Ethers and Cryptands

Crown Ethers and Oyptands.— The use of crown ethers and cryptands as chemical catalysts requires the incorporation of a suitable reactive group into the crown ether or cryptand and a substrate that contains a functional group capable of complexing with the host (5). The host molecule (6), which shows chiral selectivity, contains functional groups (R = CH H or CH OH) which catalyse the 

A = catalytic functional group B = reactive site on substrate C = binding site 

The combination of a dihydropyridine residue into a crown ether gives enhanced rates of hydride ion transfer to carbonyl substrates containing a functionality capable of complexing with the crown. For example, crown ether (8) transfers hydride ion 2700 times faster than a suitable model dihydropyridine to the carbonyl group of the sulphonium salt (9).  

Cram and co-workers have continued their elegant work on host-guest com-plexation and particularly on chiral recognition in solution. The syntheses of a large number of stereoisomeric macrocyclic polyethers that contain chiral units have been described. The general shape is as shown, (10), giving a chiral cavity. They have been used for optical resolution of racemic alkyl ammonium salts and the catalytic reduction of enantiomers.  

Secondary dialkylamines are selectively acylated in the presence of primary amines by complexation of the latter with 18-crown-6 and a proton source. As the cavity in 18-crown-6 is ca. 2.7 A , secondary dialkylammonium salts are less efficiently complexed.  

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The bulk of the work which has been performed on open-chained crown ether and cryptand equivalents, especially for application to general cation binding studies has been accomplished by Vogtle and his coworkers. Vogtle has reviewed both his own and other work in this field . 

In later work, Vogtle and his coworkers prepared analogs of both crown ethers and cryptands. These molecules are designed to have a terminal donor group which is capable of offering a complexed cation additional binding sites. Numerous... 

Heumann, K. G. Isotopic Separation in Systems with Crown Ethers and Cryptands. 127, 77-132 (1985). 

Gokel, G. W., Crown Ethers and Cryptands. Royal Society of Chemistry, Cambridge, UK, 1991. 

Related work is dedicated to compounds with (L)AuC=C-functions attached to crown ether and cryptand-type units, following the idea that the luminescence properties of the chromophores will be influenced by complexation of cations in the polyether groups.87 Scheme 15 presents two examples of the devices probed in these highly successful studies. 

The difference in behaviour between crown ethers and cryptands is also evident from the results reported for the reaction of AgNOs with aceto-bromoglucose [111) (14). In the presence of equimolar amounts of di-... 

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... 

The discovery of crown ethers and cryptands in the late sixties opened new possibilities of cation recognition with improvement of selectivity, especially for alkali metal ions for which there is a lack of selective chelators. Then, the idea of coupling these ionophores to chromophores or fluorophores, leading to so-called chromoionophores and fluoroionophores, respectively, emerged some years later l9) As only fluorescent probes are considered in this chapter, chromoionophores will not be described. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

Suitably functionalised crown-ethers and cryptands have been synthesi -ed and reacted with chloromethyl polystyrene. Initially... 

Linear AH-TAS Plot Glyme, Crown Ether, and Cryptand... 

Figure 14, Schematic drawings of conformational changes upon cation binding by glymes, crown ethers, and cryptands (D denotes donor atom) also shown are the slopes (a) and intercepts TASo oi AH-TAS plots as measures of conformational change and desolvation.

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Complexation constants of crown ethers and cryptands for alkali metal salts depend on the cavity sizes of the macrocycles 152,153). ln phase transfer nucleophilic reactions catalyzed by polymer-supported crown ethers and cryptands, rates may vary with the alkali cation. When a catalyst 41 with an 18-membered ring was used for Br-I exchange reactions, rates decreased with a change in salt from KI to Nal, whereas catalyst 40 bearing a 15-membered ring gave the opposite effect (Table 10)l49). A similar rate difference was observed for cyanide displacement reactions with polymer-supported cryptands in which the size of the cavity was varied 141). Polymer-supported phosphonium salt 4, as expected, gave no cation dependence of rates (Table 10). 

Polymer-supported onium ions are relatively unstable under severe conditions, especially concentrated alkali154). Polymer-supported crown ethers and cryptands are stable under such conditions. In practice, they could be reused without loss of catalytic activity for the alkylation of ketones under basic conditions, whereas the activity of polymer-supported ammonium ion 7 decreased by a factor of 3 after two recycles of the catalyst147). 

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