Benzenediazonium tetrafluoroborates

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. 

Treatment of aniline 1 with nitric acid in the presence of tetrafluoroboric acid leads to a relatively stable benzenediazonium tetrafluoroborate 2 by the usual diazotization mechanism. There are several variants for the experimental procedure. Subsequent thermal decomposition generates an aryl cation species 4, which reacts with fluoroborate anion to yield fluorobenzene 3 " ... 

As discussed in Section 8.10, dediazoniation in methanol or ethanol yields mixtures of the corresponding aryl ethers and arenes, except with alcohols of very low nucleo-philicity such as trifluoroethanol, in which the aryl ether is the main product. Therefore aryl ethers are, in general, synthesized by alkylation of the respective phenol. Olah and Wu (1991) demonstrated, however, that phenylalkyl and aryl ethers can be obtained in 46-88% yield from benzenediazonium tetrafluoroborate using alkoxy- and phenoxytrimethylsilanes in solution in Freon 113 (l,l,2-trichloro-l,2,2-tri-fluoroethane) at 55-60 °C with ultrasonic irradiation. As seen from the stoichiometric... 

The only author to postulate a homolytic mechanism in the last few decades was Deng (1989). His arguments are based on the formation of small amounts of fluorinated bi- and polyphenyls in thermal fluoro-de-diazoniations and in mass-spec-trometric degradations of benzenediazonium tetrafluoroborate and its substituted derivatives. However, he does not include a critical discussion of his work. 

Table 10-1 also includes the experimental results of Daasbjerg and Lund (1992) on the chloro-de-diazoniation of benzenediazonium tetrafluoroborate in tetrachloro-methane/acetonitrile (3 1) in the presence of FeCl3/FeCl2 (4 3). The reported yield is surprisingly high. 

A suitable catalyst for carboxy-de-diazoniations was found by Matsuda s group in their work on arylations of alkenes. As in the case of alkene arylations (Sec. 10.9), they used Pd11 acetate (2 mole %) and carbon monoxide (9 atm) for reactions with benzenediazonium tetrafluoroborate and sodium acetate in acetonitrile as solvent at room temperature (Nagira et al., 1980 82-85% yield). Similar results were obtained... 

With Pd(dba)2 in acetone/dichloromethane (1 1) and ethene (6-8 atm), styrene is formed from benzenediazonium tetrafluoroborate in 51% yield with seven substituted benzenediazonium salts the yields were 62-75%, but very small yields were obtained with the 2,4,6-trimethyl and the 2- and 4-nitro derivatives (Kikukawa et al., 1979). The two nitrodiazonium salts are good substrates in the Meerwein reaction... 

Becker et al. (1977b summary Becker, 1978) measured rates and quantum yields for the photolysis of benzenediazonium tetrafluoroborate under various conditions (additives, quenchers, etc.) in methanol. 

More accurate information on k3 is obtainable if the equilibrium constant K is also determined at various crown ether concentrations, as shown by Nakazumi et al. (1981, 1983). The results with benzenediazonium tetrafluoroborate and 3- and 4-substituted derivatives demonstrate that k3 is not unmeasurably small, but that ky-values are generally 1-2% of k2 for complexation with 18-crown-6, 0.1-0.5% of k2 with 21-crown-7, and 2-10% of k2 with dicyclohexano-24-crown-8. A dual substituent parameter (DSP) analysis of A 3-values (Nakazumi et al., 1987) showed that the dediazoniation mechanism of the complexed diazonium ions does not differ appreciably from that of the free diazonium ions. 

The same conclusion was reached in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols. As expected due to the difference in solvation, the effects of para substituents are smaller in protic than in dipolar aprotic solvents. Alkyl substitution of phenol in the 2-position was found to increase the coupling rate, again as would be expected for electron-releasing substituents. However, this rate increase was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect (Hashida et al., 1975 c). 

Penton and Zollinger (1979, 1981 b) reported that this could indeed be the case. The coupling reactions of 3-methylaniline and A,7V-dimethylaniline with 4-methoxy-benzenediazonium tetrafluoroborate in dry acetonitrile showed a number of unusual characteristics, in particular an increase in the kinetic deuterium isotope effect with temperature. C-coupling occurs predominantly (>86% for 3-methylaniline), but on addition of tert-butylammonium chloride the rate became much faster, and triazenes were predominantly formed (with loss of a methyl group in the case of A V-di-methylaniline). Therefore, the initial attack of the diazonium ion is probably at the amine N-atom, and aminoazo formation occurs via rearrangement. 

Benzenediazonium tetrafluoroborate, Difluoramine See Difluoramine Benzenediazonium tetrafluoroborate, etc. 

Benzenediazonium tetrafluoroborate, Caesium fluoride Baum, K., J. Org. Chem., 1968, 33, 4333... 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

Azidomethylbenzenediazonium tetrafluoroborate, 2708 Benzenediazonium tetrafluoroborate, 2223 2-Chloro-3-pyridinediazonium tetrafluoroborate, 1816 5-(4-Diazoniobenzenesulfonamido)thiazole tetrafluoroborate, 3119 5,6-Diphenyl-l,2,4-triazine-3-diazonium tetrafluoroborate 2-oxide, 3677... 

Dediazoniation itself is not dramatically dependent on the hydrogen fluoride/pyridine ratio, as shown from the dediazoniation of benzenediazonium tetrafluoroborate (previously isolated) in hydrogen fluoride/pyridine mixtures provided that the hydrogen fluoride/pyridine ratio is larger than 6, fluorobenzene is always obtained in quantitative yield (for hydrogen fluoride/ pyridine ratios below 6, byproducts result from the phenyl cation and free pyridine).10 These observations have resulted in new, very efficient syntheses of 4-fluorophenols 2 involving dediazoniation, in hydrogen fluoride/pyridine mixtures, of crystalline 4-hydroxy benzenediazonium chloride and tetrafluoroborate 3 (formed under anhydrous conditions)89,96 as well as quinone-diazide (formed from aminophenol in tetrafluoroboric acid).93,97 Such methods are more efficient than the Balz-Schiemann reaction under standard conditions. 

Concerning electrophilic side reactions, intramolecular Friedel-Crafts condensations have been reported for example, fluorenone is formed from 2-benzoylbenzenediazonium tetrafluorobo-rate.241 The strong Lewis acid boron trifluoride can also be responsible for side reactions, such as the extensive formation of tars from nitro-substituted arenediazonium tetrafluorobor-ates or the acidic hydrolysis of ester substituents, especially in the case of 2-(ethoxycar-bonyl)benzenediazonium tetrafluoroborate.105,242... 

Most of the side reactions have already been discussed in Section 26.1.3.3. (dediazoniation in organic solvents). Chlorinated aliphatic solvents, such as 1,2-dichloroethane and dichloro-methane, lead to an extensive formation of chloroaromatics, and aromatic solvents, even halogenated examples, can be arylated to some extent by arenediazonium tetrafluoroborates (vide supra). For example, during dediazoniation of benzenediazonium tetrafluoroborate in toluene, chlorobenzene, bromobenzene or anisole, 3-5 % of substituted biphenyls Ph-C6H4-X (X = Me, Cl, Br, OMe) are formed together with 0.5-0.8% of fluorobiphenyls.5 Fluorobiphenyls are formed through an ionic pathway (only 2- and 4-isomers are formed) whereas chlorobiphenyls result from a radical process (X = Cl, 2-/3-/4-isomer 26 47 27).243... 

The effect of ultrasound on the course of the Balz-Schiemann reaction has been studied using benzenediazonium tetrafluoroborate.263 In the presence of triethylamine trihydrofluoride in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113), fluorobenzene is formed in a 92-95% yield under 17 kHz sonication for 8 hours at 40 °C. Without ultrasound, only an 85 % yield is obtained after a reaction time of 16 hours. 

Cyclization of 2-(pyrid-2-yl)benzenediazonium tetrafluoroborate (451) with alkyl or aryl nitriles afforded the pyrido[l, 2-c]quinazolinium salts 452 [72GEP(O)2043665]. 

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