We report the first case of a stable TiO₂ nanorods-supported CoPc heterogeneous catalytic system for highly selective generation of quinclorac. Due to the strong interaction between CoPc and TiO₂, the Co²⁺/Co³⁺ redox cycles could be maintained. The catalytic oxidation reaction follows the free radical mechanism, while the more Co²⁺/Co³⁺ redox cycles efficiently facilitate the catalyzed pathway.

Abstract

Quinclorac is important precursor for pharmaceutical, agricultural, and synthetic chemistry. The state-of-art synthesis of quinclorac via condensation, chlorination and oxidative hydrolysis often uses homogeneous catalysts and strong acid oxidant agents to promote the catalytic oxidation, which requires huge manpower input for the late-stage purification process and is usually environmentally unfriendly. In this work, we successfully fabricated a stable cobalt phthalocyanine (CoPc) Co-based composite (CoPc/TiO₂) by anchoring CoPc on the surface of TiO₂ nanorods for the selective oxidation of 3,7-dichloro-8-dichloro methyl quinoline (3,7-D-8-DMQ) into quinclorac. More impressively, CoPc/TiO₂-2.5 %-Mn-Br exhibits a high selectivity of 91.9 % for the catalytic oxidation of 3,7-D-8-DMQ to quinclorac in acetic acid, with a quinclorac yield of 86.4 %, which is approximately 2.43 times higher than that of pristine CoPc-Mn-Br. The obtained heterogeneous catalytic system shows good reusability. Detailed mechanistic investigations reveal that the system works through a free radical mechanism via the formation of Co²⁺/Co³⁺ redox cycles. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts for selective catalytic oxidation.