A Simple and Efficient Oxidative Coupling of Aromatic Nuclei Mediated by Manganese Dioxide

Oxidative intramolecular coupling of the aryl rings of various stilbenes for direct construction of the phenanthrene ring system is promoted efficiently by manganese dioxide-trifluoroacetic acid at room temperature in excellent yields. This approach is also applied to the intermolecular biaryl coupling of 2-naphthols, 2-naphthalenethiol, 2-naphthylamine, a phenol ether and a phenol under very mild conditions. An electron-transfer mechanism is proposed in which manganese dioxide acts as a two-electron oxidant.