Spectral properties and isomerism of nitroenamines. Part 3

Abstract

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.

15N,13C and1H NMR study of azo coupling products from diazonium salts and enaminones

“…The chemical shift υ 15 N is only slightly sensitive to the change of solvent from CDCl 3 to CD 3 2 SO (Table 1). The chemical shift υ 15 N of 4-aminopent-3-en-2-one (1a) is very close to the value published 4 for 4-amino-3-methylpent-3-en-2-one ( 274.4 in C 6 D 6 ) and to the value of 274.8 found for 4-amino-3-ethylpent-3-en-2-one (1d) in CDCl 3 . Only the spectrum of 4-methylaminopent-3-en-2-one (1b) exhibited three signals (Table 1).…”

“…The 1 H and 13 C NMR spectra of 4-amino-3-(4-chlorophenylazo)pent-3-en-2-one have already been published. 2 The proportions of minor isomers in both compounds mentioned was low (Table 3), hence it was only possible, from the 1 H, 15 N GHMQC sequence, to determine reliably the coupling constants 1 J 15 N, 1 H of the major isomers because of the low solubility in CDCl 3 . Their values are close to those of the corresponding protons of 4-amino-3-phenylazopent-3-en-2-one ( Table 3).…”

“…Structurally similar to the products of azo coupling reactions with enamines areˇ-amino-˛-nitro-˛,ˇ-unsaturated ketones, whose structure was studied by Chiara et al using 1 H NMR and IR spectra. 3 The aim of this work was both to verify earlier information 2 about tautomerism of the coupling products prepared from enaminones with the use of a larger set of models and to synthesize their selected 15 N isotopomers and, by means of 15 N NMR spectra (not yet studied) of isotopically labelled and unlabelled substances, to obtain further more detailed information about their tautomerism.…”

Recent development in preparation reactivity and biological activity of enaminoketones and enaminothiones and their utilization to prepare heterocyclic compounds

“…If there is at least one hydrogen atom at the nitrogen, then the Z-isomer contains an intramolecular hydrogen bond. The E-isomer (without this intramolecular hydrogen bond) containing two different substituents at nitrogen can exist as a pair of isomers of limited stability (their stability lies between that of conformers and the geometrical isomers) as a consequence of partial double bond character of the C-N bond [6,7]. The conjugation of the system facilitates the interconversion between the Z and the E forms.…”

New Approaches for the Synthesis of Thiazoles and Their Fused Derivatives with Antimicrobial Activities

“…It is convenient to notice that compounds, 5, 8a-d, 11a,b, 14a-d, 18a-d, 21a-c, 22a-d and 23a,b can exist in either Z or E structures. However, according to the concept of push-pull alkenes that was reviewed by Sandstrom,17 where the physicochemical properties and chemical reactivity of numerous functionalized compounds of that type have been extensively studied, [18][19][20][21][22][23] and the recent studies which showed that the 4-oxothiazolidines with one and two exocyclic double bonds attached to a thiazolidine ring, respectively, exemplify typical push-pull compounds which can exist in Z/E equilibrium depending on temperature and the nature of the solvent. 24 One can say that compounds 5, 8a-d, 11a,b, 14a-d, 18a,b, 21a-c, 22a-d and 23a,b are examples of push-pull conjugated alkenes which are existing in a Z/E equilibrium mixture.…”