Potassium-containing crown ether complexes

Lehn and coworkers have profitably employed tartaric acid-containing crown ethers as enzyme models. The rate of proton transfer to an ammonium-substituted pyridinium substrate from a tetra-l,4-dihydropyridine-substituted crown ether was considerably enhanced compared to that for a simple 1,4-dihydropyridine. The reaction showed first order kinetic data and was inhibited by potassium ions. Intramolecular proton transfer from receptor to substrate was thus inferred via the hydrogen bonded receptor-substrate complex shown in Figure 16a (78CC143). 

Another example of the activation of a hydroxy acid was described by Rastetter and Phillion [60] First the 0-protected hydroxyacid 68 reacts with a thiol group containing crown ether 67. Then the resulting thioester 69 reacts with potassium mrt-butoxide to give the alkoxide. At the same time a complexation of the potassium ion by the [18]crown-6 part of the molecule occurs. Thus, the alkoxide ion comes close to the carbonyl group of the molecule, so that nucleophilic attack leading to ring formation is facilitated (cooperation of dilution principle, template effect, and ion pair interaction). 

Now let us take the example of the phase transfer reaction between an aqueous solution containing potassium chloride and a solution containing crown ether L (a complexing agent of IC ions) in an organic solvent, as shown in figure 3.7... 

K2Hg(CN)4 and the crown ether dibenzo-18-crown-6 in aqueous ethanol yield the crystalline [K(C2oH2406)]2[I Ig(CN)4]-2I I2C) the structure contains two independent potassium-crown complexes and a distorted tetrahedral Hg(CN)4 entity (rav(l Ig—C) 218.4, rav(C—N) 112.3 pm) which coordinates to one of the complexed K ions through one of the CN ligands (slight lengthening of the C—N bond).116... 

Known as crown ethers because of their crown-like shape, these ethers contain cavities that are ideal for forming complexes with metal ions. It is this property that allows ordinary salts to dissolve in organic solvents. For example, potassium permanganate is usually insoluble in benzene, but readily dissolves in benzene if [18]-crown-6 ether is added. This solution is useful because it allows oxidation with potassium permanganate to be carried out in organic solvents. The potassium ion (shown in green) is just the right size to fit into the cavity in the crown ether. 

Its X-ray structure indicates the formation of a contact ion-pair where the complexed cation [K-18-crown-6 ] , shaped as "an overturned umbrella", is connected to the anion through three bridging iodine atoms. This arrangement allows the potassium ion to interact with other possible ligands in a direction perpendicular to the plane containing the crown-ether. 

We have prepared a variety of new Schiff base bis crown ether ligands (98-100) that contain alkali and transition metal coordinating sites via the condensation of two or three equivalents of 4-formyl-benzo-15-crown-5 with an appropriate di- or triamine (Scheme 18). Homometallic copper(I), silver(I), and heteropolymetallic copper(I)-sodium, silver-(I)-sodium, and potassium complexes have been isolated. The singlecrystal X-ray structure of the [Cu(57)K](PF6)2 complex (100) is shown... 

In a limited number of cases, arylsilanes react with aldehydes as if they were aryl Grignard or aryllithium reagents. Both trimethyl(perchlorophenyl)silane and trimethyl(perfluoro-phenyl)silane react with benzaldehyde to give the corresponding 7.-(pcrhalophenyl)bcnzyl tri-methylsilyl ethers.163 Benzaldehyde reacts completely with trimethyl(perfluorophenyl)silane in diethyl ether in the presence of either a catalytic amount of the potassium cyanide/18-crown-6 complex in less than 5 hours at room temperature or potassium fluoride in dimethylform-umide.164 In the case of aryltrimcthylsilanes containing electron-withdrawing substituents in the ortho position, the reaction is observed only under the conditions of nucleophilic catalysis by potassium fluoride or cesium fluoride. 

Crown Ethers. BINOL-derived crown ethers have been reported. Crown ethers containing 3,3 -disubstituted BINOL derivatives are particularly effective for asymmetric synthesis. Thus complexes of these crown ethers (e.g. 18-Crown-6) with Potassium Amide or Potassium t-Butoxide catalyze asym-... 

At least two sources offer resins containing a crown ether function in addition to carboxyl or carboxyl and phosphonate (see Table 7.1). In particular, the crown ether function strengthens the affinity of the ion exchanger for potassium(I) by virtue of complexation. These resins are recommended for determination of trace sodium and ammonium in various treated water- or wastewater samples. 

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