Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 799
a two-phase reaction (aqueous-phase and organic-phase reaction), transfer of QCN from
aqueous phase to organic phase and transfer of QCl from organic phase to aqueous phase,
and equilibrium partition of QCN and QCl between organic and aqueous phases, respectively.
The overall reaction rate highly depends on the intrinsic rate constants in aqueous
phase and organic phase, the mass transfer rate of QCN and QBr, and the equilibrium partition
coefficient of QCN and QBr, which are all affected by the interaction of components
and their environments. The organic solvent provides the environment for the interaction of
reactants. Therefore, the organic solvent plays an important role in influencing the reaction
rate and the conversion of reactant.
Since then, Makosza used an interfacial mechanism 65-67 to describe the behavior in the
two-phase reaction. Later, Starks 107 used the extraction mechanism to explain the behavior
in the two-phase reaction and selected phase-transfer catalysis (PTC) to describe this special
chemical process. 14,161 The most important advantage of using PTC technique is in synthesizing
specialty chemicals with almost no byproducts and moderate reaction conditions.
Today, PTC is widely applied to various reactions via substitution, displacement, condensation,
oxidation and reduction, polymer modification and polymerization to synthesize specialty
chemicals. Based on the reaction mechanism, phase-transfer catalysis can be
classified as: (1) normal phase-transfer catalysis (NPTC), (2) reverse phase-transfer catalysis
(RPTC), and (3) inverse phase-transfer catalysis (IPTC). Equation [13.3.1] illustrates the
typical reaction for NPTC. The phase-transfer catalyst (Q + ) brings the nucleophilic reagent
(CN - ) from aqueous phase to organic phase. Quaternary ammonium salts, quaternary
phosphonium salts, crown ethers, polyethylene glycols (PEGs) and tertiary amines are the
common normal phase-transfer catalyst (NPTC). 17,29,94,108,109,110,128,130,152
In general, the cation transfers from aqueous phase to organic phase in the RPTC. The
principle of reverse phase-transfer catalysis (RPTC) 24,42-44,50 is that an ion pair is formed
from catalyst and cation in the aqueous phase. This ion-paired compound then transfers to
the organic phase reacting with an organic-phase reactant. Alkyl-aryl sulfonate (RSO 3Na),
such as sodium 4-dodecylbenzene sulfonate (NaDBS) and tetraarylboronate such as sodium
tetra(diperfluoromethyl)phenyl-boronate (TFPB) are the common reverse phase-transfer
catalysts. However, few results were reported using reverse phase-transfer catalysis in synthesizing
specialty chemicals. 24,42-44,50 A typical reaction mechanism in a liquid-liquid
two-phase solution is given by Equation [13.3.2]
In the NPTC and RPTC, the function of the catalyst is that it first reacts with aqueous-phase
reactant to produce an organic-soluble ion-pair compound. Mathias and Vaidya 69
found that an aqueous-soluble ion pair was produced in the organic phase from the reaction
of alanine and benzoyl chloride catalyzed by 4-dimethylaminopyridine (DMAP). This discovery
initiated the research of the field in inverse phase-transfer catalysis (IPTC), in which
the catalyst first reacts with organic-phase reactant in the organic phase to produce an aqueous-soluble
ion-paired intermediate. Then, this aqueous-soluble ion-paired intermediate
transfers to the aqueous phase, prepared for reacting with aqueous-phase reactant to produce
the desired product. Catalyst is released in the aqueous phase and transferred to the organic
phase for further regeneration. A typical IPTC mechanism of the reaction of benzoyl
chloride and sodium acetate to synthesize ester compound in the liquid-liquid two-phase reaction
is expressed by Equation [13.3.3]