Autoxidation hydrogen peroxide formation

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

Iron and copper catalyse the formation of oxyradicals. Three reactions are relevant in this context (1) Autoxidation of metal complexes may yield the superoxide radical which by itself is not very reactive, but is a precursor of more reactive radical species. (2) The one-electron reduction of hydrogen peroxide -the Fenton reaction - results in hydroxyl radicals via a higher oxidation state of iron [2]. (3) A similar reaction with organic peroxides leads to alkoxyl radicals, although a recent report alleges that hydroxyl radicals are also formed [3]. There is a fourth radical, the formation of which does not require mediation by a metal complex. This is the alkyldioxyl radical, ROO , which is formed at a... 

Calvert (2 ) has pointed out that gas-phase reactions of SO2 with ozone (O3), hydroxyl radical (OH ), and hydroperoxyl radical (HOp ) are too slow to account for the aforementioned rates of sulfate production. Consequently, the catalytic autoxidation of SO2 in deliquescent haze aerosol and hydrometeors has been proposed as a viable non-photolytic pathway for the rapid formation of sulfuric acid in humid atmospheres (30-35). In addition, hydrogen peroxide and ozone have been given serious consideration as important aqueous-phase oxidants of dissolved SO2 as discussed by Martin (35). Oxidation by H2O2 seems to be most favorable under low pH conditions (pH < 4) because of a rapid rate of reaction anc[ a negative pH-dependence that favors the facile conversion of HSO3 to sulfate. 

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

In most cases dialkyl peroxides arise from the alkyl hydroperoxides formed as primary products this is so in autoxidation of isochroman,333 phthalan,334 2-methyl-l,3-dioxolane,344 and tetrahydroacenaphthacene.345 Formation of dialkyl peroxides from alkyl hydroperoxides can occur under the influence of acids, and in this ionic dimerization 333,334 hydrogen peroxide is also formed ... 

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