NMR and theoretical investigation of the keto–enol tautomerism in cyclohexane-1,3-diones

The keto–enol tautomerization for 4,4-dimethyl-cyclohexane-1,3-dione (1) and for 4-methyl-cyclohexane-1,3-dione (2) was investigated. We observed that one of the possible keto-enolic forms for each compound is more stable than the other and thus was preferentially formed in the keto–enol tautomerism. This study was supported through NMR analysis, geometry optimization calculations and NBO analysis for keto-enolic forms (3 and 4) and (5a,b and 6a,b).