1H, 13C and 15N NMR study of 2-alkylnitrosoamino-4-nitropyridines and N-oxides: An example on restricted inversion of sp3 nitrogen

The 1H, 13C and 15N NMR chemical shifts of seven 2-alkylnitrosoamino-4-nitropyridines and seven 2-alkylnitrosoamino-4-nitropyridine N-oxides have been assigned. The GIAO/DFT 13C chemical shifts of energetically optimized structures have been calculated. The results were compared to the chemical shifts of previously studied 2-alkylamino and 2-alkylnitramino derivatives of 4-nitropyridine and 4-nitropyridine N-oxide. In sterically crowded ortho-substituted congener (2-ethylnitrosoamino-3-methyl-4-nitropyridine N-oxide) restricted inversion of pyramidal amino nitrogen forms a chiral center in the molecule and makes the geminal protons of N–CH2-moiety diastereotopic. In 5- and 6-CH3-substituted derivatives this diastereotopism disappears, but the rotation around N–CH2 sigma bond in all congeners is so slow in NMR time scale, that 1H, 13C and 15N NMR chemical shifts could be detected for two conformers. In 4-nitropyridine and 4-nitropyridine N-oxide derivatives the shielding of aromatic nitrogen is very sensitive to the ring substituents. 2-Alkylnitrosoamino group produces a significant shielding based on conjugation between the ring and the nitrosoamino group.