Crown ethers cationic complexation

Smaller, hard cations, such as alkyltrimethylammonium salts and crown ether-cation complexes, will promote the elimination of dichlorocarbene to form a hard ion pair of type Q+C1 . 

Such supercomplexes may serve as models for the transition state of crown-assisted reactions, for example nucleophilic substitutions. The observed stereo-specifity could be caused by locally ordered structures in which the crown ether cation complex could act as the substrate and as the anion, as well. 

A crown ether-cation complex is called a host-guest complex. The crown ether is the host and the cation is the guest. 

Figure 2.3 Plot of log values of crown ether/cation complexes in methanol (15). Reprinted by...

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

In our initial studies of the polymerization of butyl acrylate by solid potassium persulfate in acetone solution (2), we attempted to relate the rate of polymerization to the ability of various crown ethers to complex the potassium cation. A reasonable correlation was discovered between log Rp and log K, where K represents the binding constant of the crown ether for in methanol solution (Figure 1). This finding provided some support for the idea that a typical phase transfer process was occurring in these reacti ons. 

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

Crown ethers selectively complex various alkali metal cations and can be thus used as model systems to study interactions between a macrocycle-bound cation and the 7r-system of a sidearm arene. Alkali-metal cation-7i interactions have recently received considerable attention because of the biological importance [88, 89, 175]. These studies have focused on Na+ and K+ interacting with benzene, phenol, and indole, which are the side chain arenes of phenylalanine, tyrosine, and tryptophan, respectively. Recent work [177-180] has demonstrated the formation of stable complexes between, for example, K+... 

Molybdenum and tungsten complexes with three crown ether benzenedithio-lene ligands (21) have been reported (105) and the effect of alkali ion binding has been probed by CV (106). Upon binding with Li+, Na+, or K+, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). 

The ability of crown ethers to complex cations can be exploited in nucleophilic substitution reactions, as shown in Figure 9.5. Nucleophilic substitution reactions are usually run in polar solvents to dissolve both the polar organic substrate and the ionic nucleophile. With a crown ether, though, the reaction can be run in a nonpolar solvent under conditions that enhance nucleophilicity. 

When 18-crown-6 is added to the reaction of CH3CH2Br with KCN, for example, the crown ether forms a tight complex with that has nonpolar C-H bonds on the outside, making the complex soluble in nonpolar solvents like benzene (CeHg) or hexane. When the crown ether/K" complex dissolves in the nonpolar solvent, it carries the "CN along with it to maintain electrical neutrality. The result is a solution of tightly complexed cation and relatively unsolvated anion (nucleophile). The anion, therefore, is extremely nucleophilic because it is not hidden from the substrate by solvent molecules. 

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