Reaction of superoxide with phenoxyl-type radicals

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 pK_a 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.