Crown Ethers and Cryptands

Studies with these ligand systems mainly involve the Group One and Two metal ions. The suggested mechanism for the formation of complexes has a number of features in common with 

The k term corresponds to spontaneous dissociation and the k2 term arises from cyanide-assisted dissociation  

The formation of Li +, Cd +, Zn + and Co + (but not Cu+) complexes of 12 is biphasic. The first step (overall second-order) likely represents binding of the metal ion to one of the chelating subunits of 12. The second step (first-order) is very slow (k 10 — ls ) and possibly corresponds to the gliding motion of one ring within the other while the second phenanthroline fragment attempts to bind to the metal center. 

There have been 22 reviews of this topic and the major ones are listed.  

The aminolysis of 4-nitrophenyl acetate in chlorobenzene occurs with the intermediate formation of a tetrahedral adduct [equation (2)] causing the generation of an ammonium ion. The latter may be stabilized by hydrogen bonding to a crown or poly ether. A flexible polyether is a better catalyst for this reaction than relatively rigid crowns.  

The rates of transacylation of the functionalized crown ethers (8) with 

4-nitrophenyl ester salts, HjN(CH2) C02Ar, vary with the nature of Z and may be correlated with the distance of the catalytic site from the polyether ring.  

Carboxylates act as general base catalysts for the aminolysis of 4-nitrophenyl acetate in chlorobenzene and the rate of reaction is enhanced in the presence of crown-ether-complexed potassium carboxylates.  

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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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