Rates of Oxidation of 1-Benzyl-1,4-dihydronicotinamide by Pyrazinium, Quinoxalinium, and Phenazinium Cations

Second-order rate constants (k2) have been measured for the oxidation of 1-benzyl-1,4-dihydronicotinamide by the N-methyl pyrazinium, 3-aminocarbonylpyrazinium, quinoxalinium, and phenazinium cations in 20% acetonitrile-80% water at ionic strength 1.0 and 25°C. These rate constants are also shown to be independent of ionic strength, but to decrease by a factor of >10-fold as the solvent polarity is decreased in aqueous acetonitrile mixtures containing from 1 to 70% acetonitrile (v/v). The reactivities (kN2) of the above pyrazinium and benzopyrazinium cation oxidants are much greater than those (kC2) of the corresponding isoelectronic pyridinium and benzopyridinium cations. Both kN2 and kC2 increase dramatically in the order monocyclic < bicyclic < tricyclic cations; however, the relative reactivities (kN2/kC2) decrease systematically from 8000 to 200 with increasing reactivity in these series of cations. The current studies directly demonstrate that the reactivities of 5-deazaflavins toward reduction are indeed good chemical models for the reduction of the analogous isoelectronic flavin species.