Solvent control of <i>E</i> / <i>Z</i> 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 <i>Z</i>-alkenes were achieved with formic acid as the hydrogen source when the reaction was conducted in DMSO, while <i>E</i>-alkenes were obtained in toluene under otherwise identical reaction conditions. Detailed mechanism studies suggested that the alkyne hydrogenation gave <i>Z</i>-alkene intermediates. The <i>E</i>-alkene product was generated by a <i>Z</i> to <i>E</i> alkene isomerization process in toluene, and the <i>Z</i>-alkene was stabilized by employing DMSO as the solvent.