1.2.5- Thiadiazole, 3-amino-, ring bromination

The 1,2,5-thiadiazoles were relatively inert in electrophilic substitution reactions (see also Section 4.02.1.4). The parent ring does not react with bromine, either under irradiation or in the presence of iron(III) salts, nor does it enter the Friedel-Crafts or nitration reactions. Electrophilic deuteration has been effected in low yield under drastic conditions (250 °C in trideuterophosphoric acid). If the thiadiazole ring bears an activating group, e.g. amino or methyl, electrophilic substitution can be achieved <68AHC(9)107, 70RCR923). 

However, a 2-amino group does activate the ring towards electrophilic agents, since Bak et al. could prepare 2-amino-5-bromo-1,3,4-thiadiazole by bromination of 2-amino-l,3,4-thiadiazole in 40% hydrobromic acid. The product was not isolated but was diazotized in situ to give 2,5-dibromo-l,3,4-thiadiazole. 

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