Highly selective oxidation of 3,7-dichloro-8-dichloro methyl quinoline toward quinclorac by CoPc-Mn-Br catalytic system

Abstract

Nowadays, the use of quinoline derived synthons as precursors of quinclorac products is an important research topic in green chemistry. O2 is a cheap, stable, and eco-friendly oxidant, holding promise for organic oxidation reactions. Herein, we have pioneered the discovery of CoPc-Mn(OAc)2-HBr (CoPc-Mn-Br) catalyst system for the selective oxidation of 3,7-dichloro-8-dichloro methyl quinoline (3,7-D-8-DMQ) into quinclorac with high 3,7-D-8-DMQ conversion at 59.3 %. The CoPc-Mn-Br exhibits excellent selectivity of quinclorac (up to 60.3 %), affording high quinclorac yield of 35.7 %, which is 3.1-fold higher than the yield of Co(OAc)2-Mn(OAc)2-HBr (Co-Mn-Br). Electron paramagnetic resonance (EPR) investigations and density functional theory (DFT) calculations indicate that CoPc promotes the adsorption and activation of O2 in this oxidation process, which generates more Co3+, leading to producing the reactive intermediate species 3,7-dichloro-8-carbonyl quinoline free radicals via the radical process. The insight achieved from this fundamental study may be further applied to other organic syntheses via selective oxidation.