Structure and tautomerism of azo coupling products from N-alkylenaminones derived from acetylacetone and benzoylacetone in solid phase and in solution

A series of azo coupling products have been prepared by reaction of substituted benzenediazonium tetrafluoroborates with N-alkyl 4-aminopent-3-en-2-ones or 3-amino-1-phenylbut- 2-en-1-ones. The structure and tautomerism of the reaction products were studied by means of single-crystal X-ray study and by NMR spectroscopy in deuteriochloroform solution. The azo coupling products obtained from 4-methylaminopent-3-en-2-ones (3a-i) exist in CDCl3 solution as E/Z isomer mixtures with the Z isomer strongly predominating. The major isomer is a mixture of enamino - azo and imino - hydrazo tautomers with the former predominating. The proportion of the azo form depends on substitution of the benzene ring of the diazonium salt and decreases in the order of MeO > Me > Br > NO2. The position of tautomeric equilibrium is practically unaffected by switching from 4-methylaminopent-3-en-2-ones to 3-methylamino-1-phenylbut- 2-en-1-ones. In the solid phase, the azo form always predominates; substitution of diazonium salt and at N3 nitrogen does not significantly affect the position of the tautomeric equilibrium. The azo coupling products always exist in the form of a single Z isomer. All determined structures, in the solid state, consist of a mixture of the two tautomeric forms, amino - diazenyl and imino - hydrazone, in ratios ranging from 82/18 to 91/9%. The weighed superimposition of both the hydrogen-bonded N1=N2 - C1=C2 - N3H/HN1 - N2=C1 - C2=N3 heterodienic fragments, however, do not allow to clarify the effects of the para-substituents at the N1-phenyl ring both on the N1 center dot center dot center dot N3 hydrogen-bond distances and on the bond lengths in the heterodienic systems within the series 3a - c, f and 4a - c, e, f.