Reaction triphasic

Seddon s group described the option of carrying out Heck reactions in ionic liquids that do not completely mix with water. These authors studied different Heck reactions in the triphasic [BMIM][PFg]/water/hexane system [91]. While the [BMIM]2[PdCl4] catalyst used remains in the ionic liquid, the products dissolve in the organic layer, with the salt formed as a by-product of the reaction ([H-base]X) being extracted into the aqueous phase. 

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]. 

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... 

A particularly ingenious approach is that of triphase catalysis which was developed by Regen (1979). The catalyst is a quaternary ammonium residue which is covalently bound to an insoluble polystyrene resin, and reactions of anionic reagents are carried out in a two-phase water-organic solvent mixture. 

Bentley et al.m recently improved upon Julia s epoxidation reaction. By using urea-hydrogen peroxide complex as the oxidant, l,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the base and the Itsuno s immobilized poly-D-leucine (Figure 4.2) as the catalyst, the epoxidation of a, (3-unsaturated ketones was carried out in tetrahydrofuran solution. This process greatly reduces the time required when compared to the original reaction using the triphasic conditions. 

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... 

Half-life times of the reaction of n-C,H17Br (in toluene) with aqueous potassium salts under triphase conditions ... 

The detailed mechanism of triphase catalysed reactions still remains to be established. This holds in particular for reactions performed under liquid-... 

Although phase transfer agents have been attached to clays, silica and alumina, the vast majority of studies have used organic polymers, especially polystyrene, as the support. The earliest of these triphase catalysts was prepared from 12% chloromethylated polystyrene crosslinked with 2% divinylbenzene by reaction with a tertiary amine. A wide range of triphase catalysts has since been reported, some examples of which are shown in Figure 5.16. 

Figure 5.17 Schematic diagram of the effect of mixing on the concentration of substrate in the liquid and solid phases of a triphasic reaction a represents a reaction that is limited only by the intrinsic reactivity b represents a reaction that is limited by a combination of intrinsic reactivity and mass transport effects c represents a reaction which is limited by mass transport only...

For a triphasic reaction to work, reactants from a solid phase and two immiscible liquid phases must come together. The rates of reactions conducted under triphasic conditions are therefore very sensitive to mass transport effects. Fast mixing reduces the thickness of the thin, slow moving liquid layer at the surface of the solid (known as the quiet film or Nemst layer), so there is little difference in the concentration between the bulk liquid and the catalyst surface. When the intrinsic reaction rate is so high (or diffusion so slow) that the reaction is mass transport limited, the reaction will occur only at the catalyst surface, and the rate of diffusion into the polymeric matrix becomes irrelevant. Figure 5.17 shows schematic representations of the effect of mixing on the substrate concentration. 

The combined catalysis by 18-crown-6 and tetra-n-butylammonium bromide produces higher yields in shorter reaction times than either of the catalysts separately (Table 3.7) [21] and almost quantitative yields have been reported for solid solid liquid triphase catalysed esterification using silica impregnated with tetramethylammonium chloride [22]. 

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],... 

The catalytic systems described here are liquid triphasic ones, with a heterogeneous catalyst such as supported noble metals (Pt, Pd), or high-surface-area metals (Raney-Ni). The liquid phases are constituted by an alkane, water, and an ammonium salt. This kind of system was developed over the past 12 years, initially as an efficient and mild catalytic methodology for the hydrodehalogena-tion reaction of haloaromatics, after which it was sffidied for other kinds of reactions, and careful observation has resulted in a theory of modes of action whereby reaction rates, and selectivity, could be intensified. 

Catalysts have been bonded to insoluble polymers to allow, in principle, an appreciable simplification of PTC the catalyst represents a third insoluble phase which can be easily recovered at the end of the reaction by filtration, thus avoiding tedious processes of distillation, chromatographic separation and so on. This is of potential interest mainly from the industrial point of view, due to the possibility of carrying on both discontinuous processes with a dispersed catalyst and continuous processes with the catalyst on a fixed bed. This technique was named "triphase catalysis" by Regen (13,33,34). 

Recent efforts have concentrated on the immobilization of these materials onto both organic polymers(11) and metal oxides(12) to simplify, by filtration, the separation, recovery and recycle process. These supported catalysts now function in a triphasic environment in either a liquid-solid-liquid or solid-liquid-solid reaction mixture. In this case, the catalyst must transfer the anion from the surface of the crystal lattice to the liquid phase. Here adsorption phenomena often significantly affect the reaction rate(13). 

Kumar, R., Mukherjee, P., and Bhaumik, A. (1999) Enhancement in the reaction rates in the hydroxylation of aromatics over TS-I/H2O2 under solvent free triphase conditions. Catal Today, 49,185-191. 

It is immediately apparent that the main attraction of the biphasic method is the much shorter reaction times it affords. This is illustrated well by the chalcone system. Using triphasic conditions, the epoxidation of chalcone can take up to 8 hours, while the biphasic reaction (Table 2, Entry 1) is complete in 30 minutes... 

The biphasic method, like the triphasic method, allows the epoxidation of a broad range of substrates accommodating both aliphatic and aromatic substituents at either end of the enone moiety. However, unlike the triphasic method, the biphasic conditions allow reaction of hydroxide-sensitive systems. 

Just as for the triphasic system, the dienes and triene (Entries 7-13) undergo selective epoxidation at only one of the double bonds. The selective reactivity of the compound featured in Entry 11 is particularly noteworthy. Monoepoxida-tion of the double bond next to the phenyl-bearing carbonyl group was the only reaction observed, despite extended reaction times and use of excess oxidant. 

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