<i>Z</i>-Selective Semihydrogenation of Alkynes Catalyzed by a Co(I)PC_NHCP Pincer Complex: A Simple, Fast, and Practical Methodology

The stereoselective synthesis of alkenes from their corresponding internal alkynes is a highly desirable and atom-economical reaction that is typically performed via transfer hydrogenation. Very few practical methods have been developed that allow for the direct hydrogenation of alkynes under mild reaction conditions and with high stereoselectivities. Here, we demonstrated that a well-defined cobalt catalyst [(PCNHCP)Co(N2)][BAr4F] (1) catalyzes the semihydrogenation of alkynes under mild conditions (25 °C; 1 bar H2) and with high stereoselectivities (>99:1). The reaction does not require any additives, is fast (<60 min), and is compatible with a variety of functional groups that include aldehydes, ketones, esters, and alcohols. Even natural products can be used with our hydrogenation protocol highlighting its versatility. Our mechanistic studies indicate the presence of a transient Co(III)-dihydride that is responsible for the hydrogenation of the herein reported internal and terminal alkynes.