Types of phase transfer reactions

In the presence of 18-crown-6 the degree of conversion increases with increased solvent polarity, best results being obtained in DMF (Table 1) as expected, the influence of temperature is also quite noticeable. Table 1 shows that the nature of the catalyst and the type of phase transfer reaction, solid-liquid or liquid-liquid, are very important factors. Short-chain tetraalkyl ammonium salts (methyl, ethyl or propyl) have no catalytic activity, while tetrabutyl ammonium or phosphonium salts have good activities several other phase transfer catalysts were also included in this study but will not be reviewed here. Reactions with aqueous solutions of potassium acetate (Table 2) confirm that best results are obtained when a concentrated solution of the salt is used. The scale of catalytic activity for these liquid-liquid reactions is the following ... 

Many catalysts can be used tetrabutylammonium halides, tetrabutylammonium hydroxide, tetrabutylammonium hydrogen sulfate, tetrabutylphosphonium bromide, 18-crown-6 ether, and cryptand[2.2.2]. There have been few studies on the influence of the catalyst on the reactions. However, Nishibuko et al carried out an excellent study on the influence of experimental conditions on phase transfer catalyzed polymer modification they showed that the nature of the catalyst and the type of phase transfer reaction (solid-liquid, liquid-liquid), as well as the polarity of the solvent are very important parameters. The purity of the system must be carefully controlled thus, the presence of traces of water may have a great influence on the conversion and the occurrence of side reactions. 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

To date, this type of phase-transfer-catalyzed Michael reaction of 28 has been investigated with either acrylates or alkyl vinyl ketones as an acceptor, under the influence of different catalysts and bases. Typical results are listed in Table 4.6 in order to determine the characteristics of each system. 

The type of phase-transfer catalyst plays a key role in the phase-transfer catalytic synthesis of l-bromo-1-chlorocyclopropanes, which are formed in good yields and with high selectivity if the reaction of dibromochloromethane with an alkene is performed using a crown ether (dibenzo-18-crown-6, " 3,5-di-fer/-butylbenzo-15-crown-5, " " 3,3, 5,5 -tetra-tert-butyldiben-zo-lS-crown-b ) or tetramethylammonium chloride.For the specific effect of the tetra-methylammonium chloride on the dichlorocyclopropanation of unconjugated dienes, see Section I.2.I.4.2.I.2., and some electrophilic alkenes, see Section I.2.I.4.2.I.8.2. The reason why these catalysts exhibit peculiar properties is not clear,other crown ethers behave like typical phase-transfer catalysts (Table 25). " ... 

Okahata, Y., and K. Ariga, A New Type of Phase-Transfer Catalysts (PTC) Reaction of Substrates in the Inner Organic Phase with the Outer Aqueous Anions Catalyzed by PTC Grafted on the Capsule Membrane, J. Org. Chem., 51, 5064 (1986). 

Phase-transfer catalysis (PTC) is one of the most important methods for the enhancement of reactivity of inorganic reagents having poor solubility in organic solvents. As such, PTC is often used for reactions involving potassium fluoride, which has very low solubility in aprotic solvents. Both types of phase-transfer catalysts, quaternary ammonium salts and 18-crown-6, have been used to accelerate... 

Alternate types of phase transfer catalysts for two-phase reactions involving salts are crown ethers, cryptates and dialkylpolyethylene oxides, which form reversible complexes with many cations. For example, crown ether 18-crown-6, also strongly catalyzes reaction (1). In this case, the crown ether transfers the entire KCN molecule into the organic phase by complexation. 

This type of phase-change reaction is distinct from the dissolution-precipitation reaction which occurs in the Pb, PbOg, Ag, and Cd electrodes. In a dissolution-precipitation reaction, one soUd phase ie.g. Pb) dissolves electrochemically e.g. to form Pb " ), combines with an ion in solution, and the product precipitates e.g. PbS04). Methods for modeling mass transfer and nucleation kinetics in dissolution-precipitation reactions have been described [18,50,51,52,53,8]. 

Class (2) reactions are performed in the presence of dilute to concentrated aqueous sodium hydroxide, powdered potassium hydroxide, or, at elevated temperatures, soHd potassium carbonate, depending on the acidity of the substrate. Alkylations are possible in the presence of concentrated NaOH and a PT catalyst for substrates with conventional pX values up to - 23. This includes many C—H acidic compounds such as fiuorene, phenylacetylene, simple ketones, phenylacetonittile. Furthermore, alkylations of N—H, O—H, S—H, and P—H bonds, and ambident anions are weU known. Other basic phase-transfer reactions are hydrolyses, saponifications, isomerizations, H/D exchange, Michael-type additions, aldol, Darzens, and similar... 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic... 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

In phase transfer catalysis of the solid/liquid type, the organic phase (containing dissolved organic reactant and a small amount of the crown) is mixed directly with the solid inorganic salt. Such a procedure enables the reaction to proceed under anhydrous conditions this is a distinct advantage, for example, when hydrolysis is a possible competing reaction. Because of their open structure, crown ethers are readily able to abstract cations from a crystalline solid and are often the catalysts of choice for many solid/liquid phase transfer reactions. 

Phase transfer catalysis (1,2) has become in recent years a widely used, well-established synthetic technique applied with advantage to a multitude of organic transformations. In addition to a steadily increasing number of reports in the primary literature, there are several reviews (3-6), comprehensive monographs (7-10) and an ACS Audio Course (1 ) which describe the phase transfer process and which provide extensive compilations of phase transfer agents and reaction types. While the list of applications and in many cases the synthetic results are impressive, phase transfer catalysts (PTCs) suffer some of the same disadvantages as more conventional hetero-and homogeneous catalysts — separation and... 

Isoquinoline Reissert compounds of type 12 could be easily converted to the corresponding 1-cyanoisoquinolines (13) by simple base treatment (4,5) (Scheme 3). This transformation also takes place with high yields when type 12 compounds are oxidized with molecular oxygen in a two-phase system in the presence of phase-transfer catalysts (12-14). It should be mentioned that similar oxidation of dihydro Reissert compounds of type 14 afforded the corresponding dihydroisocarbostyril derivatives (15) (12-14). Base treatment of isoquinoline Reissert eompounds followed by intramolecular rearrangement, due to the absence of a proper intermolecular reaction partner, results in 1-acylisoquinoline derivatives (18) (3). 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

Reactions accomplished by enantioselective phase-transfer catalysis are summarized in Table 10.1 according to type of catalyst and synthetic transformation [9-81], Highest reported enantioselectivities (% ee) or optical purities (% op) are listed to give perspective to the overall field [82]. General aspects of phase-transfer systems, including catalysts are then discussed, followed by particular reaction classes. 

A number of other types of compounds have been used as chiral catalysts in phase-transfer reactions. Many of these compounds embody the key structural component, a P-hydroxyam-monium salt-type structure, which has been shown to be crucial to the success of the above described cinchona-derived quats. Although they have not been as successful as the cinchona catalysts, the ephedra-alkaloid derived catalysts (see 20, 22, 23 and 25 in Charts 3 and 4) have been used effectively in several reactions. In general, quats with chirality derived only from a single chiral center, which cannot participate in a multipoint interaction with other reaction species, have not been effective catalysts [80]. 

The aim of this book is to provide a concise and comprehensive treatment of this continuously growing field of catalysis, focusing not only on the design of the various types of chiral phase-transfer catalyst but also on the synthetic aspects of this chemistry. In addition, the aim is to promote the synthetic applications of these asymmetric phase-transfer reactions by giving solid synthetic evidence. Clearly, despite recent spectacular advances in this area, there is still plenty of room for further continuous development in asymmetric phase-transfer catalysis. 

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