¹³ C NMR spectroscopic study of the tautomeric equilibrium in p-phenyl substituted benzoylacetones

Solution 13C NMR chemical shifts are reported for a series of p-phenyl substituted benzoylacetones which undergo a fast, intramolecular proton-transfer reaction between both possible enol tautomers. This information, together with 13C NMR spectroscopic data for related non-exchanging model compounds, allows the study of substituent-induced equilibrium shifts. The results show a systematic trend: electron-withdrawing para groups shift the equilibrium towards the methyl keto form.