18-Crown 2.2.2 Cryptand

The disadvantages of these names are manifest, but one shudders to think about naming them systematically. It seems likely that for the foreseeable future, nomenclature of crowns, cryptands and related substances will bear a semblance to heiro-glyphics. 

Polyether complexation. The kinetics of formation of polyether crown, cryptand and related complexes have received considerable attention. Since formation rates are often quite fast, techniques such as temperature-jump, ultrasonic resonance, and nmr have typically been used for such studies. 

Figure 1. Clyme, crown, cryptand, and ionophore antibiotic.

Fig. 119. (Top) Crown-cryptand for the analysis of barium and ethylenediamine ligand for anion sensing (tpen). (Bottom) Dendrimer-like ligand for anion sensing.

Macrocycles are a highly topical subject. They constitute a large spectrum of compounds involving both artifical substances and natural products such as crowns, cryptands, cyclophanes, porphyrins, or macrolides. The former initiated the exiting area of host-guest supramolecular chemistry, which was highlighted by the award of the Nobel Prize for Chemistry to D. J. Cram, J.-M. Lehn, and C. J. Pedersen in 1987 but is still developing enormously. Porphyrins and macrolides are important active substances. No wonder that macrocycles are of immediate interest and everyone wants to know how they can be synthesized efficiently. 

Crown ethers, of caUxarenes 1396-1399—see also Cahx[4]crowns Cryptands—see Calixcryptands Crystal effects, on LD IR spectroscopy 368 Crystal stmcture determination, computer-based 549 C—S bond fission, homolytic 1079 Cumene, as substrate for antioxidants 859 4-Cumyl-l-naphthol, formation of 607 Cumyloxyl radical 877 Cumylphenols, formation of 607 4-Cumylresorcinol, formation of 607 Curcumin 867, 868, 870 Curie-point carbon-isotope-ratio mass spectrometry 303 Curie-point Py/GC/MS 303 Curie-point pyroUzer 938 p-Cyanophenol,... 

Soon after crown ethers came on the scene as the first synthetic host molecules capable of binding guests, cryptands followed, and soon thereafter, the spherands. The preorganization of these three classes of molecules follows the order of their invention, as does overall binding affinity, i.e., crownsNobel Laureate, was the creator of the family of hosts that he named spherands. The spherand story began shortly after the genesis of supramolecular chemistry, and it demonstrates how quickly the field matured, as complex syntheses and methods of characterization enhanced the rapid sophistication of host-guest chemistry. 

Fig. 7. Crown type and analogous receptor molecules of different varieties (1) crown ethers (2) cryptands (3) a podand (4) a spherand and (5) the natural...

The match between crown cavity diameter and cation diameter is obvious from Table 3 showing that, eg, and 12-crown-4 (la) or, respectively and 18-crown-6 (Ic) correspond. Similar are the cryptands of gradually increasing cavity size [2.1.1], [2.2.1] and [2.2.2] for and... 

Fig. 3. Crown compounds/cryptands and analogous inclusion hosts. (1 4) Crown macro rings bicyclic cryptands (5) [37095-49-17, (6) [31250-06-3J, (7) [31364-42-8] (8) [23978-09-8]-, (9) spherical cryptand [56698-26-1]-, (10) cylindrical cryptand [42133-16-4]-, (11) apodand [57310-75-5]-, and (12) a spherand...

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. 

A method for the polymerization of polysulfones in nondipolar aprotic solvents has been developed and reported (9,10). The method reUes on phase-transfer catalysis. Polysulfone is made in chlorobenzene as solvent with (2.2.2)cryptand as catalyst (9). Less reactive crown ethers require dichlorobenzene as solvent (10). High molecular weight polyphenylsulfone can also be made by this route in dichlorobenzene however, only low molecular weight PES is achievable by this method. Cross-linked polystyrene-bound (2.2.2)cryptand is found to be effective in these polymerizations which allow simple recovery and reuse of the catalyst. 

In 1967, DuPont chemist Charles J. Pedersen (21) discovered a class of ligands capable of complexing alkaU metal cations, a discovery which led to the Nobel Prize in Chemistry in 1987. These compounds, known as crown ethers or cryptands, allow gready enhanced solubiUty of sodium and other alkaU metals in amines and ethers. About 50 crown ethers having between 9—60 membered oligoether rings were described (22). Two such stmctures, dibenzo-18-crown-6 (1) and benzo-9-crown-3 (2), are shown. 

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. 

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