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Synthesis and tautomerism of two benzothiazolyl azo dyes

Azo dyes with hydroxy group in conjugation to the azo chromophore exhibit azo-hydrazone tautomerism, and the existence of this proton transfer between oxygen and nitrogen atoms is quite interesting from the theoretical and practical aspects. The position of tautomeric equilibrium affects the basic properties of azo dyes, such as color, tone and photostability, as well as technical properties of different tautomers. On the other hand, replacement of aromatic diazo component with heteroaromatic is interesting from the toxicological point of view because some of aromatic azo dyes can undergo natural reductive cleavage of azo group to potentially carcinogenic amines. We report the efficient synthesis of 6-[(2-hydroxy-1-naphthyl)diazenyl]-2-methylbenzothiazole and investigate tautomerism of two benzothiazolyl azo dyes using C–2 carbon chemical shifts. For unequivocal determination of C–2 chemical shift a complete assignment of signals in 1H and 13C NMR spectra was established by 2D NMR techniques. These azo dyes exist as a rapidly exchanging mixture of azo and hydrazone forms and the tautomeric equilibrium are slightly shifted towards hydrazone forms.

azo dyes; benzothiazole; NMR spectroscopy; tautomerism

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