Silanes, continued vinyl

Vinyl silanes continue to attract attention as intermediates for the stereoselective synthesis of olefins. Zweifel and Lewis now describe the stereoselective synthesis of both -and Z-(l-halogenoalk-l-enyl)silanes (15) from alk-l-ynyl-silanes, and show how they may then be processed to dialkyl-substituted vinyl-silanes, alkenyl halides, and trisubstituted olefins (Scheme 12). The E-l-halogenoalkenyltrimethylsilanes are readily prepared in high isomeric purity by treatment of the dialkylhydroalumination adduct (14) with N-chlorosuccinimide,... 

The explanation resembles that for the retention of stereochemistry in the reaction of vinyl silanes with electrophiles (chapter 16). To make this clearer, we have drawn the diagrams as nearly the same as those in that chapter as possible. It does not matter which surface of the alkene is attacked by the nucleophile as long the bromine atom continues to rotate in the same direction. Once it reaches the vertical it can be eliminated by the carbanion and the result is retention (<5% of the other isomer). 

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