Marc E. Halpern

Phase-Transfer Catalysis: Fundamentals I

Critical to the success of phase-transfer catalytic (PTC) processes are (1) the maximization of the rate of transfer of reactant anions from the aqueous or solid phase to the organic phase, (2) the maximization of the rate of transfer of product anions from the organic phase to the aqueous or solid phase, and (3) the related equilibrium partitioning of the reactant and product anions between the organic and aqueous or solid phases. The common organic solvents employed in phase-transfer processes are usually relatively nonpolar and usually aprotic. Because anions do not have a great affinity for such solvents and prefer to reside in an aqueous environment, the desired transfer is not a particularly favorable process. The transfer of anions from an aqueous to an organic phase, however, may be achieved by choosing a phase-transfer cation that is not strongly solvated by water and that has organic-like characteristics and is thus compatible with the organic phase. For instance, the volume-to-charge ratio (as well as the organic-like nature) of quaternary ammonium and phosphonium salts can be adjusted over a wide range of values by simply changing the length of the alkyl (or aryl) substituents bonded to the quaternary heteroatom. Tetramethylammonium salts are highly soluble in aqueous media and only slightly soluble in most organic solvents, whereas tetradoecylammonium salts are soluble in most organic media but only slightly soluble in water.

Basic Concepts in Phase-Transfer Catalysis

Phase-transfer catalysis (PTC) is a powerful tool in many areas of chemistry. It is a technique for conducting reactions between two or more reagents in two or more phases, when reaction is inhibited because the reactants cannot easily come together. A “phase-transfer agent” is added to transfer one of the reagents to a location where it can conveniently and rapidly react with another reagent. It is also necessary that the transferred species be in a highly active state when transferred; otherwise large amounts of phase-transfer agent will be required. This activation function, plus the transfer function, allows phase-transfer catalysis to occur with only a catalytic amount of phase-transfer agent.

Phase-Transfer Catalysts

In previous chapters we learned that a phase-transfer catalyst must have two particular chemical functions to be successful, that is, it must rapidly transfer one of the reactant species into the normal phase of the other reactant, and second it must make the transferred species available in a highly reactive form. The need for one or the other of these two functions to be strongly catalyzed often plays an important role in the selection of the best transfer catalyst for a particular reaction, as, for example, to pick a phase-transfer catalysis (PTC) catalyst that is especially useful for activating an anion, or for a catalyst that is especially good for facilitating anion transfer to the organic phase. Catalyst structure may also be of great significance when more than one product can be formed, to cause one product to be favored over another. Sometimes catalyst structure is not important at all, and almost any PTC catalyst will perform satisfactorily for some reactions, whereas for others it may be desirable to have two (or more) catalysts to facilitate each of the various steps in a reaction sequence. Some examples of the effect of catalyst structure are discussed in this chapter to illustrate these points.

Variables in Reaction Design for Laboratory and Industrial Applications of Phase-Transfer Catalysis

Many factors, known and unknown, contribute to the effective selection of a catalyst and other conditions for a given phase-transfer catalysis (PTC) application. Therefore, no simple single guideline exists for a universal effective choice of reaction conditions. However, several useful, though noncomprehensive, approaches exist for separately considering the optimization of various components of PTC reactions under a variety of conditions. It is recommended that the reader be familiar with the various thought approaches (presented below) and choose reaction conditions based on a combination of the approaches. In all cases, the factors that are thought to enhance, or otherwise affect, reaction rates in PTC will be put into the perspective of the PTC Rate Matrix.

Phase-Transfer Catalysis: Industrial Perspective

In the early 1990s, it was estimated that approximately 500 commercial phase-transfer catalysis (PTC) processes were being performed using at least 25 million pounds per year of catalyst. It was also estimated that sales of products manufactured by processes consisting of at least one major PTC step were at least

Phase-Transfer-Catalyzed Oxidations

10 billion/year (approximately

Phase-Transfer Catalysis: Chiral Phase-Transfer-Catalyzed Formation of Carbon-Carbon Bonds

5 billion in polymers;

Phase-Transfer Catalysis: Polymerization and Polymer Modification

3 billion in pharmaceuticals;

Phase-Transfer Catalysis-Transition Metal Cocatalyzed Reactions

2 billion in agricultural chemicals;

Phase-Transfer-Catalyzed Reductions

1 billion in monomers; and unestimated sales in general chemicals, flavors and fragrances, dyes, surfactants, explosives, and others).