A heterobimetallic mechanism for c-h borylation elucidated from experimental and computational data

Abstract

To understand better how homogeneous catalysts comprised of two base metals can mimic precious metal catalysts, we have elucidated a complete mechanistic pathway for C-H borylation with Cu-Fe catalysts that is consistent with experimental observations as well as first-principles quantum chemistry. The catalytic cycle begins with the B-H bond of the borane inserting into the Cu-Fe bond of the catalyst, followed by bimetallic oxidative B-H activation and release of the NHC-bound Cu-H group. After UV irradiation, release of CO permits the inner-sphere Fe coordination of a solvent arene molecule, which then undergoes C-H borylation via a concerted, 4-centered transition state. The resulting iron-hydride can undergo bimetallic reductive elimination with the Cu-H partner to form H 2, closing the catalytic cycle. Analysis of fragment charges during these processes confirms that the bimetallic reaction pathways resemble oxidative addition and reductive elimination steps. Spectroscopic studies are included to probe the nature of the unsupported Cu-Fe bonds of the catalyst in solution. This extensive experimental and computational investigation provides useful insight into canonical organometallic reaction mechanisms involved in bimetallic catalysts, which are generally less well understood than their monometallic counterparts.