5- Nitro-2- pyridine, tautomerism

The photoinitiated colour change of aromatic nitro compounds such as the dinitrobenzyl-pyridine (140) is due to a hydrogen transfer from the methylene bridge to the oxygen of the nitro group (aci-nitro tautomerism, Scheme 14) (60JCP(32)1888). 

Aromatic Nitro Compounds. The photoinduced isomerization of crystalline 2(2, 4 -dinitrobenzyl) pyridine was first reported by Tchit-chibabine et al. (29) who proposed the following tautomeric equilibrium ... 

A study of the tautomerism between 49 and 50s 5, which are synthesized from the condensation of 3-aminopyridines and isobutyraldehyde, shows that the equilibrium position varies drastically according to the substituents and their position on the pyridine ring. With a nitro or an ethoxy group at position-2, the equilibrium lies well to the... 

The spectroscopic (UV, IR, NMR) studies of 2-aminopyridine, 2-(phenylami-no)pyridine and 5-nitro-2-(phenylamino)pyridine indicated the presence of only amino tautomer in a solution. The tautomeric equilibrium constants, calculated from the acidity of the fixed derivatives, were found to be 2 x 105 for 2-(phenylamino)pyridine and 8 x 103 for 5-nitro-2-(phenylamino)pyridine, indicating the shift of the equilibrium toward the imino tautomer on introduction of a nitro group (76BCJ2770, 80BCJ717). The molar fractions of 2-aminopyridine and 4-aminopyridine in acetone were estimated as 92+6% and 94+6%, respectively, by 14N NMR spectroscopy... 

Benzannulation of pyridines 117 (R1 = H R2 = R3 = H) to quinolines (R2 = ben-zo) is not sufficient for realization of the tautomer 117b. However, additional stabilization by introduction of a single nitro group into the phenyl ring results in the appearance of 117b in tautomeric equilibrium with 117a (90KG75). 

An NMR study has shown that the amino form (419) is the predominant tautomer of 2-[arylamino(imino)]-4a,5,6,7,8,8a-hexahydro-(47f)-l,3,4-benzoxadiazines. The proton-transfer tautomerism of 4-azabenzimidazole, mediated by hydrogen-bonded complexes, has been studied in the ground and excited states by means of absorption and emission spectroscopy, and it has been shown that 6-nitro and 8-nitro groups in the tetrazolo[l,5-a]pyridine molecule exhibit completely different influences on the tetrazole-azide equilibrium. The solvent effect on the azo-hydrazone tautomeric equilibrium of l-phenylazo-4-naphthol has been modelled by using ab initio quantum-chemical calculations. The results have shown that the hydrazone form is more stable in water, while methanol stabilizes the azo form. ... 

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