11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

quaternary ammonium salts closely related to the cinchona alkaloid cinchonine can beused in the benzoylation of a glycineimine [10].The indan-based -amino acid derivatives can be synthesized by PTC. Kotha andBrahmachary [11] indicated that solid–liquid PTC is an attractive method that offered aneffective way of preparing optically active products by chiral PTC. They found that ethylisocyanoacetate can be easily bisalkylated in the presence of K 2 CO 3 as the base andtetrabutylammonium hydrogen sulfate as the catalyst. The advantage of isolating waterfrom the reaction medium is to avoid the formation of unwanted hydroxy compounds inthe nucleophilic substitution reaction. If liquid–liquid PTC is applied in the system withthe strong base NaOH and dichloromethane as the organic solvent, the formation ofdihydroxy or cyclic ether can be observed.2. Other ApplicationsPTC incorporated with other methods usually greatly enhances the reaction rate. Masstransfer of the catalyst or the complex between different phases is an important effect thatinfluences the reaction rate. If the mass transfer resistance cannot be neglected, animprovement in the mass transfer rate will benefit the overall reaction rate. The applicationof ultrasound to these types of reactions can be very effective. Entezari andKeshavarzi [12] presented the utilization of ultrasound to cause efficient mixing of theliquid–liquid phases for the saponification of castor oil. They used cetyltrimethylammoniumbromide (CTAB), benzyltriethylammonium chloride (BTEAC), and tetrabutylammoniumbromide (TBAB) as the catalysts in aqueous alkaline solution. The more suitablePT catalyst CTAB can accumulate more at the liquid–liquid interface and produces anemulsion with smaller droplet size; this phenomenon makes the system have a high interfacialsurface area, but the degradation of CTAB is more severe than that of BTEAC orTBAB because of more accumulation at the interface of the cavity under ultrasound.Recently, electron-transfer catalysis by viologen compounds has attracted muchattention. The compounds function as mediators of electron transfer and have beenapplied in the reduction of aldehydes, ketones, quinines, azobenzene, acrylonitrile,nitroalkenes, etc., with zinc or sodium dithionite in a monophase or a two-liquid phasesystem [13]. Noguchi et al. [13] found that a redox-active macrocyclic ionene oligomer,cyclobis(paraquat-p-phenylene), acted as an electron phase-transfer catalyst for the reductionof quinines, as compared with acyclic benzyl viologen. The enhanced activity of thiscompound is due to the inclusion of the substrate into the catalyst cavity.One of the important applications of PTC is in the field of pollution control. Anearly utilization was to apply the PTC method to recover phenolic substances from aqueousalkaline waste streams [14]. The methodology is based on the reaction of phenolicsubstances in the aqueous solution with materials such as benzoyl chloride, p-toluenesulfonylchloride, etc., dissolved in the organic solvent in the presence of PT catalysts:ð3ÞCopyright © 2003 by Taylor & Francis Group, LLC