Hydroxycyclohexadienylperoxyl radical

Whereas base-induced deprotonation at a heteroatom is very fast (practically diffusion-controlled), deprotonation at carbon is generally much slower (Eigen et al. 1964, 1965). Thus, this type of 02 -elimination is observed at higher pH values compared to the reactions discussed before. The elimination of HO2 is subject to steric restrictions, but the OH -induced 02 -elimination is not, and at high pH all hydroxycyclohexadienylperoxyl radicals eliminate 02 bringing the phenolate yield close to 100% [reactions (9) and (14)/(15)] competing reactions (see below) are thereby suppressed. 

In the series of hydroxycyclohexadienylperoxyl radicals, one encounters the competition between the H02-/02- elimination leading to phenol [reactions (9) and (14)/(15)] and fragmentation of the ring (Pan et al. 1993). That latter has been attributed to an intramolecular addition of the peroxyl radical function to a diene double bond [reaction (24)]. This reaction is reversible [reaction (-24)], but when 02 adds to the newly created carbon-centered radical the endoperoxidic function is locked in [reaction (25)]. In analogy to reaction (24), the first step of the trichloromethylperoxyl-radical-induced oxidation of indole is its addition to the indole C(2)-C(3) double bond (Shen et al. 1989). 

Thus the rate of HO2 -elimination must be considerably slower (nearer to 1 s 1), but still fast enough to play a role in DNA free-radical chemistry. Reaction (172) is reminescent of the H02--elimination from 3-hydroxycyclohexadienylperoxyl radicals (derived from the reaction of OH with benzene in the presence of 02 Chap. 8.4). In this system, the reaction is much faster (k = 800 s 1 Pan et al. 1993b) possibly due to the gain in energy in the course of the re-aromatiza-tion]. 

In this context it is worth considering that in the free-radical chemistry of DNA the C(6)- OH-adducts radicals are certainly of a greater importance than the C(6)-H-adduct radicals investigated here. If the benzene system is a good guide H02--elimination from hydroxycyclohexadienylperoxyl radicals is noticeably slower than that from cyclohexadienylperoxyl radicals (Pan et al. 1993a), i.e., the rate of H02--elimination from 5,6-dihydro-6-hydroxy-thymidine-5-per-oxyl radicals may even be slower. 

In the abatement of aromatic pollutants, the main initial transient species is the substrate-OH-radical adduct, hydroxycyclohexadienyl. Depending on the nature of the substituents in the ring, the addition of dioxygen to these radicals may be reversible [34]. The behaviour of the hydroxycyclohexadienylperoxyl... 

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See also in sourсe #XX -- [ ]

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