Vibrational spectroscopy and conformations of 2-alkoxycarbonyl-3-aminoacrylic esters

Abstract

The i.r., Raman, and ¹H n.m.r. spectra of 2-alkoxycarbonyl-3-aminoacrylic esters (IV; R¹= H, alkyl, aryl) and those of their N-deuteriated derivatives show that these substances exist in the enamino-ester form with the a strong intramolecular hydrogen bond between the cis-NH (or -ND) and CO₂R² groups. Some complexities in the i.r. and Raman bands, and their dependence on solvent polarity, temperature, and aggregation state, indicate the existence of rotational isomerism: conformers (IVa and b) predominate in non-polar media; an additional rotamer (IVd) is also probably present in polar solvents. Conformation (IVb) is preferred in the solid state. Some features of the ν(CO) bands suggest a weak vibrational coupling between the carbonyls. The validity of the criteria established by Smith and Taylor to ascertain the planarity of these and related substances is discussed, and no significant difference in this respect is considered to exist between compounds(IV) with primary and secondary amino-groups.