Tautomerism thione-thiol

For hydroxy- and mercaptoazoles, hydroxy-oxo and thiol-thione tautomeric equilibria depend strongly on the orientation of the substituents and heteroatoms. Most 2- and 5-hydroxy(mercapto)azoles exist mainly as 0X0 (thione) tautomers, but the corresponding 3- and 4-isomers often prefer the hydroxy (mdrcapto) forms. 

Kabachnik and co-workers have determined the pKa s of a number of dithio- and monothio-organophosphorus acids and considered the thiol-thione tautomeric equiUbria of monothiophosphoric acids. For most monothio acids the tautomeric equlibrium is shifted toward the thione form. There are slight changes in the pKa s of O, O -dialkyldithiophosphoric acids with substituent variation. Other data U4,29i) indicates that the pKavalues are rather substituent dependent, e.g., Ref. gives pKa values of -1.10 and +0.22 for HSP(S)(OC2Hs h... 

Thiol-thione tautomerism is similar to that exhibited by the oxygen analogues but with the a- and y-thiones, e.g. (129), slightly more favoured than the corresponding a- and y-oxo compounds. UV and other data have indicated the predominance of the thione forms of quinoline-2- and -4-thione, e.g. (130), isoquinoline-1- and -3-thione, e.g. (131), and acridine-9-thione (132) (76AHC(S1)144>. 

Another interesting problem related to the thiol/thione tautomerism in thioformic acid is the relative stability of the s-cis (4a) and s-lrans (4b) conformers of the thiol form. As we have mentioned in the preceding section, the former is 0.7 kcal mol 1 more stable than the latter. A much larger energy difference is predicted to exist, at the ab initio level, between the s-cis (23a) and s-trans (23b) forms of formic acid (oc6.2 kcal mol 1) or between the same conformers of the thione form of thioformic acid (4c, 4d) (oc6.5 kJmol l)llx. 

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 most complete investigation on the thiol/thione tautomerism of thiopyridines and thiodiazines was carried out by Adamowicz and coworkers61 63 123, who presented combined theoretical and experimental IR spectroscopic studies of Ar and N2 matrix-isolated compounds for the whole series. 

Iv) Thiadiazoles. The thiol-thione tautomerism of some thiadiazoles, such as 2,5-dimercapto-l,3,4-thiadiazole (DMTD) and related compounds, has attracted some attention because these compounds are of potential use in the production of antiwear additives for engine lubricating oils. 

Hydroxy- and 5-mercaptotetrazoles and their mono-N1 -substituted derivatives can be involved into keto-enol and thiol-thione tautomeric equilibria (Equation 7). This is the reason for significant differences in chemical and physical properties of these compounds from other 5R-tetrazoles. 

In the complex Cp Rhm(PyS)2 (PyS = pyridine-2-thiolato), one PyS ligand is bound to the rhodium ion in an A-monodentate mode, while the other ligand chelates to the metal in an TVA -bidentate mode. In this complex, the thiol thione tautomerism occurs in solution at ambient temperature, so its NMR spectra are solvent-dependent (00ICA(299) 100). 

The tautomeric equilibrium of 2-thiouracil in dioxane, water and acetonitrile was studied by ab initio calculations using the SCRF theory. The oxo-thione form was predicted to be predominant in all the solvents studied, but the order of tautomer stability was shown to depend on the level of theory and the dielectric constant of the solvent (00MI243). The tautomerism of protonated forms of 2-thiouracil has also been studied (00JPC(A)5122). Semiempirical AMI, PM3 and MNDO calculations of the thiol-thione tautomeric equilibrium 188c 188d predicted the mercapto form... 

A theoretical study has been performed on the influence of solvent on the thiol-thione tautomeric equilibrium of monothioformic acid [see (417) (418)], and... 

---