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 H2 O as the H2 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. D2 O has also been used as a D2 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-PdL2 -OAc as the crucial step leading to the presence of two pathways involving H-Pd-B(OR)2 and molecular H2 as active intermediates.