Amination Arenediazonium tetrafluoroborates

The preparation of an aryl fluoride—e.g. fluorobenzene 3—starting from an aryl amine—e.g. aniline 1—via an intermediate arenediazonium tetrafluoroborate 2, is called the Schiemann reaction (also called the Balz-Schiemann reaction) The diazotization of aniline 1 in the presence of tetrafluoroborate leads to formation of a benzenediazonium tetrafluoroborate 2 that can be converted into fluorobenzene 3 by thermolysis. 

Doyle and Bryker (1979) reported high yields of arenediazonium tetrafluoroborates when aromatic amines were reacted with tert-butyl nitrite and trifluoroboro etherate in CH2C12. It is likely that nitrosyl fluoride is formed as nitrosating reagent by fluoride-alkoxy exchange. 

Due to the corrosive and toxic nature of volatile hydrogen fluoride, the fluorodediazoniation of aromatic and heteroaromatic amines in anhydrous hydrogen fluoride (see Section 26.1.2.), though very efficient, inexpensive and easy to scale up, needs special apparatus and safety measures which are not always available in every laboratory. Thus, the Balz-Schiemann reaction remains the most popular way to substitute aromatic amino groups for fluorine on a laboratory scale. Moreover, special techniques have been developed during the last decade to control formation, storage and decomposition of arenediazonium tetrafluoroborates on a large scale. 

Arenediazonium tetrafluoroborates 3 arc formed from aromatic amines 1 either by precipitation (shown schematically as a dropping arrow) of the diazonium tetrafluoroborate from an aqueous solution of another soluble diazonium salt 2 or through direct diazotization in aqueous tet-rafluoroboric acid. 

The decomposition of arenediazonium tetrafluoroborates in the resulting fluoroarene as solvent has also been utilized.179 This method is very fruitful in the synthesis of fluorobenzene (96% yield) or fluorotoluenes (85-88% yield for the three isomers) but fails with substrates such as anisolediazonium tetrafluoroborates which probably arylate the reactive fluoroaromatic solvent. In contrast, perfluorinated tertiary amines (EF-L 174, F-C 40), perfluorinated alkyl tetrahydropyran (EF-L 102), perfluorodecahydronaphthalene (Flutec PP7) or other per-fluorocycloalkanes (Flutec s) are claimed to be the best media for fluorodediazoniation of all types of diazonium tetrafluoroborates.224 These media seem to be more efficient, at least in the synthesis of 1,2-difluorobenzene, than silicone oil which has also been claimed to improve fluorodediazoniation.225... 

Usually it is possible to isolate and characterize the arenediazonium tetrafluoroborate intermediate, e.g. 8, and then, in the next step, carry out the thermal decomposition of that intermediate, c.g. the preparation of 2.6-disubstituted 3-fluoropyridines 9 from the corresponding pyridin-3-amines 7 (Tabic 7, see also Vol. If lOa, p 711... 

A new synthetically useful procedure for the preparation of arenediazonium tetrafluoroborates from the corresponding amines involves the in situ generation of nitrosyl fluoride (Scheme 53). The diazotization reactions are performed under mild conditions in an anhydrous solvent, preferably methylene chloride. Excess boron trifluoride etherate traps the alcohol and water produced, and the diazonium salts are precipitated from the reaction medium and isolated by simple filtration. 

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