Phase-Transfer Catalysis: An Overview

Abstract

Uniquely interesting, complex and useful activities and phenomena occur at interfaces: one need only to look at the interfaces between the land, the atmosphere, and the sea to find this truth. The same truth occurs in chemical interfaces, although sometimes it is the lack of activity that draws our attention. In many chemical situations where two species cannot collide and therefore cannot react because they are separated by an interface, the lack of activity has been overcome by use of the technique of PHASE TRANSFER CATALYSIS (PTC), which not only allows reaction to occur, but often to occur in very selective ways.An early and clear example of PTC(1) demonstrated that the lack of reactivity between a mixture of 1-chlorooctane and aqueous sodium cyanide (without organic solvent) could be overcome by the use of a phase transfer agent, whose function was to transfer cyanide ion in reactive form from its normal aqueous phase into the chlorooctane phase. Use of a small amount of phase transfer agent makes the system catalytic, since the phase transfer agent can repeatedly transfer active cyanide ions into the organic phase for reaction with 1-chlorooctane. This sequence of steps is represented by equation 1, where Q+ represents a quaternary salt containing sufficiently long alkyl groups or other organic structure as to make QCN predominantly soluble in the organic phase. organicCgHiyCl + Q+CN" ψ CgHijCN + Q+Cl" phase f \ (1) I aqueous Cl" + Q+CN" r CN" + Q+Cl" phase ^ Other classic examples illustrating the use of quaternary salts as phase transfer catalysts were published by Makosza(2), and by Brandstrom(3.) . Subsequent development of crown ethers(4-7) and cryptands(7-8) as phase transfer catalysts gave PTC an entirely new dimension since now the inorganic reagent, as sodium cyanide in the above equation, need no longer be dissolved in water but can be used

A Novel and Chemoselective Process ofN-Alkylation of Aromatic Nitrogen Compounds Using Quaternary Ammonium Salts as Starting Material

“…Since it would be very difficult to carry out the attack on the long chain due to steric effects, the pyrrolyl anion would proceed to attack the less hindered electrophilic carbon, leading to N-methylation in the methyl group [11,12]. Tables 1 and 2 indicate that the quaternary ammonium salts are the main substrate in the reaction that results in Nalkylation and the formation of a tertiary amine.…”

A Kinetic Study of Phase Transfer Catalyzed Benzylation of 2,4,6-Tribromophenol in Liquid-Liquid Two-Phase Solution

“…Phase transfer catalysis (PTC) is an important technique in enhancing the reactions of two immiscible reactants (Dehmlow and Dehmlow, 1993;Freedman, 1986;Starks, 1985;Starks et al, 1994;Weber and Gokel, 1977). It is now well recognized as an invaluable methodology for synthesizing organic chemicals from immiscible reactants (Keller, 1986(Keller, , 1987(Keller, , 1992.…”

Kinetic study of S-alkylation of 2-mercaptobenzimidazole by allyl bromide in the presence of potassium hydroxide

“…Furthermore, byproducts usually accompany generation of the main products. This difficulty was overcome by the development of phase-transfer catalysis (PTC) for the synthesis of organic chemicals from two immiscible reactants (Dehmlow and Dehmlow, 1993;Freedman, 1986;Keller, 1986Keller, , 1987Keller, , 1992Starks, 1985;Starks et al, 1994;Weber and Gokel, 1977). Quaternary salts are conventionally employed as the phase-transfer catalysts, dramatically increasing the conversion and the reaction rate.…”