KETO-ENOL-TAUTOMERISM OF MONOSUBSTITUTED PHENYLPYRUVIC ACIDS AS STUDIED BY NMR AND PM3 CALCULATION

Keto-enol tautomerism of mono-substituted phenylpyruvic acids has been studied by the H-1 and C-13 NMR spectra. The equilibrium constants an d the kinetic parameters for the tautomerism were obtained from the sp ectral data. The equilibrium constants are strongly dependent on the p osition of the substitution; the values for the o-substituted PPAs are several times greater than those of the m- or p-substituted derivativ es. The PM3 calculations have been carried out to obtain the informati on on the preferred conformations of the tautomers and on the mechanis m for the tautomerism. The results suggest the involvement of a solven t molecule in the equilibrium process.