Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO(2-700)) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO(2-700) yielded 1-butene with ≥97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy(3)) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO(2-700) provided a selectivity to 2-butene in the range of 72–100% depending on the reaction conditions, while the catalyst modified with PCy(3) again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2–0.6% for unmodified Cu/SiO(2-700) and ≥2.7% for Cu/SiO(2-700) modified with PCy(3), highlighting the effect of surface modification with the tricyclohexylphosphine ligand.