| Abstract
| - The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)2 (Ar =4-Z-C6H4 with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fullyelucidated. The peroxo complex (η2-O2)PdL2 (L = PPh3), generated in the reaction of dioxygen with thePd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate anadduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronicacid) characterized by 31P NMR spectroscopy and ab initio calculations. This adduct reacts with a secondmolecule of arylboronic acid to generate trans-ArPd(OH)L2 complexes. A transmetalation by the arylboronicacid gives trans-ArPdArL2 complexes. The biaryl is then released in a reductive elimination. This reactionis at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki−Miyaura reactionswhen they are not conducted under oxygen-free atmosphere.
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