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(C 6 F 5 ) 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.