Selective oxidation of amines using O2 catalyzed by cobalt thioporphyrazine under visible light

Abstract

A highly efficient oxidation of amines including primary and secondary amines into imines using O2 as the oxidant was realized by cobalt tetra(2-hydroxymethyl-1,4-dithiin)porphyrazine (CoPz(hmdtn)4) under irradiation of visible light (λ ≥ 420 nm). Particularly, the photocatalytic selective oxidation of benzylamine into N-benzylidenebenzylamine was thoroughly studied in order to get more insight into the reaction kinetics and mechanism for the oxidation of amine. The photocatalytic oxidation of benzylamine was observed to follow pseudo-first-order reaction kinetics. Moreover, a reasonable linear relationship between the log(kX/kH) values and the Brown-Okamoto constant (σ+) parameters was obtained for oxidation of para- and meta-substituted benzylamines. Besides imine, the presence of other intermediate products such as NH-imine and H2O2 was further evidenced in this photocatalytic system. Additionally, the reactive oxygen species (ROSs) generated in this photocatalytic process were confirmed by electron spin resonance (ESR) technique. The possible photocatalytic transformation pathway of amine into imine was also proposed, involving the dehydrogenation of amine to form NH-imine and the nucleophilic addition of NH-imine by free amine to afford the final imine.