Solvent control of E / Z selectivity in palladium-catalyzed semi-hydrogenation of alkynes

A solvent-controlled stereoselective semi-reduction of alkynes with excellent stereoselectivity and chemoselectivity was accomplished by palladium catalysis. Various Z-alkenes were achieved with formic acid as the hydrogen source when the reaction was conducted in DMSO, while E-alkenes were obtained in toluene under otherwise identical reaction conditions. Detailed mechanism studies suggested that the alkyne hydrogenation gave Z-alkene intermediates. The E-alkene product was generated by a Z to E alkene isomerization process in toluene, and the Z-alkene was stabilized by employing DMSO as the solvent.