Gas-phase tautomerism in hydroxy azo dyes - from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol

Abstract

The tautomeric constants of a series of azo dyes were estimated in the gas phase by using electron ionization mass spectrometry. It was shown that the relative amount of the keto tautomer increases from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol, thus confirming the quantum-chemical predictions. The existence of the enol tautomer of 4-phenylazo-anthracen-1-ol is shown for the first time by mass spectrometry in the gas phase. This finding is supported by flash photolysis measurements in solution.

Tautomerism in Azo and Azomethyne Dyes: When and If Theory Meets Experiment

“…The proton acceptor/donor sites in the tautomers of 1 are accessible for the solvents, which allows for a large scale of the tautomeric constant change [38]. In 6, due to aromatic reasons, the tautomeric equilibrium is shifted almost fully to the keto form [13,39,40]. The structural effects are also different; substitution on para position of the phenyl ring in the azodyes leads to a shift of the tautomeric equilibrium towards the enol form when electron donor group is implemented and vice versa.…”

Spectroscopic studies of keto–enol tautomeric equilibrium of azo dyes

“…Electron ionization-mass spectrometry (EI-MS) technique was used to estimate the tautomeric forms of a series of azo dyes (Scheme 3: 22, 23, 24) [19]. The gas-phase tautomerism in 4phenylazo-1-phenol, 4-phenylazo-naphthalen-1-ol and 4-phenylazo-anthracen-1-ol has been studied.…”

(E)-1-(4-Methoxyanthracen-1-yl)-2-phenyldiazene

“…For investigations of the tautomerism of azodyes, see: Kelemen (1981). For the synthesis of the title compound, see: Nedeltcheva et al (2010). Table 1 C-HÁ Á Á interactions (Å , ).…”

“…In such cases model compounds, possessing the characteristics of the corresponding end-structures, are usually applied. As a part of our interest in sensing molecules based on tautomeric switches (Nedeltcheva et al, 2009, the equilibrium in 4-phenylazoantracene-1-ol was studied in the gas phase by mass spectrometry, and in solution by flash photolysis (Nedeltcheva et al, 2010). The corresponding O-methyl and N-methyl derivatives were used as model enol and keto tautomers, respectively, and the tautomeric constant was estimated.…”

“…Simple methylation of (E)-4-(phenyldiazenyl)anthracen-1-ol in basic media gave an easy separable mixture of the title compound and the corresponding N-methyl derivative, with yields of 32% and 43%, respectively (Nedeltcheva et al, 2010). Dark red block-like crystals of the title compound, suitable for X-ray diffraction analysis, were grown by slow diffusion of hexane into a chloroform solution of the title compound.…”