Borenium-Catalyzed Hydroborations of Silyl-Substituted Alkenes and Alkynes with a Readily Available N-Heterocyclic Carbene-Borane

Borenium-catalyzed hydroboration reactions of a stable, readily available N-heterocyclic carbene-borane with allyl-, alkenyl-, and alkynylsilane substrates provides either standard 1,2-hydroboration products or rearranged 1,1-hydroboration products, depending on the structure of the substrate. A competent catalyst can be generated in situ by addition of bis(trifluoromethane)sulfonimide or diiodine. In a typical 1,2-hydroboration, reaction of 1,3-dimethylimidazol-2-ylidine borane (diMe-Imd-BH3) with 1,2-bis(trimethylsilyl)ethene provides 1,3-dimethylimidazol-2-ylidine (1,2-bis(trimethylsilyl)ethyl)borane (diMe-Imd-BH2CH-(TMS)CH2TMS)) as a stable product. In a typical 1,1-hydroboration, the reaction of diMe-Imd-BH3 with bis(trimethylsilyl)-ethyne provides 1,3-dimethylimidazol-2-ylidine-bis(2,2-bis(trimethylsilyl)ethenyl)borane (diMe-Imd-BH(CH=C(TMS)(2))(2) again as a stable product.