Selective Carboxylation of Benzoic Acid Using Cyclodextrin as Mediator

Abstract

ABSTRACT:The selective synthesis ofterephthalic acid from benzoic acid with carbon tetrachloride, copper powder, and aqueous alkali was achieved by the usc of P-cyclodextrin (jJ-CyD) as a mediator at 60°C under nitrogen of atmospheric pressure, producing terephthalic acid in 74 mol% yield with 100% selectivity. When iX-CyD or y-CyD was used instead of {J-CyD, the reaction hardly proceeded. The addition of p-benzoquinone to the reaction mixture decreased the yield ofterephthalic 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 CyDbenzoate inclusion complexes in aqueous alkaline solution were determined by the nuclear magnetic resonance spectroscopy using 1 H homonuclear Overhauser enhancement on the rotating frame. The benzoate anion was axially included in the cavity of P-CyD with orientation such that the carboxylate group was at the primary hydroxyl side of P-CyD. The essential factor of the carboxylation by the use of P-CyD was the inclusion complex formation of P-CyD with benzoate anion and fJ-CyD with carbon tetrachloride in the reaction mixture. The very high selectivity was ascribed to the conformation of the P-CyD-benzoate inclusion complex.

Selective synthesis of aromatic dicarboxylic acid using cyclodextrin as catalyst

“…The inclusion complex formation of ␤-CyD with carbon tetrachloride in alkaline aqueous solution was confirmed by the 400 MHz 1 H-NMR spectroscopy [14]. The inclusion formation constants of the ␣-CyD-carbon tetrachloride complex and of the ␥-CyD-carbon tetrachloride complex were much smaller than that of the ␤-CyD-carbon tetrachloride complex [21].…”

“…The promotion of carboxylation by ␤-CyD is probably attributable to inclusion complex formation [14]. The solubilities of carbon tetrachlo- ride and benzoic acid in aqueous alkali in high concentration increase by the formation of the ␤-CyD-carbon tetrachloride complex and ␤-CyDbenzoic acid complex.…”

“…When ␥-CyD was used instead of ␤-CyD, the yield of terephthalic acid, phthalic acid and isophthalic acid were 0.4, 0.0 and 0.0 mol%, respectively. Figure 3 exhibits the relationship between the yield of terephthalic acid and the initial mole ratio of ␤-CyD to benzoic acid at the optimal amounts of carbon tetrachloride and copper powder II (average particle diameter 1 m), which were determined by the previous study [14]. The yield of terephthalic acid has a maximal value (74 mol% with 100% selectivity) at a mole ratio of ␤-CyD to benzoic acid of 0.7, where both the yield of phthalic acid and isophthalic acid were 0.0 mol%.…”

“…The carboxylation mechanism can be proposed as shown in Scheme 4, on the basis of the results of the previous study [14]. Carbon tetrachloride reacts with copper powder to form the trichloromethyl radical, as the active species.…”

“…The benzoate anion is axially included in the cavity of ␤-CyD with an orientation such that the carboxylate group is at the primary hydroxyl side of ␤-CyD and the para position of benzoate at the secondary hydroxyl side of ␤-CyD. Carbon tetrachloride included in ␤-CyD with copper powder forms the trichloromethyl radical, as the active species [14]. The trichloromethyl radical should predominantly attack the para position of the benzoate anion included in ␤-CyD.…”