Nitration of pyridine by dinitrogen pentaoxide in sulfur dioxide: investigation of the reaction mechanism

Abstract

The nitration of pyridines with dinitrogen pentaoxide (DNP) in liquid sulfur dioxide has been investigated. ¹H NMR spectroscopy showed that a pyridinium–SO₂–DNP complex (1) was formed in SO₂ .On reaction of the SO₂ solution with water, 1 reacted with nucleophiles present to give a 1,4-dihydropyridine complex (2). This reacted by a first-order reaction to give 3-nitropyridine or substituted nitropyridines. The reaction of 2 was unaffected by addition of sodium nitrate to the water solution and addition of H¹⁵NO₃ did not result in any incorporation of ¹⁵NO₂ into 3-nitropyridine. The reaction showed no primary deuterium isotope effect. From this it was proposed that the reaction proceeded either by a tight ion pair or by a six-electron six-membered transition state (3).