Homo aryldiazonium salts

Cohen has shown that copper(l) salts readily induce the decomposition of the aryldiazonium salt to generate the aryl radical which subsequently reacts with Cu(l) to give an arylcopper(ll) species [99,100]. This reacts with another aryl radical to produce the key diarylcopper(III) intermediate. The latter undergoes reductive elimination of the biaryl with regeneration of Cu(l), similar to other copper-based homo-coupling reactions, see Chapter 3, [100] Scheme 26. 

Although the copper-catalysed homo-coupling reaction of aryldiazonium salts is still unemployed, the Cohen s work is a good basis for further achievements of this perspective approach. 

Further important organometallics which offer ready access to the homo-coupling products are arylmercuric halides. This can be easily prepared by several ways, but the most convenienty from the corresponding aryldiazonium salts via the Nesmeyanov reaction, see Chapter 4. Arylmercuric chlorides derived from electron-rich arenes such as thiophene, which easily undergoes the direct mercuration reaction at 2-position, can be readily coupled with an equimolar amount of palladium(II) chloride in the presence of triethylamine as the base in high yield [35], Scheme 16. 

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