Semi-reduction of alkynes catalyzed by a tri-palladium complex

Upon activation by ammonia-borane, a tri-palladium complex [(bpnp)2Pd3Cl2]Cl2 (Pd3) was found to be catalytically active for hydrogenation of alkynes to render the corresponding cis-olefins. Typically, a mixture of diphenylacetylene (0.80 mmol), Pd3 (0.2 mol %), and NH3.BH3 (10 mol%) in THF/H2O (10:1, 1 mL) was stirred at 30 °C under H2 (1 atm) for 2 h, giving (Z)-stilbene in 95 % yield with a turnover frequency (TOF) ∼240 h−1. This catalyst is applicable to both internal and terminal alkynes with aryl or alkyl substituents.