Ethers cryptands

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 ... 

Of these terms, the names crown ether, cryptand and cryptate are in general usage. 

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

Compounds which produce a complex with Li+ ions have been investigated. The compounds examined were N,N,N, N tetramethylethylenediamine (TMEDA), eth-ylenediamine, crown ethers, cryptand [211], diglyme, triglyme, tetraglyme, eth-ylenediamine tetraacetic acid (EDTA) and EDTA-Li+ (n=l, 2, 3) complexes [59]. The cycling efficiency was improved by adding TMEDA, but the other additives did not show distinct effects. 

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

Crown ether, cryptand, and poly(ethylene glycol) catalysts are more stable in base than the quaternary ammonium and phosphonium ions. Only the polyethylene glycols) are likely to meet industrial requirements for low cost, although a number of more efficient, lower cost crown ether syntheses have appeared recently, such as those of sila-crowns 64 bound to silica1B9). 

X-Ray crystallography has provided a large amount of data on the structures of crown ethers, cryptands and their complexes in the crystal (B-78M152102). The uncomplexed... 

Miscellaneous molecules involving crown ethers, cryptands and related moieties... 

Since the discovery of crown ethers, cryptands, and other macrocyclic ligands by Cram, Lehn, and Pedersen, who were awarded the 1987 Nobel Prize in chemistry for their development and use of molecules with structure-specific interactions of high selectivity [1], a completely new research field was opened supramolecular chemistry [2-4-]. Since then, this research field has been extended in many fields such as molecular recognition, organic sensing, and liquid crystals. 

Wipff, G. (1992) Molecular Modeling Studies on Molecular Recognition - Crown Ethers, Cryptands and Cryptates - from Static Models in Vacuo to Dynamic Models in Solution, J. Coord. Chem. 27, 7-37 and chapter in this volume. 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

Since the pioneering work of Pedersen (1), Lehn (2), and Cram (3) on synthetic macrocyclic and macropolycyclic host systems such as the crown ethers, cryptands, and spherands, there has been an enormous development of the field of host-guest or supramolecular chemistry. Molecular hosts designed to bind inorganic and organic, charged and neutral guest species via cumulative, noncovalent interactions have all been reported and extensive reviews on this subject have appeared (4-8). 

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