Autoxidation of substituted phenols catalyzed by cobalt Schiff base complexes in supercritical carbon dioxide

This first study of O(2) oxidation (autoxidation) of substituted phenols catalyzed by a dioxygen carrier in supercritical carbon dioxide (scCO(2)) provides additional insights into the established mechanism of reactions that have been much studied in conventional solvents. As has been long believed, the cobalt(II) dioxygen carriers of the class represented by [[N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato(2-)]cobalt(II)], Co(salen), show both oxidase and oxygenase activities during oxygenation of substituted phenols in scCO(2). The catalytic autoxidation of 2,6-di-tert-butylphenol (DTBP) and 3,5-di-tert-butylphenol (35-DTBP) in scCO(2) was studied by analysis of products in batch reactions with carefully controlled variables, in the presence of a large excess of O(2), at 207 bar of total pressure and a reaction temperature of 70 degrees C. The oxidation of 35-DTBP yielded only traces of products under the same experimental conditions that converted DTBP totally to a mixture of the oxygenation product 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and the related product of radical coupling 3,5,3',5'-tetra-tert-butyl-4,4'-diphenoquinone (TTDBQ). The effects on conversion of DTBP to products and on selectivity between the two products were studied for variations in temperature and the concentrations of catalyst, oxygen, and methylimidazole. Selectivity in favor of the O-transfer product DTBQ over the self-coupling of the phenoxy radical was observed upon changing the oxygen concentration. In contrast, selectivity remained unaffected over a wide range of temperatures and catalyst concentrations. The oxygen dependence of both the conversion and selectivity showed saturation effects identifying the dioxygen complex as the effective oxidant in both the initial radical formation step and the oxygenation of that radical. No direct reaction is observed between the electrophilic phenoxy radical and O(2).