Crown ethers structures

The relative order of the catalytic activities of the crown ethers ([20] + [21]) > [9] > [ 11 ] > [ 13] > [8]) is the same as the relative order of their capacities to bind K+ (Table 4). However, the intrinsic reactivities of the ion pairs were also dependent on crown-ether structure, as was shown by experiments in which the alkylation rates were determined at various crown/phenoxide molar ratios. The curve obtained (Fig. 2) is similar to the curves found in titration experiments (Live and Chan, 1976 De Jong et al., 1976b), and shows that the rate constant reaches a maximum (called plateau kinetics in the literature) when all of the salt is complexed. 

Effect of the symmetry of the crown-ether structure on chiral recognition of RCH(COOCH3)NH,X atO°C ... 

FIGURE 16. Part of the polymeric chain of inverse crown ether structure 15... 

The binding of N02 groups on the calixarene took place on the aromatic rings in the para position (never on the crown moiety) (68, 69, 72), and mononitro-calixarene was always the most abundant compound, with the presence, to a lesser extent, of dinitro-compound. Despite the large number of compounds, the crown ether structure was conserved, which is an indirect indication of such ligands good stability. 

Pedersen s reports of the compounds he called crown ethers (Pedersen, 1967) began a world wide synthetic effort to prepare novel macrocycles, to define the limits of crown ether structure, and to assess the range of their biological and chemical properties. Among the latter, great effort was expended to define and understand cation complexation by these remarkable molecules. On the biological side, the toxic effects of crown ethers to cell lines and animals were assayed to understand their inherent safety or danger and the... 

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. 

It can be concluded that stereoselective (and particularly enantioselective) separation can proceed by the simple apphcation of a chiral organic phase or in most cases by incorporation of carriers in the membrane phase. Sometimes, their structure is very complex and these molecules can act as real receptors for the enantiomers. Different types of transport mechanisms are involved in the separation and the most popular one is cotransport. This is a result of the fact that most frequently used carriers are based on crown ethers structure. The stereoselectivities by apphcation of carrier-mediated SLM separation are very different and depend on the structure of the guest and host molecules. The magnitudes of the stereoselectivity are in most cases moderate but similar to other membrane-based separation techniques for stereoisomers. 

Carbohydrates have often been applied as starting materials for the synthesis of crown ethers, as they already contain hydroxy functions which are easy to incorporate in the crown ether structures. Crown ethers derived from commercial disaccharide D-lactose (7) have been used for the catalysis of the Michael addition. Benzyl 2,3,2 -tri-0-benzyl-3, 4 -0-isopropylidene-6,6 -0-(3,6,9-trioxaundccane-1,1 l-diyl)-/ -lactoside (8) was prepared by careful use of protection and benzyla-tion techniques as outlined11 similar crown ethers, e.g., 9, were obtained by analogous techniques12 13. 

Apparently the compounds 56) deviate so strongly from classical crown ether structures that other complexation factors — leading nevertheless to high enantio-selectivity — become important. Because only weak complexes are formed in solution interpretation of the results remains speculative. We ascribe, however, the formation of S-alcohols from L-amino acid derivative (56) to the formation of a ternary complex having the structure crudely represented in (65). The main stereochemical... 

After the diseovery of crown ethers, Lehn et al. synthesized cryptands, whieh eontain a double-cyclic crown ether structure (see Figure 1.3). Owing to the double-cyclic structure, cryptands can capture potassium ions selectively and strongly. This complexation behavior has received a great deal of attention because this system is very similar to the encapsulation of potassium ions by the antibacterial agent valinomycin. 

Analysis of electrolytes based on P(EO-EM) + 20 wt% Lil, 2 wt% I2 and 70 wt% GBL with different CE contents (a) Nyquist plots of the electrochemical impedance spectroscopy and (b) ionic conductivity values. The crown ether structure is shown in the inset. 

Doddi, G., Ercolani, G., Pegna P.L., Mencarelh, P. Remarkable catalysis by strontium ion in Sfj2 and E2 reactions occurring in proximity to a crown ether structure. J. Chem. Soc. Chem. Commun. 1994, 1239—1240. 

---