11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

the cycloalkylation of phenylacetonitrile (PAN) with an excess of 1,4-dibromobutaneusing aqueous sodium hydroxide as the base, and the following pseudo-first-order kineticswas observed:C 6 H 5 CH 2 CN þ BrðCH 2 Þ 4 Br Dq-Br !ð0:75 mol%Þ C 6 H 5 CðCH 2 Þ 4 CN ð6ÞHwang et al. [24] studied the Wittig reaction of benzyltriphenylphosphonium(BTPP) salts and benzaldehydes via ylide-mediated PTC. They concluded that the reactionof benzylidenetriphenyl phosphorane and the benzaldehyde in the organic phase is thedecisive step for stereoselectivity. The order of effectiveness of substituents isCF 3 > ðCl; BrÞ > MeO > F > NO 2 . Satrio and Doraiswamy [25] proposed a case studyfor the production of benzaldehyde in a possible industrial application of PTC. Thereaction between benzyl chloride and hypochlorite anion isC 6 H 5 CH 2 Cl ðorgÞþOCl ðaqÞ !C 6 H 5 CHO ðorgÞþHCl ðaqÞþCl ð7ÞThey show that the conventional route is the preferred one for a large-scale organicintermediate, and the improvements in merely one or two PTC steps can greatly enhancethe prospects of the PTC route.II.LIQUID–LIQUID PHASE TRANSFER CATALYSISA necessary condition for a reaction is to cause the collision of two reactant molecules. Itis obvious that the reaction rate of two immiscible reactants is low due to their lowsolubilities. A general method for overcoming this difficulty was to employ a protic oran aprotic solvent in order to improve their mutual solubilities. Nevertheless, thisimprovement was not very significant. The problems of two-phase reactions were notsolved until Jarrouse [26] discovered the catalyzing effect of quaternary ammonium saltin the aqueous–organic phase reaction system. PTC is an effective tool for synthesizingorganic chemicals from two immiscible reactants [27–32]. It has been extensively applied tothe synthesis of special organic chemicals by displacement, alkylation, arylation, condensation,elimination, oxidation, reduction, and polymerization. The advantage of PTC inthe synthesis of organic chemicals are fast reaction rate, high selectivity of product, moderateoperating temperature, and applicability to industrial-scale production.A. Mechanism of Liquid–Liquid Phase Transfer Catalysis (LLPTC)Quaternary salts, crown ethers, cryptands, and polyethylene glycol (PEG) are the mostcommon agents used for LLPTC. Over the last few decades, the two reaction mechanismsused to describe the phenomenon of a two-phase PTC reaction were the Starks extractionmechanism and Mąkosza interfacial mechanism.1. Starks Extraction MechanismThis reaction mechanism described by Starks [28,33] is widely accepted for a catalysttransferring between the two phases. Reactions occurring in such systems involve: (1) thereactant reacting with catalyst in the normal phase to form an intermediate catalyticreactant, (2) transfer of the intermediate catalytic reactant from its normal phase intothe reaction phase, (3) transferred intermediate catalytic reactant reacting with untransformedreactant in the reaction phase to produce the product and catalyst, and (4)Copyright © 2003 by Taylor & Francis Group, LLC