The preferential direction of enolization of some asymmetric 1,3-dicarbonyl compounds in solution: a study by multinuclear NMR spectroscopy

Previously described methods for determining the relative concentrations of the two cis-enol tautomers of asymmetric β-diketones using 13C and 17O NMR were compared and applied to a wider range of compounds. From the chemical shifts of the enol peaks and model shifts for the pure enol forms, the equilibrium constant for the enol—enol equilibrium was obtained. The small but significant effects of the β-substituents on the direction of enolization are in the following order of stabilization of the enol form at the adjacent carbonyl carbon: COOC2H5 > CF3 > C6H5 > C(CH3)3 > CH3. This relative order does not totally coincide with the preferred location of the regioselective attack of those compounds to form monodithioacetals. This indicates that, besides electronic effects, steric effects may also be quite important in determining regioselectivity in this type of compound.