Diboron-Assisted Copper-Catalyzed Z-Selective Semihydrogenation of Alkynes Using Ethanol as a Hydrogen Donor

We herein describe a B(2)Pin(2)-assisted copper-catalyzed semihydrogenation of alkynes. A variety of alkenes were obtained in good to excellent yields with Z-selectivity under mild reaction conditions. Mechanistic studies indicated that a transfer hydrogenation process was involved and ethanol acted as both a solvent and a hydrogen donor in this reaction. The present protocol enabled convenient synthesis of deuterium-substituted Z-alkenes such as Z-Combretastain A4-d(2) in a high deuteration ratio by using readily available ethanol-d(1) as the deuterium source.