Hydroboration of vinyl silanes with bis-(pentafluorophenyl)borane: ground state beta-silicon effects.

Hydroboration of aliphatic vinyl silanes with the highly electrophilic bis- (pentafluorophenyl)borane, HB(C6F5)(2), gives predominantly the thermodynam ically favored regioisomer with boron and silicon on the same carbon. Therm odynamic product mixtures are obtained because equilibration of isomers thr ough boryl migration is facile in the products of hydroboration with HB(C6F 5)(2). The 1,1-substituted isomers are the most stable by virtue of a groun d state beta-silicon effect involving hyperconjugation between the Me3Si gr oup and the electrophilic borane center. More complex thermal rearrangement s are observed when aromatic vinyl silanes are hydroborated with HB(C6F5)(2 ). Experiments using C-13-labelled and para-substitituted substrates provid e mechanistic information on these rearrangements which appear to be driven by the formation of compounds which can engage in the ground state beta-si licon interaction. The mechanistic proposals given are supported by computa tional results performed at the AM1 level, (C) 1998 Elsevier Science Ltd. A ll rights reserved.