Phase-transfer catalysis triphase system

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

Another important asymmetric epoxidation of a conjugated systems is the reaction of alkenes with polyleucine, DBU and urea H2O2, giving an epoxy-carbonyl compound with good enantioselectivity. The hydroperoxide anion epoxidation of conjugated carbonyl compounds with a polyamino acid, such as poly-L-alanine or poly-L-leucine is known as the Julia—Colonna epoxidation Epoxidation of conjugated ketones to give nonracemic epoxy-ketones was done with aq. NaOCl and a Cinchona alkaloid derivative as catalyst. A triphasic phase-transfer catalysis protocol has also been developed. p-Peptides have been used as catalysts in this reaction. ... 

Phase transfer catalysis and the use of crown ethers are also of particular advantage in alkanenitrile synthesis (Table 1). Usually quaternary ammonium and phosphonium salts serve quite well as catalysts. Another modification is represented by the use of a solid catalyst, which is insoluble in the two-phase system, for instance alumina or anion-exchange resins (triphase catalysis). Crown ethers again capture the cations and generate naked cyanide ions in fairly nonpolar solvents, leading to exceptionally mild reaction conditions. 

An emerging system similar to the preceding employs what has come to be known as triphase catalysis, in which a phase-transfer catalyst is immobilized on a solid support for use in a liquid-liquid reacting system. In view of the potential importance of such a system, it is considered at greater length in Chapter 19 on phase-transfer catalysis. 

Leznoff has published further on the solid-phase synthesis of insect sex attrac-tants. The advantages and uses of enzymes attached to solid supports have been reviewed. Aspects of triphase catalysis (organic layer-water-polymer) have been discussed by Regen, while advances in phase-transfer catalysis have been reviewed. A crown ether NAD(P)H mimic has been described,bringing synthetic chemists nearer to the objective of artificial enzyme systems. 

Crown ethers attached to insoluble polymeric substrates (see the following discussion for examples) have been used as phase transfer catalysts for liquid/liquid systems. In using such systems, the catalyst forms a third insoluble phase the procedure being referred to as triphase catalysis (Regen, 1979). This arrangement has the advantage that, on completion of the reaction, the catalyst may be readily separated from the reaction solution and recycled (Montanari, Landini Rolla, 1982). As... 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

Nevertheless, the separation of the catalyst at the end of the reaction and, if possible, its recycling is often a limiting factor for the application of PTC. The chemical nature of the catalyst makes it at least partially soluble both in polar and apolar solvents and higher catalyst loadings are often used to maximize the effects on the reaction rates. This led very soon to the development of polymer-supported phase transfer catalysts [222], When using insoluble supports, an additional phase is added to the former biphasic system and, accordingly, the term triphase catalysis was coined (Figure 10.8) [223-225],... 

Mechanism of Triphase Catalysis.. Although the activity of a supported PT catalyst is usually less than that of the corresponding soluble catalyst, it is believed (Molinari et al., 1979 Montanari et al., 1983, Anelli et al., 1984) that the mechanism of the phase-transfer cycle remains the same. However, there are certain characteristics typical of heterogeneous catalysts that make supported PTC different from soluble PTC. For example, in a triphase catalytic system, one does not consider the planar phase boundary as in a classical two-phase system. Instead, a volume element which incorporates the catalytic active sites as well as the two liquid phases has to be considered. Diffusion of both the aqueous and organic phases within the solid support is important. Various mechanisms have been proposed for triphase catalysis, some of which are touched upon here. However, it should be noted that no single mechanism has been verified completely, and it is quite possible that the true mechanism involves a combination of the various mechanisms proposed so far. 

The technique of "triphase catalysis", where liquid-liquid reactions are catalyzed by "phase transfer catalysts" chemically fixed on inert polymer supports, is an extremely interesting example of three-phase systems. They may be potentially of great interest technically since the catalysts can be recovered or used continously (56). 

In liquid-liquid-solid (three phase) systems such as triphase catalysis, mass transfer in the aqueous and organic phases occurs serially in a single PTC cycle, that is, ion exchange in the aqueous phase followed by the main reaction in the organic phase. The design of a contactor for getting these values simultaneously has to be quite different from that for two-phase systems. 

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