Crown ethers as phase-transfer catalysts

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). 

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,... 

Landini D, Montanar F, Pirisi FM (1974) Crown ethers as phase-transfer catalysts in 2-phase reactions. J Chem Soc Chem Commun 879-880... 

Scheme 11.15 Chiral crown ethers as phase-transfer catalysts.

In many PTC systems, a third phase can form where reaction occurs and rates are enhanced. This third phase can form in many multiphase systems, and has been termed the omega phase [18]. The first example of an omega phase was discovered in the ITC reaction of cyanide ion on a benzyl halide in the presence of a crown ether as phase transfer catalyst. This liquid-solid PTC system was found to have rates dependent on the amount of water present. It was determined that in the absence of either water or the... 

Furthermore, the hydrolysis of butyl acetate and methyl pivalate in benzene in the presence of KOH at 25 °C as well as the reaction of potassium phenolate with benzyl chloride in boiling acetonitrile are accelerated by addition of polyoxyethylene [183]. The catalytic effect of POE is augmented by an increase in the number of oxyethylene units, i.e. 1 <6< 12. PEO is also an interfacial catalyst of the reaction of phenol and 2,4,6-trimethylphenol with methyl iodide in water-chloroform and dichloromethane. The kinetic study of the reaction between benzyl chloride and potassium acetate in the presence of PEO of variable molecular weight in toluene and butanol has been performed with IR spectroscopy [184]. The dissolution of a reagent of poor solubility is apparently a rate-limiting step of the reaction in a solution of low polarity (toluene). The presence of PEO impurities in toluene has been detected. Moreover, effect of PEO and crown ethers as phase transfer catalysts has been compared. In a low-polarity solvent, oligoethylene oxides are more effective catalysts, while in a polar solvent (butanol) the effectiveness of PEO and crown ethers as phase transfer catalysts is similar. 

The in situ procedure as proposed by Sonnet et al. (18) is much more attractive for synthetic applications. With the use of only a moderate excess of monopersulfate (C=C KHSO5 = l 2-2.4), they achieved an 80% yield for the epoxidation of oleic acid methyl ester and 81-96% for the epoxidation of various plant oils. It is a twophase reaction with a crown-ether as phase-transfer catalyst yet a considerable amount of inorganic waste (six times the weight of the product) is produced. In a recent work (21), the phase-transfer catalyst was replaced by acetonitrile as a polar solvent. In summary, epoxidation by dioxiranes is a promising new method for oleo-chemistry, especially because it also works in combination with metal catalysts to influence diastereoselectivity (22) an enantioselective epoxidation with sugardioxiranes has also been reported (23). 

Aliphatic hydrocarbons are immiscible wifli PEGs. Therefore, it is important to select a good organic-phase solvent such as aromatic hydrocarbons, chlorinated hydrocarbons, or acetonitrile. In toluene, PEGs are more effeetive eatalysts than crown ethers for the reaetion of benzyl chloride and soUd potassium acetate. In butanol, the effeetiveness of PEGs and crown ethers as phase-transfer catalysts were the same for die reaction of benzyl chloride and solid potassium acetate. ... 

One final note regarding the use of crown ethers as phase transfer catalysts there is little literature which directly compares quaternary ammonium catalysts with crown ethers in liquid-liquid processes (see Sect. 1.10) [48]. There are examples where both have been tried and are effective. In general, however, it appears that for solid-liquid phase transfer processes, the crowns are far better catalysts than are the quaternary ammonium ions. In order for a solid-liquid phase transfer process to succeed, the catalyst must remove an ion pair from a solid matrix. The quaternary catalysts have no chelating heteroatoms with available lone pairs which would favor such a process. The combination of a quaternary catalyst and some simple coordinating amine or ether would probably succeed [28, 32, 34]. It seems likely, as mentioned above, that it is the combination of diamine and quaternary catalyst generated in situ which accounts for the success of Normanf s catalysts [28]. It is interesting to speculate on the possibility of using a quaternary ammonium compound and a drop of water as a catalytic system. 

Cram s pioneering work had a significant impact in the design of new chiral crown ethers as phase-transfer catalysts. Koga synthesized a series of C2-symmetric chiral crown ethers 2-4 from optically active diols, and systematically investigated their performance in the asymmetric conjugate addition of methyl phenylacetate to methyl acrylate [8]. Recently, a novel BINOL-based aza-crown ether 5 was developed by Jaszay et al. [9], which was proved to be effective for the asymmetric conjugate addition reaction. Takizawa and coworkers applied a chiral spiro crown ether 6 to the asymmetric benzylation... 

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