Experimental and Computational Evidence for a Boron-Assisted, Sigma-Bond Metathesis Pathway for Alkane Borylation

Photoejection of one CO ligand from isolated CpM(CO)_n+1BR₂ (n = 1:  M = Fe, Ru; n = 2:  M = Mo,W; R₂ = catecholate or pinacolate) compounds produces a coordinatively unsaturated 16 e^- intermediate, a cyclic dioxaboryl transition metal complex, that can efficiently and selectively initiate regioselective C−H bond activation and can be used in the functionalization of alkanes. This chemistry appears distinct from that reported previously for related CpM(CO)_n complexes of alkyl and aryl ligands. We show here by a combination of experimental and theoretical studies that the “unoccupied” p orbital of dioxaboryl ligands are intimately involved in the C−H bond activation step and that this hydrogen transfer to boron occurs by a boron-assisted, metal-mediated σ-bond metathesis. The “unoccupied” p orbital of boron lowers the energy of the transition state and the intermediates by accepting electron density from the metal. The metal-bound borane then rotates, transfers back through a σ-bond metathesis to capture the alkyl, and leaves the metal hydride. Read more on publisher's site.