Metal–Substrate Cooperation Mechanism for Dehydrogenative Amidation Catalyzed by a PNN-Ru Catalyst

The pyridine-based PNN ruthenium pincer complex (PNN)­Ru­(CO)­(H) can catalyze the well-known dehydrogenative amidation reaction, but the mechanism is not fully understood. In this work, we find there exists an alternative metal–substrate cooperation mechanism in this reaction system, which is more favorable than the aromatization–dearomatization mechanism. The possible reaction of the excess base <i>t</i>-BuO^– with catalyst species (PNN)­Ru­(CO)­(H) is studied, indicating <i>t</i>-BuO^– is able to facilitate the ligand substitution and enhance catalytic activities. With the bifunctional Ru–N moiety, the imine-substituted species (PN)­(imine)­Ru­(CO)­(H) <b>5</b> could serve as an alternative catalytic species and efficiently facilitate some elementary steps such as the hydrogen transfer, hydrogen elimination, and C–N coupling. Meanwhile, the C–N coupling step proceeds via the split of aldehydic C–H bond across the Ru­(II)–imine bond, which results in an amide bond directly. The hemiaminal is uninvolved in the C–N coupling process. Finally, the formation of linear peptides and cyclic dipeptides are unveiled by the newly proposed mechanism. The metal–substrate cooperation could widely exist in transition metal catalyst systems with a large influence on the reaction activity