Mechanisms of antioxidant action. Part 3. The decomposition of 1-methyl-1-phenylethyl hydroperoxide by OO′-dialkyl(aryl)phosphorodithioate complexes of cobalt, nickel, and copper

Abstract

OO′-dialkyl(aryl)phosphorodithioates of cobalt(II), nickel(II), and copper(I) have been used as promoters for the decomposition of 1-methyl-1-phenylethyl hydroperoxide (cumene hydroperoxide). The results have been compared with previous work using zinc bis-[OO′-dialkyl(aryl)phosphorodithioates] as promoters for cumene hydroperoxide decomposition. The comparison shows that the product distribution and activation parameters for the reactions promoted by the cobalt, nickel, and copper complexes are identical with the product distribution and activation parameters for the reactions promoted by the corresponding zinc complexes. On this evidence it is proposed that the metal complex promoters form the corresponding OO′-dialkyl(aryl) hydrogen phosphorodithioate and it is this acid that is the catalyst for hydroperoxide decomposition. The order of the reaction with respect to the hydroperoxide depends upon the promoter and is second order for all the zinc complexes but first order for all the nickel complexes. This can be explained by assuming that there are two mechanisms for the formation of the acid catalyst. The relative importance of the two mechanisms depends upon the experimental conditions used and this provides an explanation for the three-stage reaction observed in hydroperoxide decompositions carried out at low temperatures in the presence of relatively high concentrations of OO′-dialkyl(aryl)phosphorodithioate promoters.