Silicon-Carbon Bond Cleavage from a Hydroboration Sequence

The reactions of internal pyridyl/trialkylsilyl alkynes with the potent hydroboration reagent bis(1-methyl-ortho-carboranyl)borane (HBMeoCb2) results in unanticipated Si-Calkyl cleavage following a regioselective 1,1-hydroboration under mild conditions. The rare Si-Me bond rupture of the trimethylsilyl group occurs by an intramolecular frustrated Lewis pair system forming a silylium heterocycle with a pendant borate bearing the methyl group. Investigating a methyl/ethyl mixed silyl substituent, SiEtMe2, revealed the reaction is completely selective in breaking the Si-Me bond over the Si-Et bond. Exposure of the silylium/borate zwitterion to methanol resulted in O-H bond cleavage and ring opening to a zwitterionic pyridinium/borate. These findings provide insight into the challenging selective functionalization of Si-Calkyl bonds.