Metal-and Chemical-Oxidaet-Free C-H/C-H Cross-Coupling of Aromatic Compounds: The Use of Radical-Cation Pools

摘要

The direct oxidative C-H/C-H cross-coupling, also known as the dehydrogenative cross-coupling, of unactivated aromatic compounds has fascinated many chemists because it does not require prefunctionalization of starting aromatic compounds and it serves as a straightforward, atom-, and step-economical method for connecting two aromatic rings by a single C-C bond. In 2007, a metal-catalyzed C-C cross-coupling reaction involving two C-H activations and employing a stoichiometric amount of oxidant was achieved and, since then, the method has been studied exten-sively.In 2008, a metal-free biaryl compound synthesis that uses a stoichiometric amount of an organoiodine(III) oxidant was developed. A new synthetic method that enables metal-and chemical-oxidant-free C-H/C-H cross-coupling of two aromatic compounds is needed. Electrochemical oxidation serves as an efficient method that does not use metal catalysts and chemical oxidants for activating aromatic compounds. In fact, the subjection of a mixture of two unactivated aromatic compounds to electrochemical oxidation can give the cross-coupling products, although yields are usually low.'' Recently, Waldvogel and co-workers reported a selective electrochemical phenol-arene cross-coupling reaction using boron-doped diamond electrodes. This method, however, cannot be applied to aromatic compounds that do not have a hydroxy group because the phenoxyl radical intermediate plays a crucial role.