Enols of Amides Activated by the 2,2,2-Trichloroethoxycarbonyl Group

Reaction of isocyanates XNCO (X = Ar, i-Pr, t-Bu) with CH2(Y)CO2CH2CCl3 (Y = CO2Me, CO2CH2CCl3, CN) gave 15 amides XNHCOCH(Y)CO2CH2CCl3 (6) or enols of amides XNHC(OH)C(Y)CO2CH2CCl3 (5) systems. The amide/enol ratios in solution depend strongly on the substituent Y
and the solvent and mildly on the substituent X. The percentage of enol for group Y increases
according to Y = CN > CO2CH2CCl3 > CO2Me and decreases with the solvent according to CCl4 >
C6D6 > CDCl3 > THF-d8 > CD3CN > DMSO-d6. With the most acidic systems (Y = CN) amide/enol
exchange is observed in moderately polar solvents and ionization to the conjugate base is observed
in DMSO-d6. The solid-state structure of the compound with Y = CN, X = i-Pr was found to be
that of the enol. The reasons for the stability of the enols were discussed in terms of polar and
resonance effects. Intramolecular hydrogen bonds result in a very low δ(OH) and contribute to the
stability of the enols and are responsible for the higher percentage of the E-isomers when Y =
CO2Me and the Z-isomers when Y = CN. The differences in δ(OH), δ(NH), Kenol, and E/Z enol ratios
from the analogues with CF3 instead of CCl3 are discussed.