Thiadiazoles thiol-thione 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). 

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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