Ir(III)/Ir(V) or Ir(I)/Ir(III) Catalytic Cycle? Steric-Effect-Controlled Mechanism for the para-C-H Borylation of Arenes

Density functional theory method N12 was used to study the mechanism of the [Ir(cod)OH](2)/Xyl-MeO-BIPHEP-catalyzed para-selective C-H borylation reaction. The results revealed that the use of a bulky diphosphine ligand such as Xyl-MeO-BIPHEP was unfavorable for the previously proposed iridium(III)/iridium(V) catalytic cycle because it resulted in considerable steric repulsion in the hepta-coordinated iridium(V) intermediate. Inspired by this steric effect, we have proposed a novel iridium(I)-/iridium(III)-based catalytic cycle for this transformation and shown that it can be used to account for the experimental results. The iridium(I)/iridium(III) catalytic cycle induced by this steric effect consists of several steps, including (i) the oxidative addition of the C-H bond of the substrate to an: active iridium(I) boryl complex; (ii) the reductive elimination of a C-B bond; (iii) the oxidative addition of B(2)pin(2) to an iridium(I) hydride complex; and (iv) the reductive elimination of a B-H bond. Notably, the computed regioselectivity of this reaction was consistent with the experimental observations. The high para-selectivity of this reaction was also explained using structural analysis and a 2D contour model, which revealed that the strong steric repulsion between the diphosphine ligand and the meta-substituents resulted in a higher energy barrier for meta-C-H activation.