Highly active palladium-based catalyst system for the aerobic oxidative direct coupling of benzene to biphenyl

A highly efficient Pd-containing catalytic system for the intermolecular direct C-H homocoupling of benzene to biphenyl has been developed. The catalytic system was composed of Pd(OAc)₂ with trifluoromethanesulfonic acid (TfOH) as an additive and O₂ as the sole oxygen source in the absence of any metal-containing cocatalyst. An excellent efficiency of Pd^II with the acidic additive was attained in the aerobic oxidation of benzene to biphenyl. A high yield (25.3 %) and selectivity (98 %) were achieved by using a small amount of Pd(OAc)₂ (0.07 mol %) and TfOH, which gave a high turnover number (180) for Pd species. Theoretical calculation by DFT and UV/Vis absorption spectra illustrated that the formation of electropositive Pd^II species in the presence of TfOH contributed to the high efficiency of the catalytic system. The acids have it: A highly efficient Pd-based catalytic system is constructed for the direct oxidative coupling of benzene with dioxygen to biphenyl with the C-H activation strategy. The system exhibits an extremely high turnover number of 180, which is accomplished with the aid of CF₃SO₃H without a metal-bearing cocatalyst.