Aromatic diazonium tetrafluoroborates

Fukuhara, T., Sekiguchi, M., Yoneda, N. Facile preparation of aromatic fluorides by thefluoro-dediazoniation of aromatic diazonium tetrafluoroborates using HF-pyridine solution. Chem. Lett. 1994,1011-1012. 

In the classical procedure, base is added to a two-phase mixture of the aqueous diazonium salt and an excess of the aromatic that is to be substituted. Improved yields can be obtained by using polyethers or phase transfer catalysts with solid aryl diazonium tetrafluoroborate salts in an excess of the aromatic reactant.177 Another source of aryl radicals is A-nitrosoacetanilides, which rearrange to diazonium acetates and give rise to aryl radicals via diazo oxides.178... 

The photolysis of crystalline diazonium tetrafluoroborates and hexa-fluorophosphates affords a convenient route to aromatic fluorides<53) ... 

Diazotization in the presence of boron trifluoride enables diazonium tetrafluoroborates to be isolated from the reaction mixture and purified. Subsequent controlled decomposition produces the required fluoroaromatic. Although explosion hazards and the toxicity of the isolated salts are significant concerns with this process, known as the Balz-Schiemann process, 4,4 -di-fluorobenzophenone (BDF. 6) has been prepared by this route as a monomer for the production of the engineering plastic poly(ether ether ketone) , or PEEK , by condensation with 1,4-dihydroxybenzene in the presence of potassium carbonate. BDF 6 is superior to its chlorine analog because in aromatic systems the nucleophilic displacement of fluorine is more facile than that of chlorine, leading to a shorter polymerization time and a better quality product containing less degradation impurities. 

Synthesis of Aromatic and Heteroaromatic Fluoro Compounds via Diazonium Tetrafluoroborates... 

Arenediazonium tetrafluoroborates 3 arc formed from aromatic amines 1 either by precipitation (shown schematically as a dropping arrow) of the diazonium tetrafluoroborate from an aqueous solution of another soluble diazonium salt 2 or through direct diazotization in aqueous tet-rafluoroboric acid. 

As diazonium tetrafluoroborates are more soluble in strongly acidic media, the precipitation of the diazonium tetrafluoroborate from an auxiliary diazonium salt and alkaline tetrafluoroborate is preferred when the diazonium tetrafluoroborate is thought to be too soluble an increase of the ionic strength tends to decrease its solubility.3-129 However, some alkaline chloride can co-precipitate with the diazonium tetrafluoroborate. Consequently, the diazonium tetrafluoroborate yields can be a little optimistic and some chlorinated aromatics can be produced together with fluorinated aromatics during the dediazoniation step.3,171 To prevent such contamination, the diazonium tetrafluoroborate is washed with the minimum amount of chilled water, then with another polar solvent, such as an alcohol, and finally with a water-insoluble solvent, such as diethyl ether. When their decomposition points are not too low, diazonium tetrafluoroborates can also be purified by rccrystallization (in water or acetic acid) or by dissolution in acetone then salting out with diethyl ether.3... 

When performing the decomposition of diazonium tetrafluoroborates stabilized by nitro or chloro groups, in aromatic solvents (1,2-dichlorobenzene, nitrobenzene, chloronaphthalene), arylation of the solvent is observed.137... 

Though safer than the decomposition of pure, solid diazonium tetrafluoroborates, dediazoni-ation of these compounds mixed with inert solid salts cannot be scaled up to a large extent since heat exchange through large quantities of powdered solids rapidly becomes difficult. Thus, dediazoniation of arenediazonium tetrafluoroborates suspended in inert fluids is an alternative proposition.13105 141 175-219 220 In addition to the safety improvement, lower quantities of tar are formed using this technique. The inert fluid can be ligroin,143 petroleum ether,147 Decalin,3 or simple aromatic compounds,1-3,5,131-221 such as toluene, xylene, biphenyl, nitrobenzene,177 or quinoline. Simple esters have also been used as solvents in the dediazoniation... 

The triazene linker can be used for the immobilization of aromatic diazonium salts, and therefore for aromatic amines, but not for aliphatic amines due to the instability of their diazonium salts. Cleavage of the linker can be achieved under mild acidic conditions to yield the benzylamine resin and the corresponding diazonium salt [136,145,146]. The main difference between the preparation of triazenes in solution and triazenes on solid support is the respective amine, namely bisben-zylamine and polymer-supported benzylamine 114. In solution, it was used in excess to quench unstable diazonium salts and force the reaction to completion. In the solid-phase approach it was immobilized and cannot be used in excess with respect to low loadings. A simple three-step procedure (Scheme 36) starting from benzylamine resin 114 via carboxylate 115 led to the successful preparation of ester resins 116 in essentially higher loadings. Treatment of resin 114 with 4-carboxy-benzene diazonium tetrafluoroborate yielded benzoic acid resin 115. 

Diazotization/Fluorodediazoniation of Aromatic Amines via Complex Diazonium Fluorides Other than Diazonium Tetrafluoroborates... 

A general method of replacement of the primary aromatic amino group by hydrogen is reduction of aromatic diazonium salts, prepared by diazotization, typically by treatment of the primary amine or its salts with sodium or potassium nitrite in strongly acidic aqueous media. Conversion to the diazonium salts may also be accomplished in nonaqueous media using acetic acid, or using alkyl nitrites (equation 86). Most diazonium salts decompose at temperatures above 0-5 C. Only certain salts such as naphthalene-1,5-sulfonates,tetrafluoroborates ° ° and hexafluorophosphates ° ° are stable at room temperature. 

Although aryl and vinyl halides have found vast applications in Heck coupling, aromatic diazonium salts, particularly the isolable tetrafluoroborates, proved successful in the work of Kikukawa et al. [23 a, b] and a one-pot reaction with the C-C-coupling step [23 c-e]. Extensions of this tandem diazotation Heck reaction technique were reported by Beller et al. (eq. (5)) [23 f], including heterogeneous Pd/carbon catalysts [23 g]. 

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. 

One of the earliest means of introducing fluorine selectively into specific positions of aromatic compounds is the Balz-Schiemann reaction [77] which dates back to the 1920s. An isolated arene diazonium tetrafluoroborate is thermolyzed at up to 120 °C to yield the corresponding fluoroaromatic compound. Because of the infamously hazardous nature of isolated diazonium salts the scope of the classical variant of the Balz-Schiemann reaction was limited to the small scale. The high exothermicity of the reaction is most conveniently controlled by diluting the diazonium salt with a solid inert medium such as sea sand. In addition to the danger to the experimenter, the reproducibility of the reaction yield is quite poor. 

Figure 26.11 Reductive electrochemical modification (rECM) of individual contacted m-SWCNTs utilising an aromatic diazonium salt, (a) Electrical transport measurements and (b) Raman spectra before and after rECM on the same contacted tube. The rECM was performed at -1.3 V versus Pt for 120 s in a solution of 10 mM 4-nitrobenzene diazonium tetrafluoroborate in M,M-dimethylformamide.

Boron trifluoride forms addition compounds that incorporate an sp hybridized boron into a tetravalent structure. Salts of Bp4 are readily formed with BF3 and a suitable fluoride donor. Halogen fluorides such as chlorine trifluoride react with BF3 to generate interhalogen cations such as [C1F2]" [BF4]. " Some further examples are shown in equations (43) and (44). In an organic application, the Schiemann reaction provides an entry into fluorinated aromatics by thermal decomposition of a diazonium tetrafluoroborate (equation 45). 

Potassium acetate, phase-transferred by 18-crown-6 from the solid into nonpolar solvents, catalyses the formation from arene diazonium tetrafluoroborates of aryl radicals (Scheme 53),-which in aromatic solvents couple with the solvent to... 

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