Manganese(III) porphyrins as catalysts for the oxidation of aromatic substrates: An insight into the reaction mechanism and the role of the cocatalyst

Manganese porphyrins bearing electron-withdrawing nitro substituents on the beta-positions catalyse the oxidation of activated aromatic compounds by hydrogen peroxide. The reaction yields based on the starting compounds, are largely lowered by complexation of the metal by the final or starting phenolic compounds and depend on the concentration of the added co-catalyst, e.g. imidazole.Using hindered silyl or sec-butyl ether derivatives of cresol, the reaction gives better results. In this case, it is possible to isolate the corresponding benzyl alcohol and benzoic acid derivatives in a moderate yield for such substrates. For the sec-butyl ether derivative, the corresponding quinone has also been isolated. The conversion of the benzyl alcohols into the benzoic acid derivatives under the experimental conditions was also demonstrated.

The oxidation of aromatic compounds by hydrogen peroxide catalysed by manganese porphyrins is inhibited by the final phenolic products. When the reaction is performed on the sec-butyl or tert-butyldimethylsilyl ethers of 4-methylphenol, the oxidation gives a mixture of the benzyl alcohol and benzoic acid derivatives in moderate yield. In the case of the sec-butyl ether a quinone derivative is also produced. Figure optionsDownload high-quality image (36 K)Download as PowerPoint slide