Oxalamides are among the best ligands for Cu-catalyzed cross-coupling reactions that form C-O and C-N bonds, but no mechanistic understanding exists to explain why they outperform ligands from other classes. Our mechanistic studies on the cross-coupling between phenols and aryl halides reveal that the catalyst resting state is a Cu(II) bis-oxalamide complex. Furthermore, the aforementioned complex reacts directly with aryl halide in the turnover-limiting step. Radical trap experiments demonstrate that no aryl radical is formed during the turnover-limiting reaction between the Cu(II) bis-oxalamide complex and aryl halide. The mechanism described above significantly diverges from the mechanistic paradigm established for Ullmann and Goldberg reactions catalyzed by copper with other ligand classes, which proceed through a Cu(I)/Cu(III) cycle. The mechanistic understanding gained will facilitate the design of next-generation copper catalysts for cross-coupling reactions, and has been used to develop a method for coupling phenols and aryl chloride electrophiles at low catalyst loading (0.25 mol %).

Unique mechanism of the Ullmann C-O cross-coupling reaction catalyzed by copper with oxalamide ligands

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