Vibrational spectroscopy and conformations of 2,2-diacylethenamines

Abstract

The i.r., Raman, and ¹H n.m.r. spectra of 2,2-diacetylethenamines (3) and 2-aminomethylene-5,5-dimethylcyclohexane-1,3-diones (4) show that these substances exist exclusively in the chelated enamino-diketone form, and that the conjugated system contained in them is essentially planar. The open-chain enamino-diketones (3) exist either in the solid state or in solution in the EZE-conformation (7). A similar conjugated core, having the fixed ZZE-conformation, contained in 5-aminomethylene-2,2-dimethyl-1,3-dioxane-4,6-diones (5) is most probably planar or nearly planar. The EZE- and ZZE- alignments of the enamino-diketones can be distinguished by their spectral properties, and particularly, by their vibrational spectra. Some features of the i.r. and Raman spectra indicate vibrational coupling between the two ν(CO) vibrations, the effect being more marked in the cyclic enamino-diketones (4) and -diesters (5) than in their open-chain analogues. On the other hand, the coupling of the ν(CO) with the ν(CC) and the δ(N–H) modes seems to be less efficient in these enaminodione systems than in the related simple enaninones. Further arguments are presented questioning the criteria established to ascertain the planarity of enamino-diones and of other enamines having two strong electron-acceptor groups at C-2.