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 ν_a(NO₂), and a strong IR (medium or weak Raman) band, mainly, ν_s(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.