Synthesis and tautomerism of new 2-substituted benzothiazolyl azo derivatives of 2-naphthol

The azo dyes with a hydroxyl group in conjunction with an azo chromophore exhibit azo-hydrazone tautomerism. The investigation of proton transfer between oxygen and nitrogen atoms is interesting both from the theoretical and practical points of view. The position of the tautomeric equilibrium affects the basic properties of azo dyes such as colour, tone and photostability, as well as their technical properties. We have recently reported the study of azo-hydrazone tautomerism of 6-[(2-hydroxy-1-naphthyl)diazenyl]benzothiazole and 6-[(2-hydroxy-1-naphthyl)diazenyl]-2-methyl benzothiazole jointly by 13C NMR spectroscopy and quantum-mechanical calculations [1]. The Kazo-hydrazone(H)/Kazo-hydrazone(CH3) ratio of 1.3 suggests that the electron-donating substituents move the position of the equilibrium towards the azo form.Here, we report the synthesis and the tautomerism investigation of several new derivatives which exhibit more pronounced equilibrium shift. Using 13C NMR spectroscopy and DFT calculations we estimate the relative proportions of azo and hydrazone forms present at the equilibrium, as well as draw general conclusions about electron density redistribution upon the substitution.