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

CO2-expanded acetonitrile and methylene chloride have been used in this first detailed study of catalytic O2 oxidations in these remarkably effective reaction media. The autoxidation of 2,6-di-tert-butylphenol (DTBP) with the cobalt Schiff-base (Co(salen*) in these so-called CO2-expanded liquids (CXLs) has been extensively studied using precisely controlled and monitored batch reactions. The dependence of conversion, selectivity and turn-over-frequency on various reaction parameters, including temperature, [O2], [catalyst], and solvent composition has been evaluated. The rates of O2-oxidation in CXLs are typically 1–2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO2 as reaction media. In keeping with the proposed mechanism, the dependence of both the selectivity and conversion on O2 concentration and catalyst concentration indicates that the O2 adduct, and not free O2, serves as oxidant in two critical steps in these systems. The increase in conversion with increasing temperature supports formation of the phenoxy radical as the rate determining step. In contrast, the temperature independence of selectivity is as expected for two competing radical coupling reactions. The balance between O2 solubility and mixed-solvent dielectric constant determines some of the benefits of the CXLs. Because of the greatly increased solubility of O2 in CXLs, the conversion in those media is substantially greater than that in either scCO2 or the neat organic solvent. However, conversion eventually decreases with increasing CO2 content of the solvent because of the decreasing dielectric constant of the medium. The solubilities of O2 and Co(salen*) have been determined in CXLs based on methylene chloride.