Thiadiazolin-3-ones, tautomerism

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). 

Alkoxy-5-amino-1,3,4-thiadiazoles (41) are not particularly stable to hydrolysis. They were hydrolyzed by hot hydrochloric acid, first to 2-amino-l,3,4-thiadiazolin-5(4)-one (108) (or the tautomeric imine) and then to thiosemicarbazide. On diazotization in hydrochloric acid 41 gave 2-chloro-l,3,4-thiadiazolin-5(4)-one (107), and by hot alkali it was cleaved to 1-alkoxythiocarbonylsemicarbazide (109). ... 

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