NMR of Enaminones

Natural abundance 17O NMR spectra of 55 primary and secondary enaminones with various substituents at the C-1, C-3 and N-positions are reported. Analyses of the 17O chemical shifts of these enaminones give the values of the increments for the substituents at these positions. The influence of the substituents on the 17O chemical shifts of the carbonyl groups of the enaminones is additive. It is found that the 17O chemical shifts of the enaminones can be calculated by the sum of the substituent increments. The differences between the calculated and the experimental data are generally less than ±2 ppm. The large deviations observed for the enaminones with a primary amino group are explained in terms of the formation of intermolecular hydrogen bonds. The chemical shift differences between the E- and Z-forms of enaminones is mainly attributed to intramolecular hydrogen bonding. The contribution coming from intramolecular hydrogen bonding (ΔδHB), ranging from -14 to -47 ppm, depends on the donor properties of the amino groups and the type of substituents. The 17O chemical shifts of these enaminones correlate well with each other and with the pKa values of the corresponding alkylamines. Fair correlations of the 17O data of carbonyl groups with the 1H-2 chemical shifts are observed, suggesting that the change of electron density on the carbonyl O atom parallels that on C-2.