Tetrahedron, 60, 6791-6805 (2004)


On the tautomerism of pyrazolones: the geminal 2J[pyrazole C-4,H-3(5)] spin coupling constant as a diagnostic tool

The tautomerism of pyrazolones unsubstituted at position 3(5) has been investigated by 13C- and 1H NMR spectroscopic methods. Apart from chemical shift considerations and NOE effects the magnitude of the geminal 2J[pyrazole C-4,H3(5)] spin coupling constant permits the unambiguous differentiation between 1H-pyrazol-5-ol (OH) and 1,2-dihydro-3H-pyrazol-3-one (NH) forms. Whereas 1H-pyrazol-5-ols and 2,4-dihydro-3H-pyrazol-3-ones (CH-form) exhibit 2J values of approximately 9–11 Hz, in 1,2-dihydro-3H-pyrazol-3-ones this coupling constant is considerably reduced to 4–5 Hz. This can be mainly attributed to the removal of the lone-pair at pyrazole N−1 in the latter due to protonation or alkylation. According to the data obtained, 2-substituted 4-acyl-1,2-dihydro-3H-pyrazol-3-ones exist predominantly as pyrazol-5-ols in CDCl3 or benzene-d6 solution, whereas in DMSO-d6 also minor amounts of NH tautomer may contribute to the tautomeric composition. 2,4-Dihydro-2-phenyl-3H-pyrazol-3-one (1-phenyl-2-pyrazolin-5-one) exists in benzene-d6 solely in the CH-form, in CDCl3 as a mixture of CH and OH-form, whereas in DMSO-d6 a fast equilibrium between OH and NH isomer (with the former far predominating) is probable. For 11 compounds, including neutral and protonated molecules, we have calculated at the B3LYP/6-311++G** level, the2J(1H,13C) coupling constants which are in good agreement with those measured experimentally.