An Alkene-Promoted Borane-Catalyzed Highly Stereoselective Hydrogenation of Alkynes to Give Z- and E-Alkenes

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z-alkenes, the hydrogenation of alkynes to give E-alkenes is still not well resolved. In particular, selective preparation of both Z- and E-alkenes by the same catalytic hydrogenation system using molecular H-2 has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C6F5)(2)- catalyzed metal-free hydrogenation of alkynes was adopted. Significantly, both Z- and E-alkenes can be furnished by hydrogenation with molecular H-2 in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H-2 play an essential role in promoting the hydrogenolysis reaction.