Crown ethers catalytic activity

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. 

Increasing the hydrophobicity of quaternary ammonium salts increases the apparent extraction constants for the ion pair and therefore leads to a higher catalytic activity (Brandstrom, 1977). The same phenomenon has been observed by Cinquini and Tundo (1976) for crown ether catalysis (Table 35). The catalytic activity of 18-crown-6 [3] and alkyl-substituted derivatives [117]—[ 119] in the reaction of n-CgH17Br with aqueous KI follows the order [117], [118] > [119] s> [3]. The alkyl-substituted [2.2]-cryptand derivatives are also much more efficient than the parent compound [86]. Increasing the hydrophobicity of [2.2.2]-cryptand (Cinquini et al., 1975) and even of polypode ligands (Fornasier et al., 1976) leads to higher catalytic activity. The tetradecyl-substituted compounds show the reactivity sequence [2.2.2]-cryptand at 18-crown-6 > [2.2]-cryptand on the reactivity scale that can be distilled from Table 35. 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Polymer-bound catalysts containing both quaternary ammonium centres and oligo-(oxyethylene) links of the type shown in Scheme 1.10 have been synthesized [34]. There is an increase in catalytic activity resulting from a cooperative effect of the two types of catalyst upon nucleophilic reactions, compared to that of simple quaternary ammonium catalysts and crown ethers. 

More recently, 6-astatomethyl-19-norcholest-5(10)-en-3/S-ol (V) has been prepared rapidly, in high yield (60-70%), and at high specific activity by halogen exchange (At /I) in the presence of crown ethers. The crown ethers may fulfill a catalytic role by acting as specific cationic sinks, and thus facilitating rapid nucleophilic exchange. The product was identified by TLC with an Rp value very close to that of the iodinated derivative (95). 

For adequate reaction rates, a high concentration of iodide anion is necessary. The cation portion of the salt appears to have little or no effect on catalytic activity or reaction selectivity. Inorganic iodides (such as potassium iodide) are the obvious first choice based on availability and cost. Unfortunately these catalysts have very poor solubility in the reaction mixture without added solubilizers or polar, aprotic solvents. These solubilizers (e.g., crown ethers) and solvents are not compatible with the desired catalyst recovery system using an alkane solvent. Quaternary onium iodides however combine the best properties of solubility and reactivity. 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Polymer-supported onium ions are relatively unstable under severe conditions, especially concentrated alkali154). Polymer-supported crown ethers and cryptands are stable under such conditions. In practice, they could be reused without loss of catalytic activity for the alkylation of ketones under basic conditions, whereas the activity of polymer-supported ammonium ion 7 decreased by a factor of 3 after two recycles of the catalyst147). 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

The effect of cation-complexing agents on the barium(II)-assisted basic ethanolysis of phenyl acetate has been looked at.184 Addition of various crown ethers yields ternary complexes of 1 1 1 crown-metal-ethoxide composition and a definite cation activation takes place. Cryptand 222 removes the catalytic activity. 

Kuwamura, T. Yoshida, S., (1980) Surface-active crown ethers. 2. Synthesis, surfactant properties, and catalytic action of macrocyclic polyethers possessing a 4-alkyl-l,3,5-triazine subunit Nippon Kagaku Kaishi 427 [Chem. Abstr. 493 28168e],... 

Under the same reaction conditions, catalytic activity is greater for cryptands than crown ethers or quaternary onium salts (111, 113). A series of polymer cryptands derived from the vinyl-containing monomer 16 have been discussed and could well find useful application (114). 

Much more impressive rate accelerations for several Diels-Alder (and other) reactions have been observed by employing solutions of lithium perchlorate (up to 5 m) in diethyl ether (LPDE solutions) [802-806]. The dramatic rate accelerations found for Diels-Alder reactions in LPDE solutions appear to stem from Lewis acid catalysis by the coordinative unsaturated Li+ ion (see the end of Section 3.1). The Lewis acid catalysis by LPDE is applicable to those Diels-Alder reactions in which the lithium cation can coordinate with suitable functional groups in the reactants e.g. Li+---0=C). Addition of lithium-specific crown ethers e.g. [12]crown-4) leads to a loss of the catalytic activity of the Li+. For a recent extensive review of salt effects on Diels-Alder reactions, see reference [802]. 

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