E -Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H 2 Directly Employed or In Situ-Generated.

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac -[Mn(dippe)(CO) 3 (CH 2 CH 2 CH 3 )] (dippe = 1,2-bis(di- iso -propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH 4 with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E- alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z- isomer undergoes fast isomerization to afford the thermodynamically more stable E- isomer.