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)-basedcatalytic systems bearing a redox non-innocent amine functionalizedazo-aromatic pincer-like ligand for the synthesis of quinoline bydehydrogenative oxidation of 2-amino benzyl alcohol and subsequentcoupling with a ketone. All the precatalysts were characterized thoroughlyusing various spectroscopic techniques and DFT studies. The catalyticprotocol was efficient and versatile, providing major advances inthe catalytic synthesis of quinoline. It was also found to be applicableto the gram-scale synthesis of quinolines as well. Several controlexperiments, kinetic studies, and spectroscopic studies have proposeda plausible reaction mechanism where the redox non-innocent azo-pyridinemoiety acts to activate the & alpha;-C-H bond of alcohol andthe subsequent Co(II) & RARR; Co(I) reduction leads to the formationof aldehyde. The generated aldehyde undergoes a base-catalyzed C-Cbond formation reaction to result in the quinolines. The redox potentialof the non-innocent azo-pyridine moiety controls the efficiency ofthe precatalysts. Thus, complex 3 with the most anodicreduction potential of the azo-pyridine moiety was found to be thebest precatalyst among complexes 1-4.