Aryl diazonium salts, Balz-Schiemann reaction

The Balz-Schiemann and Wallach reactions The Balz-Schiemann reaction (the thermal decomposition of an aryl diazonium salt. Scheme 46) was for many years the only practical method for the introduction of a fluorine atom into an aromatic ring not bearing electron-withdrawing substituents. This reaction, first reported in the late 1800s, was studied in fluorine-18 chemistry as early as 1967 [214]. It involves the generation of an aryl cation by thermal decomposition, which then reacts with solvent, nucleophiles or other species present to produce a substituted aromatic compound. Use of fluorine-18-labelled... 

More prevalent than diazonium fluorosilicates in so-called modified Baiz-Schiemann reactions have been diazonium hexafluorophosphates.16 In some cases these latter diazonium salts give higher yields of aryl fluorides. Some examples are shown below, with the normal Balz-Schiemann reaction yields using diazonium tetrafluoroborates given in parentheses.163... 

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