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

Abstract

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 η³-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(η³-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.