Reaction of superoxide with phenoxyl-type radicals

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Abstract
Radiolytically generated phenoxyl radicals derived from the structurally similar phenols cresol, tyrosine, tyramine and tyrosol were reacted with O2˙− [k = (1–4.5) × 109 dm3 mol−1 s−1, by pulse radiolysis] and the consumption of the phenol determined. Although the reduction potentials of these phenoxyl radicals are very close, ranging between +0.64 and +0.68 V, the yields of phenol consumption vary considerably [between 8% (tyrosol) and 90% (tyrosine)]. This indicates that electron transfer with restitution of the phenol does not necessarily occur to a major extent, although this reaction is thermodynamically favoured (reduction potential of O2˙−, −0.33 V). The reactivity of phenoxyl radicals from phenols that are structurally different from the above has also been studied. In the case of the reaction of O2˙− with 2,4,6-trimethylphenoxyl, hydroperoxides are formed which revert to the phenol by eliminating dioxygen (perhaps in the singlet state) in a slow reaction on a time-scale of many minutes. The rate of this reaction increases with increasing pH and increasing temperature. From these data it is calculated that the hydroperoxides from 2,4,6-trimethylphenol have a pKa of 11.3 and that the reaction requires an activation energy of 105 kJ mol−1 (frequency factor, 3.5 × 1014 s−1). In competition with the elimination of dioxygen, depending on the presence of suitable substituents, such hydroperoxides can undergo other reactions. In the case of tyrosine, there is a cyclization reaction followed by the elimination of hydrogen peroxide. Phenoxyl radicals that carry hydrogen in the ortho or para-position (e.g. such as those derived from 4-methylphenol, 2,6-dimethylphenol or 2-methoxy-4-methylphenol) give rise to hydroperoxides which can eliminate water to produce the corresponding quinones. These subsequently suffer stepwise reduction by O2˙− to the corresponding catechols or hydroquinones.

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