18-Crown-6 ether phase transfer catalyst

Alkaline hydrolysis rates of a series of thiophenyl 4-X-benzoates (47 X = H, Me, N02) was significantly enhanced in the presence of cyclodextrins (CDs), and this was attributed to strong binding of the benzoyl moiety within the CD cavity and covalent catalysis by secondary hydroxy groups of the CDs (48).63 The effect of MeCN and MeOH on the alkaline hydrolysis of acetylsalicylic acid in aqueous micellar solutions was reported.64 Butylaminolysis of p-nitrophenyl acetate in chlorobenzene in the presence of different kinds of phase-transfer catalysts (crown ethers and gly-mes) supported the existence of a novel reaction pathway exhibiting a first-order dependence on the concentration of the phase-transfer catalyst and a second-order... 

SoHd-liguid phase-transfer catalysis. Crown ethers have commonly been used as catalysts for reactions between a solid-liquid interface, and quaternary ammonium and phosphonium salts have been used only as catalysts for reactions in two-phase liquid liquid reactions. However, several laboratories have reported that the latter catalysts are also satisfactory for two-phase solid liquid reactions. Thus dichlorocarbene can be generated from chloroform and solid sodium hydroxide under catalysis from benzyltriethylammonium chloride in yields comparable to those of the classical Makosza method. Another example of this type of catalysis is the oxidation of terminal and internal alkynes by solid potassium permanganate in CH2CI2 with Adogen 464 as catalyst. Aliquat 336 has been found to be as satisfactory as a crown ether for certain displacement reactions with NaOAc, KSCN, KNOa, and KF in CH3CN or CHaCla. ... 

Both phase transfer and crown ether catalysts have been used to promote a eliminations from chloroform and other haloalkanes. " Various trialkylammonium hydroxides catalyze formation of dichlorocarbene in the organic phase of two phase systems consisting of CHCI3 and 50% sodium hydroxide solution. 

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). 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

Both of these structures are open-chained compounds corresponding to crown ethers in function if not exactly in structure (see Chap. 7). They have repeating ethyleneoxy side-chains generally terminated in a methyl group. Montanari and co-workers introduced the polypodes 22 as phase transfer catalysts . These compounds were based on the triazine nucleus as illustrated below. The first octopus molecule (23) was prepared by Vogtle and Weber and is shown below. The implication of the name is that the compound is multiarmed and not specifically that it has eight such side-chains. Related molecules have recently been prepared by Hyatt and the name octopus adopted. For further information on this group of compounds and for examples of structures, refer to the discussion and tables in Chap. 7. 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Difluoromethoxy-2-chloro-l,l,l-trifluoroethane and potassium fluoride produce 2-difluoromethoxy-1,1,1,2-tetrafluoroethane [50] The yield of the latter reaction is improved by adding a phase transfer catalyst or crown ether, tetra-methylammonium chlonde, tetrabutylammonium chloride, or 18-crown-6 with a solvent like sulfolane can be used for this purpose [5/] (equation 32)... 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

Crown ether (Section 18.7) A large-ring polvether used as a phase-transfer catalyst. 

Aprotic solvents may also dissolve a phase-transfer catalyst to realize higher rate.sAselectivities. Thus, in a claim, dichloronitrobenzene was reacted with KF in an aprotic solvent like 2-chlorotoluene, in the presence of hexadecyltributyl phoshonium bromide /crown ether/ PEG-dimethyl ether, to give chlorofluoronitrobenzene. 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

Chiral monoaza-crown ethers containing glucose units have been applied as phase-transfer catalysts in the Michael addition of 2-nitropropane to a chalcone to give the corresponding adduct in up to 90% ee. (Eq. 4.138).202... 

Crown ethers are also phase-transfer catalysts and are able to transport ionic compounds into an organic phase. 

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... 

Triorganotin fluorides can be prepared by the use of new fluorinating systems. 18-Crown-6 or dibenzo-24-crown-8 can act as solid-liquid phase transfer catalysts for CsF. Trialkyltin mercaptides can be fluorodestannylated by CsF in the presence of crown ethers or alkyl bromides358 ... 

The closely related Todd reaction, useful for the preparation of dialkyl phosphorochloridates and phosphoramidates, also involves the use of amines with dialkyl phosphites.146152 180-182 Although the reaction proceeds using preformed salts of the dialkyl phosphites,183 the use of tertiary amines facilitates the reaction by allowing all reagent materials to be in solution. Biphasic reaction systems utilizing phase-transfer catalysts and crown ethers have also been successful for this reaction.158-161... 

The Darzens reaction can also proceed in the presence of a chiral catalyst. When chloroacetophenone and benzaldehyde are subjected to asymmetric Darzens reaction, product 89 with 64% ee is obtained if chiral crown ether 88 is used as a phase transfer catalyst (Scheme 8-30).69... 

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. 

Chiral crown ethers such as 13 are suitable alternatives to the ammonium salts and not decomposed under alkaline conditions. They usually have higher catalyst turnover than the chiral ammonium salts, and the design of catalysts will be much easier. However, they are, in general, costly and difficult to prepare on large scale. Polyols (eg., (RR)-TADDOL14) also serve as phase transfer catalysts. 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

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... 

For synthetic purposes, crown ethers have been used frequently as phase-transfer catalysts under conditions where the sodium or potassium hydroxide is present as a concentrated aqueous solution. Various anions of C—H acids have been generated this way (Makosza and Ludwikow, 1974). The use of such conditions for the generation of carbenes will be dicussed in a separate section. 

Crown ethers have been used successfully as phase-transfer catalysts for liquid-liquid and liquid-solid oxidation reactions. Sam and Simmons (1972) observed that potassium permanganate can be solubilized in benzene by dicyclohexyl-18-crown-6 to yield concentrations as high as 0.06 M. From... 

The cocatalytic effects of pinacol in the phase transfer catalysis (PTC) of dihalocarbene additions to alkenes were noted by Dehmlow and co-workers who showed that pinacol accelerates the PTC deprotonation of substrates up to pKa 27.7 Dehmlow also studied the effects of various crown ethers as phase transfer catalysts in the addition of dibromocarbene to allylic bromides.8 In Dehmlow s study, elevated temperature (40°C) and dibenzo-18-crown-6 did not give the highest ratio of addition/substitution to allyl bromide. However, the submitters use of pinacol,... 

Figure 5.16 Examples of phase transfer catalysts attached to polystyrene, containing (a) a tetraalkylammonium group, (b) a crown ether group and (c) a cryptand group...

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

Other classic examples illustrating the use of quaternary salts as phase transfer catalysts were published by Makosza(2), and by Brandstrom(3). Subsequent development of crown ethers(4-7) and crvptands(7-8) as phase transfer catalysts gave PTC an entirely new dimension since now the inorganic reagent, as sodium cyanide in the above equation, need no longer be dissolved in water but can be used... 

For use of chiral phase transfer catalysts see the following references and references contained therein J. W. Verbicky Jr., and E. A. O Neil, J. Org. Chem.. 50, 1786 (1985) E. Chiellini, R. Solaro, and S. D Antone, Polvm. Sci. Technol. (Plenum), 24 (Crown Ethers Phase Transfer Catal. Polym. Sci.) 227 (1984). 

Cation-Binding Properties of Crown Ethers, Lariat Ethers, Bibracchial Lariat Ethers, and Poly(ethylene glycols) as Potential Phase-Transfer Catalysts... 

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