Oxidative Kupplung CH‐acider Verbindungen mit p‐Phenylendiaminen. VI. 13C‐NMR‐Untersuchung der Tautomerie und des Substituenteneffektes 4‐substituierter Pyrazolin‐5‐one

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Abstract
Oxidative Coupling of CH‐acid Compounds with p‐Phenylendiamines. VI. 13C‐N.M.R. Study of Pyrazolin‐5‐on Tautomerism and the Effect of 4‐SubstituentsThe 13C‐n.m.r. spectra of a series of 4‐substituted 3‐methyl‐1‐phenyl‐pyrazolin‐5‐ones 1–26 have been recorded and assigned. Examination of the chemical shifts of the heterocyclic ring carbons C‐3, C‐4, C‐5 and the 1‐phenyl carbons Cipso and Cortho permits to determine the preferred tautomeric forms of the compounds. In polar solvents like DMSO the OH‐form is dominant, sometimes besides a small component of the CH‐form. In nonpolar solvents like CDCl3 the CH‐form is preferred at room temperature besides a small component of the NH‐form. Cooling or addition of CH3OH to the solution shifts the tautomeric equilibrium to a higher amount of the NH‐form. The influence of 4‐substituents on the C‐3, C‐4 and C‐5 chemical shifts have been compared with the effect of these substituents on the Cipso and Cortho carbons in benzene. A linear dependence between C‐4 and Cipso shows, that the effect of substituents in pyrazolin‐5‐ones and benzene is controlled by their electronegativity. The substituent effects are influenced remarkably by intermolecular hydrogen bondings to polar solvents and by intramolecular hydrogen bondings, if possible.