Decomposition to fluoroarenes

Ab initio and MNDO-d calculations have been reported for some diaryliodonium fluorides (8) and their decomposition pathways, which may lead to fluoroarenes. ... 

ArNHj remains (as it is a weak base), but the concentration is low enough to prevent as yet undiazotised amine undergoing a coupling reaction with the first formed ArNj (c/. p. 147). Aromatic diazonium chlorides, sulphates, nitrates, etc., are reasonably stable in aqueous solution at room temperature or below, but cannot readily be isolated without decomposition. Fluoroborates, ArN2 BF4 , are more stable cf. stabilising effect of BF on other ion pairs, p. 136) and can be isolated in the dry solid state thermolysis of the dry solid is an important preparative method for fluoroarenes ... 

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... 

The amine was introduced into a slurry of commercial NO+ BF4 (10% mol excess) in CH2C12 cooled in an ice bath. Sufficient time, usually 30 min, was allowed for completion of the diazotization, then an inert, relatively high boiling diluent, such as 1,2-dichloroben/ene, was added and the mixture was heated. CH,C12 was allowed to distill from the system and decomposition of the diazonium tetrafluorobonite occurred to yield the fluoroarene. 

---