Stereodivergent Alkyne Reduction by using Water as the Hydrogen Source

A homogeneous Pd‐catalyzed stereodivergent reduction of alkynes to Z and E alkenes by using H 2 O as the H 2 source is presented. Mediated by a diboron reagent, the transfer hydrogenation has been accomplished to yield the desired geometrical isomer by rational ligand selection. The switchable stereoselectivity achieved using simple phosphine ligands is generally excellent. D 2 O has also been used as a D 2 source for synthesizing the corresponding deuterated olefins. Supported by a gram‐scale synthesis, the reaction can easily be scaled up making it an efficient way to prepare alkenes commercially as well. Mechanistic studies suggest formation of H−PdL 2 −OAc as the crucial step leading to the presence of two pathways involving H−Pd−B(OR) 2 and molecular H 2 as active intermediates.