NMR of enaminones .3. H-1, C-13 and O-17 NMR spectroscopic studies of acyclic and cyclic N-aryl enaminones: Substituent effects and intramolecular hydrogen bonding

O-17, C-13 and H-1 NMR spectra for para- and meta-substituted 4-arylaminopent-3-en-2-ones (acyclic enaminones, 1 and 2) and 3-arylaminocyclohex-2-en-1-ones (cyclic enaminones, 3 and 4) are reported. The O-17, C-13 and H-1 shift values of these enaminones correlate well with sigma(m)(0) and sigma(p)(-) constants in the correlations for meta and para derivatives, and with pK(a) values of the corresponding anilines. Dual substituent parameters analyses were also performed. Correlations of O-17 and C-13 chemical shifts of the carbonyl groups with those of the corresponding N-acylanilines indicate that the enaminone moiety as a whole has electronic properties similar to those of the RCONH group. The O-17 shift values of the carbonyl O atoms of enaminones correlate well with their H-1 and C-13 data. Shieldings of 33-45 ppm for O atoms are observed for 1 and 2 compared with 3 and 4, respectively. This is attributed to intramolecular hydrogen bonding. (C) 1997 by John Wiley & Sons, Ltd.