Tautomerism vs lone pair metal ligand cooperation in selective Mn-PNN catalyzed hydrogenation of amides

Abstract

Metal-ligand cooperation (MLC) is an important strategy in transition-metal (TM) catalysis. Tautomerism-driven MLC has recently been emerging as a novel type of catalyst design strategy. However, the difference between the conventional lone-pair-driven MLC and tautomerism-driven MLC in mechanism and selectivity remains unclear. Herein, a DFT study was performed to unveil the difference between them in the (iPrPHNN)Mn catalyzed selective hydrogenation of amide. The double deprotonated (iPrPHNN)Mn could lead to an active species with two functional sites, i.e., the N site and the C site, which would promote the lone-pair-driven MLC and the tautomerism-driven MLC for the catalytic hydrogenation, respectively. Systematic investigation of the full catalytic cycles, including hemiaminal formation, hydrogenation of amide, and the generation of alcohol and amine, reveals that the addition of H2 onto amide prefers the lone-pair-driven MLC mechanism assisted by a proton shuttle. Then, the formed hemiaminal intermediate would competitively undergo the C-O bond cleavage to the secondary amine product or the C-N bond cleavage to the primary amine and alcohol products. Both the C-N bond cleavage and the C-O bond cleavage prefer the tautomerism-driven MLC, with the free energy barrier of the former being lower than that of the latter by around 12 kcal/mol. In the tautomerism-driven MLC mechanism, the lone-pair electrons on the innocent amido would increase the electron density of the metal center, which would lead to an easier electron transfer from the Mn center to the substrate, causing the favorable cleavage of the C-N bond. In the following hydrogenation step, the generation of aldehyde to alcohol or the hydrogenation of imine to amine can occur via either the lone-pair-driven MLC or the tautomerism-driven MLC. The driving force of the tautomerism-driven MLC is of key importance for selectivity. The results of the lone-pair-driven MLC and the tautomerism-driven MLC mechanisms in selectivity would be helpful for the design of MLC catalysis.