Observation and Mechanistic Study of Facile C-O Bond Formation between a Well-Defined Aryl-Copper(III) Complex and Oxygen Nucleophiles

A well-defined macrocyclic aryl-Cu-III complex (2) reacts readily with a variety of oxygen nucleophiles, including carboxylic acids, phenols and alcohols, under mild conditions to form the corresponding aryl esters, biaryl ethers and alkyl aryl ethers. The relationship between these reactions and catalytic C-O coupling methods is demonstrated by the reaction of the macrocyclic aryl-Br species with acetic acid and p-fluorophenol in the presence of 10 mol% Cu-I. An aryl-Cu-III-Br species 2(Br) was observed as an intermediate in the catalytic reaction. Investigation of the stoichiometric C-O bond-forming reactions revealed nucleophile-dependent changes in the mechanism. The reaction of 2 with carboxylic acids revealed a positive correlation between the log(k(obs)) and the pK(a) of the nucleophile (less-acidic nucleophiles react more rapidly), whereas a negative correlation was observed with most phenols (more-acidic phenols react more rapidly). The latter trend resembles previous observations with nitrogen nucleophiles. With carboxylic acids and acidic phenols, UV-visible spectroscopic data support the formation of a ground-state adduct between 2 and the oxygen nucleophile. Collectively, kinetic and spectroscopic data support a unified mechanism for aryl-O coupling from the Cu-III complex, consisting of nucleophile coordination to the Cu-III center, deprotonation of the coordinated nucleophile, and C-O (or C-N) reductive elimination from Cu-III.