Tautomeric and conformational properties of β-diketones

✍ Scribed by Natalya V. Belova; Valeriy V. Sliznev; Heinz Oberhammer; Georgiy V. Girichev

• Publisher: Elsevier Science
• Year: 2010
• Tongue: English
• Weight: 582 KB
• Volume: 978
• Category: Article
• ISSN: 0022-2860
• DOI: 10.1016/j.molstruc.2010.02.070

✦ Synopsis

Available literature data about keto-enol equilibrium in b-diketones with different substituents in b-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 b-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 b-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 b-diketones with at least one substituent from group I the enol tautomer is preferred. Obviously, the strong hydrogen bond and p-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.