Phase transfer catalytic reactions

Catalysts immobilized on resins are quite often used for catalytic reactions. Phase-transfer conditions, guaranteeing separation and recovery of the catalysts by use of water/organic solvent biphasic systems, are another possibility. Unlike homogeneous processes, these kind of catalytic processes take account of product loss and substrate selectivity. Furthermore,... 

The products of the catalytic reaction are transferred by convection and diffusion to the bulk liquid phase. 

Having optimized the catalytic enantioselective phase-transfer alkylation system, the group explored the scope and limitations. A variety of electrophiles were reacted with the benzophenone imine glycine tert-butyl ester 1 catalyzed by 5 mol% of the selected chiral dimeric PTCs, benzene-linked-l,3-dimeric PTC 37, 2 -F-benzene-linked-1,3-dimeric PTC 41, and naphthalene-linked-2,7-dimeric PTC 39, at reaction temperatures of 0°C or — 20 °C (Scheme 4.8). 

Catalytic Asymmetric Phase-Transfer Mannich-Type Reaction... 

Scheme 6.5 Catalytic asymmetric phase-transfer Mannich-type reaction.

In catalytic reactions mass transfer from the fluid phase to the active phase inside the porous catalyst particle takes place via transport through a fictitious stagnant fluid film surrounding the particle and via diffusion inside the particle. Heat transport to or from the catalyst takes the same route. These phenomena are summarized in Fig. 8.15. 

For example Shibuguchi, T., Fukuta, Y, Akachi, Y. et al. (2002) Development of new asymmetric two-center catalysts in phase-transfer reactions. Tetrahedron Letters, 45,9539-9543. Ohshima, T., Shibuguchi, T., Fukuta, Y. and Shibasaki, M. (2004) Catalytic asymmetric phase-transfer reactions using tartarate-derived asymmetric two-center organocatalysts. Tetrahedron, 60, 7743-7754. 

ShibasaM M, Ohshima T, Shibuguchi T, Fukuta Y (2004) Catalytic Asymmetric Phase-Transfer Reactions Using Tartrate-Derived Asymmetric Two-Center Organocatalysls. Tetra-hedrrai 60 7743... 

Shibuguchi T, Mihara H, Kuramochi A, Ohshima T, Shibasaki M (2007) Catalytic Asymmetric Phase-Transfer Michael Reaction and Mannich-Type Reaction of Glycine SchiffBases with Tartrate-Derived Diammonium Salts. Chem Asian J 2 794... 

There are, of course, many catalysts that are only available either as a dissolved comj und or as a solid. Solid surfaces are often essential to make certain reactions possible, particularly stereospecific reactions. However, in recent times homogeneous catalysts have been developed that promote stereospecific reactions. Phase transfer catalysis is a form of heterogeneous catalysis where the catalytic phase is a dispersed liquid phase, containing a dissolved catalyst. The reactants penetrate the dispersed phase, react there, and the reaction product dissolves again in the continuous phase, where it is protected from consecutive reactions. 

Fukuta Y, Ohshima T, Gnanadesikan V, Shibuguchi T, Nem-oto T, Kisugi T, Okino T, Shibasaki M. Enantioselective syntheses and biological studies of aeruginosin 298-A and its analogs application of catalytic asymmetric phase-transfer reaction. PAAS 2004 101 5433-5438. 

A and its analogues using a catalytic asymmetric phase-transfer reaction and epoxidation. J. Am. Chem. Soc. 2003 125(37) 11206-11207. 

Shibuguchi T, Mihara H, Kuromachi A, Ohshima T, Shibasaki M. Catalytic asymmetric phase-transfer Michael reaction and Mannich-type reaction of glycine Schiff gases with tartrate-derived diammonium Salts. Chem. Asian J. 2007 2(6) 794-801. 

Ohshima T, Shihuguchi T, Fukuta Y, Shihasaki M. Catalytic asymmetric phase-transfer reactions using tartrate-derived asymmetric two-center organocatalysts. Tetrahedron 2004 60 7743-7754. 

Ohshima, T., Gnanadesikan, V., Shibuguchi, T, Fukuta, Y., Nemoto, T., and Shibasaki, M. (2003) Enantioselective syntheses of aeruginosin 298-A and its analogues using a catalytic asymmetric phase-transfer reaction and epoxidation./. Am. Chem. Soc., 125,11206-11207. 

Halex rates can also be increased by phase-transfer catalysts (PTC) with widely varying stmctures quaternary ammonium salts (51—53) 18-crown-6-ether (54) pytidinium salts (55) quaternary phosphonium salts (56) and poly(ethylene glycol)s (57). Catalytic quantities of cesium duoride also enhance Halex reactions (58). 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

When two reactants in a catalytic process have such different solubiUty properties that they can hardly both be present in a single Hquid phase, the reaction is confined to a Hquid—Hquid interface and is usually slow. However, the rate can be increased by orders of magnitude by appHcation of a phase-transfer catalyst (40,41), and these are used on a large scale in industrial processing (see Catalysts, phase-TRANSFEr). Phase-transfer catalysts function by faciHtating mass transport of reactants between the Hquid phases. Often most of the reaction takes place close to the interface. 

There are examples of preferential arylation of Af-substituted pyrroles, thiophenes and furans in the 2-position. A preparatively useful reaction of this type is the o-nitrophenylation of thiophene (Scheme 40). A phase transfer catalytic technique has been recommended for this reaction (77TL1871). 

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

The most widely accepted mechanism of reaction is shown in the catalytic cycle (Scheme 1.4.3). The overall reaction can be broken down into three elementary steps the oxidation step (Step A), the first C-O bond forming step (Step B), and the second C-O bond forming step (Step C). Step A is the rate-determining step kinetic studies show that the reaction is first order in both catalyst and oxidant, and zero order in olefin. The rate of reaction is directly affected by choice of oxidant, catalyst loadings, and the presence of additives such as A -oxides. Under certain conditions, A -oxides have been shown to increase the rate of reaction by acting as phase transfer catalysts. ... 

Catalytic asymmetric synthesis with participation and formation of heterocycles (including asymmetric phase transfer reactions and asymmetric reactions with chiral Lewis catalysts) 93MI1. 

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

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