Spectral properties and isomerism of nitroenamines. Part 4.1 β-Amino-α-nitro-α,β-unsaturated ketones

Abstract

A set of 4-alkyl(aryl)amino-3-nitrobut-3-en-2-ones (5), 4-ethylamino-3-nitropent-3-en-2-one (6), the related 3-alkyl(aryl)amino-2-nitro-2-cyclohexenones (7) with fixed geometry, and the N-deuteriated derivatives of some of them, were prepared and studied by vibrational (IR, Raman), NMR and, for some of the compounds, dynamic NMR spectroscopy. The spectra, considered together with the results of theoretical studies, provide a fairly accurate quantitative picture of the isomerism affecting 5 and 6. These compounds exist in solution as a mixture of the Z-isomer, having a strong hydrogen bond between the cis-related NO2 and the NH groups, in equilibrium with the E-isomer having a still stronger hydrogen-bond between the cis CO and NH groups. The proportions of the two isomers depend on concentration, solvent polarity, number of substituents around the CC bond and temperature, though the E-isomer is always the predominant one. The CH3CO group of the Z-isomers adopts, in the case of compounds 5, a planar s-cis conformation around the (C)C–C(O) single bond; in the case of the more sterically crowded compound 6, adopts a non-planar quasi-s-cis conformation. A low energy barrier between the configurational isomers was measured for compound 6 by dynamic 1H NMR spectroscopy. Vibrational couplings occur inside these strongly electron-delocalised systems, the extent of which depends on the molecular geometry, affecting mainly the ν(CC) and ν(C–N) modes, δ(N–H), and to a lesser extent ν(CO) and νa(NO2). The two isomeric forms can be readily distinguished and quantified by the spectra, and the energies of the intramolecular hydrogen bonds estimated by the large two-bond isotope effect, 2Δ13C(2/1H), observed on the C(1) chemical shifts on partially N-deuteriated samples. The spectral results for these compounds are discussed in comparison with those obtained for the simpler enaminones 1, nitroenamines 2, as well as for the related β-amino-α-nitro-α,β-unsaturated esters 4.