Tautomeric and conformational properties of β-diketones

Available literature data about keto-enol equilibrium in beta-diketones with different substituents in beta-Positions (R1C(O)-CH(2)-C(O)R2) has been analyzed. It was concluded that substituents from group I (R = H, CH(3), CF(3), C(CH3)(3)) strongly favour the enol tautomer, whereas substituents from group II (R = F, Cl, OCH(3), NH(2)) favour the keto form. To understand the influence of the nature of the substituents on the keto-enol tautomerism, quantum chemical calculations (B3LYP/aug-cc-pVTZ) were performed for the series of beta-diketones with substituents from both groups, with R1 = R2 or R1 not equal R2. Equilibrium structures of enol and keto forms and vibrational spectra were analyzed for all investigated molecules. The electron density distribution was studied by NBO-analysis. Experiments and quantum chemical calculations demonstrate, that the keto form is preferred only in beta-diketones with R1 and R2 from group II. This result can be explained by hyperconjugation between lone pairs of these substituents and the C = O double bond in both R-C = O fragments. Thus, lone pairs at the atom, connected with the C(O)-C-C(O) skeleton, which are present in the substituents from group II, are the reason for favouring to the keto form rather than electronegativity or other properties. In beta-diketones with at least one substituent from group I the enol tautomer is preferred. Obviously, the strong hydrogen bond and pi-conjugation in the ring are the reasons for stabilizing the enol form. Optimized geometrical parameters of the molecules are in a good agreement with the experimental gas-phase structures. The calculations predict reasonably well the energetically most preferred tautomer. However, exact prediction of the keto/enol equilibrium composition apparently requires methods, more sophisticated than B3LYP or MP2. (C) 2010 Elsevier B.V. All rights reserved.