Stereoselective Synthesis Using iVee-Membered Carbon Ring and Silyl Groups

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Reactions of acylsilanes with sulfur methylides proceed with cationotropy or anionotropy of silicon in the betaine intermediate to give the corresponding enolsilanes or β-ketosilanes. The product's ratio can be controlled by manipulating the reaction conditions. The reaction with sulfur ethylides and that with sulfur cyclopropylide result in the preferential formation of the corresponding α,β-epoxysilanes and cyclopropylidene(siloxy)methane derivatives, respectively. In contrast, various acylsilanes react with α-sulfinyl carbanions such as α-lithioalkyl phenyl sulfoxide to give the regiochemically pure enolsilanes produced by cationotropy of silicon to β-oxygen in the α-silyl-β-oxysulfoxide intermediate. The product's selectivity in these reactions depends on the leaving ability of sulfur compounds from the intermediate. In the reaction using sulfur ylides the elimination of neutral sulfur compound from the betaine intermediate occurs prior to the silicon migration, whereas in the reaction with α-sulfinyl carbanions the cationotropy of silicon assists the elimination of sulfenate ion from the β-oxysulfoxide intermediate.On the other hand, rearrangement of cyclopropylsilylcarbinols derived from cyclopropylacylsilanes with TsOH in methanol proceeds stereoselectively to give 4-silylhomoallyl methyl ether. The cyclopropylsilylcarbinols bearing n- or s-alkyl group on the carbinyl carbon produce E-4-silylhomoallyl methyl ethers, and those bearing f-butyl group afford Z-4-silylhomoallyl methyl ethers. E- and Z-4-silylhomoallyl methyl ethers are protiodesilylated with TBAF to yield the corresponding Z- and E-olefins, respectively.The asymmetric reduction of acylsilanes, in the presence of catalytic amounts of borane-(-)- diphenylhydroxymethyl pyrrolidine complex, with borane leads to the corresponding optically active silylcarbinols in quantitative chemical yield and in high enantiomeric excess.