Lewis Base Activation of Lewis Acids: Group 13. In Situ Generation and Reaction of Borenium Ions

A variety of Lewis bases were combined with 9-BBN-NTf₂ to establish the requirements for the generation of borenium cations. Five different types of behaviors were found, but the most interesting was the combination of Et₃N, DABCO, 2,6-lutidine, or Ph₃PS, which formed borenium ions exclusively even in sub- or superstoichiometric quantities. The 9-BBN borenium ion complex of 2,6-lutidine rapidly catalyzes the hydrosilylation of a variety of ketones in the presence of Et₃SiH. Preliminary mechanistic experiments suggest that the reduction involves borenium ion activation of Et₃SiH and not the ketone.