Selective Semi-Hydrogenation of Alkynes over Bulk Palladium via Direct Mechanocatalysis

Bulk palladium is identified as a selective catalyst for the semi-hydrogenation of alkynes under mechanochemical conditions. Employing Pd balls or a Pd-coated milling vessel in combination with a simple organic nitrogen base, alkynes are converted selectively to alkenes without additional catalyst modification. The method affords cis-stilbene in 98% selectivity at 97% conversion, exceeding the turnover frequency of the classical Lindlar catalyst while maintaining comparable selectivity. Mechanistic investigations using XPS, DFT calculations, surfaceenhanced IR absorption spectroscopy and kinetic studies suggest the selectivity to be enabled by competitive adsorption, supported by enhanced masstransport through ball milling. This mechanochemical protocol is broadly applicable to terminal and internal, aromatic and aliphatic alkynes, underscoring the potential of bulk Pd as a selective catalyst for semihydrogenation.