g em-Silylborylation of an sp Carbon:  Novel Synthesis of 1-Boryl-1-silylallenes

Novel synthesis of 1-boryl-1-silylallenes involving gem-silylborylation of 3-chloro- or 3-alkoxyalkyn-1-yllithiums with (dimethylphenylsilyl)(pinacolato)borane has been established. The reaction proceeds via 1,2-migration of the silyl group from the negatively charged boron atom of an intermediary borate complex to the terminal acetylenic carbon and is accelerated by the addition of chlorotrimethylsilane in the case that methanesulfonyloxy is employed as a leaving group. Furthermore, axially enantioenriched products could be prepared from mesylates of optically active propargylic alcohols.