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

Abstract 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.