Direct observation of Cu-I/Cu-III redox steps relevant to Ullmann-type coupling reactions

A series of aryl-copper(III)-halide complexes have been synthesized and characterized by NMR and UV-visible spectroscopy, cyclic voltammetry and X-ray crystallography. These complexes closely resemble elusive intermediates often invoked in catalytic reactions, such as Ullmann-Goldberg cross-coupling reactions, and their preparation has enabled direct observation and preliminary characterization of aryl halide reductive elimination from Cu-III and oxidative addition to Cu-I centers. In situ spectroscopic studies (H-1 NMR, UV-visible) of a Cu-catalyzed C-N coupling reaction provides definitive evidence for the involvement of an aryl-copper(III)-halide intermediate in the catalytic mechanism. These results provide the first direct observation of the Cu-I/Cu-III redox steps relevant to Ullmann-type coupling reactions.