Mechanistic study of the ligand controlled regioselectivity in iridium catalyzed C-H borylation of aromatic imines

Abstract

As a major challenge in C-H borylation, how to control the selectivity has attracted lots of attention, however, the related mechanistic information still needs to be uncovered. Herein, density functional theory (DFT) has been used to study the mechanism for the ligand controlled regioselectivity in the iridium-catalyzed C-H borylation of aromatic imines, which is inspired by experimental observations (R. Bisht, B. Chattopadhyay, J. Am. Chem. Soc., 2016, 138, 84-87). The proposed Ir(i)-Ir(iii) catalytic cycle includes (i) the oxidative addition of the C-H bond to iridium(i); (ii) the reductive elimination of a C-B bond; (iii) the oxidative addition of B₂pin₂ to an iridium(i) hydride complex; and (iv) the reductive elimination of a B-H bond. The oxidative addition of a C-H bond to the iridium center is the determining step. For the ligand AQ, ortho-selectivity is proposed to be attributed to the decreased steric hindrance and increased electron donating effect of AQ (8-aminoquinoline) which promotes proton-transfer in the ortho-transition state of C-H activation. While, for the TMP ligand, the steric repulsion between the TMP (4,5,7,8-tetramethyl-1, 10-phenanthroline) ligand and the ortho-substituted imine hinders the ortho C-H activation and favors meta borylation. Our calculations provide insights into further ligand design to achieve different regioselective borylation of aromatics. Guided by the results, the regioselectivity in the borylation of aromatics may be achieved by accordingly modifying the electronic and steric substituents of the ligand.