IV-Nitropyridinium nitrates

Nitropyridine itself, and some of its substituted derivatives, can now be prepared efficiently by reaction with dinitrogen pentoxide as shown below. The initially formed iV-nitropyridinium nitrate suffers addition of a nucleophile - sulfur dioxide when this is used as solvent or co-solvent, or sulfite, added subsequently - forming a 1,2-dihydropyridine. Transfer of the nitro group to a 3- or 5-position, via a [1,5]-sigmatropic migration, is then followed by elimination of the nucleophile regenerating the aromatic system. ... 

Registration of radical cations by the EPR method in the presence of nitrogen dioxide could provide direct experimental evidence that the initiation proceeds through scheme (36). However, because of high reactivity and fast decomposition [45], these particles are difficult to detect by this mediod. Nevertheless, the formation of radical cations can be revealed indirectly in the act of their decomposition with emission of a proton. Pyridine is known to be capable of accepting protons to yield p5rridinium cations. Hence, if protons are formed in during decomposition of radical cations by Eq. (36), they can be detected easily from IR spectra typical of pyridinium cations. Note that pyridine can be nitrated only imder quite severe conditions. For example, iV-nitropyridinium nitrate was obtained only when pyridine was treated with a N02-ozone mixture in an inert solvent [46], This is evident from the IR spectrum (Fig. 5) of 1 1 mixture... 

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