1.2.4- Triazole-5-thiones, tautomerism

In solutions of 3-mercapto-l, 2,4-triazoles the tautomeric equilibrium is shifted to the thione forms 181b (Scheme 65) [76AHC(S1), pp. 404, 415 96UK326]. Such an equihbrium was observed for 3-mercapto-5-ferrocenyl-4-phenyl-1,2,4-triazole 182 (94MI1121 96UK326). The thione tautomers 183 of 5-mercapto-l,2,4-triazoles are predominant [76AHC(S1), pp. 405, 414 97SA(A)699]. 

A comprehensive study of the vibrational spectra of 1,2,4-triazole in comparison to those of 1,2,3-triazole and tetrazole has been carried out <2000JST(530)183>. The tautomerism displayed by simple 1,2,4-triazole thiones has also been studied using vibrational spectroscopic techniques and the preferred tautomers present in the solid state determined <1997SAA699>. 

The tautomerism of 3-mercapto-l,2,5-triazole 184 has not been studied thus far. Very little information is available on the structure of 2-mercapto-1,3,4-triazole and its selenium analog, but apparently the thione 185 and se-lenone 186 forms are preferred for these compounds [76AHC(S1), pp. 408, 414]. 

Several papers have dealt with 1,2,4-triazolo[3,4-fc][ 1,3,4]thiadiazines including synthesis of new derivatives and tautomeric studies <02SC3455 02ZN(B)552 02PS487> and a one-pot synthesis starting from 4-amino-5-substituted-l,2,4-triazole-3-thiones involving a solid acid-induced cyclocondensation <02PS2403>. 

Characteristic examples of the use of IR spectra are structural studies on urazoles and triazolinones (66T(S7)213) and triazolinethiones (7UCS(C)ioi6). Tautomeric problems of oxo-hydroxy and thione-thiol systems are simplified by JV-alkylation (73CPBI342). Although earlier views in favour of 0x0 and thio rather than hydroxy and thiol forms have been confirmed, counter-examples have been found 3-hydroxy-2-methyl-5-phenyl-1,2,4-triazole (25) is preferred to the 0x0 tautomer (26) while the isomeric 4-methyl compound (27) follows the general rule (Scheme 9). 

---