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

Abstract

Second-order rate constants ( k 2) 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 ( k N 2) of the above pyrazinium and benzopyrazinium cation oxidants are much greater than those ( k C 2) of the corresponding isoelectronic pyridinium and benzopyridinium cations. Both k N 2 and k C 2 increase dramatically in the order monocyclic < bicyclic < tricyclic cations; however, the relative reactivities ( k N 2/ k C 2) 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.