Highly Active and Selective Semihydrogenation of Alkynes with the Palladium Nanoparticles‐Tetrabutylammonium Borohydride Catalyst System

Palladium nanoparticles are prepared from palladium(II) acetate and 2 equivalents of potassium tert‐butoxide in the presence of 4‐octyne. The palladium nanoparticles‐tetrabutylammonium borohydride system shows excellent catalytic activity and selectivity in the semihydrogenation of alkynes to the [(Z)‐]alkenes. The hydrogenation of 4‐octyne is conducted with the catalyst system at a substrate‐to‐palladium molar ratio of 10,000–200,000 under 8 atm of hydrogen to give (Z)‐4‐octene in>99% yield. Isomerization and over‐reduction of the Z‐alkene are very slow even after consumption of the alkyne.