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 cat alyzed by a dioxygen carrier in supercritical carbon dioxide (scCO(2)) prov ides additional insights into the established mechanism of reactions that h ave 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-cyclohexanedianlinato(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-b utylphenol (DTBP) and 3,5-di-tert-butylphenol (35-DTBP) in scCO(2) was stud ied by analysis of products in batch reactions with carefully controlled va riables, in the presence of a large excess of O-2, at 207 bar of total pres sure and a reaction temperature of 70 degreesC. The oxidation of 35-DTBP yi elded only traces of products under the same experimental conditions that c onverted DTBP totally to a mixture of the oxygenation product 2,6-di-tert-b utyl-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 cataly st, oxygen, and methylimidazole. Selectivity in favor of the O-transfer pro duct DTBQ over the self-coupling of the phenoxy radical was observed upon c hanging the oxygen concentration. In contrast, selectivity remained unaffec ted over a wide range of temperatures and catalyst concentrations. The oxyg en dependence of both the conversion and selectivity showed saturation effe cts identifying the dioxygen complex as the effective oxidant in both the i nitial radical formation step and the oxygenation of that radical. No direc t reaction is observed between the electrophilic phenoxy radical and O-2.