Diversity of Reactions of Isomeric Aminopyridine N‐Oxides with Chloronitropyridines: An Experimental and Theoretical Study

Reaction of 2-aminopyridine N-oxides 1a, 1b, 1c, 1d with chloronitropyridine 7a gave 2-[(pyridin-2-yl)amino]pyridine N-oxides 8a, 8b, 8c and 9 in good yield. The reactions of 4- and 3-aminopyridine N-oxides 12a,12b and 24 with 7a, 7b, 7c proceed in the different manner involving initial formation of the intermediary 1-pyridyloxypyridinium salts 13a, 13b, 13c, 13d and 26, which rearrange to 4-[(5-nitropyridin-2-yl)amino]pyridine N-oxide 22 and 1-(3-aminopyridin-2-yl)pyrid-2-one derivatives 27a,27b, respectively. However, N-protected 2-aminopyridine N-oxides 17 gave quaternary 1-pyridyloxypyridinium salts 18a,18b, which upon treatment with aqueous ammonia afforded 2-[(pyridin-2-yl)amino]pyridine N-oxides 8a and 20. Quantum chemical calculations at the DFT/B3LYP/6-311++G(d,p) level were performed to explain the differences in properties of the frontal orbitals and atomic charge distribution in isomeric aminopyridine N-oxides.