Tautomeric equilibria of para-bromophenyl substituted arylhydrazones of β-diketones

New arylhydrazone derivatives of β-diketones, 2-(4-bromophenylhydrazono)-5,5-dimethylcyclohexane-1,3-dione (1), 5-(4-bromophenylhydrazono)pyrimidine-2,4,6(1H,3H,5H)-trione (2) and (E)-2-(4-bromophenylhydrazono)-1-phenylbutane-1,3-dione (3), were prepared by coupling of β-diketone with the respective para-bromophenyldiazonium chloride, and characterized by IR, 1H and 13C NMR spectra. The IR and NMR spectral data disclose an effective intramolecular hydrogen bonding in all the cases. The single crystal X-ray analysis of 3-(4-bromophenyl-hydrazono)pentane-2,4-dione (4) evidences the intramolecular resonance-assisted hydrogen bonding (RAHB) which is significantly strengthened in view of the planarity of the six atoms involved in the resonance cycle. Compound 3 derived from the unsymmetric 1-phenylbutane-1,3-dione exists in solution as a mixture of the enol-azo and hydrazone tautomeric forms and an increase of the solvent polarity shifts the tautomeric balance to the enol-azo form, while for 1, 2 and 4, derived from the symmetric β-diketones, the hydrazone form is dominating under all the studied conditions.

► Three new para-Br substituted arylhydrazones of β-diketones (AHBD) were synthesized and fully characterized.
► Unsymmetrical bromo-substituted AHBD exist in the hydrazone and enol-azo tautomer forms in solution.
► Tautomeric balance shifts from hydrazone to the enol-azo form with increase of the solvent polarity.