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Enantioselective Hydrosilylation of Prochiral Alkenes Using Homochiral Thiols As Polarity-Reversal Catalysts

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Enantioselective Hydrosilylation of Prochiral Alkenes Using Homochiral Thiols As Polarity-Reversal Catalysts Enantioselective Hydrosilylation Of Prochiral Alkenes Using Homochiral Thiols As Polarity-Reversal Catalysts A Thesis Presented to the University of London in Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy Muhammed Bodrul Hague March 1998 Christopher Ingold Laboratories Department of Chemistry University College London London WCIHOAJ i ...= ] ProQuest Number: U642176 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 U642176 Published by ProQuest LLC(2015). 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 To My Parents Abstract It has been shown that the radical-chain hydrosilylation of alkenes by simple triorganosilanes is promoted by thiols, which behave as polarity-reversal catalysts. The radical-chain hydrosilylation of alkenes of the type H 2C=CR^R^, catalysed by small amounts of homochiral thiol (R*SH), affords fimctionalised organosilanes in moderate to high enantiomeric purity by a mechanism which involves enantioselective hydrogen-atom transfer from the homochiral thiol to a prochiral p-silylalkyl radical [eqn. (i)]. + R*SH --------► I.....k + R*S- (i) CH2SiR3 F^"'^CH2SiR3 First, hydrosilylations of acyclic prochiral alkenes were investigated, then the corresponding reactions of cyclic prochiral alkenes were studied. All hydrosilylation reactions were first carried out using achiral thiol catalysts and then with homochiral thiols. All the homochiral thiols investigated were derived from naturally-occurring homochiral molecules and a number of new enantiomerically-pure thiols have been prepared. The fimctionalised organosilanes obtained, could be oxidatively desilylated to give fimctionalised alcohols and other fimctionalised derivatives. The enantiomeric excesses were generally low at the beginning of this project, but progressively increased as more was understood about the important factors leading to higher enantioselectivities. Enantiomeric excesses of up to 95 % could be obtained in one-pot reactions at 60 °C when using sterically-hindered cyclic prochiral alkenes with the bulky triphenylsilane and catalysed by homochiral monosaccharide carbohydrate thiols. The enantiomeric purities were generally determined by chiral-stationary-phase HPLC analysis, otherwise by NMR analysis using homochiral shift reagents. Enantioselective atom abstraction reactions are relatively rare and the selectivities obtained in the present work are the highest obtained to date. Furthermore, high enantioselectivities can be achieved at relatively high temperature (60 °C). Acknowledgements I would like to thank my supervisor Dr. B.P. Roberts for all the help and advice throughout the project. I would also like to thank the members of the group past and present for making life in the lab pleasant (most of the time). A big thank you goes to all the people in the Department for their time when pointing me in the right direction, especially Dr. H.-S. Dang for useful tips and some initial advice on operating the NMR spectrometers, Steve Corker for help with the HPLC instruments and Dr. J. Cai for his help and advice when I first started. Finally, thank you to my parents for their support for the duration of my studies. Ill Abbreviations Ac acetyl ACHN azobiscyclohexanecarbonitrile AIBN azobisisobutyronitrile ATPH aluminium tris( 2,6-diphenylphenoxide) DEAD diethyl azodicarboxylate Dibal-H diisobutylaluminium hydride DMA? 4-dimethylaminopyridine DMF #,#-dimethylfbrmamide DMPU 1,3-dimethyl-3,4,5,6-tetrahydro-2( 1 H)-pyrimidinone DMSO dimethyl sulphoxide ee enantiomeric excess ether diethyl ether HPLC high performance liquid chromatography HMPA hexamethylphosphoramide IPA isopropyl alcohol LDA lithium diisopropylamide petroleum petroleum spirit 40-60 °C Piv pivaloyl PRC polarity-reversal catalysis NMR nuclear magnetic resonance RT retention time TBAF tetra-M-butylammonium fluoride TBHN di-rerr-butyl hyponitrite THF tetrahydrofuran I f trifluoromethanesulfonyl (trifyl) TfO trifluoromethanesulfonate (triflate) tic thin layer chromatography TTMSS tris(trimethylsilyl)silane Ziram zinc 7V,#-dimethyldithiocarbamate IV Contents Abstract i Acknowledgements ii Abbreviations iii Introduction 1 Radical-chain hydrosilylation of alkenes 2 Aspects of the radical-chain process 2 Thiols as polarity-reversal catalysts 3 Hydrosilylation of alkenes using thiols as polarity-reversal catalysts 4 Other applications of polarity-reversal catalysis (PRC) by thiols 5 Silanes as reducing agents with PRC by thiols 5 Intramolecular hydrosilylation using PRC by thiols 7 Further applications of PRC by thiols 7 Stereoselective radical reactions 8 Enantioselective radical reactions 10 Kinetic resolution 10 Catalytic kinetic resolution: enantioselective hydrogen-atom abstraction by homochiral amine-boryl radicals 11 Enantioselective hydrogen-atom abstraction by homochiral silanethiyl radicals 12 Transfer of chirality to prochiral radicals 13 Catalytic transfer of chirality to prochiral radicals 16 Desilylation. Oxidative cleavage of the carbon-silicon bond 17 Aims of the project 19 Results and Discussion 20 Preparation of d\-tert-hu\y\ hyponitrite (TBHN) 20 Preparation of acyclic prochiral alkenes 21 Hydrosilylation of acyclic prochiral alkenes using ^erf-dodecanethiol as catalyst 22 Standard experimental procedure 24 Table 1: Hydrosilylation of acyclic prochiral alkenes using /-C 12H25SH as catalyst 25 Hydrosilylation of acyclic prochiral alkenes using other potential catalysts 27 Initiator and solvent effects on the standard reaction 30 Table 2: Hydrosilylation of isopropenyl acetate 18 with PhMe2SiH using /-C12H25SH as catalyst 30 Preparation of thiols derived from camphor and from menthol 31 Enantioselective hydrosilylation of acyclic prochiral alkenes using homochiral thiols as catalyst 34 Table 3: Enantioselective hydrosilylation of acyclic prochiral alkenes with PhMe2SiH using homochiral thiols as catalysts 35 Consideration of cyclic prochiral alkenes 36 Preparation of cyclic prochiral alkenes 39 Hydrosilylation of cyclic prochiral alkenes using achiral thiols 42 Table 4: Hydrosilylation of cyclic prochiral alkenes using achiral thiols as catalysts 44 Enantioselective hydrosilylation of cyclic prochiral alkenes using homochiral thiols 46 Table 5: Enantioselective hydrosilylation of lactone 60 at 60 °C using homochiral thiols as catalysts 46 X-ray structure of adduct 84 49 Improvement of enantioselectivity 50 Increasing thiol concentration 50 Low temperature hydrosilylation reactions 50 Lewis acid-mediated hydrosilylation reactions 52 Effects of Lewis acids on enantioselective reactions 53 Table 6: Lewis acid-mediated enantioselective hydrosilylation of prochiral cyclic alkenes 60 and 67 with PhgSiH57 as catalyst 55 Using ATPH as Lewis acid 56 Homochiral carbohydrate-derived thiols 57 Preparation of 2,3,4,6-tetra-O-acetyl-1 -thio-p-D-galactopyranose 95 59 Preparation of glucofiiranose and allofuranose-derived thiols 96 and 97 60 VI Preparation of 2,3,4,6-tetra-O-pivaloyl-1 -thio-P-D-glucopyranose 98 61 Preparation of a- and P-mannose thiols 99 and 100 62 X-ray structure of the P-mannose thiol 100 66 Enantioselective hydrosilylations using homochiral carbohydrate thiols as catalysts 67 Table 7: Enantioselective hydrosilylation of cyclic prochiral alkenes using homochiral carbohydrate thiols as catalysts 68 Table 8: A summary of the determinations of enantiomeric excess and optical rotation 74 Factors governing enantioselectivity for homochiral carbohydrate thiols 75 Control experiments 79 General points 79 Side reactions 80 Desulphurization of thiol catalysts 82 Mixed thiol catalysis 83 Racemization of silane adducts 85 Large scale hydrosilylation reaction 86 Hydrosilylation of miscellaneous alkenes 87 Desilylation 90 Experimental 95 X-Ray crystallography 95 General procedures 96 Preparation of TBHN and prochiral alkenes 97 Preparation of homochiral thiols 104 Hydrosilylation of prochiral alkenes 122 Control experiments 153 Desilylation 157 References 161 Appendix 168 Introduction 1 Introduction Organosilicon compounds play an important role in organic synthesis.^ They have a variety of uses, including silicon-containing perfumes, medicines, adhesives and have been used in the pharmaceutical industry.^ After oxygen, silicon is the most abundant element in the Earth’s crust (28 % by weight).^ Two important types of reaction first observed involving silicon, (a) the direct synthesis of halosilanes which was first reported in 1945"^ and (b) hydrosilylation, were first observed many years ago. Hydrosilylation is the name given to the reaction in which a hydrosilane {e.g. RgSiH) adds to an imsaturated compounds such as an alkene, alkyne, ketone or imine [eqns. (l)-(4)] to give an alkylsilane, vinylsilane, alkoxysilane or aminosilane, respectively.^ The hydrosilylation of alkenes [eqn. (1)] is an important method for the SiR —C = C — + RgSiH H—C—O—SiR] C = N R + RgSiH H—C - N formation of Si-C bonds.^ This hydrosilylation reaction can proceed by a radical-chain mechanism or under the influence of various transition-metal complexes as catalysts.^
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