Singlet oxygen disproportionation

As shown in Figure 13.3, oxidation of ascorbic acid, for example, by the reduction of superoxide to hydrogen peroxide or Fe + to Fe +, and similar reduction of other transition metal ions, proceeds by a one-electron process, forming the monodehydroascorbate radical. The radical rapidly disproportionates into ascorbate and dehydroascorbate. Most tissues also have both nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione-dependent monodehydroascorbate reductases, which reduce the radical back to ascorbate. Ascorbate is thus an effective quencher of singlet oxygen and other radicals. 

It should be noted that the tetroxide intermediate proposed as the mechanism for peroxyl radical disproportionation remains somewhat controversial. If it exists, it has been argued that the oxygen should be released as O2 to avoid spin restrictions (291). Some studies claim to have detected O2 from lipid hydroperoxides (366), but the evidence has not been conclusive. One of the difficulties in determining when 02 is produced is that 02 reduces singlet oxygen when water sufficient to provide a hydration shell of five water molecules is present (373). 

The present discussion will focus first on one-electron- (free-radical) and two-electron- (nucleophilic) transfer reactions of I" with these oxidants and then on singlet oxygen. The first intermediates, I2 and HOI, are considered during these discussions. HOI reactions, including disproportionation, are also presented in further detail in a separate section. 

HOCl is a strong oxidizing agent ( hoci/ci- = 1-49 V, pH = 0) in acid but not in alkaline solution. Hence, oxidation goes only from HOCl or its protonized form HOCIH" with the direct transfer of Cl (chlorination), similar to the reaction of HOBr and HOI. It disproportionates into chloride and chlorate singlet oxygen is then partly formed, which can oxidize organic compounds (the biocide application of hypochlorite is based on this) ... 

The main reactions of phenolic antioxidants take place with peroxy radicals, oxygen, singlet oxygen, phenoxy radical disproportionation, etc. 

Wahlen, J. Hertogh De S. Vos, De D. E. et al. Disproportionation of hydrogen peroxide into singlet oxygen catalyzed by lanthanum-exchanged zeolites. J. Catal. 2005, 233,422-433. 

Whichever mechanism operates, it appears to be generally true that singlet aromatic carbenes react with the lower alcohols to form ethers at rates approaching the diffusion limit. On the other hand, aromatic carbenes that are clearly triplets do not give any ether at all from reaction with alcohols. Instead, these triplets behave as is expected of biradicals and abstract a hydrogen atom from the oxygen bearing carbon of the alcohol. The stable products of this reaction are those due to the combination and disproportionation (10) of the pair of radicals (Lapin et al., 1984). The more com-... 

Oxidase activation is the pivotal event with respect to microbicidal metabolism in that its products, 02 and H, serve as reactants in a variety of disproportionation and radical reactions yielding H2O2 and highly reactive oxidizing species such as hydroxyl radical (-OH), and either singlet molecular oxygen ( O2) or an oxidant having the reactive po-... 

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