H-1, C-13 and N-15 NMR study of 2-alkylnitrosoamino-4-nitropyridines and N-oxides: An example on restricted inversion of sp(3) nitrogen

摘要

The H-1, C-13 and N-15 NMR chemical shifts of seven 2-alkylnitrosoamino-4-nitropyridines and seven 2-alkylnitrosoamino-4-nitropyridine N-oxides have been assigned. The GIAO/DFT C-13 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-ethyinitrosoamino-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 H-1, C-13 and N-15 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. (c) 2006 Elsevier B.V. All rights reserved.