Theoretical investigations on the methylation of NH bond using CO2 and hydrosilane catalyzed by ZincII complexes: Mechanism and ligand effect

The detailed catalytic mechanisms of zincII complexes in the methylation of the N−H bond of N-methylaniline with CO2 and hydrosilane (PhSiH3) were studied using density functional theory (DFT) calculations. Among all the examined catalytic reaction pathways, we found that the most favorable pathway is composed of two stages: the formation of the formamide (FA) species and the reduction of the FA species to the target product methylamine (MA). Over the zinc complexes, the FA species formation begins with the reaction of N-methylaniline with CO2, followed by the reduction of CO2 with PhSiH3 to the formoxysilane (FOS) species. The formation of the FOS species prefers to proceed through the “SN2@Si-Acceptor” SiH bond activation mechanism and is predicted to be the rate-determining step (RDS) for the overall reaction. The ammonium carbamate (AC) zwitterion generated in the reaction of N-methylaniline with CO2 is the nucleophilic species to promote SiH bond activation, and the zinc complex is demonstrated to be effective in facilitating the CN bond formation in the reaction of N-methylaniline with CO2 and stabilizing the nucleophilic AC zwitterion. The calculations reveal that the N-heterocyclic carbene (NHC) ligand plays two main roles in the reaction. (i) Preventing the coordination of the FA species to the zinc center and avoiding a decrease in the nucleophilicity of the carbonyl oxygen atom in the FA species, and (ii) lowering the Lewis acidity of the zinc center, which makes the activity of the SiH bond less decreased.