NITRATION OF PYRIDINE BY DINITROGEN PENTAOXIDE IN SULFUR-DIOXIDE - INVESTIGATION OF THE REACTION-MECHANISM

The nitration of pyridines with dinitrogen pentaoxide (DNP) in liquid sulfur dioxide has been investigated. H-1 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 (HNO3)-N-15 did not result in any incorporation of (NO2)-N-15 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).