Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane

The reaction mechanism of the rhodium - phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C - H bond activation have been examined. It was found that [Rh(PR3)(2)(H)] is the active species which oxidatively adds the C - H bond leading to an eta(3)-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)(2)(eta(3)-benzyl)(H)(Bpin)] complex from which B - C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)(2)(boryl)(3)-based catalysts, for which Ir(III) - Ir(V) cycles have been proposed, the Rh(I) - Rh(III) cycle is operating with the system addressed herein.