Arene diazonium tetrafluoroborates

Ahern and Gokel (1979) claim that diazocyanides can be prepared by stirring arene-diazonium tetrafluoroborates with one equivalent of KCN (solid) and 0.05 equivalent of 18-crown-6 in CH2C12 for some hours. The authors did not, however, investigate whether the products, obtained in good yields, are the (Z)- or the (ii)-isomers. 

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. 

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... 

In solvents from which a hydrogen atom can be easily abstracted (methanol, dioxane),195 dediazoniation of diazonium tetrafluoroborates RC6H4N2+ BF4 becomes a radical process and arenes RC6H4H are significant byproducts for example, when R = 4-f-Bu, the ratio ArF/ ArH ranges from 5 13 (MeOH) to 55 26 (dioxane). The same phenomenon is observed in aprotic polar solvents,5 179,226 227 such as acetonitrile, hexamethylphosphoric triamide or dimethylformamide, especially when a stabilizing substituent (N02, Cl, OMe) is present in the ortho position. 

Firstly, various side reactions result from impurities left in the diazonium tetrafluoroborate (water, sodium nitrite, washing solvents). As already discussed, residual water or alcohols (often used to wash the solid diazonium tetrafluoroborate after filtration) substitute the diazo group in an ionic process to produce phenols or alkoxyarenes as byproducts. Residual alcohol can also lead to arenes through a radical hydrodediazoniation process, especially when electron-withdrawing groups are present, as in 3-fluoro-2-methoxy-5-(methoxyearbonyl)benzenediazo-nium tetrafluoroborate (ArF/ArH 4 1)227 or in 2-methyl-5-nitrobenzenediazonium tetrafluoroborate.240 Such radical phenomena increase when sodium nitrite is present as impurity... 

Reduction of aryl diazonium salts Aryl diazonium tetrafluoroborates are reduced to arenes by thiophenol in 85-100% yield. Replacement by deuterium can be effected with CsH SD. 

In work recently reported [20] it was found that reduction of aryldiazonium tetra-fluoroborates in nonpolar solution by aqueous hypophosphorous acid and catalytic cuprous oxide, good to excellent yields of arenes were obtained under very mild conditions. The single exception in the group of salts studied was 4-methoxybenzene-diazonium tetrafluoroborate which yielded only 67% of arene. Addition of 7.5 mole-9 of 18-crown-6 increased the yield to 88%. The effect of crown in this system is not understood but believed to involve crown enhanced solubility of the salt (see Eq. 15.6) [20]. 

Keumi et al. (1989) describe hydro-de-diazoniations of arenediazonium tetrafluo-roborates using chlorotrimethylsilane, (CH3)3SiCl), in tetrahydrofuran or tetra-hydrofuran/A A-dimethylformamide mixtures. Excellent yields were obtained with polycyclic arene derivatives such as 2-fluorene-, 2-fluorenone-, and 1-pyrenediazo-nium tetrafluoroborate and other similar diazonium salts. In a modification of this method 2-halogenofluorenones can be synthesized (see Sec. 10.6). 

The Gomberg coupling reaction of aryl diazonium salts with arenes is catalysed by quaternary ammonium salts [49], Although yields are variable, they are generally >50% [49, 50]. When aromatic solvents other than benzene are used, the appropriate biaryls are produced, e.g. 4-chlorobenzenediazonium tetrafluoroborate reacts with chlorobenzene to produce the 2,4 -, 3,4 - and 4,4 -dichlorobiphenyls in a ca. 67 18 15 ratio. 

The major limitation of the all GBH reactions is that the arene has to be in the liquid form to serve as the solvent. All attempts to arylate solid arenes under the GBH reaction conditions were unsuccessful. Furthermore, the presence of radical-sensitive substituents in either diazonium salts or in arene results in lower yields, or the reaction fails completely. Generally, some alkyl, alkoxycarbonyl, formyl, or iodo-substituted arenes or diazonium salts are susceptible for hydrogen- or iodo-abstraction by free-radicals and are not common GBH reactants. For example, 2-tolyldiazonium tetrafluoroborate in the ptGBH reaction with benzene gives indazole in 74% yield, and only 2% of biaryl product [61]. 

This reaction is the most simple method for synthesis of these structures beside several recent methods based on cross-coupling reactions with different organometallic reagents. The starting materials are very inexpensive and easily available, whereas an excess of arene can be regenerated by distillation of the filtrate after the separation of potassium tetrafluoroborate. This can be also reused as a precipitation reagent for preparation of the parent aryldiazonium tetrafluoroborate from the diazonium chloride... 

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