Hypervalent iodine induced metal-free C-H cross couplings to biaryls

Abstract

Hypervalent iodine reagents, such as phenyliodine diacetate (PIDA) and phenyliodine bis(trifluoroacetate) (PIFA), are promising alternatives to metal-based oxidants for developing environmentally benign oxidation reactions, due to their low toxicities, mild reactivities, ready availability, high stabilities, and easy handling. During the study on the hypervalent iodine chemistry, we first determined in the 1990's the single-electron-transfer (SET) oxidation ability of the hypervalent iodine(III) reagent toward electron-rich aromatic rings, such as the phenyl ether compounds, affording the corresponding aromatic cation radicals. This discovery led us to develop a series of metal-free carbon-hydrogen (C-H) bond functionalizations of aromatic compounds using the hypervalent iodine oxidants. We have extended the strategy to the carbon-carbon bond formation of biaryls based on the double aromatic C-H bond couplings, which can now provide a novel technology to the synthetic area of metal-free cross couplings for producing mixed biaryls from two molecules of unfunctionalized aromatic compounds. These achievements found by us during the development of hypervalent iodine-induced metal-free C-H cross couplings are now summarized in detail.