Catalytic Chemoselective and Stereoselective Semihydrogenation of Alkynes to E-Alkenes Using the Combination of Pd Catalyst and ZnI2

An efficient E-selective semihydrogenation of internal alkynes was developed under low dihydrogen pressure and low reaction temperature from commercially available reagents: Cl2Pd(PPh3)(2), Zn-0, and ZnI2. Kinetic studies and control experiments underline the significant role of ZnI2 in this process under H-2 atmosphere, establishing that the transformation involves syn-hydrogenation followed by isomerization. This simple and easy-to-handle system provides a route to E-alkenes under mild conditions.