Thermal Rearrangement of 1,4-Dinitroimidazole to 2,4-Dinitroimidazole: Characterization and Investigation of the Mechanism by Mass Spectrometry and Isotope Labeling

The thermal rearrangement of 1,4-dinitroimidazole to 2,4-dinitroimidazole has been investigated by differential scanning calorimetry and mass spectrometry techniques. When mixtures of independently prepared deuterium-and {sup 15}N-labeled samples of the 1,4-isomer were subjected to thermal rearrangement, the resulting 2,4-dinitroimidazole failed to show isotope-scrambled molecular ions in its mass spectrum, suggesting that the reaction was intramolecular in nature. This was interpreted to mean that the mechanism was of the (1,5)-sigmatropic type rearrangement. Extensive NMR measurements were used to obtain unequivocal evidence for the identity of the assumed structures of the isomeric dinitroimidazoles. Two byproducts (4-nitroimidazole and a trinitroimidazole), formed during the rearrangement reaction, have also been identified. Plausible mechanisms for their formation are discussed. 15 refs., 3 figs., 3 tabs.