Crown ether counterions

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

Crown ethers are cyclic polyethers. Larger crown ethers contain a cavity that can partially engulf atomic ions. 18-crown-6 actually binds so tightly that it can extract this ion into benzene from water, driving counterions, like MnOc, into the benzene layer, i.e. 

The influence of the counterion on the stability of crown-ether complexes in general was reviewed in detail in one of the preceding sections. There it was shown to be an important parameter. The nature of the counterion in diastereomeric complexes of chiral crown ethers with primary ammonium salts also influences the chiral recognition. First of all it greatly determines whether salt can be extracted into the organic phase where the chiral discrimination takes place. In a series of experiments (Kyba et al., 1978) it was shown that when S,S -6zs(dinaphthyl)-22-crown-6 [284] in chloroform was equilibrated with racemic er-phenylethylammonium salts the type of anion also influences the degree of enantiomeric differentiation (Table 70). The highest... 

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. 

The crown ethers and the related cryptates (cryptates were first reported by J-M Lehn (J ) of France in 1969 (Figure 3)), have been used in a variety of synthetic procedures primarily because of their ability to solvate ionic materials in organic solvents (1, 14). Recently, it has been shown that crown ethers in the solid state form "sandwich" like complexes with most metals and that the counterion is also tightly bound (1 ). It is this evidence that suggested their use as ion traps in silicone RTV formulations. 

The most interesting point is that extraction of amino acids into IL occurs without addition of a counterion. Typically, for amino acid extraction into conventional solvents (including extraction with crown ether), a hydrophobic counterion is required. Moreover, in most cases, even the presence of such a counteranion does not provide an efficient recovery. 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [18]-crown-6... 

The selective cation binding properties ol crown ethers and cryptands have obvious commercial applications in the separation of metal ions and these have recently been reviewed (B-78MI52103.79MI52102, B-81MI52103). Many liquid-liquid extraction systems have been developed for alkali and alkaline earth metal separations. Since the hardness of the counterion is inversely proportional to the extraction coefficient, large, soft anions, such as picrate, are usually used. 

The idea that the stereochemical outcome of an intramolecular enolate alkylation is determined by chelation in the transition state was recently demonstrated by Denmark and Henke, who observed a marked preference for a "closed transition state (coordination of the cationic counterion to an enolate and the developing alcohol) resulting in a syn product. For example, the highest syn anti ratio (89 11) was obtained in toluene and the lowest syn.anti ratio (2 98) was obtained with a crown ether. These observations parallel the facial selectivities described herein and in ref 11 on the intramolecular SN2 reaction see (a) Denmark, S. A. Henke, B. R. J. Am. Chem. Soc. 1991, 113, 2177. (b) Denmark, S. A. Henke, B. R. J. Am. Chem. Soc. 1989, 111, 8022. 

Increasing die effective nucleophilicity of an ion allows S 2 substitution reactions to occur under milder conditions. An anion will become a better nucleophile when it is less effectively solvated and when it is further separated from its counterion. Methods that can achieve these changes include selection of a tetraafldammomum counterion [see Eqs. (6) and (6)], addition of a crown ether or a cryptand [see Eq. (7)], and use of a solvent that effectively solvates cations [see Eqs. (1) and (2)]. 

Most popular schemes used to collect analytes are based on coordination reactions and electrostatic attraction. Common examples include the accumulation of nickel onto dimethylglyoxime-containing surfaces [39], the uptake and voltammetry of mercury on a diphenylcarbazide-carbon paste electrode [40], the use of surface-bound crown ethers for the collection and measurements of lead [41], or of trioctylphosphine oxide for the preconcentration of uranium [42], and the utility of polyelectrolyte-coated electrodes for the electrostatic collection of counterionic reactants [43,44], Bioaccumulation through binding to surface-bound microorganisms [45] or biocatalytic processes [46] can also offer the desired sensitivity and selectivity enhancements. 

The Na+, K+, Rb+, and Cs+ complexes were simulated in the cone, partial-cone, and 1,3-alternate conformations, first without counterion. It was found that the smallest Na+ lies in a very deep position, surrounded by phenoxy-oxygens, not involving crown ether oxygens. As M+ gets bigger, it moves up to the crown. Picrate as the counterion remains an intimate pair with the complexed M+. The structure of the Na+ complex is different, compared with the host-M+ complex, because the picrate counterion pulls Na+ more exo to the crown region. On the contrary, for Cs+, the structures with and without picrate are very close, which demonstrates the good fit between Cs+ and the 1,3-alternate host. 

Several metal-organic polymers assembled by dithioether ligands, which are functionalized by heteroelements such as O, N, or Si in the spacer unit, have been recently described and reacted with Cu(I) salts. For example, the 2D square-grid coordination polymer 37 incorporating the 16-membered dithiaoxa crown-ether L37 has been prepared by a self-assembly process involving the interaction of the dithiaoxa macrocycle with CuCN (Scheme 17) the parallel reaction of the dithiaoxa crown-ether with Cul afforded the ID double-stranded coordination polymer 37 . This example demonstrates, how the nature of the counterion can control the form of the network.157... 

In a recent paper, Soumillion and co-workers [49] were able to identify CIP and SSIPin the P-naphtholate anion/alkali cation/tetrahydrofuran system. They found out that with lithium, a CIP is formed whereas with sodium/crown ether, a SSIP results. Using uncomplexed sodium or potassium counterion, mixtures of CIPs and SSIP s were detected. All their conclusions are based on spectral shifts in the transient absorption and emission spectra which were gained using laser flash spectroscopy. 

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