Ethers, crown macrocyclic

An interesting footnote to the general synthesis of macrocyclic crown ether-type esters is found in the work of Ors and Srinivasan . These workers used cinnamate esters and diesters as precursors to large ring lactones. Depending on orientation, either truxil-... 

More than 50 macrocyclic crown ethers were synthesised by Pedersen, and many were found to solublise alkali metal salts in non-polar solvents. He isolated 1 1 complexes with metal salts (87) and also showed that if the cation was too large to fit in the central hole, complexes with ratios of 1 2 or 2 3 (metal ether) could be obtained (88). Some of the larger ethers have been shown to complex two metal atoms simultaneously (89). Stability constants in solution are affected by the nature of the anion and the solvent. Both are also important in obtaining crystalline products. 

It should be stressed that there is not alwaysjustice in reseach evaluation. The selective formation of inclusion complexes by cyclodextrins (such as 11) was established by Cramer [6] at least 15 years earlier than that by crown ethers. However, cyclodextrin studies forming an independent branch of host-guest chemistry seem underestimated in spite of their considerably greater practical importance at present than that of other host macrocycles (crown ethers 17, calixarenes 18, etc.). Sometimes they are even totally neglected by discussing inclusion phenomena [7]. 

Liquid-liquid PTC would seem to be convenient for the synthesis of crown ethers and derivatives. But most still use conventional conditions,28,29 probably because the synthesis of macrocyclic crown ethers gives better results when Na+ or K+ ions are involved. These ions can induce a template effect with the polyethyleneoxy chain 63. 

An additional nuance in the nomenclature of these compounds concerns their complexes. The open-chained compounds are often referred to as podands and their complexes as podates. The cyclic ethers may also be called coronands and their complexes are therefore coronates. Complexed cryptands are cryptates. The even more complicated structures known as spherands, cavitands, or carcerands are called spherates, cavitates, or carcerates, respectively, when complexed. The combination of a macrocycle (crown ether or coro-nand) and a sidechain (podand) is typically called a lariat ether. 

However, a large number of complexing agents of all kinds with chemistries designed for specific metal ions have been reported in the literature. The tertiary amine Alamine 336 is widely used to transport anions such as U02(S04)4- and CfiOj [44-46], The macrocyclic crown ether family has also been used to transport alkali and rare earth metals [47,48] ... 

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. 

Only An(III) and An(IV) ions have been inserted into macrocyclic crown-ethers, e.g., in [UCl3(18-crown-6)]2[U02Cl3(OH)H20]. Hydrogen bond interactions between etheric oxygen and coordinated water molecules were only observed in the corresponding U02+ complexes [282]. The infrared spectrum and isotope effect of uranium-crown-ether complexes were reported [435], as well as the crystal structures... 

PEGs have been extensively investigated as PTCs in many commercial processes to replace expensive and environmentally harmful PTCs [46, 47], Compared with the commonly used PTCs, linear PEGs are much cheaper than analogous macrocyclic crown ethers and cryptands [48]. PEGs are also more stable at high temperatures, up to 150-200 °C, and show higher stability to acidic and basic conditions than quaternary onium salts [49]. 

We will discuss alcohol and ether functionalised NHC first, followed by furan functionalised ones. After a brief glimpse into Bertrand s noncyclic biaryl amino carbenes, we will turn towards macrocyclic, crown ether type examples and finally to NHC bearing a carbonyl group in the sidechain. 

Biehl MP, Izatt RM, Lamb JD, and Christensen JJ. Use of a macrocyclic crown ether in an emulsion (Uquid surfactant) membrane to effect rapid separation of Pb from cation mixtures. Sep Sci Technol 1982 17 289-294. 

Carbenes, generated by photolysis of di- and tetrachloro-o-quinone diazides, react with oxetane in a 1 3 ratio to afford 15-membered crown ethers. Benzocrown ether 675 was obtained in 16% yield (91CB1865). Derivatives of macrocyclic crown ethers with four or five oxygen atoms in a ring were synthesized by Cu(acac)2-catalyzed cyclization of a,polyethylene glycols. 20-26-Membered crown-4(5) ethers 676 were prepared from the above-mentioned diazo ketones with tri- or tetra-ethylene glycols in 7-26% yields. Treatment of l,8-bis(diazoacetyl)octane with dodecane-l,12-diol under the same conditions results in a mixture of 52-membered tetraether 646 (40%) and compound 645 (81CC616). 

Kobuke et al. [40] demonstrated that the podand 28 only forms weak complexes with alkali metal ions. However, when a chiral macrocyclic crown ether structure is formed by complexation with boric acid, alkali metal ions are bound much more strongly (Scheme 10). Stiffening, provided by covalent B-O bonds, improves the preorganization of the ligands that is required for the complexation of metal ions. 

The third class of ligands we mention here is that of polyaza-macrocycles, crown ethers, and calixarenes bearing polycarboxylate arms.293 294 299-301... 

Another plausible strategy consists of the functionalization of CNTs via covalent attachment of selected groups. This is the case of thiol and thiol-plus macrocyclic ///-crown-/ ether ( = 12, n = 4, and zn = 15, = 5)-functionalized single-wall CNTs (// C SI la-SWNT) prepared by Alvaro et al. (2007). The functionalization process and the structure of pendant units are depicted in Figure 7.21. Attachment of such... 

Macrocycles (crown ethers, cryptands) with chromogenic groups combine the natural selectivity of macrocycles with the possibility of direct spectrophotometric determination of some metals (e.g., K, Ca) in an organic phase after extraction [127-129]. 4-Picrylamlnobenzo-15-crown-5 crown ether (formula 4.41) is applied in the extraction and spectrophotometric determination of potassium. The determinations are based on extractable ion-associates of metals (e.g., Li, Na, K, Pb) with crown ethers and xanthene or sulphophthalein dyes [130]. 

Table 9.2 Some Typical Macrocyclic Crown Ethers Used in Coupled Transport. ...

Macrocyclic crown ethers have, almost from the very beginning, been modified by the incorporation of novel subunits thus this trend continues with diverse cage compounds,... 

Specific complexation of cations by polyethylene oxide chains (PEGs 1), macrocyclic (crown ethers 2-4) or macrobicyclic (cryptands 5) polyethers facilitates salt dissolution in low-polarity solvents, at the same time providing nonsolvated highly reactive anions by increasing ion-pair dissociation (Chart 1) [37],... 

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