Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Tautomerism of aromatic β‐ketoaldehydes <jats:italic>p</jats:italic>‐XPhCOCH<jats:sub>2</jats:sub>CHO (<jats:bold>1</jats:bold>, X = NMe<jats:sub>2</jats:sub>, OMe, Me, H, Br, NO<jats:sub>2</jats:sub>), aliphatic β‐ketoaldehydes and benzoylacetaldehyde RCOCH<jats:sub>2</jats:sub>CHO (<jats:bold>2</jats:bold>, R = Me, <jats:italic>i</jats:italic>‐Bu, <jats:italic>t</jats:italic>‐Bu, Ph), RCOCH(Me)CHO (<jats:bold>3</jats:bold>, R = Me, Et, <jats:italic>i</jats:italic>‐Pr) and methyl 2‐formylpropionate MeOCOCH(Me)CHO (<jats:bold>4</jats:bold>) has been studied by the <jats:sup>1</jats:sup>H NMR technique. In basic solvents both <jats:italic>cis</jats:italic>‐ and <jats:italic>trans</jats:italic>‐enol forms of these compounds co‐exist. <jats:italic>trans</jats:italic>‐Enolisation, which occurs exclusively at the formyl group, is most favoured in compound (<jats:bold>4</jats:bold>) and least favoured in compounds (<jats:bold>1</jats:bold>) and (<jats:bold>2</jats:bold>). The increasing electron‐attracting property of the substituent X in the aromatic β‐ketoaldehydes (<jats:bold>1</jats:bold>), as well as increasing solvent basicity in the series propanediol‐1, 2‐carbonate, acetone < dimethylformamide < dimethylacetamide < pyridine, also shifts the equilibrium towards the <jats:italic>trans</jats:italic>‐enol form. The <jats:italic>trans</jats:italic>‐enol form is absent in aprotic solvents of low basicity such as CCl<jats:sub>4</jats:sub>, C<jats:sub>2</jats:sub>HCl<jats:sub>3</jats:sub> and toluene. The thermodynamic parameters of the <jats:italic>cis</jats:italic>‐<jats:italic>trans</jats:italic>‐enol (C ⇌ T) and <jats:italic>cis</jats:italic>‐enol‐enolic (C ⇌ C') equilibria have been estimated from the temperature dependences. The transition from the <jats:italic>cis</jats:italic>‐to the <jats:italic>trans</jats:italic>‐enol form is accompanied by an entropy decrease of about 10 cal mol<jats:sup>−1</jats:sup> degree<jats:sup>−1</jats:sup>. Nevertheless the <jats:italic>trans</jats:italic>‐enol form is stabilised due to its lower enthalpy. The <jats:italic>cis</jats:italic>‐<jats:italic>trans</jats:italic>‐enol equilibrium is determined by the relative strength of the intramolecular hydrogen bond in the <jats:italic>cis</jats:italic>‐enol form and the intermolecular hydrogen bonds with basic solvent molecules of the <jats:italic>trans</jats:italic>‐enol form.</jats:p><jats:p>The enthalpy difference of the two <jats:italic>cis</jats:italic>‐enolic forms does not exceed 1.0 kcal/mol, in rough agreement with the data calculated by the CNDO/2 approximation. Polar solvents favour the hydroxymethyleneketone form (C) for both groups of compounds <jats:bold>2</jats:bold> and <jats:bold>3</jats:bold>. The content of the hydroxymethyleneketone form is about the same within series <jats:bold>2</jats:bold> where R = Me, <jats:italic>i</jats:italic>‐Bu, Ph and is a little higher for the <jats:italic>t</jats:italic>‐Bu derivative. A decrease of temperature only slightly shifts the equilibrium of compounds <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold> to the hydroxymethyleneketone form, while in the case of 2‐methyl‐β‐ketoaldehydes (3) this effect is markedly pronounced.</jats:p>