Amine Functionalized Pincer-like Azo-aromatic Complexes of Cobalt and Their Catalytic Activities in the Synthesis of Quinoline via Acceptorless Dehydrogenation of Alcohols

Herein we disclose a series of phosphine-free cobalt(II)-based catalytic systems bearing a redox non-innocent amine functionalized azo-aromatic pincer-like ligand for the synthesis of quinoline by dehydrogenative oxidation of 2-amino benzyl alcohol and subsequent coupling with a ketone. All the precatalysts were characterized thoroughly using various spectroscopic techniques and DFT studies. The catalytic protocol was efficient and versatile, providing major advances in the catalytic synthesis of quinoline. It was also found to be applicable to the gram-scale synthesis of quinolines as well. Several control experiments, kinetic studies, and spectroscopic studies have proposed a plausible reaction mechanism where the redox non-innocent azo-pyridine moiety acts to activate the α-C–H bond of alcohol and the subsequent Co(II) → Co(I) reduction leads to the formation of aldehyde. The generated aldehyde undergoes a base-catalyzed C–C bond formation reaction to result in the quinolines. The redox potential of the non-innocent azo-pyridine moiety controls the efficiency of the precatalysts. Thus, complex 3 with the most anodic reduction potential of the azo-pyridine moiety was found to be the best precatalyst among complexes 1–4.