Stoichiometric and catalytic isomerization of alkenylboranes using bulky Lewis bases.

1,1-Carboboration of alkynes with boranes R'B(C6 F5 )2 (R' = (CH2 )3 Ph, C6 F5 ) affords mixtures of the E and Z isomers of RCH = CR'B(C6 F5 )2 (R = (CH2 )2 Me, (CH2 )3 Me, (CH2 )2 Ph, CH2 CHMe2 , CHMe2 , CMe3 , CH2 OMe, CH2 N(CO)2 C6 H4 ), treatment of these alkenylboranes with stoichiometric or catalytic amounts of tBu2 PH or other donors are shown to induce isomerization. The proposed mechanism involving a zwitterionic borataalkene intermediate is supported by computations.