Thione–thiol tautomerism and stability of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines

Abstract

The possible thione–thiol tautomerism of 2- and 4-mercaptopyridines, 2-mercaptopyrimidine, and 4,6-dimethyl-2-mercaptopyrimidine in solution is studied by means of absorption (UV–VIS) spectroscopy. In accordance with earlier observations, polar solvents and self-association shift the apparent tautomeric equilibrium significantly towards the thione form. In dilute solutions of nonpolar solvents the thiol form predominates. On standing, significant changes are observed in the absorption spectra of these tautomeric compounds in ethanol, dioxane, and water. The time course of the tautomerization, followed spectrophotometrically, reveals quantitative transformation of the thiol form to the corresponding symmetrical disulfides. The influence of concentration, temperature, and irradiation with indirect sunlight are discussed. This thione–disulfide process is reversible in water, starting either from the tautomeric thione or from its symmetrical disulfide, implying a possible importance in biological systems. Keywords: thione–thiol tautomerism, absorption spectra, thiol–disulfide oxidation, mercaptopyridines and pyrimidines.

Concomitant Effects of Transition Metal Chelation and Solvent Polarity on the First Molecular Hyperpolarizability of 4-Methoxyacetophenone Thiosemicarbazone: A DFT Study

“…From the tot values of the thione-thiol tautomers of MAPTSC, it is clear that the second-order NLO response of MAPTSC is potentially switchable especially in polar solvents, due to the presence of a tautomeric form (thiol form) with a higher CT capability. Since thione-thiol equilibrium is readily influenced by factors such as the nature of the solvent, temperature, and concentration [38], the switchable NLO response of MAPTSC can be easily fine-tuned by modifying these factors. The coordination of L1 to different transition metal centers results in significant modifications of its tot value.…”

Synthesis and Structure of the Group 12 Pyrimidinethiolate Complexes ∞3[Zn(S‐2‐N2C4H3)2], ∞2[Cd(S‐2‐N2C4H3)2], [Hg(S‐2‐N2C4H3)2] and[Cd(S‐2‐N2C4H3)2(tmeda)]

“…[21] In solution, pyrimidines and pyridines that are substituted at the 2-or 4-position by thiol groups can exist in two tautomeric forms I (thiol) and II (thione; Scheme 3). [22] In the solid state, 2-mercaptopyridine and 4-mercaptopyridine have been found to exist as Hbonded dimers of the thione form. [23,24] Similarly, it might be assumed that 2-mercaptopyrimidine also exists in the solid state as an H-bonded dimer, displayed in Scheme 3 as form III.…”

Synthesis and Biological Evaluation of Novel Isopropyl 2-thiazolopyrimidine-6-carboxylate Derivatives

“…13,14 Base on this approach, 2-TPs have attracted significanat interest of synthetic-biochemists. 15,16 A patent 17 revealed the application of 2-TP derivatives in preparation of cardiotonic drugs. Pathak et al have evaluated primary activity of 2-TP derivatives against Mycobacterium tuberculosis (Mtb).…”