Single Electron-Catalyzed Coupling Reactions of Aryl Halides

Transition metal catalysts have inevitably been used for C(sp(2))-C(sp(2)) bond forming reactions of aryl halides with sp(2)-carbon nucleophiles. On the other hand, we have recently developed such a kind of reactions with no aid of transition metal catalysis, utilizing single electron transfer (SET) mechanism. Here, a single electron acts as a catalyst to promote the reaction of aryl halides (Ar-X) with arenes, styrenes or aryl Grignard reagents (ArMgBr) to give biaryls, stilbenes or biaryls, respectively. The common intermediates are anion radicals ([Ar-X](center dot-)) of aryl halides and those ([Ar-R](center dot-)) of coupling products. [Ar-X](center dot-), generated through SET from a base or ArMgBr to Ar-X, reacts directly with ArMgBr, whereas less reactive arenes and styrenes react with Ar-center dot, generated by decomposition of [Ar-X](center dot-). All the reactions include, in the last step, SET from [ArR](center dot-)- to Ar-X to give Ar-R and regenerate [Ar-X](center dot-).