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.