Diazonium fluoroborates introduction

Introduction of a fluorine atom is achieved using the Schiemann reaction. Originally, the reaction involved gently heating the solid diazonium fluoroborate (Scheme 8.18), but improved yields result from the thermal decomposition the hexafluorophosphate, ArN2 PF, or hexa-fluoroantimonate, ArN2 SbFg, salts. 

Nucleophilic displacements of imidazole diazonium salts have some synthetic utility, particularly for the introduction of groups at C-2. Thus 2-nitroimidazoles" and 2-azidoimidazoles" are available via the diazonium fluoroborates. The formation of halogen-substituted imidazoles by the photolytic (but not thermal) decomposition of diazonium fluoroborates has proved useful. " " While 2- and 4-fluoro- and 2-chloro-imidazoles can be prepared in this way, the reaction fails for iodo- and bromo imidazoles." ... 

Selective introduction of fluorine into an aromatic ring is often achieved by thermal decomposition of diazonium fluoroborates, which is called the Balz-Schiemann reaction (Scheme The scope of... 

Nucleophilic introduction of nitro functions seems to be confined to reactions of diazonium salts which offer a useful entry, particularly to 2-nitroimidazoles (from the diazonium fluoroborates see Section 3.02.7.1.6). This method, though, depends on the ease of synthesis, isolation, and stability of aminoimidazoles. The 4- and 5-amino isomers in particular are frequently unstable in air, and aminoimidazoles are usually made either by ring synthetic methods or by reduction of nitro compounds. Oxidation of amino groups to nitro is seldom a useful option. 

The Schiemann reaction is the most common method for the introduction of fluorine onto an aromatic ring. It is carried out by treatment of a primary aromatic amine with sodium nitrite in aqueous HCl followed by addition of HBF or NaBF. The diazonium fluoroborate salt precipitates and is collected and dried. Heating the dry salt brings about its decomposition to an aryl fluoride, nitrogen, and boron trifluoride. The Schiemann reaction is also thought to involve an aryl cation intermediate. 

Selective introduction of fluorine into an aromatic ring is often achieved by thermal decomposition of diazonium fluoroborates, which is called the Balz-Schiemann reaction (Scheme 37). 25 i26 jjie scope of the reaction is wide and includes heterocyclic compounds (equation 47). The Bp4 salts are unusually stable for diazonium salts and yields are usually high. In some cases, however, use of Pp6 or Sbp6 salts gives improved yields. ... 

The introduction of fluorine into the aromatic nucleus can readily be accomplished by first converting the diazonium chloride with fluoroboric acid to an insoluble borofluoride, which is isolated and then decomposed by heat (Schiemann reaction). 

The Balz-Schiemann reaction has been used for the preparation of the 4-iluoro-derivatives of pyridine and 2,5-, 2,6-, and 3,5-lutidine it has also been used to obtain 5-fluoronicotinic acid, required for conversion into the corresponding pyridylmethanol via LiAlH reduction of the ethyl ester. The introduction of F into fluoroaromatic compounds has been achieved via isotopic exchange in diazonium tetrafluoroborates. U.v. irradiation of aqueous solutions of the appropriate diazonium tetrafluoroborates has been used to procure the first ring-fluorinated imidazoks, e.g. photolysis of the diazonium solution obtained by adding sodium nitrite to 2-amino-imidazole in aqueous fluoroboric acid provides 2-fluoroimidazole contaminated with only a small amount of 2-azidoimidazole, the sole product of thermal decomposition of imidazole-2-diazonium tetrafluoroborate. 

---