The reaction mechanism of the nitration of pyridine compounds by N2O5-NaHSO3

Reaction of pyridine compounds in organic solvents with N2O5 gives the corresponding N-nitropyridinium nitrate. On reaction of this with an aqueous solution of NaHSO3 unstable 1,2- and 1,4-dihydropyridine compounds are formed which react to give beta-nitropyridine compounds. The reaction of the 1,2-dihydropyridine compounds have been studied. From pyridine itself compound 2 is obtained which reacts with Delta H double dagger = 18(1) kcal mol(-1) and Delta S double dagger = - 5(4) cal mol(-1) K-1. Its rate of reaction Is only marginally affected by the polarity of the reaction medium. From 3-acetylpyridine the 1,2-dihydropyridine compound 8a is obtained from attack of the nucleophile in the 2-position. This then reacts with a regioselective migration of the NO2 group from the 1- to the 3-position. Analogous results are obtained from 3-methyl- and 2,5-dimethyl-pyridine. These results do not support a reaction via the formation of an ion pair (Route B, Scheme 1) but are in accordance with a reaction by a [1,5] sigmatropic shift of the NO2 group (Route A).