Abstract-Tautomerism of aromatic [j-ketoaldehydes p-XPhCOCHzCHO (1, X = NMe,, OMe, Me, H, Br, NOz), aliphatic B-ketoaldehydes and benzoylacetaldehyde RCOCH,CHO (2, R = Me, i-Bu, f-Bu, Ph), RCOCH(Me)CHO (3, R = Me, Et, i-Pr) and methyl 2-formylpropionate MeOCOCH(Me)CHO (4) has been studied by the 'H NMR technique. In basic solvents both cis-and trans-enol forms of these compounds co-exist. trans-Enolisation, which occurs exclusively at the formyl group, is most favoured in compound (4) and least favoured in compounds (1) and (2). The increasing electron-attracting property of the substituent X in the aromatic p-ketoaldehydes (l), as well as increasing solvent basicity in the series propanediol-l,2-carbonate, acetone < dimethylformamide < dimethylacetamide < pyridine, also shifts the equilibrium towards the frans-enol form. The trans-en01 form is absent in aprotic solvents of low basicity such as CCI,, C2HCI, and toluene. The thermodynamic parameters of the cis-trans-enol (C Z T) and cis-enol-enolic (C z C') equilibria have been estimated from the temperature dependences. The transition from the cis-to the trans-enol form is accompanied by an entropy decrease of about 10 cal mol-l degree+. Nevertheless the frans-enol form is stabilised due to its lower enthalpy. The cis-trans-enol equilibrium is determined by the relative strength of the intramolecular hydrogen bond in the cis-enol form and the intermolecular hydrogen bonds with basic solvent molecules of the trans-enol form.The enthalpy difference of the two cis-enolic forms does not exceed 1.0 kcal/mol, in rough agreement with the data calculated by the CND0/2 approximation. Polar solvents favour the hydroxymethyleneketone form (C) for both groups of compounds 2 and 3. The content of the hydroxymethyleneketone form is about the same within series 2 where R = Me, i-Bu, Ph and is a little higher for the I-Bu derivative. A decrease of temperature only slightly shifts the equilibrium of compounds 1 and 2 to the hydroxymethyleneketone form, while in the case of 2-methyl-P-ketoaldehydes (3) this effect is markedly pronounced.
