Tautomeric and conformational properties of β-diketones

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• 作者：Natalya V. Belova ; Valeriy V. Sliznev ; Heinz Oberhammer ; Georgiy V. Girichev
• 关键词：Keto– ; enol tautomerism ; β ; -Diketones ; Gas-phase structures ; Quantum chemical calculations ; NBO-analyses
• 刊名：Journal of Molecular Structure
• 出版年：2010
• 出版时间：20 August 2010
• 年：2010
• 卷：978
• 期：1-3
• 页码：282-293
• 全文大小：581 K

文摘

Available literature data about keto–enol equilibrium in β-diketones with different substituents in β-positions (R1C(O)–CH₂–C(O)R2) has been analyzed. It was concluded that substituents from group I (R = H, CH₃, CF₃, C(CH3)₃) strongly favour the enol tautomer, whereas substituents from group II (R = F, Cl, OCH₃, NH₂) 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 β-diketones with substituents from both groups, with R1 = R2 or R1 ≠ 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 β-diketones with R1 and R2 from group II. This result can be explained by hyperconjugation between lone pairs of these substituents and the CO double bond in both R–CO 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 β-diketones with at least one substituent from group I the enol tautomer is preferred. Obviously, the strong hydrogen bond and π-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.