The Influence of Crown Ethers

Even more surprising is the finding that the reactivity picture exhibited by the various ethoxide species in the ethanolysis of phenyl acetate is very similar to that found in the ethanolysis of the activated amide N-methyl-2,2,2-trifluoroacetanilide [Eq. (3)], despite the different mechanisms of the two reactions [13]. Schowen ef al. [14] showed that C-N bond breaking in the rate-limiting decomposition of the tetrahedral intermediate is assisted by proton transfer from a general acid to the leaving group. 

Accordingly, the simplest mechanism that accounts for the enhanced ethanolysis rates in the presence of metal ions is shown in IV, where a metal-bound ethanol molecule, made acidic by metal coordination, serves as a general acid catalyst for C—N bond cleavage. Here again enhanced reactivity is exhibited by the 18C6-complexed metal ethoxide species [13]. 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

---

In the literature, there are numerous studies about the influence of different additives upon the electrical percolation of microemulsions [45-51]. Recently, the influence of crown ethers and aza crown ethers on the percolation temperature... 

Dehmlow, E. W., and Lissel, H., Reactions conditions of alkyne synthesis under the influence of crown ether. Tetrahedron, 37, 1563, 1981. 

Numerous other methods exist for making the simple crowns. Many of these methods are compared in Sect. 3.1 which deals with variations in the synthesis of 18-crown-6. Further commentary on the influence of templating ions on the synthesis of crown ethers may be found in Chap. 2, which deals with various aspects of the template effect. 

The small effect on reaction rates of the addition of crown ethers to the lower alcohols was also observed in the reaction of potassium acetate with 1-bromobutane in ethanol (Hirao et al., 1978a,b). The displacement of fluorine in either o-nitro- or p-nitro-fluorobenzene by a methoxy group, by reaction with potassium methoxide in methanol was hardly influenced by the presence of dicyclohexyl-18-crown-6 (Del Cima et al., 1973). Mariani et al. (1978), too,... 

The influence of the counterion on the stability of crown-ether complexes in general was reviewed in detail in one of the preceding sections. There it was shown to be an important parameter. The nature of the counterion in diastereomeric complexes of chiral crown ethers with primary ammonium salts also influences the chiral recognition. First of all it greatly determines whether salt can be extracted into the organic phase where the chiral discrimination takes place. In a series of experiments (Kyba et al., 1978) it was shown that when S,S -6zs(dinaphthyl)-22-crown-6 [284] in chloroform was equilibrated with racemic er-phenylethylammonium salts the type of anion also influences the degree of enantiomeric differentiation (Table 70). The highest... 

The conductance of lithium salt solutions in aprotic solvents generally shows a maximum as the concentration of electrolyte is increased, as illustrated in Fig. 4.5. Such maxima can be interpreted on the basis of the opposing influence of an increasing number of charge carriers on the one hand, and increasing viscosity and increasing ion association with the formation of non-conducting ion pairs, on the other. It has been shown that the conductance can be increased by the addition of crown ethers, such as 12-cr own-4 ... 

Cavities of macrocyclic hosts. For example, the cavities of crown ethers, cyclodextrins, calixaranes and other synthesized and naturally occurring macrocycles are several angstroms in size and are capable of recognizing the guest molecules. These microreactors can dramatically influence the rate and product distribution of certain reactions. 

Separation of cations can be influenced by their interaction with crown ethers, which depends on the sizes of the cation and the crown ether cavity. The concentration of a crown ether in the running buffer also plays a role. The best results have been obtained with 18-crown-6-ether where the selectivity changes were largest. The use of crown ethers makes it possible to separate, e.g., potassium from ammonium. Electrolyte containing 4mmoll 18-crown-6, 4 mmol 1 copper sulfate, and 4 mmol 1 formic acid was successfully applied to complete separation of all alkali and alkaline earth cations including ammonium (Figure 2). 

J. Dasilva-Carvalhal, L. Garcia-Rio, D. Gormez-Diaz, J.C. Mejuto, and P. Rodriguez-Dafonte 2003 Influence of crown ethers on the electric percolation of AOT/isooctane/ water (w/o) microemulsions, Langmuir 19, 5975-5983. 

Theoretical as well as practical contributions have been made regarding the substitution chemistry with alkoxy and aryloxy nucleophiles. The former include kinetic investigations dealing with influence of crown ether additives on the reactivity of alkoxides with l-halogene-2,4-dinitrobenzene [220]. 

Another type of phase-transfer catalysts is synthetic macro-cyclic polyethers, so-called cro m ethers, and poly(ethylene glycol) derivatives. Since the discovery of crown ethers and their complex-ing capabilities toward metal and ammonium ions in 1967 by Pedersen, crown ethers and modified compounds such as cryptates > have been attracting ever increasing interest among scientists having different applications in their mind. Complexes formed between these compounds and cations correspond to Q" " shown in Fig. 1 and possess phase-thransfer capacity for anions. The nature of cations is known to greatly influence the complexation with a given crown either or cryptate. ... 

Becker and Wagner have found that the addition of crown ethers leads to an essential acceleration of the polymerization rate when potassium is used as counterion. However, there is an insignificant influence on the rate of chain-transfer reac-tion. Although it has been stated that the average MW is limited to 6000 at temperatures around 90 °C because of the high rate of chain-transfer (fep/fe < 100) studies by Price and coworkers have shown that number-average MWs as high as 13 000 can be obtained at 40 ° C in neat PO in the presence of 18-crown-6 ether. ... 

No significant close contacts are observed between ILs and crown ethers based on the study of co-crystallisation of imidazolium based salts with 18-crown-6 (18C6), though coulomb interactions between the ionic (liquid) components and hydrogen bonding are important (Gjikaj et al., 2008). However, because of the influence of ILs to the system of crown ethers, they are being used in the extraction of metal ions, amino acid and so on. 

The influence of the ring size of crown ethers on the complexation equilibrium constant K was investigated by Zollinger s group (Nakazumi et al., 1983) for four... 

The influence of additional functional groups on the complexing abilities of 1,3-xyly 1-crown ethers [28] and [50]—[53] and binaphthyl-crown ethers... 

---