethiols by a radical-chain mechanism 22 2.1.2 Applications of organosilanethiols 23 2.1.2.1 Silanethiols as H 2S equivalents 24 2.1.2.2 Silanethiols as reducing agents 24 2.1.2.3 Silanethiols as protic polarity-reversal catalysts 24 2.1.3 Aims of the project 26 2.2 RESULTS AND DISCUSSION 27 2.2.1 Effect of Lewis acids on enantioselective hydrosilylation 28 2.2.2 Computer-assisted design of homochiral thiols as catalysts for 31 enantioselective hydrogen-atom transfer reactions 2.2.2.1 The molecular mechanics method 31 2.2.22 Parameterisation of the force field 32 2.2.2.3 Calculation of enantiomeric excess 33 2.2.2.4 Modelling of hydrogen transfer reactions using homochiral silanethiols 34 2.2.2.5 Problems with this approach 40 2.3 CONCLUSION 40 2.4 EXPERIMENTAL 41 2.4.1 General procedures 41 2.4.2 Materials 41 2.4.2.1 Di-tgrf-butyl hyponitrite (TBHN) 41 2.4.2.2 4,4-Dimethyl-5-oxohexanenitrile 42 2.4.2.3 4,4-Dimethyl-5-oxohexanoic acid 43 2.4.2.4 5,5-Dimethyl-6-methylenetetrahydropyran-2-one 43 2.4.2.5 Typical procedure for hydrosilylation 44 2.4.2.6 rerr-Butyldiphenylsilanethiol 45 2A.2.1 Tri-orr/io-tolylsilane 45 2.4.2.8 T ri-<7At/io-tolylsilanethiol 46 2.4.2.9 T ri-( 1/?, 25,5R)-menthoxy silanethiol 47 2.4.2.10 Tri-(15,2R,55)-menthoxysilanethiol 48 2.4.2.11 Tri-( 15,2R, 5/?)-menthoxy silanethiol 49 2.4.2.12 Tri-( 15,25,5R)-menthyloxysilanethiol 50 2.4.2.13 Tri- [( \S)-endo\-bomyloxy silanethiol 50 2.4.2.14 Di- [( 1 S)-endo\-homy loxypheny Ichlorosilane 52 2.4.2.15 Di- [( 1 S)-endo\-bomy loxypheny Isilanethiol 53 2.4.2.16 Tri-(lR,2R,3R,55)-isopinocampheyloxysilanethiol 54 REFERENCES 552.5 CHAPTER 3 GENERATION AND REACTIONS OF SILANETHIYL RADICALS STUDIED BY EPR SPECTROSCOPY 3.1 INTRODUCTION 57 3.1.1 Addition of alkyIthiyl radicals to alkenes 57 3.1.2 Thiophosphoranyl radicals 57 3.1.3 The reactions of thiyl radicals with trialkylboranes 58 3.1.4 Addition of alkylthiyl radicals to alkyl isocyanides 59 3.1.5 Radical-chain reactions involving silanethiyl radicals 59 3.1.6 Aims of the research 60 3.2 RESULTS AND DISCUSSION 61 3.2.1 Preparation of XaSiSSSiXs 61 3.2.2 UV/Visible spectra 61 3.2.3 Addition of silanethiyl radicals to ethene 61 3.2.4 Reaction of silanethiyl radicals with cyclopentene 66 3.2.5 Addition of silanethiyl radicals to propene 70 3.2.6 Abstraction of weakly-bound hydrogen 73 3.2.7 Addition of silanethiyl radicals to triethyl phosphite 73 3.2.8 Reactions of silanethiyl radicals with tributylphosphine 76 3.2.9 Addition of silanethiyl radicals to tris(dimethylamino) phosphine 78 3.2.10 The reactions of silanethiyl radicals with trialkylboranes 81 3.2.11 Addition of silanethiyl radicals to isocyanides 85 3.2.12 Addition of silyl radicals to isothiocyanates 90 3.3 CONCLUSION 92 3.4 EXPERIMENTAL 93 3.4.1 EPR Spectroscopy 93 3.4.2 Materials 94 3.4.2.1 T riisopropylsilanethiol 95 3.4.2.2 Bis(triisopropylsilyl) disulfide 95 3.4.2.3 Tri-terr-butoxysilanethiol 96 3.4.2.4 Bis(tri-rerr-butoxysilyl) disulfide 96 3.4.2.5 Bis(rert-butyldiphenylsilyl) disulfide 97 3.4.2.Ô Tributylphosphine sulfide 98 3.5 REFERENCES 99 CHAPTER 4 ISOMERISATION OF ALLYL SILYL ETHERS TO SILYL ENOL ETHERS UNDER CONDITIONS OF POLARITY-REVERSAL CATALYSIS 4.1 INTRODUCTION 101 4.1.1 Preparation of silyl enol ethers 101 4.1.1.1 Preparation from enolates 101 4.1.1.2 Preparation from hydrosilanes 102 4.1.1.3 Preparation by transition-metal catalysed rearrangement of an allyl 102 silyl ether 4.1.2 Uses of silyl enol ethers 103 4.1.3 Aims of research 104 4.2 RESULTS AND DISCUSSION 105 4.2.1 Isomérisation of rerr-butyldiphenyl(2-methylallyloxy)silane 105 4.2.2 Inhibitor studies 105 4.2.3 Pentafluorothiophenol as catalyst 110 4.2.4 Electron paramagnetic resonance spectroscopy of siloxyallyl radicals 110 4.2.5 Isomérisation of other primary allyl silyl ethers 117 4.2.6 Isomérisation of Z-1 -rerr-butyldiphenylsiloxypropene to show 118 thermodynamic control 4.2.7 Relative stabilities of (£)- and (Z)-l-rerr-butyldiphenylsiloxypropene 119 4.2.8 Isomérisation of 16 to show thermodynamic equilibrium with 14 120 4.2.9 Isomérisation of secondary allyl silyl ethers 123 4.2.10 Photochemically initiated isomérisation of allyloxy-?er/-butyl- 128 diphenylsilane 4.2.11 Isomérisation of allyl alkyl ethers to alkyl vinyl ethers 132 4.2.12 Alternatives to thiols as polarity-reversal catalysts 135 4.3 CONCLUSION 136 4.4 EXPERIMENTAL 137 4.4.1 General procedures 137 4.4.2 Materials 137 4.4.3 Preparation allyl silyl ethers and allyl alkyl ethers 137 4.4.3.1 ^err-Butyl-(2-methylallyloxy)diphenylsilane 137 4.4.3.2 Allyloxy-?eAt-butyldiphenylsilane 138 4.4.3.3 fert-Butyl(3-methylbut-2-enyloxy)diphenylsilane 138 4.4.3.4 1 -rerr-B utyldimethyl(3 -phenylally loxy) silane 139 4.4.3.5 [(l/?,55)-6,6-Dimethylbicyclo[3.1.1]hept-2-en-2-ylmethoxy]- 139 trimethylsilane 4.4.3.Ô [( l/?,55)-6,6-Dimethylbicyclo[3.1.
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