Mechanism of the Palladium-Catalyzed Borylation of Aryl Halides with Pinacolborane

With the aid of DFT calculations, we have examined the mechanism of the widely employed Pd-catalyzed borylation of aryl halides using HBpin (pin = OCMe 2 CMe 2 O) developed by Masuda et al. In contrast to their original proposed mechanism involving an ammonium boride ion pair in the transmetalation step that follows aryl halide oxidative addition to Pd(0), our calculations support a transmetalation process that involves σ-bond metathesis between HBpin and a cationic [L 2 Pd(Ar)] + species as the ArBpin product generating step. The new mechanism is also applicable to the related boration reaction employing R 2 NBH 2 reagents developed by Alcaraz et al.