<i>E</i> -Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H ₂ Directly Employed or In Situ-Generated

Abstract

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst <i>fac</i>-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-<i>iso</i>-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield <i>E-</i>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 <i>Z-</i>isomer undergoes fast isomerization to afford the thermodynamically more stable <i>E-</i>isomer.