Phase transfer catalysis triphase catalysts

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. 

Phase Transfer Catalysis. Initial evaluations of the ability of polymer 2 to function as an insoluble or triphase catalyst involved examination of relative rates compared to 18-crown-6. 

R. A. Sawicki, Triphase Catalysis in Organometallic Anion Chemistry, in Phase Transfer Catalysis New Chemistry Catalysts, and Applications (Ed. Ch. M. Starks), ACS Symposium Ser. No. 326, American Chemical Society, Washington, DC, USA, 1987, Chapter 12, p. 143. 

While phase-transfer catalysis (PTC) is a well established method with diverse applications in organic synthesis, conventional catalysts suffer several drawbacks including hygroscopicity, low thermal stability and difficulty in separation and recovery. Ironically, the high solubilities of conventional catalysts are a drawback to recovery and a problem to product purification. The concept of triphase catalysis, whereby the catalyst is immobilised onto a support material and the resulting supported PTC is then used in a biphasic aqueous-organic solvent reaction mixture is recognised as a viable solution to many of these problems.144-146... 

Baeyer-Villiger oxidation of alkyl- and aryl-substituted C -C, cycloalkanones, steroid ketones and branched chain aliphatic ketones is catalysed by arsonated polystyrene resins [53], Larger size cycloalkanones and linear ketones react much slower. Water miscible and immiscible solvents can be used. With the latter, the resin behaves as an effective catalyst and a phase-transfer agent (triphase catalysis). The same compounds are also epoxidation catalysts. More recently, a method for the preparation of phenols by the oxidation of aromatic aldehydes and ketones has been reported. The most convenient catalysts are nitro-substituted arylseleninic acids and corresponding diselenides [54]. 

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. 

Triphase catalytic methods are complementary to phase transfer catalysis. Polystyrene based triphase catalysts exhibit a degree of selectivity which is attributed to the relative solubility (absorption) of the reactants within the catalyti-cally active regions of the resin. ... 

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... 

The simplest C-C bond formation reaction is the nucleophilic displacement of a halide ion from a haloalkane by the cyanide ion. This was one of the first reactions for which the kinetics under phase-transfer catalysed conditions was investigated and patented [l-3] and is widely used [e.g. 4-12], The reaction has been the subject of a large number of patents and it is frequently used as a standard reaction for the assessment of the effectiveness of the catalyst. Although the majority of reactions are conducted under liquiddiquid two-phase conditions, it has also been conducted under solidrliquid two-phase conditions [13] but, as with many other reactions carried out under such conditions, a trace of water is necessary for optimum success. Triphase catalysis [14] and use of the preformed quaternary ammonium cyanide [e.g. 15] have also been applied to the conversion of haloalkanes into the corresponding nitriles. Polymer-bound chloroalkanes react with sodium cyanide and cyanoalkanes under phase-transfer catalytic conditions [16],... 

Polystyrene has been used most often as the support for phase transfer catalysts mainly because of the availability of Merrifield resins and quaternary ammonium ion exchange resins. Although other polymers have attrative features, most future applications of polymer-supported phase transfer catalysts will use polystyrene for several reasons It is readily available, inexpensive, easy to functionalize, chemically inert in all but strongly acidic media, and physically stable enough for most uses. Silica gel and alumina offer most of these same advantages. We expect that large scale applications of triphase catalysis will use polystyrene, silica gel, or alumina. 

Phase-transfer catalysts are used to facilitate reactions between reagents that are in two different phases (e.g., 1-bromooctane in toluene with aqueous potassium iodide to form 1-iodooctane). They are usually quaternary ammonium or phosphonium salts or crown ethers. They can complicate the workup of the reaction and may be difficult to recover for reuse. When they are insoluble polymeric ones, workup and recycle can be done by simple filtration.192 The process is called triphase catalysis. In favorable cases, their activity can be comparable with that of their lower molecular weight analogues. They are often based on cross-linked polystyrene, for which spacers between the aromatic ring and the quaternary onium salt can increase activity two- to fourfold. Copolymerization of 4-vinylben-zyl chloride with styrene or N, N- d i m e Ihy I a c ry I a m i d e, followed by treatment with tri-/ -butylphosphine produced catalysts that were used in the reaction of benzyl chloride with solid potassium acetate (5.43).193... 

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. 

A full study of some 5n reactions of alkyl halides to give other halides, nitriles, ethers, etc. under triphase catalysis conditions i.e. organic and aqueous liquid phases with a heterogeneous phase-transfer catalyst) has appeared. 

Ohtani et al. used polystyrene-supported ammonium fluoride as a phase transfer catalyst (triphase catalysis) for several base-catalyzed reactions, such as cyanoethylation, Knoevenage reaction, Claisen condensation and Michael addition. The catalytic activity of the polystyrene-supported ammonium fluid was comparable to that of tetrabutylammonium fluoride (TBAF). The ionic loading and the ammonium structure of the fluoride polymers hardly affected the catalytic efficiency. The reaction was fast in a non-polar solvent (e.g., octane or toluene) from which the rate-determining step of the base-catalyzed reaction is very similar to that of the nucleophilic substitution reactions. 

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). 

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