Transfer semihydrogenation of alkynes catalyzed by a zero-valent palladium N-heterocyclic carbene complex

Beyond Lindlar and without hydrogen: Transfer hydrogenation of internal alkynes catalyzed by a palladium(0) catalyst containing an N-heterocyclic carbene ligand gives Z alkenes without overreduction to alkanes (see scheme). Contrary to most transfer hydrogenations, ketones are not reduced. As such, this is the first catalyst that shows excellent stereo- and chemoselectivity for the semihydrogenation of alkynes without the need for hydrogen gas.