Pd-catalyzed aerobic oxidative coupling of arenes: evidence for transmetalation between two Pd(II)-aryl intermediates.

Pd-catalyzed aerobic oxidative coupling of arenes provides efficient access to biaryl compounds. The biaryl product forms via C-H activation of two arenes to afford a Pd(II)ArAr' intermediate, which then undergoes C-C reductive elimination. The key Pd(II)ArAr' intermediate could form via a "monometallic" pathway involving sequential C-H activation at a single Pd(II) center, or via a "bimetallic" pathway involving parallel C-H activation at separate Pd(II) centers, followed by a transmetalation step between two Pd(II)-aryl intermediates. Here, we investigate the oxidative coupling of o-xylene catalyzed by a PdX2/2-fluoropyridine catalyst (X = trifluoroacetate, acetate). Kinetic studies, H/D exchange experiments, and kinetic isotope effects provide clear support for a bimetallic/transmetalation mechanism.