One-Electron Reduction of Nitrobenzenes by OH and H Radical Adducts to 6-Methyluracil and 6-Methylisocytosine via Electron Transfer and Addition/Elimination. Effect of Substituents on Rates and Activation Parameters for Formation and Heterolysis of Nitroxyl-Type Thetrahedral Intermediate

Article abstract of DOI:10.1021/j100312a025

The radicals formed by OH radical addition to C-5 of 6-methyluracil or of 6-methylisocytosine, i.e., 5-hydroxy-5,6-dihydro-6-methyluracil-6-yl or 5-hydroxy-5,6-dihydro-6-methylisocytosin-6-yl, or that produced by H addition to C-5 of 6-methyluracil (or by H abstraction from C-6 of 5,6-dihydro-6-methyluracil), i.e., 5,6-dihydro-6-methyluracil-6-yl, reacts in aqueous solution with para-substituted nitrobenzenes to give both nitrobenzene radical anions and nitroxyl-type radicals with rate constants that vary from ca. 7E7 to (2-5)E9 M^-1s^-1, depending on the pyrimidine radical and on the nitrobenzene.The nitroxyl radicals undergo a spontaneous unimolecular heterolysis to yield (additional) nitrobenzene radical anion and oxidized pyrimidine with rate constants of 1E3 to 5E5 s^-1 depending on the structure of the pyrimidine and of the nitrobenzene.This reaction is characterized by activation enthalpies of 30-40 kJ mol^-1 and by activation entropies of -7 to -89 J mol^-1 K^-1 (entropy control).The addition/elimination sequence constitutes a case of inner-sphere electron transfer.The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidin-6-yl radicals.On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition.