The structure and tautomerism of azo coupled β-Enaminones

This paper concerns a systematic structural study of the products of the reaction between diazonium salts and β-enaminones both in solution and in solid state. Azo coupling proceeds solely at the α-carbon atom of the β-enaminone. The structure of the azo coupling products is affected mainly by that of the starting enaminone; in the case of acyclic substrates, the directing influence is the nature of the amino group. Compounds having primary and secondary amino groups exhibit azo-hydrazone tautomerism, the position of which, depends on the nature of the N-substitution (i.e. H, alkyl, aryl). A novel type of azo coupling that results in a new class of azo coupling products is described and discussed. For cyclic enaminones the structure of the azo coupling products is affected mainly by ring size and either the presence or absence of an annelated benzene ring. Owing to the push-pull effect of the carbonyl and amino groups, some products exhibit, besides azo-hydrazone tautomerism, E/Z isomerism connected with various kinds of hydrogen bond.Results of the structural studies on azo coupled enaminones are summarized with regard equilibria, structure of the starting β-enaminone (acyclic, cyclic) and type of amino group (primary, secondary, tertiary). Studies were made in CDCl3 solution using 1H, 13C and 15N NMR spectroscopy and in the crystalline state using X-ray crystallography. Structural features leading to multiple azo coupling are analysed.