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 R<sup>1</sup>R<sup>2</sup>CHCONHPh systems. When R<sup>1</sup>R<sup>2</sup>CH<sub>2</sub> = Meldrum's acid (MA), the solid-state structure is that of the enol R<sup>1</sup>R<sup>2</sup>CC(OH)NHPh (<b>7</b>). In CDCl<sub>3</sub> solution the structure is <b>7</b>, 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 R<sup>1</sup>R<sup>2</sup>CC(OH)NH<sub>2</sub> (<b>12a</b>) (R<sup>1</sup>R<sup>2</sup>C = MA moiety) and a small preference for (MeO<sub>2</sub>C)<sub>2</sub>CC(OH)NHPh (<b>11b</b>) over the amide structures. However, solid <b>11</b> has the amide structure (MeO<sub>2</sub>C)<sub>2</sub>CHCONHPh (<b>11a</b>). NMR spectra in CDCl<sub>3</sub> show >90% of <b>11a</b>, but a minor species, probably <b>11b</b>, is also present. In DMSO this species is not observed. The analogous dimedone-substituted anilide <b>10</b> exists both in the solid state and in solution as an enol of a ring carbonyl. Calculations show that HC(CO<sub>2</sub>Me)<sub>3</sub> 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.