Vibrational, N M R and dynamic N M R 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 ) R'=C(2) H-NO, (R' = H, Me). The compounds with primary or secondary amino groups ( R2 and/or R3 = H) exist as solvent-dependent equilibrium mixtures of the intramolecularly hydrogenbonded Z-form and the €-form; the latter isomer can adopt the Z and/or the € conformation around the C(1)-N single bond when R2 # R3. The compounds with a tertiary amino group exist solely in the Eform. Vibrational couplings occur inside the mesomeric system leading to an IR strong (medium or weak Raman) 'enamine' band at 1650-1550 cm-', the result of the asymmetrical coupling of the C=C and C( 1 )-N stretching modes, and when R' and R2 = H, with contributions of the in-plane N-H and C( 1 )-H bending modes. The N-0 stretchings do not contribute to the enamine band, but couple with other vibrations to give a weak IR and Raman band at 1530-1 480 cm-', with a main contribution of the v,(NO,), and a strong IR (medium or weak Raman) band, mainly v,(NO,), at 1280-1230 cm-'. The energy barriers to rotation around the C(l)=C(2) and C(1)-N bonds, and the AGt 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.Nitroenamines have attracted interest because of their potential use in organic synthesis and their biological a ~t i v i t y . ~ A knowledge of the isomerism and electron distribution inside these mesomeric systems is of paramount importance in understanding their properties and reactivity. As in similar push-pull ethylenes, spectroscopic techniques combined with theoretical studies can provide information on these matters, and we have previously reported on the NMR and vibrational spectra of 3-amino-2-nitroacrylic (1) and 3-amino-2-nitro-NO2 C02Me R2 R3N C( R1 ) ==( 1 R ' = H 2 R L M ~ NO2 R2R3NC( R 1 ) 7 ( 3 R ' = H 4 R'=Me * The symbols indicate, in the order shown, the configuration around the C(l)=C(2) bond and the conformation around the C(1)-N single bond.
A set of 3‐alkylaminoacroleins (R1NHCH=CHCH=O, R1=alkyl) were studied by 13C and 1H NMR spectroscopy in solutions of different solvents and, for the simplest representative of the series, 3‐methylaminoacrolein, at different temperatures. The equilibrium solutions of these compounds consists of mixtures of the chelated ZZE form (Z geometry around the =C—C=O single bond and the C=C bond, and E geometry around the C—N bond) and the two E configurational isomers having the E disposition around the =C—C=O single bond and differing in the disposition around the C—N bond (EEZ and EEE forms). The relative proportions of the three isomers depend on solvent polarity, concentration, bulk of substituent R1 and temperature. The EEZ isomer is the most abundant in polar solvents, while the concentration of the ZZE form increases in non‐polar solvents and when increasing the bulk of R1. A lineshape 1H NMR study for 3‐methylaminoacrolein in CDCl3 gave a ΔG‡ value of 66.0 kJ mol‐1 at 303 K for the EEZ→EEE conversion. The presence of other tautomeric forms was not observed. © 1998 John Wiley & Sons, Ltd.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.