Spectral properties and isomerism of nitroenamines. Part 3

Abstract

Vibrational, NMR and dynamic NMR spectra, considered together with the results of theoretical studies, provide a complete and fairly accurate quantitative picture of the isomerism affecting the nitroenamines R2R3N–C(1)R1C(2)H–NO2(R1= H, Me). The compounds with primary or secondary amino groups (R2 and/or R3= H) exist as solvent-dependent equilibrium mixtures of the intramolecularly hydrogen-bonded Z-form and the E-form; the latter isomer can adopt the Z and/or the E conformation around the C(1)–N single bond when R2≠ R3. The compounds with a tertiary amino group exist solely in the E-form. Vibrational couplings occur inside the mesomeric system leading to an IR strong (medium or weak Raman)‘enamine’ band at 1650–1550 cm–1, the result of the asymmetrical coupling of the CC and C(1)–N stretching modes, and when R1 and R2= H, with contributions of the in-plane N–H and C(1)–H bending modes. The N–O stretchings do not contribute to the enamine band, but couple with other vibrations to give a weak IR and Raman band at 1530–1480 cm–1, with a main contribution of the νa(NO2), and a strong IR (medium or weak Raman) band, mainly, νs(NO2), at 1280–1230 cm–1. The energy barriers to rotation around the C(1)C(2) and C(1)–N bonds, and the ΔG‡ values for the ionization of the N–H group, indicated that the E-Z isomerization takes place by a thermal mechanism with dipolar transition state, with the contribution, in some of the compounds with an NH group, of an anionic mechanism.