1, 2, 4, Thiadiazole diazonium salts with amines

In Section 3.4 we discussed the problem of reversibility of diazotization of aromatic and heteroaromatic amines. Simple stoichiometric considerations indicate that the reverse reaction (ArNJ -> ArNH2) may take place under strongly acidic conditions. Experimentally the reverse reaction was found only with heteroaromatic diazonium salts (Kavalek et al., 1989). Reaction conditions of hydroxy-de-diazonia-tion are comparable to those used for the reverse reactions of diazotization (e.g., 10 m H2S04, but at 0°C for the formation of 2-amino-5-phenyl-l,3,4-thiadiazol from the corresponding diazonium salt, Kavalek et al., 1979). So far as we know, however, amines have never been detected in aromatic hydroxy-de-diazoniations, not even in small amounts. 

Research on the fluorination of aromatic amines has provided access to 4-fluoro-2,l,3-benzo-thiadiazole (77). Treatment of 4-amino-2,l,3-benzothiadiazole (1) with nitrosonium tetra-fluoroborate (NOBF4), followed by heating of the resulting diazonium salt, gave the aryl fluoride (77) in 23% yield <92SC73>. 

The first 1,2,3-thiadiazole synthesized, 1,2,3-benzothiadiazole, was prepared by diazotiz-ation of o-aminothiophenol with nitrous acid (equation 31) (B-61MI42400), and recently sodium nitrite-acetic acid has been substituted for nitrous acid (B-79MI42400),. Another modification, thermal decomposition of diazonium acetate (34), affords benzothiadiazole in good yield in contrast to the variable yields usually experienced in the diazotization of o-aminothiophenols (equation 32) (78SST(5)43l). Benzothiadiazoles are also available directly from aromatic amines (equation 33) (70JCS(C)2250). Sulfur monochloride reacts with the amine to form a benzothiazothiolium salt which reacts with nitrous acid to yield a chlorinated 1,2,3-benzothiadiazole (35). This process, depending on the aromatic ring substitution, may afford a number of products, and yields are variable. 

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