1.2.4- Thiadiazole 5-diazonium salts

In their detailed study of thiadiazole diazonium salts (292) Goerdeler and his co-workers have succeeded in isolating these compounds in the solid state by the following preparative methods ... 

The stability and relatively ready accessibility of thiadiazole diazonium salts has encouraged their use in the production of numerous monoazo-dyes that have proved particularly suitable for dyeing polymers.192,196... 

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>. 

Chloro- and bromo-l,3,4-thiadiazoles are usually prepared by nucleophilic processes, e.g., Sandmeyer reactions of diazonium salts [56CB1534 68AHC(9)165 86CHE1148], and reactions of thiadiazolinones with phosphorus halides [68AHC(9)165]. The halogeno derivatives are important... 

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... 

The equilibrium existing between nitrosamines and diazonium salts of 1,2,4-thiadiazoles in acid media was mentioned in Section III,E. Diazonium salt solutions may be directly prepared from the amines in concentrated sulfuric, phosphoric, or acetic acid.6 80> 17°-191 Most of... 

These and other experiments show that diazonium salts incorporating the 1,2,4-thiadiazole ring are undoubtedly among the most reactive examples of their kind.80 Their reactivity is attributable to the electron deficiency of the terminal nitrogen of the diazo group and depends in turn on the electron-attracting nature of the heterocyclic nucleus.80... 

Substituted l,2,4-thiadiazol-5-yl diazonium tetrafluoroborates (314) react with potassium halides in acetonitrile in the absence of catalysts to afford the corresponding 5-halogeno compounds (315) in excellent yields.169 The isomeric 5-phenyl-l,2,4-thiadiazol-3-yl diazonium salts, however, resist substitution by this procedure, except with iodide ions by taking advantage of the catalytic effect of copper... 

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). 

Thiadiazoles amination, 6, 562 diazonium salts coupling reactions, 6, 484 electrophilic attack at carbon, 5, 56... 

By contrast, 3-hydrazino-l,2,4-thiadiazoles (163), which are prepared by ring closure methods, are very sensitive to acids and undergo elimination of sulfur with formation of 5-arylamino-l,2,4-triazoles (164) as indicated in Scheme 61 (63JCS4566). The mechanism of this conversion is not known but presumably involves the attack of a hard nucleophile at the 5-position followed by ring opening, loss of sulfur and recyclization as has been observed with the quaternary salts (77JCS(P1)1791) (see Schemes 22, 23, and 24). Hydrazine (163) reacts with aldehydes to produce hydrazones (165) and with dimethyl malonate to yield pyrazoles (166). Diazonium salts of type (153) have been prepared by the methods... 

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>. 

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