Borrowing Hydrogen-Mediated N‑Alkylation Reactions by a Well-Defined Homogeneous Nickel Catalyst, ACS Catal. 2019, 9, 9051−9059

A well-defined, cheap and bench stable azo-phenolate ligand-coordinated nickel catalyst which can efficiently execute N-alkylation of a variety of anilines by alcohol. Redox-active nature of azo ligand can store hydrogen generated during alcohol oxidation and redelivers the same to an in-situ-generated imine bond to result in N-alkylation of amines. Various substrates and functional group were tolerated. Preliminary mechanistic studies including deuterium labeling to the substrate establishes the borrowing hydrogen method from alcohols. Isolation of the ketyl intermediate in its trapped form with a radical quencher and higher k_H/k_D for the alcohol oxidation step suggest altogether a hydrogen-atom transfer (HAT) to the reduced azo backbone to pave alcohol oxidation as opposed to the conventional metal− ligand bifunctional mechanism. This example clearly demonstrates that an inexpensive base metal catalyst can accomplish an important coupling reaction with the help of a redox-active ligand backbone.