PNNH-Ru-Catalyzed Highly E-Selective Semi-Hydrogenation of Alkynes Using MeOH as a Hydrogen Donor

Herein, we describe the selective semi-hydrogenation of alkynes utilizing methanol as a safe and readily accessible hydrogen donor, catalyzed by a newly developed PNNH-Ru complex. This approach offers a practical and attractive alternative to traditional hydrogenation methods that depend on pressurized H2 gas. A wide range of alkynes are smoothly converted into (E)-alkenes in high yields, exhibiting excellent stereoselectivity under relatively mild reaction conditions. Detailed kinetic investigations combined with mechanistic analysis reveal that an active ruthenium dihydride species is generated in situ, which plays a pivotal role in driving the transfer hydrogenation process. The presence of both the base and methanol is essential for converting the pre-catalyst into the catalytically active dihydride species. The excellent selectivity observed in the reaction is attributed to the Z-E isomerization of the initially generated alkene, which is promoted by the in situ-formed ruthenium dihydride complex. The method tolerates diverse functional groups, including electron-rich and electron-poor substituents such as halides, esters, and heterocycles, delivering products with high efficiency and stereoselectivity. The reaction proceeds efficiently on a gram scale, highlighting its practical utility.