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.