SPECTRAL PROPERTIES AND ISOMERISM OF NITROENAMINES - PART 4 - BETA-AMINO-ALPHA-NITRO-ALPHA,BETA-UNSATURATED KETONES

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-cy clohexenones (7) with fixed geometry, and the N-deuteriated derivative s of some of them, were prepared and studied by vibrational (IR, Raman ), NMR and, for some of the compounds, dynamic NMR spectroscopy. The s pectra, considered together with the results of theoretical studies, p rovide a fairly accurate quantitative picture of the isomerism affecti ng 5 and 6. These compounds exist in solution as a mixture of the Z-is omer, having a strong hydrogen bond between the cis-related NO, and th e NH groups, in equilibrium with the E-isomer having a still stronger hydrogen-bond between the cis C=O and NH groups. The proportions of th e two isomers depend on concentration, solvent polarity, number of sub stituents around the C=C bond and temperature, though the E-isomer is always the predominant one. The CH3CO group of the Z-isomers adopts, i n 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 compou nd 6, adopts a non-planar quasi-s-cis conformation. A low energy barri er between the configurational isomers was measured for compound 6 by dynamic H-1 NMR spectroscopy. Vibrational couplings occur inside these strongly electron-delocalised systems, the extent of which depends on the molecular geometry, affecting mainly the nu(C=C) and nu(C-N) mode s, delta(N-H), and to a lesser extent nu(C-O) and nu(a)(NO2). The two isomeric forms can be readily distinguished and quantified by the spec tra, and the energies of the intramolecular hydrogen bonds estimated b y the large two-bond isotope effect, (2)Delta(13)C(H-2/1), observed on the C(1) chemical shifts on partially N-deuteriated samples. The spec tral results for these compounds are discussed in comparison with thos e obtained for the simpler enaminones 1, nitroenamines 2, as well as f or the related beta-amino-alpha-nitro-alpha,beta-unsaturated esters 4.