13C NMR and crystallographic evidence of push-pull effects in furoindolic ?-enamino esters

A series of cyclic enamine derivatives, stabilized through conjugation with an electron-attracting CO2Me group at the ?-carbon of the enamine system, has been prepared. The influence of substituents at the vinylogous donor NH2 group on 13C chemical shifts has been investigated, and the results analyzed on the basis of resonance and steric effects on the dipolar charge distribution in the molecule. Intramolecular H-bonding was established for the N-methylated enamines 4a and 4b based on the high-frequency shift of the NH proton and its solvent immutability. Several deuterium-induced isotope effects on 13C chemical shift [n?C(ND)] due to deuteration at the enamine nitrogen have been measured. The crystal structures of the enamine 1b and the N,N-diacetylated enamine 3a, determined by X-ray analyses, show that these compounds, in the solid state, adopt an s-trans disposition around the C-C=O bond, with the carbonyl ester group and the C=C double bond in an almost coplanar conformation. Comparison of bond lengths of 1b and 3a in the crystalline state provides information on changes in the extent of n-? delocalization arising from the nature of the N-substituent