4- Mercaptopyrimidine, tautomerism

For the particular case of 2- and 4-mercaptopyridines and 2-mercaptopyrimidines and by means of absorption UV-VIS spectroscopy, Stoyanov and collaborators122 have shown that polar solvents shift the thiol/thione tautomerism towards the thione form, while in dilute solutions of nonpolar solvents the thiol form predominates. However, one of the most significant contributions of this work122 is the observation of self-association. It also favors the thione forms and is followed by quantitative transformation of the thiol form into the corresponding symmetrical disulfides (see Scheme 6). More importantly thione-disulfide process is reversible in water, which can be of some relevance in biological systems. 

The 3/(ch) values for the C-N-CH, C-N = CH, and C = N-CH structural elements were found to a valuable criterion for estimation of the tautomeric equilibrium in mercaptopyrimidines in solution (83OMR20). 

Dimethyl-2-mercaptopyrimidine exists as a mixture of thiol and thione tautomers in a solution with the tautomeric ratio depending on the solvent polarity. As expected, the more polar thione form predominates in the polar solvents (dichloromethane, methanol, and DMSO-J6), whereas the thiol tautomer is favored in non-polar solvents (cyclohexane, 1,2-dichloroethane). Addition of cyclohexane to the dichloromethane solution of 4,6-dimethyl-2-mercaptopyrimidine shifts the equilibrium toward the thiol form. The opposite effect was observed on addition of dichloromethane to the... 

The ab initio calculations at the SCF/3-21G level predicted the predominance of the thiol form of unsubstituted 2- and 4-mercaptopyrimidines in the gas phase in accordance with the experimental IR spectra (91JPC2404, 91SA(A)339). Ab initio SCRF calculations have been carried out for tautomerism of 2-mercaptopyrimidine in the gas phase and in solution. In the gas phase and non-polar solvents, the thiol tautomer was found to be the most stable (AE — 8.16kcal/mol in the gas phase and 5.91kcal/mol in cyclohexene), whereas in polar solvents the thione tautomer becomes more stabilized (AE — —5.51 kcal/mol in DMSO and —5.83 kcal/mol in water) (95CPL(232)61). 

Tautomerism of 4,6-dimethyl-2-mercaptopyrimidine in gas phase and in solution has been studied using density functional theory calculations. While the thiol was... 

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