And cryptands

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

Often poly(ethylene glycol)s or derivatives thereof can be used instead of crowns or onium salts advantageously, although their catalytic activity frequently tends to be somewhat lower. The possible toxicity of crowns and cryptands and the price difference between these compounds and onium salts (100 1 to 10 1) are other important factors to be considered. Thus (1) [17455-13-9] (2) [14187-32-7] and (3) [16069-36-6] and cryptands are used more often in laboratory work, whereas onium salts are more important for industrial processes. 

The need for simple descriptions of complicated organic ligands has led to the evolution of some trivial nomenclature systems, such as those for crown ethers (e.g. 76) 72AG(E)16) and cryptands 73MI10200), which have become quite elaborate 8OMII0200). These systems are intended primarily to indicate topology, and the positions of potential donor atoms, and are not particularly appropriate for general use. 

The terms crown and cryptand have been universally adopted. A number of other terms have enjoyed less widespread recognition as noted above. Recently, Vogtle and Weber have proposed use of the terms crown ether, cryptand and podand according to the following scheme. Their suggested definitions are as follows ... 

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... 

Krespan has utilized these substances in the preparation of a variety of azacrowns and cryptands . Note that this material is also mentioned by Coxon and Stoddart . The strategy and techniques utilized with 14 are similar to those discussed above and are not noted here in detail. An overall transformation is illustrated in Eq. (8.12). 

A number of bi- or polycyclic structures have been prepared over the years which are somewhat difficult to classify within the narrow confines of crown and cryptand nomenclature. Nevertheless, these molecules deserve mention and are noted here. 

There is some evidence that Cs + can be formed by cyclic voltammetry of Cs+[OTeF5] in pure MeCN at the extremely high oxidizing potential of 3 V, and that Cs + might be stabilized by 18-crown-6 and cryptand (see pp. 96 and 97 for nomenclature). However, the isolation of pure compounds containing Cs + has so far not been reported. 

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

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Not all quaternary salts and cryptands work equally well in all situations. Some experimentation is often required to find the optimum catalyst. 

Crown ethers (Fig. 3.57) and cryptands (Fig. 3.58) can solubilize organic and inorganic alkali metal salts even in nonpolar organic solvents they form a complex with the cation (see Fig. 3.57c), and thus act as an organic mask (Gates, 1992). 

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. 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

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... 

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