Enols of Carboxylic Acid Amides with β-Electron-Withdrawing Substituents

The effect of stabilizing enols of carboxamides by several two β-electron-withdrawing substituents was studied with the R¹R²CHCONHPh systems. When R¹R²CH₂ = Meldrum's acid (MA), the solid-state structure is that of the enol R¹R²CC(OH)NHPh (7). In CDCl₃ solution the structure is 7, but there may be some exchange on the NMR time scale with a tautomer. B3LYP/6-31G** calculations show a significant preference for the enol R¹R²CC(OH)NH₂ (12a) (R¹R²C = MA moiety) and a small preference for (MeO₂C)₂CC(OH)NHPh (11b) over the amide structures. However, solid 11 has the amide structure (MeO₂C)₂CHCONHPh (11a). NMR spectra in CDCl₃ show >90% of 11a, but a minor species, probably 11b, is also present. In DMSO this species is not observed. The analogous dimedone-substituted anilide 10 exists both in the solid state and in solution as an enol of a ring carbonyl. Calculations show that HC(CO₂Me)₃ has a lower energy than its tautomeric enol. The effects of the push−pull structures of the enols on structural and spectrometric parameters, of the β-substituents, of the planarity of the system, of the acid derivative group (ester or anilide), and of the solvent as enol-stabilizing factors are discussed. Destabilization of the acid form contributes to the increased relative stability of the enols.