Cyano-, nitro-, and alkoxycarbonyl-activated observable stable enols of carboxylic acid amides

A search for the enol structures of several amides YY'CHCONHPh with Y,Y' = electron-withdrawing groups (EWGs) was conducted. When Y = CN, Y' = CO2Me the solid structure is that of the enol (8b) MeO2CC(CN)=C(OH)NHPh, whereas in solution the NMR spectrum indicate the presence of both the amide MeO2CCH(CN)CONHPh (8a) and 8b. When Y = NO2, Y' = CO2Et the main compound in CDCl3 is the amide, but <10% of enol(s); presumably EtO2CC(NO2)=C(OH)NHPh (9b), are also present. When Y = COEt, Y' = CO2Me or Y = COMe, Y' = CO2Et (10 and 11) enolization in solution and of 11 also in the solid state occurs at the carbonyl rather than at the; ester site. With Y = Y' = CN a rapid exchange between the amide(NC)(2)CHCONHPh (12a) and a tautomer, presumably the enol, take place in several solvents on the NMR time scale. With YY' = barbituric acid moiety the species in DMSO-d(6) is an enol of an amide although which CONH group enolizes is unknown. B3LYP/6-31G** calculations showed that the enol (NC)(2)C=C(OH)NH2 (13b) is more stable by <Delta>G of 0.4 kcal/mol than (NC)(2)CHCONH2 (13a) due to a combination of stabilization of 13b and destabilization of 13a and both are much more stable than the hydroxyimine and ketene imine tautomers. The effect of Y,Y' and the solvent on the relative stabilization of enols of amides is discussed.