A phosphonium-based deep eutectic solvent promotes the stereoselective semi-reduction of internal alkynes to ( Z )-alkenes.

Treatment of internal alkynes with Al powder, Pd/C and basic water in a methyltriphenylphosphonium bromide/ethylene glycol eutectic mixture resulted in highly chemoselective formation of ( Z )-alkenes in a yield of up to 99% with Z / E stereoselectivity ranging from 63/37 to 99/1. The unusual catalytic activity of Pd/C is thought to be modulated by the in situ formation of a phosphine ligand.