Triphase catalytic system

Further evidence of the acceleration enjoyed by the Heck reaction in an ionic phase (2V-methyl-7V,7V,7V,-trioctylanimonium chloride or Aliquat-336) has been reported by Tundo and co-workers in a triphasic catalytic system The arylation of electron-poor olefins was catalysed by palladium supported on charcoal (Pd/C) in the heterogeneous z-octane/Aliquat-336/water system (Scheme 1.58). The use of phosphines was not necessary. Aliquat-336 trapped the solid-supported catalyst, ensuring an efficient mass transfer between the bulk phases, which resulted in an increase in reaction rate of an order of magnitude, compared to the reaction in the absence of the ionic liquid. 

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. 

In a triphase catalytic system, reagents from two immiscible liquid phases are transferred to the interface of a solid where they react. The products formed at the interface are then returned to the liquid phases in which they are soluble. Typically, for a nucleophilic displacement reaction carried out under triphase conditions an... 

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. 

The combination of base and alumina, in essence, represents a triphasic solid-liquid-solid catalytic system particularly when highly insoluble bases are used. While the results suggest that... 

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... 

Julia, S., Guixer, J., Masana, J., Rocas, J., Colonna, S., Annuziata, R. and Molinari, H. Synthetic enzymes. 2. Catalytic asymmetric epoxidation by means of polyamino-acids in a triphase system, J. Chem. Soc., Perkin Trans. 1, 1982, 1317-1324. 

Neumann and Miller (360) reported catalytic epoxidations with analogous P-W materials in a triphasic mode. The activity in the solvent-free system is influenced by the length of the hydrocarbon spacer between the silica and the ammonium group. Cyclooctene, for example, is epoxidized with only 10% conversion when a trimethyl propyl ammonium salt is used, whereas a conversion of 45% can be obtained in the presence of an immobilized octyldimethyl benzyl ammonium salt. The enhanced conversion is probably the result of a nearly ideal hydrophilic-lipophilic balance at the active site. 

Polystyrene-bound phenylseleninic acid has been used in catalytic amounts in a triphasic system consisting of the polymer, aqueous hydrogen peroxide, and dichloromethane, to catalyze the oxidation of al-... 

Quaternary ammonium salts of alkaloids have been used for the synthesis of optically active oxiranes from electron-poor olefins under phase-transfer conditions. The enantiomer yield is inversely proportional to the dielectric constant of the solvent,Asymmetric epoxidation in the presence of catalytic amounts of poly-(S)-amino-acids in a triphase system has been described with optical yields up to 96% ... 

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