On the tautomerism of pyrazolones: the geminal J[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 ²Jpyrazole 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, the ²J(¹H,¹³C) coupling constants which are in good agreement with those measured experimentally.