Selective Carboxylation of Benzoic Acid Using Cyclodextrin as Mediator

The selective synthesis of terephthalic acid from benzoic acid with carbon tetrachloride, copper powder, and aqueous alkali was achieved by the use of β-cyclodextrin (β-CyD) as a mediator at 60°C under nitrogen of atmospheric pressure, producing terephthalic acid in 74 mol% yield with 100% selectivity. When α-CyD or γ-CyD was used instead of β-CyD, the reaction hardly proceeded. The addition of p-benzoquinone to the reaction mixture decreased the yield of terephthalic acid remarkably. An oxygen atmosphere also suppressed the reaction appreciably. Thus, the active species formed from carbon tetrachloride and copper powder in the reaction mixture is proposed to be trichloromethyl radical. The conformations of CyD-benzoate inclusion complexes in aqueous alkaline solution were determined by the nuclear magnetic resonance spectroscopy using 1H homonuclear Overhauser enhancement on the rotating frame. The benzoate anion was axially included in the cavity of β-CyD with orientation such that the carboxylate group was at the primary hydroxyl side of β-CyD. The essential factor of the carboxylation by the use of β-CyD was the inclusion complex formation of β-CyD with benzoate anion and β-CyD with carbon tetrachloride in the reaction mixture. The very high selectivity was ascribed to the conformation of the β-CyD-benzoate inclusion complex.