Tautomerism of Pyrazolidine-3,5-diones. III. Nuclear magnetic resonance spectra

The tautomerism of some N1- and N1,N2-substituted pyrazolidine-3,5-diones has been studied by N.M.R. spectroscopy. The diketo form is the only unambiguously detectable tautomer of N-phenylpyrazolidine-3,5-diones in CDCl3 and (CD3)2SO whereas N-(1H-1,2,4-triazol-5-yl)pyrazolidine- 3,5 diones exist as a mixture of the diketo form with other tautomers. Polar solvents shift the equilibrium towards the keto-enol form; the diketo form is favoured at raised temperatures.