Z-alkenes are versatile building blocks in organic synthesis and often found in pharmaceuticals, perfumes, and natural products. Although a number of methods for stereoselective synthesis of Z-alkene have been developed so far, the Lindlar semihydrogeneation of alkynes is a long-standing synthetic transformation and still remains a privileged system. However, it used toxic Pb additive and a large amount of quinolone, and has other limitations such as narrow substrate scope. Therefore, development of a powerful and reliable system for stereoselective semihydrogenation of alkyne to Z-alkene as alternative to the Lindlar catalytic system is still challenging target.

Under these backgrounds, in this work, we have demonstrated electrocatalytic hydrogenation in a proton exchange membrane reactor so called PEM reactor to develop a powerful and reliable stereoselective semihydrogenation system (Figure 1). This system has several merits such as easy separation and recycle use of catalysts, operation under mild and benign conditions, and easy reaction control by applied voltage. Indeed, the use of PEM reactor with Pd catalysts gave Z-alkenes with excellent current efficiencies in our demonstrations.