Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst.

Copper salts find widespread use in Pd-catalyzed oxidation reactions, and they are typically used as oxidants or redox-active cocatalysts. Here, we probe the origin of a dramatic acceleration effect of Cu(OTf)2 in the C-H/C-H aerobic oxidative coupling of o-xylene. NMR spectroscopic analysis of the PdII catalyst in the presence of Cu(OTf)2, together with other experimental and DFT computational studies of the catalytic reaction, show that Cu(OTf)2 activates the PdII catalyst for C-H activation via a non-redox pathway and has negligible impact on catalyst reoxidation. These observations led to the testing of other metal triflate salts as cocatalysts, the results of which show that Fe(OTf)3 is even more effective than Cu(OTf)2.