Autoxidation of Substituted Phenols Catalyzed by Cobalt Schiff Base Complexes in Supercritical Carbon Dioxide

Abstract

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).

Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over T(p-Cl)PPMnCl/Co(OAc)2 Under Moderate Conditions

“…A possible reason related to this phenomenon was that metalloporphyrin could react with oxo-radical RO to produce the non-radical product, and the chain initiation was terminated. In addition, the metalloporphyrins tend to form catalytically inactive dimeric l-oxo-and/or l-peroxobridged dimetal species when its concentration surpassed the critical value, which was not in fovor of the catalysis Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over T(p-Cl)PPMnCl/Co(OAc) 2 157 [20]. Similar phenomena were also observed in our previous works [6,15].…”

Recyclability of water-soluble ruthenium–phosphine complex catalysts in multiphase selective hydrogenation of cinnamaldehyde using toluene and pressurized carbon dioxide

“…Liquid phases (substrates or solvents) pressurized with CO 2 , called as CO 2 -dissolved expanded liquid phases [25], have some merits in chemical transformations, which are not only to promote the dissolution of gaseous reactants (H 2 , O 2 , for example) into the liquid phases but also to modify the chemical reactions [16][17][18]26].…”

Autoxidation of 2,6-di-tert-butylphenol with cobalt Schiff base catalysts by oxygen in CO2-expanded liquids

“…7 The same reaction has now been studied in two families of CXLs using precisely controlled and monitored batch reactions. The influences of reaction parameters (oxygen concentration, catalyst concentration, reaction temperature, and expansion ratio) on DTBP conversion and DTBQ selectivity during the oxidation reaction were determined.…”