Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)_2 and Discovery of Fe(OTf)_3 as a Highly Effective Cocatalyst

摘要

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 Pd~(II) 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 Pd~(II) 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.