Cryptands catalytic activity

Often poly(ethylene glycol)s or derivatives thereof can be used instead of crowns or onium salts advantageously, although their catalytic activity frequently tends to be somewhat lower. The possible toxicity of crowns and cryptands and the price difference between these compounds and onium salts (100 1 to 10 1) are other important factors to be considered. Thus (1) [17455-13-9] (2) [14187-32-7] and (3) [16069-36-6] and cryptands are used more often in laboratory work, whereas onium salts are more important for industrial processes. 

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. 

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 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. 

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). 

The synthesis of cu-amino-substituted 18-crown-6 and [2.2.2]cryptand readily bonded with chloromethylated polystyrene cross-linked by different amounts of DVB, is described [86]. Such bonded polyesters are used as interfacial transfer catalysts promoted by anions. As in the case of analogous soluble systems, the catalytic activity of cryptands is higher than that of crown ethers and quaternary onium salts. Because of their high chemical stability, such catalysts can be regenerated without chemical decomposition. However, the impairment of mechanical properties caused by comminution of the polymer matrix remains to be solved. 

Polymeric cryptands with different porosity, polarity, network density and copolymer compositions have been obtained by polymerization and copolymerization of 5,6-(vinylbenzo)-4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane [87]. It was found that copolymer composition and porosity do not affect the catalytic activity of polymeric cryptands in the nucleophilic substitution reaction between benzyl chloride and solid potassium acetate. Presumingly the reaction occurred on the polymer surface. These polymeric catalysts were more active during the nucleophilic substitution reaction between solid KCN and 1-4-dichlorobutane. 

There are also many uses for nonenzymatic polymeric catalysts. For instance, polymer-bound crown ethers, cryptates, and channel compounds behave as polymeric phase-transfer catalysts. The catalytic activity is based on selective complex formation. An example is the use of polystyrene-attached oxygen heterocycles [18]-crown-6 or a cryptand[222] to catalyze replacements of bromine in n-octyl bromide by an iodine or by a cyanide groups... 

The main use of crown ethers in PTC is in solid-liquid phase-transfer. In particular, it has been emphasized that they should be the catalysts of choice under such conditions. Due to its particular structure, the crown ether can approach the crystalline lattice so that the extraction and subsequent complexation of the cation require very little cation displacement, while the anion is contemporaneously associated to the complex. In the case of quaternary salts, on the other hand, the steric hindrance around the cationic center makes its interaction with the surface of the crystal difficult and the solution mechanism more complicated in this case only the anion must be extracted to displace the one originally associated with the lipophilic cation. These conclusions met with some scepticism Indeed, onium salts, cryptands, polypodes, polyamines, etc. have been successfully employed as solid-liquid PTC catalysts (see Sects. 4, 5.2, 5.3, 5.4). Moreover, when the catalytic activity of quaternary salts, crown ethers and polyamines was compared with respect to the extraction of anions from the crystalline state into an organic solvent, crown ethers were found to be the best system for the transport of CN . The catalytic effectiveness is completely reversed in the case of other anions, such as F and CHjCOO , the quaternary salt being the most efficient in these cases... 

Catalytic activity apparently follows an order similar to that for soluble catalysts, i.e. cryptands > crown ethers and quaternary salts, under the same reaction conditions As expected, crown ethers differ in activity from quaternary salts,... 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

In preparative chemistry the use of a macrocycle or a cryptand immobilized on an insoluble support presents many advantages easy work up, easy product purification and recycling of an expensive reagent. However the ion binding and the catalytic activity of the supported macrocycle depend on many variables spacer length, fonctionnality, polarity and structure of the resin . ... 

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... 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

Phase-transfer catalysts, such as the classic onium salts, crown ethers, and cryptands, have been immobilized on insoluble polymer matrices with various degrees of cross-linking. Their activity remains reasonably high if the catalytic centre is sufficiently far from the polymer backbone or if the resin is very porous. However, with phosphonium salts immobilized on silica gel die length of the hydrophobic chain between the active centre and the matrix and the solvent determine the adsorption capacity of the polar support, which then controls the rate of reaction. ... 

Although DSCBs were less prone to ring opening under the action of nucleophiles, such compounds as hydroxides (Na, K) and their derivatives (specifically, silanolates), catalyzed the AROP of 1,3-DSCBs at moderate temperatures (even at 20°C) but very slowly [73, 74]. The addition of cryptands activated this process, albeit its rate was also very small [75, 76]. Accordingly, the molecular weight of polysilmethylenes (as polysiltrimethylenes) was much smaller compared to that obtained by other catalytic polymerization procedures. 

Hao H-G, Zheng X-D, Lu T-B (2010) Photoinduced catalytic reaction by a fluorescent active cryptand containing an anthracene fragment. Angew Chem Int Ed 49 8148-8151... 

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