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 NHCbound 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.

Bimetallic catalysis for C–C and C–X coupling reactions

“…Furthermore, crossover experiments are consistent with equilibrium concentrations of [(NHC)Cu] + [M CO ] – pairs forming in solution. 99 One derivative, a (NHC)Cu–FeCp(CO) 2 species, was shown to catalyse dehydrogenative borylation of unactivated arenes upon photochemical activation ( Scheme 32 ). 100 Based on stoichiometric reactivity studies and computational analysis, 99 it was proposed that the bifunctional Cu/Fe catalyst reacts with the boron source to generate CpFe(CO) 2 (Bpin) as the active borylating species (HBpin = pinacolborane).…”

Cobalt Pincer Complexes in Catalytic C–H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

“…Such mechanisms at various level of detail have been reported from both experiments and computations. 20,23,33,34,38–45 Furthermore, efforts to elucidate the origin of chemo-, regio-, and stereo-selectivity have also had some success. 4,46–54 Generally, catalytic borylations involve C-H bond cleavage by oxidative addition and C-B bond formation by reductive elimination, routes that utilize the inherent two-electron redox property of heavier transition metals.…”

Mechanistic Studies of Cobalt-Catalyzed C(sp2)–H Borylation of Five-Membered Heteroarenes with Pinacolborane

“…Obtaining insights into the turnover-limiting step and nature of catalyst deactivation pathways in this class of C−H functionalization reaction is crucial for the rational design of next generation catalysts. Information on the speciation of iridium, 17,18 ruthenium, 19 iron, 6a,d heterobimetallic copper, 6c,20 and frustrated Lewis pair (FLP) 21 catalysts in the presence of HBPin have been reported; however, the turnover-limiting step and possible catalyst deactivation pathways have not been identified, and principles for future catalyst development are not evident.…”