Reactions of γ-bromo-α,β,γ,δ-unsaturated acylsilanes with KCN under phase-transfer catalyst conditions using n-Bu4NBr afforded 2-cyano-2-siloxyvinylallenes via a tandem process that involves a nucleophilic attack of a cyanide ion and a Brook rearrangement induced conjugate vinylic 1,4-elimination. Use of a chiral cyanide ion source, derived from KCN and quaternary ammonium bromide derived from cinchona alkaloids, provided nonracemic allene derivatives. Based on this result and the reaction using a chiral hydride ion source, we propose a reaction pathway in which a Brook rearrangement mediated vinylic conjugate 1,4-elimination occurs in a syn alignment between the C-Br bond and C-Si bond in the silicate intermediate.
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