Direct observation of CuI/CuIIIredox steps relevant to Ullmann-type coupling reactions

Abstract

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 (¹H 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.