Novel redox reactions of diazonium fluoroborates. The formation of copper saltazo-compound complexes: water-induced free radical aromatic arylation
Whereas certain diazonium fluoroborates (ArN2BF4) are stable in benzene at room temperature in the presence of equivalent amounts of copper powder, they react slowly on addition of acetone (10% v/v) to the mixture to give precipitates of red complexes of the azo-compounds (ArN:NAr) with ionic copper. In addition, the free azo-compounds and small (<10%) amounts of biaryls (ArPh) can be isolated. The complexes decompose in water or in polar solvents to give the corresponding azo-compounds. In contrast, when the reaction of the diazonium salt is carried out in the presence of water (2 mol. equiv.), either free or bound in the form of hydrated salts, e.g. magnesium sulphate monohydrate, high yields of biaryls (ArPh) formed by a free radical path are obtained. Azo-complexes are not formed and are not precursors in this reaction, in which copper acts as catalyst.
These contrasting observations represent extremes of behaviour of diazonium salts in the presence of copper; the use of smaller quantities of water leads to mixtures of products.
The stoicheiometry has been established in each of these reactions and explanations in terms of a series of one-electron, redox reactions of the Waters type are presented.
Similar reactions occur with p-chlorobenzenediazonium hexafluorophosphate.