Mechanisms of antioxidant action. Part 2. Reactions of zinc bis-[OO-dialkyl(aryl)phosphorodithioates] and related compounds with hydroperoxides

Abstract

Zinc bis-[OO-dialky(aryl)phosphorodithioates](1), hexakis-[OO-dialkyl(aryl)phosphorodithioate]-µ₄-tetraoxozincs (2), and OOOO-tetra-alkyl(aryl) thioperoxydiphosphates (3) promote the decomposition of 1-methyl-1-phenylethyl hydroperoxide (cumene hydroperoxide). The reaction is second-order with respect to the hydroperoxide when it is promoted by the zinc complexes (1) and (2), but first-order with respect to the hydroperoxide when promoted by the diphosphate (3). The results that we have obtained are not consistent with the accepted reaction mechanisms for the decomposition of cumene hydroperoxide. Our results lead us to propose that, under our experimental conditions, (i) acetophenone (10) is formed from cumene hydroperoxide by a free-radical reaction that is independent of the promoter used, (ii) 2- phenylpropan-2-ol (11) and 2-phenylpropene (12) are formed by an ionic deomposition of the hydroperoxide and not by the free-radical mechanism normally accepted, (iii) the ionic decomposition of cumene hydroperoxide takes place by a catonic chain reaction, and (iv) the catalyst formed from the promoters (1)–(3) may be the OO-dialkyl(aryl) hydrogendithiophosphate (4). We have proposed a mechanism for the ionic decomposition of cumene hydroperoxide, and suggest possible schemes for the formation of the acid (4) from the promoters (1)–(3) which are consistent with the observed kinetics and our proposed mechanism.