Mechanism of the Oxidation of o-Nitro- and o-Benzoylbenzeneselenenic Acids by Peracids, Hydroperoxides, and Hydrogen Peroxide

Article abstract of DOI:10.1021/jo00214a020

The oxidations of o-nitro- (1a) and o-benzoylbenzeneselenenic (1b) acids by peroxybenzoic acids (eq.4), hydroperoxides (eq.2, R=t-Bu or PhCMe2), and hydrogen peroxides (R=H) have been studied kinetically over a range of pH in 60percent dioxane.With the peroxybenzoic acids oxidation of 1a and 1b occurs at comparable rates, is not acid catalyzed, and is believed to take place by a mechanism (eq.7) involving nucleophilic attack by the selenium of the selenenic acid on peroxide oxygen that is analogous to the mechanism for oxidation of alkyl sulfides by peracids.With the hydroperoxides and H2O2 the oxidations are subject to acid catalysis, and the o-benzoyl coumpond (1b) is oxidized ca. 10⁴ times faster than 1a.The much faster rate of oxidation of 1b is due to the fact that it reacts by a mechanism (eq 13) where there is reversible formation of a peroxyhemiketal (4), followed by intramolecular attack of selenium of one of the peroxide oxygens in protonated 4.Such a mechanism is not avalaible to 1a, which therefore must react by much slower bimolecular pathways (eq 10 and 11) that are equivalent to the mechanisms for the acid-catalyzed and uncatalyzed oxidations of alkyl sulfides by hydroperoxides and hydrogen peroxide.At pH>2 the oxidation of 1a by hydrogen peroxide exhibits marked autocatalysis.This is due to equilibrium formation from H2O2 and the oxidation product, o-O2NC6H4SeO2H, of some o-nitroperoxybenzeneselenenic acid, o-O2NC6H2Se(O)OOH, which, like a peroxybenzoic acid, is a much more reactive oxidizing agent for 1a than is hydrogen peroxide itself.Comparison of the previously determined (ref 5) rates of reaction of 1a and 1b with thiols with their rates of oxidation by hydroperoxides suggests that the selenenic acid functionality thought (ref 3) to be an intermediate in the reaction cycle for the enzyme glutathione peroxidase should be consumed under physiological conditions almost exclusively by reaction with glutathione rather than be oxidized further by hydroperoxides to a seleninic acid.