Синтез, спектральне та теоретичне дослідження 5-(2-гідоксифеніл)-4-металіл-1,2,4-триазол-3-тіону

Abstract

The investigation of tautomeric transformations of heterocyclic compounds is an important step for understanding and predicting the reactivity of such systems. Triazoles with a 2-hydroxyphenyl substituent have already been used in medicine due to their chelating properties. Also, such derivatives are interesting objects for numerous physico-chemical studies. We, in turn, decided to investigate the possibility of keto-enol tautomeric transformations in the structural pair of phenol-1,2,4-triazole on the model object – 4-methallyl-5-(2-hydroxyphenyl)-1,2,4-triazole-3-thione. The thione-thiol tautomerism of 1,2,4-triazole-3-thiones is well described in numerous literature data. Instead, in this work, we decided to investigate the transport of the proton between the phenol hydroxyl and the atom nitrogen in the first position of the triazole cycle. As the main prerequisites for this kind of tautomerism, we consider the acidic properties of phenol and the basic properties of the triazole ring. For reliable establishment of the structure of the triazole model in the DMSO solution, the 1HNMR spectrum was recorded and analyzed in detail. A quantum-chemical study was performed to establish a stable form of target triazole in DMSO solution. Namely, the calculation of 1H-NMR spectra of the enol and zwitterionic/keto forms was carried out. Initially, the geometry optimization of the two forms (enol and keto) was performed by density functional theory method B97-D3 in combination with a double-zeta basis set 6-31G(d, p), which contains one polarization function for all atoms. The choice of this method is due to its ability to well simulate systems in which weak dispersion interactions and hydrogen bonds are strongly influenced. In addition, the total energy of two structures was calculated on the basis of previously optimized structures, but using larger basis set 6-311++G(2d,2p). The total energy for two enol and keto-forms was –691879.1 kcal/mol and –691858.3 kcal/mol, respectively. That is why, in the DMSO solution, the considered here triazole exists in the enol form, which is for 20.8 kcal/ mol more preferable than keto-form. Keywords: triazole; DFT; tautomer; NMR.