Regioselective carboxylation of aromatic compounds using cyclodextrin as mediator

The direct introduction of a carboxylic group into an aromatic carboxylic acid has been considered difficult theoretically. Recently, the regioselective syntheses of terephthalic acid, 4,4′-biphenyldicarboxylic acid and 2,6-naphthalene-dicarboxylic acid have been achieved by the carboxylation of benzoic acid, 4-biphenylcarboxylic acid and 2-naphthalenecarboxylic acid, respectively, with carbon tetrachloride and copper powder in aqueous alkali, using β-cyclodextrin (β-CyD) as mediator under mild conditions. The one-pot syntheses of terephthalic acid, 4,4′-biphenyldicarboxylic acid and 2,6-naphthalene-dicarboxylic acid have been attained by the carboxylation of benzene, biphenyl and naphthalene, respectively, with carbon tetrachloride and copper powder in aqueous alkali, using β-CyD as mediator. The essential factor of the carboxylation by the use of β-CyD is the inclusion complex formations of β-CyD with aromatic hydrocarbon, β-CyD with aromatic monocarboxylate and β-CyD with carbon tetrachloride, respectively, in the reaction mixture. The conformations of CyD–aromatic monocarboxylate inclusion complexes in aqueous alkali have been determined by the nuclear magnetic resonance spectroscopy using 1H homonuclear Overhauser enhancement on the rotating frame. The high selectivity is ascribed to the conformation of the β-CyD–aromatic monocarboxylate inclusion complex. The reaction mechanism is discussed on the basis of inclusion complex formation.