Transients of the oxidation of pyrimidines with SO4•-:: Structure and reactivity of the resulting radicals

Depending on the methylation pattern, the reaction of SO4.- with various methylated pyrimidines yields radicals deprotonated at N(1) and C(5)-OH or C(6)-OH adduct radicals and probably allyl radicals by subsequent deprotonation at the C(5)-methyl group of the thymines. All of these radicals are derived from the initial SO4.- adducts on the pyrimidines that have lifetimes of several microseconds in the case of N(1)-methylated thymines. These transients could be taken for long lived pyrimidine radical cations. However, spectral and kinetic comparison with the pyrimidine radical cations generated by electron transfer in nonpolar solvents (butyl chloride, acetone) reveals the adduct nature of these transients. In the course of sulfate adduct decay, pyrimidine radical cations could be formed as nondetectable short-lived (tau < 20ns) intermediates rapidly reacting to the above-mentioned radical products. The reactivity of the SO4.- adducts, as well as that of the other pyrimidinyl radicals observed, was characterized by the subsequent oxidation of triphenylamine and di- or trimethoxybenzene, which supports the mechanistic interpretation given above. Hence there is no indication of long-lived pyrimidine radical cations under the conditions of the SO4.- oxidation of pyrimidines in aqueous solution.