Enantioselective hydrosilylation of prochiral alkenes using homochiral thiols as polarity-reversal catalysts

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 H2C=CR1R2, catalysed by small amounts of homochiral thiol (R*SH), affords functionalised organosilanes in moderate to high enantiomeric purity by a mechanism which involves enantioselective hydrogen-atom transfer from the homochiral thiol to a prochiral β-silylalkyl radical [eqn. (i)]. 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 functionalised organosilanes obtained, could be oxidatively desilylated to give functionalised alcohols and other functionalised 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 1H 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)