Oxidation of Phenols and Hydrazones with t-Butyl Hydroperoxide and Catalysis by Co(salen)

Abstract

(t-BuOO)Co(salen)prepared by the oxidation of Co(salen)with t-butylihydroperoxide oxidized 2, 6-di-t-butylphenols to generate the corresponding phenoxyl radicals, which were detected and identified by ESR. The oxidation of the phenols seemed to take place by hydrogen abstraction by t-butylperoxyl radical produced by homolysis of the Co-O bond in(t-BuOO)Co (salen), although a possibility of homolytic cleavage of a (phenolato)Co^III(salen) complex formed by an acid-base reaction between the phenols and (t-BuOO)Co(salen)may not be ruled out. Addition of t-butyl hydroperoxide to the resulting phenoxyl radical solution resulted in the quantitative incorporation of the t-butyldioxy group into the radicals, giving 4-tbutyldioxy2, 6-di-t-butylcyclohexadienones. These results suggest that (t-BuOO)Co(salen)can catalyze the t-butylperoxylation of the phenols with t-butyl hydroperoxide. Actually, when a solution of 4-substituted 2, 6-di-t-butylphenol and t-butyl hydroperoxide (mol/mol=1/2) in dichloromethane was stirred in the presence of a catalytic amount of (t-BuOO)Co(salen)(0.1 eq.) at room temperature, the reaction was completed normally in 1 h to give 4-substituted 4-t-butyldioxy-2, 6-di-t-butyl-2, 5-cyclohexadienone in an excellent yield. The same results were obtained when Co(salen) was used as the catalyst in place of (t-BuOO)Co(salen).
Similar chemical behavior of (t-BuOO)Co(salen) was observed for 4-nitro phenylhydrazones of ketones, where the t-butyldioxy group was incorporated into. the carbon atom of the C=N group in the hydrazones to give 1-(t-butyldioxy)-1-(4-nitrophenylazo) compounds as the main product along with small amounts of 1-(t-butyldioxy)-1-(4-nitrophenylazoxy) compounds. Interestingly, the reaction of (t-BuOO)Co(salen) with 4-methoxyacetophenone 4nitrophenylhydrazone under nitrogen gave an appreciable amount of a stereoisomer (Z-form)of the starting hydrazone (E-form) in addition to the azo peroxidic product. The results are rationalized, by a strong interaction between the substrate radical and the cobalt complex, since no reaction took place when the hydrazone substrate was treated with Co(salen) itself under nitrogen or with t-BuOK.