Thermal decomposition, 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... 

The Wallach reaction, the thermal decomposition of an aryl triazene (Scheme 46), has also been attempted as an alternative to the Balz-Schiemann reaction [54,220,221]. Although a wide variety of experimental conditions have been tried, the radiochemical yields remain low. 

The Balz-Schiemann reaction, thermal decomposition of arenediazonium fluoroborates, is still a favorite approach to the laboratory preparation of fluoro aromatics [16, 17 Caution must be exercised in handling and decomposing mtroarenediazomum fluoroborates and pyridinediazomum fluoroborates because detonations have been reported [J8, 19]... 

The controlled thermal decomposition of dry aryldiazonium fluoroborates to yield an aryl fluoride, boron trifluoride and nitrogen is known as the Balz-Schiemann reaction (Expt 6.77). 

Thermally Induced Decomposition of Arenediazonium Tetrafluoroborates (Thermal Balz-Schiemann Reaction)... 

Besides thermal decomposition of diazxvnium tetrafluoroborate salts (fluorodediazoniation), the Balz-Schiemann reaction can be carried out under either photochemical or ultrasonic irradiation. 

Another alternative to the Balz-Schiemann reaction is now available through the thermal decomposition of aryl fluoroformates or aryl thiofluoroformates [36-38], as indicated in Figure 9.13. Fluoroformates are obtained in high yields but the pyrolysis step is quite variable. 

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... 

Various partly fluorinated pyridines can be synthesized by diazotization of the amino derivatives in aqueous hydrofluoric acid,50 or by using anhydrous hydrogen fluoride followed by thermal decomposition of the diazo-nium fluoride.51,52 There are also many examples of the application of the classical Balz Schiemann reaction to heteroaromatic systems,53,54 and the interesting photolysis of diazonium tetrafluoroborates has given fluoro-imidazole derivatives55,56 and fluorazoles.57... 

The Balz-Schiemann reaction, the classic synthesis of fluoro-aromatic compounds, involves diazotisation of an aromatic amine, isolation of the diazonium fluoroborate or hexafluorophosphate, then thermal decomposition of the dry salt, usually diluted with sand for safety. It is a useful method, with application to a number of heterocyclic systems, for example methyl 3-aminothiophene-2-carboxylate. ... 

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. 

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. ... 

Whilst Sn2 displacement is a viable strategy for aliphatic fluorination, nucleophilic fluorination of aromatic systems is much more challenging. The traditional approach to aromatic fluorination is the Balz-Schiemann reaction, which is the thermal decomposition of aiyl diazonium tetra-fluoroborate salts, which are potentially explosive reagents. This decomposition also gives the toxic, fuming gas BF3 as a by-product and this should be removed in a basic aqueous scrubber. Modern alternatives should seek to provide safer and cleaner aromatic fluorination protocols. 

On the other hand, 3,3-dialkyl-l-aryltriazenes can be easily obtained from the corresponding aromatic amines (by quenching the diazotized arylamine with a secondary amine, such as piperidine). Therefore, this reaction constitutes a mild alternative to the classic Balz-Schiemann thermal decomposition of aryldiazonium tetrafluoroborate salts, a method which in many cases is too drastic and tedious (see Section 1.1.8.5.). 

The Balz-Schiemann (BS) reaction (often called the Schiemann reaction) describes the preparation of an aromatic diazonium fluoborate 1 followed by its thermal decomposition to afford the corresponding aryl fluoride 2.1"4... 

---