Diazonium fluoroborates fluorides

Diazotization of an arylamine followed by treatment with fluoroboric acid gives an aryl diazonium fluoroborate salt Heating this salt converts it to an aryl fluoride... 

The Balz-Schiemaim reaction is a useful laboratory and industrial method for the preparation of fluoroaromatics. The water-insoluble diazonium fluoroborate is filtered, dried, and thermally decomposed to give the aryl fluoride, nitrogen, and boron trifluoride (28—30). 

Preparation of aryl fluorides Addition of fluoroboric acid to a solution of a diazonium salt causes the precipitation of an aryl diazonium fluoroborate. When the dry aryl diazonium fluoroborate is heated, an aryl fluoride results. This is the Schiemann reaction it is the most general method for the preparation of aryl fluorides. 

Schiemann reaction (Section 22.17) Preparation of an aryl fluoride by heating the diazonium fluoroborate formed by addition of tetrafluoroboric acid (HBF4) to a diazonium ion. 

There have been few reports of indole fluorination. 2-Methylindole was largely destroyed by cobalt(V) fluoride treatment, giving perfluorocyclo-hexane and perfluoromethylcyclohexane among the products [70MI2 72HC(25-2)127]. 4-Fluoro-3-indoleacetonitriles have been prepared from the diazonium fluoroborates (85CPB3696). 

Fluorination. Attention has been focused on the direct fluorination of isoquinolines activated by conversion into 2-methylisocarbostyril (80). With gaseous fluorine (diluted to 10% with argon) in acetic acid a 54% yield of the 4-fluoro derivative was obtained. (Scheme 40). With methylene chloride as the solvent, only the 4-chloro analogue was formed [82H( 17)429]. Fluoroisoquinolines have also been made by displacement of nitro groups, and from diazonium fluoroborates (87JHC181). Hepta-chloroisoquinoline was converted into a perfluoro derivative by heating it in an autoclave with anhydrous potassium fluoride [66JCS(C)2328]. 

Aryl fluorides also may be prepared from arenamines by way of dia-zonium salts if the procedure is slightly modified. The amine is diazotized with nitrous acid in the usual way then fluoroboric acid or afluoroborate salt is added, which usually causes precipitation of a sparingly soluble diazonium fluoroborate. The salt is collected and thoroughly dried, then carefully heated to the decomposition point—the products being an aryl fluoride, nitrogen, and boron trifluoride ... 

Cuprous-catalyzed replacement reactions are called Sandmeyer reactions aryl chlorides, bromides, cyanides, and nitro compounds are prepared in this way formation of aryl iodides requires no catalyst, fluorides are obtained by heating diazonium fluoroborates (i.e., Schiemann reaction) benzenols are obtained by warming aqueous diazonium salt solutions. 

Replacement of the Diazonium Group by Fluoride and Iodide When an arenediazonium salt is treated with fluoroboric acid (HBF4), the diazonium fluoroborate precipitates out of solution. If this precipitated salt is filtered and then heated, it decomposes to give the aryl fluoride. Although this reaction requires the isolation and heating of a potentially explosive diazonium salt, it may be carried out safely if it is done carefully with the proper equipment. There are few other methods for making aryl fluorides. 

In their landmark publication, Balz and Schiemanns reported the syntheses of fluorobenzene, 4-fluorotoiuene, 2,4-dimethylfluorobenzene, 1-fluoronaphthalene, and 4,4 -difluorodiphenyl. Diazotization of the appropriate aromatic amine with nitrous acid (NaNC>2, HC1), followed by the addition of fluoroboric acid gave the corresponding aryl diazonium salts (62-67% yields). Thermal decomposition of the dry aryl diazonium salts gave the aryl fluoride (97-100% yields). Only 2-, 3-, and 4-nitrophenyl diazonium fluoroborates, which were prepared in 74-100% yields, failed to yield the expected nitrofluorobenzenes upon thermolysis. 

There is strong evidence that the mechanism of the decomposition of the aryl diazonium fluoroborates involves an aryl cation intermediate that rapidly collapses to the aryl fluoride and boron trifluoride by reaction of the aryl cation with tetrafluoroborate.8-12 Aryl radicals do not appear to be involved. For example, whereas iodoacetic acid is an effective aryl radical trap in several hydrodediazonization reactions, it does not interrupt the normal (ionic) pathway involved in the Schiemann reaction.1 Ib... 

Aromatic fluorides. Aromatic diazonium fluoroborates are readily transformed into the fluorides by treatment with HF-pyridine, induced either thermally or photochemically. The method is particularly useful for accessing those fluorides having polar substituents (e.g., OH, OMe, CFi, halogens). 

The diazonium group can be replaced by fluorine by treating the diazonium salt with fluoroboric acid (HBF4). The diazonium fluoroborate that precipitates is isolated, dried, and heated until decomposition occurs. An aryl fluoride is produced ... 

Formation of diazonium fluoroborates by diazotization of aromatic amines in the presence of fluoroborates, followed by their thermal decomposition to aryl fluorides ... 

Diazotization. Diazonium salts of benzothiazoles have been useful in interconversions involving amino-derivatives of this ring system. 2-Fluoro-benzothiazoles are accessible by the diazotization of the corresponding 2-amino-compounds, followed by cautious pyrolysis of the isolated diazonium fluoroborate with potassium fluoride and sand under reduced pressure. The four isomeric sulphonic acids of 2-methylbenzothiazoles are obtainable from the amino-compounds by diazotization, followed by treatment with acetic acid saturated with sulphur dioxide in the presence of cupric chloride the resulting sulphonyl chlorides serve as source of the... 

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. 

Boron fluoride Diazonium fluoroborates from amines and N-nitrosamines 1,2,4-Thiadiazolediazonium fluoroborates... 

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