Radical type of reaction

Although the estimated oxidation potentials of the a-carbonyl radicals 90 are lower than those of the corresponding enols 86, a strong oxidant is needed to prevent radical type of reactions. According to the rates calculated on the basis of the Marcus theory [188], one-electron oxidation of the radicals can only compete with other radical processes when the electron transfer step is strongly exergonic. Thus, for radicals 90a and 90b the one-electron oxidant TTA" is too weak an oxidant, as reflected by both the yields of the a-Umpolung (Table 14) and the slow electron transfer rates (Table 13). 

From comparison of antitumor activity and toxicity of hetero-ligand vanadium(V) complexes, Djordjevic and Wampler (79) arrived at the conclusion that the hetero-ligand is able to affect the redox potential of the V(V)/V(IV) couple in such a way that intramolecular electron transfer can occur within the V(V)-peroxoadduct. As a consequence, vanadium(V) is reduced to the IV state, and the peroxo group is oxidized to a superoxide radical. It is conceivable that such a species is present also during the reaction of vanadium bromoperoxidase with H2O2. However, there is no evidence for a radical type of reaction with bro-moperoxidases. 

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