Ru-Catalyzed Migratory Geminal Semihydrogenation of Internal Alkynes to Terminal Olefins.

Semihydrogenation of alkynes to alkenes represents a fundamentally useful transformation. In addition to the well-known cis- and trans-semihydrogenation, herein a geminal semihydrogenation of internal alkynes featuring 1,2-migration is described, which provides new access to the useful terminal vinylsilanes. This process also presents a new mode of reactivity of silyl alkynes. With the proper choice of the cationic [CpRu(MeCN)3]PF6 catalyst and a suitable silyl group, both aryl- and alkyl-substituted silyl alkynes can participate in this highly efficient gem-selective process. Furthermore, dedicated condition optimization also allowed switching of selectivity from gem to trans by using a combination of parameters, including the suitable silyl group, additive, and H2 pressure. A systematic DFT study on the reaction mechanism revealed that the formation of the gem-H2 Ru-carbene might be the key intermediate in both gem- and trans-addition reactions, rather than the Ru-vinylidene intermediate. The DFT results were further supported by carefully designed control experiments. This uncommon gem-addition combined with 1,2-silyl migration in the metal-carbene intermediate should open up a new synthetic avenue for alkyne transformations.