Journal of the Chemical Society. Perkin transactions II p. 787 - 798 (1992)
Update date:2022-08-02
Topics:
Chiara, Jose Luis
Gomez-Sanchez, Antonio
Bellanato, Juana
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)R1=C(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 C=C and C(1)-N streching 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 $vs(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(excit.) 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.
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