DFT Study on Mechanism of Ni–Al Bimetallic-Catalyzed C–H Cyclization to Construct Tricyclic Imidazoles: Roles of NHC Ligand and AlMe3

The mechanism of the Ni–Al bimetallic-catalyzed C–H cyclization to construct tricyclic imidazoles is investigated using density functional theory calculations. The calculation result shows that the reaction mechanism involves sequential steps of substrate coordination, ligand-to-ligand hydrogen transfer (LLHT), and C–C reductive elimination to produce the final product tricyclic imidazole. The LLHT step is calculated to be the rate-determining step. The oxidative addition of the benzimidazole C–H bond to the Ni center and the insertion of the alkene into the Ni–H bond occur concertedly in the LLHT step. The effects of N-heterocyclic carbene (NHC) ligands and AlMe3on the reactivity and regioselectivity were also analyzed. These calculation results shed light on some ambiguous suggestions from experiments.