Quantum-Chemical Investigation of Keto-enol-enethiol Tautomerism in 3-oxo-3-R1-N-R2-propanethioamides

Abstract

The keto-enol-enethiol tautomerism in 3-oxo-3-R 1 -N-R 2 -propanethioamides under vacuum and in acetone was studied in terms of density functional theory. It was established that the equilibrium depends on the structure of the 3-oxo-3-R 1 -N-R 2 -propanethioamides and on the nature of the solvent, but the most stable form is as a rule the keto form stabilized by an intramolecular hydrogen bond.Key words: 3-oxo-3-R 1 -N-R 2 -propanethioamides, keto-enol-enethiol tautomerism, quantum-chemical calculations, density functional theory.Like 1,3-thioxocarbonyl compounds, 3-oxo-3-R 1 -N-R 2 -propanethioamides are susceptible to the formation of intramolecular hydrogen bonds, as a result of which they can exist in ketone, enol, and enethiol forms, which are in dynamic equilibrium with each other. Unlike the keto-enol-enethiol tautomerism of 1,3-thioxo ketones, which have been studied in tens of papers while the results were summarized in the review [1], the keto-enol-enethiol tautomerism of 3-oxo-3-R 1 -N-R 2 -propanethioamides has only been discussed in [2][3][4]. In these papers the tautomeric equilibrium of 3-oxo-3-R 1 -N-R 2 -propanethioamides was studied by 1 H NMR and IR spectroscopy. In [3] the authors suggested that 3-oxo-propanethioamides can be present in solutions in the keto (K 1-3 ), enol (E 1-2 ), and enethiol (T 1-2 ) forms in dynamic equilibrium. These investigators established by 1 H NMR spectroscopy that the dominant form for 3-oxo-3-R 1 -N-R 2 -propanethioamides (R 1 = CH 3 , R 2 = Ar) in deuterochloroform solution is the keto form, in which the K 1 tautomer makes the largest contribution. The enol forms of 3-oxo-3-R 1 -N-R 2 -propanethioamides exist as two conformers (E 1 and E 2 ), the presence of which was demonstrated by low-temperature (-60°C) 1 H NMR spectroscopy [3]. In our paper [4] it was shown that the ratio of the keto and enol forms of 3-oxo-3-R 1 -N-R 2 -propanethioamides depends not only on the structure of the 3-oxo-3-R 1 -N-R 2 -propanethioamides but also on the nature (polarity) of the solvent. However, the existence of the enethiol conformations was not proved either by 1 H NMR spectroscopy [3] or by IR spectroscopy [2][3][4].It should be noted that 3-oxopropanethioamides, on the one hand, are convenient models for the study of prototropic tautomerism on account of their asymmetric structure but, on the other, there are experimental difficulties in determining the contributions from each tautomeric form. In addition, investigation of the tautomeric transformations of 3-oxo-3-R 1 -N-R 2 -propanethioamides is complicated by the presence of Z/E isomerism, which arises as a result of retarded 284 0040-5760/05/4105-0284