Arenes oxidative homo-couplings

The formation of a C-C bond resulting from the coupling of two C-H bonds is a particularly attractive target, since the only formal by-product would be hydrogen, or water in an oxidative system. However, substantial hurdles impede the conception of a catalytic arene cross-coupling process that does not involve any substrate pre-activation at all. Aside from issues of reactivity and regioselectivity, the prevention of homo-coupling is a key factor for the development of this important class of reaction. The catalyst must be able to react with one arene in the first step of the catalytic cycle and then invert its selectivity in the second step to react exclusively with a different arene (Scheme 29). 

The synthesis of relevant biaryls has also been reported with aryltrifluoroborates directed by the carboxylic acid functional group in addition to oxidative dimerization. Sanford has described the preparation of biaryls via two discrete C—H functionalizations through the intermediacy of a Pd(IV) fcis-cyclopalladated moiety which reductively eliminates to afford the new C—C bond (Scheme 3.3). An analogous procedure affording the homo-coupled arene from the unactivated starting material has also been reported. ... 

---