Solution and solid-state study of the structure of azo-coupling products from isomeric enaminones possessing tert-butyl group: An unprecedented observation of pure hydrazo form in azo coupled N-alkyl β-enaminones

Abstract The structure of the azo-coupling products from enaminones derived from 4,4-dimethyl-1-phenylpentane-1,3-dione has been studied by means of solution-state 1 H, 13 C and 15 N NMR spectroscopy and X-ray diffractometry. The presence of bulky tert -butyl group hinders or even prevents from the formation of planar conjugated heterodiene system H N C C N N with an intramolecular hydrogen bond N H⋯N which is the prerequisite for fast tautomeric exchange imino-hydrazo – enamino-azo. The minor amount of azo compounds is formed by a proton exchange through a hydrogen bond N H⋯N, which is either intramolecular (in solution) or intermolecular (solid state). The intermolecular exchange proceeds via the dimers of the azo coupling products. This is unprecedented result among the similar molecules hitherto studied.