Additives crown-ether

Receptor 14b was designed for ion pair recognition as it also contains a cavity suitable for cation complexation (ester groups on the lower rim). Similar recognition ability [17] was found for derivative 15 bearing additional crown ether units. In both cases the relationship between the cation present and the binding of anions was studied. 

Equation 8.28 shows only the anionic nucleophile explicitly, since the counterion does not appear to take part in the reaction. Nevertheless, the counterion affects the solubility of a nucleophilic salt, which therefore can influence the polarity of the solvent needed for the reaction. An alternative to the use of a more polar solvent to dissolve a salt for nucleophilic substitution is to use crown ether additives. Crown ethers are cyclic polyethers that can coordinate with cations and therefore increase their solubility in organic solvents. The nomenclature provides the total number of atoms and the number of oxygen atoms in the ring. Compoimd 51 is 12-crown-4, and 52 is 18-crown-6 (Figure 8.32). Coordination of a crown ether with a cation helps to dissolve the salt in a less polar solvent and leaves the anion relatively unsolvated. The activation energy for substitution therefore does not include a large term for desolvation of the nucleophilic anion, and the reactions are fast. For example, adding dicyclohexano-18-crown-6 (53) to a solution of 1-bromobutane in dioxane was found to increase its reactivity with potassium phenoxide by a factor of 1.5 x 10. Moreover, Liotta and Harris were able to use KF solubilized with 18-crown-6 (52) to carry out Sn2 reactions on 1-bromooctane in benzene. ... 

Ever since Petersen reported the complexing ability of the crown ether with alkali, alkaline earth and other cations, crown ether became a major subject for researchers. This is due to the fact that crown ether possesses the ability to form complexes with a variety of inorganic salts and also the ability to solubilize these salts in aprotic solvents The complexation between metal cation and crown ether is believed to involve ion-dipole interactions and therefore is similar in nature to ordinary solvation. In addition, crown ether/metal cation complexes can serve as catalysts in reactions involving ionic intermediates. Polymerization of diene with crown ether/metal cation complexes is a typical example of this subject, since this reaction involves ionic intermediates. The detailed information including a brief history, chemical properties of crown ether and its application in the anionic polymerization and copolymerization have been discussed. 

Crown Ether Order of Crown Ether Addition [Crown Ether] Conv.l % X10 ... 

Alkali metal fluorides are well known as traditional halogen exchange or nucleophilic substitution reagents (7-5). Typically polar aprotic solvents such as dimethylformamide, A, A dimethylacetamide, acetonitrile, tetrahydrofuran, glymes, etc. are required. In addition, crown ethers are often added to increase the solubility of the alkali metal fluorides and thereby the halogen exchange reaction rate. The order of reactivity is Cs > Rb > K > Na for the alkali metals and lo > 2 for the electrophilic carbon center. 

TBDPS ether was employed at the primary position of glucose and lactose to increase solubility in common organic solvents [16]. Scheme 13 shows the results of alkylation of these nucleophiles (47 and 50). Anomeric O-alkylation of the glucose (47) in toluene afforded a- and P ecylglycoside (48a, b) in 58% yield. The coupling of lactose (50) needed additional crown ether to yield the glycoside (51a, b). THF was not suitable because of formation of an undesired product (49a, b). 

The effects of the counterion on the reactivity of the enolates can be important Reactivity Li+ < Na+ < K+ < ITiN+ addition of crown ethers... 

Aryl, heteroaryl, and alkenyl cyanides are prepared by the reaction of halides[656-658] or triflates[659,660] with KCN or LiCN in DMF, HMPA, and THF. Addition of crown ethers[661] and alumina[662] promotes efficient aryl and alkenyl cyanation. lodobenzene is converted into benzonitrile (794) by the reaction of trimethylsiiyl cyanide in EtiN as a solvent. No reaction takes place with aryl bromides and chlorides[663]. The reaction was employed in an estradiol synthesis. The 3-hydroxy group in 796 was derived from the iodide 795 by converting it into a cyano group[664]. 

Chips, semiconductor Chiral additives Chiral-AGP Chiral auxiliaries Chiral crown ethers Chiral hydrogenation Chirality... 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

Okahara and his coworkers have made a number of contributions to the synthesis of crown ethers using a one-pot method (see Sect. 3.13). These methods have been applied largely to the preparation of simple aliphatic crown ether systems. In addition, this group has prepared macrocyclic ester compounds using a one-pot procedure. Although... 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

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... 

In addition, several oj-hydroxyacids have been prepared. The systems prepared by Yamazaki (8) have been evaluated for ion transport action. Those prepared at Upjohn have been reported to have Ca activity comparable to the natural antibiotic X-537A (9) and to be more active than crown ethers. The most active of their structures is shown as 10. 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Addition of a chiral carrier can improve the enantioselective transport through the membrane by preferentially forming a complex with one enantiomer. Typically, chiral selectors such as cyclodextrins (e.g. (4)) and crown ethers (e.g. (5) [21]) are applied. Due to the apolar character of the inner surface and the hydrophilic external surface of cyclodextrins, these molecules are able to transport apolar compounds through an aqueous phase to an organic phase, whereas the opposite mechanism is valid for crown ethers. 

A certain crown ether having additional coordination sites for a trasition metal cation (71) changes the transport property for alkali metal cations when it complexes with the transition metal cation 76) (Fig. 13). The fact that a carrier can be developed which has a reversible complexation property for a transition metal cation strongly suggests that this type of ionophore can be applied to the active transport system. 

Dipotassium phthalocyanine (PcK2) can be prepared analogously to the dilithium compound by refluxing phthalonitrile and potassium pentoxide in pentan-l-ol.58 With additional oxygen-donor ligands (e.g., crown ethers) it forms crystals with the potassium bulging outside of the phthalocyanine ring.133134... 

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. 

Furthermore, the molecular size of the Li+ -solvating solvents may affect the tendency for solvent co-intercalation. Crown ethers [19, 152-154, 196, 197] and other bulky electrolyte additives [196] are assumed to coordinate Li+ ions in solution in such a way that solvent co-intercalation is suppressed. The electrochemical formation of binary lithiated graphites Li tC6 was also reported for the reduction... 

One of the most important factors affecting Qsei [76, 78, 87] is graphite-anode exfoliation, as a result of intercalation of solvated lithium ions. Factors that are reported to decrease (9lR are increasing the EC content in organic carbonates or di-oxolane solutions [98, 991 addition of C02 [31, 87, 99] or crown ethers [8, 71, 78] and increasing the current density [73] (this also lowers <2SE [14] as a result of decrease in (2s P ) ... 

Addition of 15-crown-5 to the higher-order cuprate led to a reagent that is totally unrcac-tive towards 2-phenylpropanal even at room temperature18. If, however, boron trifluoride-diethyl ether complex was added as additional ingredient, the reactivity was restored and, furthermore, the Cram selectivity increased to 90 10 (Table 4). Analogous results could be obtained by placing the crown-ether effect within the cuprate itself, as in reagent 10. 

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