Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water.

A base-assisted metal species modulation mechanism enables Ni-catalyzed stereodivergent transfer semihydrogenation of alkynes with water, delivering both olefinic isomers smoothly using cheap and nontoxic catalysts and additives. Different from most precedents, in which E-alkenes derive from the isomerization of Z-alkene products, the isomers were formed in orthogonal catalytic pathways. Mechanistic studies suggest base as a key early element in modulation of the reaction pathways: by adding different bases, nickel species with disparate valence states could be accessed to initiate two catalytic cycles toward different stereoisomers. The practicability of the method is showcased with nearly 70 examples, including internal and terminal triple bonds, enynes and diynes, affording semi-hydrogenated products in high yields and selectivity.