Multitechnique Approach to Reveal the Mechanism of Copper(II)-Catalyzed Arylation Reactions

Abstract

Multiple in situ and time-resolved spectroscopic techniques (EDXAFS, UV−vis, EPR, and NMR), with a focus on simultaneously acquired EDXAFS and time-resolved UV−vis, are described to reveal detailed structural and electronic information on reaction intermediates of an important Cu(II)-catalyzed N-arylation of imidazole. The N-arylation of imidazole was performed in a NMP/H2O solvent mixture, at ambient temperature and atmosphere, using the commercially available Cu catalyst [Cu(OH)(TMEDA)]2Cl2 (I). The spectroscopic study resulted in the characterization of most reaction intermediates, and a novel mechanism for the Cu(II)-catalyzed arylation reaction is proposed. The first and selectivity-determining step is the reaction of the dimeric Cu(II) starting complex with imidazole, forming a mononuclear Cu(II)(imidazole) intermediate, II. After subsequent addition of phenylboronic acid, we propose the formation of a Cu(III)(imidazolate)(phenyl) intermediate, III, which after reductive elimination forms the phenylimidazole product, and a known Cu(I) monomeric species, IV, is identified. Finally, this Cu species is reoxidized, forming back an equilibrium mixture of Cu(II) mononuclear and dinuclear complexes. Inhibition of the reaction by imidazole and phenylimidazole is observed. The phenylboronic acid is, in combination with H2O, involved in the oxidation and reoxidation steps in the described catalytic cycle.