Aryl fluoride, Balz-Schiemann reaction

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. 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

The second product in this reaction, the difluorotris(perfluoroalkyl)-A5-phosphane, does not dissolve in aryl fluorides and, because of its greater density, forms a lower layer which can be easily separated. In this way the difluoro-A5-phosphanes are regenerated almost quantitatively and can be used in the synthesis many times. An example is the decomposition of 4-nitrobenzene-diazonium trifluorotris(heptafluoropropyl)phosphate at 87 C which provides l-fluoro-4-ni-trobenzene in 82% yield and difluorotris(heptafluoropropyl)-A5-phosphane in quantitative yield.6 This method is a convenient modification of the Balz-Schiemann reaction (see Section 26.1.3.). 

The story of the Balz-Schiemann reaction began with Bart102 who reported in 1913 on the insolubility of arenediazonium tetrafluoroborates which he found insensitive to shocks and friction and could isolate and dry. Then, in 1927, Balz and Schiemann observed that these solids can be thermally decomposed to aryl fluorides with loss of boron trifluoride.103 This paper was the starting point of intensive research which led, before the 1940s, to almost all the still-valid principles governing this method.104-119 Thus, Roe was able to provide, as early as 1949, a splendid detailed review1 which is still beneficial nowadays. Two other updated reviews were published in the 1960s (see Houben-Weyl, Vol. 5/3, pp 213-246).3... 

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). 

Fluoroalkanes can be obtained by fluoride transfer from complex fluoride anions to carbocat-ions in the nitrosative decomposition of certain aliphatic azides. Several complex nitrosonium salts, such as NO " BF4, NO PF(, , and NO ShF ". " have been used for this purpose. This reaction is similar to the decomposition of arenediazonium tetrafluoroborate salts to form aryl fluorides (the Balz-Schiemann reaction, discussed in Sections 1.1.8.5. and Vol. F 10a, p686ff). in which fluoride is transferred from a very weak nucleophile, such as the complex tetrafluoroborate anion, to an electron-deficient center. 

Since dialkyltriazenes were first used in aromatic fluorination by Wallach, ° many fluoroaro-matic compounds have been obtained in high yield by the decomposition of 3,3-dialkyl-l-aryl-triazenes with various fluorides in acidic media (see Vol. ElOa, p 725fT). Aryltriazenes are a potential source of aryldiazonium salts under controlled, mild, acid conditions. Therefore, this replacement (Route A) can be considered as a type of Balz-Schiemann reaction (see Section... 

The synthesis of 7-azaindoles is a challenging task and there are few efficient routes to substituted derivatives. In the laboratory of C. Thibault, the concise and efficient synthesis of 4-fluoro-1/-/-pyrrolo[2,3-jb]pyridine was achieved. The fluorination was carried out using the Balz-Schiemann reaction. The aromatic amine precursor was prepared via the Buchwald-Hartwig coupling of the aryl chloride with A/-allylamine followed by deallylation. The diazonium tetrafluoroborate intermediate was generated at 0 C and it decomposed spontaneously in 48% HBF4 solution to afford the desired aromatic fluoride. 

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... 

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... 

Historically, aryl fluorides were prepared by either Balz-Schiemann-type reactions or though the Halex process [70-75], Although a number of these procedures were quite successful, many of them required the use of toxic reagents or harsh conditions. To address these issues and provide practical routes to aryl fluorides, a host of alternative protocols have been developed [6, 76-78]. In addition to this work, a number of methods for the synthesis of aryl halides have been developed. The following sections will highlight several practical versions. 

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