Enols of amides. the effect of fluorine substituents in the ester groups of dicarboalkoxyanilidomethanes on the enol/amide and E-enol/Z-enol ratios. A multinuclei NMR study

Condensation of phenyl isocyanate substituted by 4-MeO, 4-Me, 4-H, 4-Br, an d 2,4-(MeO)(2) with esters CH2(CO2R)CO2R', R = CH2CF3, R' = CH3, CH2CF3, CH (CF3)(2), or R = CH3, R' = CH(CF3)(2) gave 17 "amides" ArNHCOCH(CO2R)CO2R' containing three, six, or nine fluorines in the ester groups. X-ray crystal lography of six of them revealed that compounds with greater than or equal to6 fluorine atoms exist in the solid state as the enols of amides ArNHC(OH )=C(CO2R)CO2R' whereas the ester with R = R' = CH3 was shown previously to have the amide structure. In the solid enols, the OH is cis and hydrogen bo nded to the better electron-donating (i.e., with fewer fluorine atoms) este r group. X-ray diffraction could not be obtained for compounds with only th ree fluorine atoms, i.e., R = CH2CF3, R' = CH3 but the C-13 CP-MAS spectra indicate that they have the amide structure in the solid state, whereas est ers with six and nine fluorine atoms display spectra assigned to the enols. The solid enols show unsymmetrical hydrogen bonds and the expected feature s of push-pull alkenes, e.g., long C-alpha=C-beta bonds. The structure in s olution depends on the number of fluorine atoms and the solvent, but only s lightly on the substituents. The symmetrical systems (R = R' = CH2CF3) show signals for the amide and the enol, but all systems with R not equal R' di splayed signals for the amide and for two enols, presumably the E- and Z-is omers. The [Enol I]/[Enol II] ratio is 1.6-2.9 when R = CH2CF3, R' = CH3, C H(CF3)(2) and 4.5-5.3 when R = CH3, R' = CH(CF3)(2). The most abundant enol display a lower field delta (OH) and a higher field d(NH) and assigned the E-structure with a stronger O-H . . .O=C(OR) hydrogen bond than in the Z-i somer. delta (OH) and delta (NH) values are nearly the same for all systems with the same cis CO2R group. The [Enols]/[Amide] ratio in various solvent s follows the order CCl4 > CDCl3 > CD3CN > DMSO-d(6). The enols always pred ominate in CCl4 and the amide is the exclusive isomer in DMSO-d(6) and the major one in CD3CN. In CDCl3 the major tautomer depends on the number of fl uorines. For example, in CDCl3, for Ar = Ph, the % enol (K-Enol) is 35% (0. 54) for R = CH2CF3, R' = CH3, 87% (6.7) for R = R' = CH2CF3, 79% (3.8) for R = CH3, R' = CH(CF3)(2) and 100% (greater than or equal to 50) for R = CH2 CF3, R' = CH(CF3)(2). O-17 and N-15 NMR spectra measured for nine of the en ols are consistent with the suggested assignments. The data indicate the im portance of electron withdrawal at C-6, of intramolecular hydrogen bonding, and of low polarity solvents in stabilizing the enols. The enols of amides should no longer be regarded as esoteric species.