Preparation of biaryls by the Ullmann coupling reaction

The two alkyl groups bonded to copper in dialkyllithium cuprate complexes are coupled by oxidizing agents such as oxygen or nitrobenzene  

Vinyl copper and vinyl silver systems, which can be prepared by metal-metal exchange from the corresponding lithium compounds, undergo thermal decomposition, resulting in coupling to butadiene, and deposition of the metal occurs near room 

The mechanism of these reactions is not known in detail. They may involve one-electron oxidation of the alkyl moiety to the equivalent of a radical, followed by combination of two such groups. Both the oxidation and coupling probably take 

The view of the mechanism by which transition metals catalyze such reactions that has gained widest acceptance pictures those reactions as being initiated by electrophilic (oxidative) attack on one of the strained cr-bonds. Such oxidative attack is then followed by carbonium-ion-like migrations and bond cleavages, which eventually are concluded by the expulsion of the metal ion. Some evidence for carbonium-ion intermediates has been obtained from results of reactions in nucleophilic solvents in which the intermediate carbonium ion can be trapped. 

Many details concerning these reactions have been established, but there are also unanswered questions regarding relationships between the catalytic metal ion and the product composition. For example, the course of isomerization of tricyclo[4.1.0.0 ]heptane, 2, with more than a dozen transition-metal derivatives 

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Scheme 5.8. Preparation of Biaryls by the Ullmann Coupling Reaction...

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