Spectromagnetic investigation of the active species in the oxidation of propenoidic phenols catalysed by [N,N′-bis(salicylidene)ethane-1,2-diaminato]cobalt(II)*

Abstract

The oxidation of propenoidic phenols by molecular oxygen, catalysed by [N,N′-bis(salicylidene)ethane-1,2-diaminato]cobalt(II) [Co(salen)], was studied in different solvents and for variously substituted phenols to find processes alternative to those reported for the removal of polyphenols from waste waters in the paper industry. The reaction of methyl E-4-hydroxy-3-methoxycinnamate (E-methyl ferulate) selectively gave the corresponding 4-hydroxybenzoic acid and 4-hydroxybenzaldehyde; yields are highest in chloroform, good in methanol and very low in pyridine. Conversion was high with E-methyl ferulate, lower with methyl E-4-hydroxycinnamate, while the other phenols, methyl E-3-hydroxy-4-methoxycinnamate (E-methyl isoferulate) and methyl E-3-chloro-4-hydroxycinnamate, did not react. An EPR investigation of the reaction mixtures, performed on samples taken at different reaction times, demonstrated that the most probable mechanism involves reactions (i)–(iii). The superoxocobalt radical, [Co^III(salen)(ROH)(O₂)]^–, and the phenoxy cobalt radical, [Co^III(salen)(RO^–)(RO˙)], are the EPR-active species; RO˙ is suggested easily to dissociate from [Co^III(salen)(RO^–)(RO˙)] and, reacting with O₂ at the β-carbon of the allyl substituent, probably gives a dioxethane. This decomposes to aldehyde and acid. The phenol electron-donor properties promote the formation of the superoxocobalt derivative and consequently favour the formation of the phenoxy cobalt radical; a too high stability of this radical does not favour its evolution into oxidation products.