Aryl chlorides tetrafluoroborate

Good yields of chlorides have also been obtained for reaction of isolated diazonium tetrafluoroborates with FeCl2-FeCl3 mixtures.100 It is also possible to convert anilines to aryl halides by generating the diazonium ion in situ. Reaction of anilines with alkyl nitrites and Cu(II) halides in acetonitrile gives good yields of aryl chlorides and bromides.101... 

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. 

In 2008, Buchwald and colleagues developed an efficient procedure for the carbonylation of aryl chlorides, aryl tosylates and aryl mesylates [202-204]. Under their reaction conditions, carboxylic acid derivatives were prepared in good yields (Scheme 2.23). The advantages of this procedure are (1) l,3-bis(dicyclohexyl-phosphino)propane bis(tetrafluoroborate) as the ligand used is stable and easily available and (2) the reactions were carried out in a reaction tube under 1 bar of CO, avoid the using of autoclave. 

Conversion of diazonium compounds to aryl chlorides, bromides, or cyanides is usually accomplished using cuprous salts, and is known as the Sandmeyer reaction. Since a CN group is easily converted to a CO2H group (eq. 10.13), this provides another route to aromatic carboxylic acids. The reaction with KI gives aryl iodides, usually not easily accessible by direct electrophilic iodination. Similarly, direct aromatic fluorination is difficult, but aromatic fluorides can be prepared from diazonium compounds and tetrafluoroboric acid, HBF4. 

Scheme 7.11. The formation of a vinylic aryl iodonium tetrafluoroborate and its Sn2 displacement by chloride.

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. 

Samsonova and Nikiforov, 1984), and porphyrin and phthalocyanine metal complexes (Becker et al., 1985a, 1986b Becker and Grossmann, 1990) were tested. That a series of relatively simple anions such as the oxalate monoanion, tetraphenyl bor-anate (Ph4B ), bromide, chloride, and even tetrafluoroborate can act as donors is, at least for the last mentioned anion, surprising, but Becker et al. (1985 b) were able to trap aryl radicals and in some cases also donor radicals (Cl, COO ) by spin trapping with nitrosodurene and phenyl-tert-butylnitrone. The photochemical effect is postulated to be due to ion pairs ArNJ X-. 

Corresponding thiatriazolium tetrafluoroborates (161 R = Ar) salts are formed by Meerwein alkylation (triethyloxonium tetrafluoroborate in methylene chloride) of the 5-olates (173) <76CC306>. This type of salt (161) generally serves as starting material for other types of mesoionic compound <76AHC(l9)i>. The l,2,3,4-thiatriazolium-5-olates (173) are reported to be stable towards acid hydrolysis while aryl azide is formed in boiling aqueous sodium hydroxide <76CC306>. 

Aldehydes can also be prepared by the carbonylation of aryl and alkenyl halides and triflate, and benzyl and allyl chlorides using tin hydride as a hydride source and Pd(Ph P)4 as a catalyst[377], Hydrosilancs arc used as another hydride source[378]. The arenediazonium tetrafluoroborate 515 is converted into a benzaldehyde derivative rapidly in a good yield by using Et.SiH or PH MS as the hydride source[379]. 

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