Copper(I) as a homogeneous catalyst for the Ullmann reaction in aqueous solutions - The transformation of 2-bromobenzoate into salicylate

The Cu^I-catalyzed transformation of 2-bromobenzoic acid (2-Br-BA) into salicylic acid (main product), benzoic acid, and diphenoic acid was studied. The reaction was monitored by UV/Vis spectroscopy, HPLC, and ESR. Up to 25 turnovers were achieved for low Cu^I concentrations. An intermediate was observed that was assigned to a complex between copper and a follow-up product of the aromatic acid, probably a complex with a copper-carbon σ-bond. The process is probably initiated by the formation of a d-π* complex of Cu^I with the aromatic ring of 2-Br-BA followed by the reaction with a second Cu^I ion. The effect of the addition of CH₃CN or NH₃ on the catalytic process is reported. Both were found to accelerate the process when present at relatively low concentrations and slow it down at higher concentrations. A side product, 2-aminobenzoate was observed in the presence of NH₃.