1.3.4- Thiadiazole amino-, diazonium salts

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 Balz-Schiemann synthesis can be applied not only to substituted anilines but also to aminobiphcnyls1,131 or amino-substituted fused polyaromatic compounds, such as naphthalene,1114,119,129 anthracene,136 phenanthrene,1135 acenaphthene,133 fluorene,1,131,134 benzanthracene,130 136 pyrene,136 chrysene,136 fluoranthene,131 fluorenone,1,131 anthra-quinone,1,137,139,140 benzanthrone,1,117,118 phenanthraquinone,138 or xanthone.132 Fluorinated pyridines,1,141"146 methylpyridincs,126,147 149 pyridinecarboxylic acids,150 quinolines,1,151 isoquinolines,152 quinazolone,1 thiazoles,153,154 isothiazoles,156 benzothiazoles,157 thiadiazoles,155 and thiophenes154 can also be obtained from the corresponding aminated heterocycles. Modified Balz-Schiemann methods are recommended for amino nitrogen-containing heterocycles, the diazonium salts of which are rather water-soluble and unstable (a violent explosion was reported for pyridine-3-diazonium tetrafluoroborate).159 These new techniques have also been specially adapted for pyrazol-, imidazol-, or triazolamines which fail to react under classical conditions.158... 

Diazonium Salts from 3-Amino-1,2,4-thiadiazoles The 3-amino group in 1,2,4-thiadiazoles (e.g. in the 5-phenyl homolog) is also capable of being diazotized, preferably in concentrated phosphoric acid. The resulting diazonium salt may be coupled in the usual way, but with sufficiently reactive partners only (e.g. phenol and 0-naphthol).126... 

Amino-l,2,4-thiadiazoles are usually prepared by treating amidine hydrochlorides with halogen and thiocyanates to yield the 3-alkyl derivatives or treating alkylisothiouronium salts similarly to yield the 3-alkylthio derivatives (56CB2742). Diazonium salts derived from 5-amino-l,2,4-thiadiazole derivatives, which are prepared in acetic acid, are extremely reactive and are capable of coupling with m -xylene (60CB397). 

Azine approach. Diazotization of 2-amino-3-hydroxypyridines might be expected to yield the bicyclic ring system (631) by bond formation between the oxygen and the diazonium group in the first formed diazonium salt fused 1,2,3-thiadiazoles and -triazoles are formed by this type of reaction. Compounds prepared by this method, however, are highly unstable and have spectroscopic properties consistent with the pyridodiazooxide structure (630) (74CS(6)222>. 

Amino-l,2,4-thiadiazoles are diazotized at — 10°C in phosphoric acid, and couple with naphthol to give moderate yields of diazo dyes.182,388 In their limited stability, the 3-diazonium salts resemble those of 3-aminoiso-thiazole, but differ from the more stable 1,2,4-thiadiazole- and isothiazole-5-diazonium salts a consideration of the 7t-electron densities leads to the opposite conclusion.182... 

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