Spectral properties and isomerism of nitro enamines. Part 2. 3-Amino-2-nitrocrotonic esters

Abstract

The IR Raman, ¹H and ¹³C NMR spectra of a series of 3-amino-2-nitrocrotonic esters (3), with both primary and secondary amino groups, have been studied. The spectral data show that these compounds exist in solution as equilibrium mixtures of the two enantiomeric quasi-s-cis-(Z) isomers (3A′)+(3A″) and the planar, and less polar s-cis(E)-(3C) isomer, the ratio [(3A′)+(3A″)]: (3B) being strongly dependent on the polarity of the medium. The E- and Z-forms can be easily distinguished by their vibrational and NMR spectra, and are separated by very low energy barriers (ΔG^‡50–70 kJ mol^–1), the magnitude of which decreases on increasing the electron-donor capacity of the substituent at the amino function. The spectral data and AM1 calculations indicate a strong polarization of the nitro enamine system with a large negative charge accumulation at C-2 and, to a lesser extent, at the NO₂ group. This group acts as a weak π- and a strong σ-acceptor, and the ester group acts as a weak π-acceptor group and a σ-donor.