1.3.4- Thiadiazole-2-thiones, tautomerism

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. 

Tautomerism was reviewed quite extensively in CHEC(1984) <1984CHEC(6)545> and CHEC-II(1996) <1996CHEC-II(4)379>. The tautomeric ability of the 2-mercapto-5-methyl-l,3,4-thiadiazole 9 was studied by its reaction with the electrophilic Cl3 FnCSCl <2003JP01>. 2-Mercapto-5-methyl-l,3,4-thiadiazole 9 was considered to exist mainly as the thione tautomer however, electrophilic substitution occurred on the thiol (Scheme 1). 

The 1,3,4-thiadiazole ring system, with three heteroatoms, does not exhibit tautomerism in its fully conjugated form. However, when certain substituents are present, tautomerism is possible. l,3,4-Thiadiazolin-2-ones (39 X = 0) and -2-thiones (39 X = S) exist in the oxo and thione forms, respectively, as shown by spectroscopic and LCAO-MO calculations. 2-Amino-l,3,4-thiadiazoles exist in the amino form in solution and in the solid state the Kt value is 10s as shown by basicity measurements. UV spectroscopy and LCAO-MO calculations show that the amino tautomer is also the main species when there is an alkoxy group in the 5-position (40 R1 = alkoxy, R2 = NH2), or if the exocyclic nitrogen atom is part of a hydrazone group (40 R2 = NHN=CR2). 

A thione-thiol structure in the crystalline state and in aprotic solvents was established for the [l,2,4]triazolo[3,4- >][l,3,4]thiadiazole (37), in accordance with calculations of the 7r-bonding and solvation energies by the Pariser-Parr-Pople (PPP) method and also on the basis of IR and UV spectral information (75CHE304). The electron-density distribution in four tautomeric forms, calculated by the PPP method, also indicates that the ring-junction carbon atom is the most reactive one with respect to nucleophilic attack (75CHE500). 

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