Stoichiometric and catalytic isomerization of alkenylboranes using bulky Lewis bases.

1,1-Carboboration of alkynes with boranes R'B(CF) (R' = (CH)Ph, CF) affords mixtures of the E and Z isomers of RCH = CR'B(CF) (R = (CH)Me, (CH)Me, (CH)Ph, CHCHMe, CHMe, CMe, CHOMe, CHN(CO)CH), treatment of these alkenylboranes with stoichiometric or catalytic amounts of tBuPH or other donors are shown to induce isomerization. The proposed mechanism involving a zwitterionic borataalkene intermediate is supported by computations.