Cryptands immobilized

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 . ... 

A number of other cryptand-bound polymers have been synthesized using similar procedures to those discussed previously for immobilization of crown molecules. Apart from their use in phase transfer catalysis, such polymers have been studied extensively as chromatography reagents for the separation of a range of metal-ion types (Blasius Janzen, 1982) in a number of instances quite useful separations have been achieved. 

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... 

Immobilization of phase-transfer catalysts on polymeric matrices avoids the problem of separating and recycling the catalysts. In this case the chemical stability of the immobilized catalyst becomes very important quaternary salts often decompose under drastic reaction conditions whereas polydentate ligands are always stable. However, the difficult synthesis of cryptands, despite their high catalytic efficiency, can hardly justify their use. Synthesis of crown-ethers is much easier, but catalytic efficiences are often too low. 

In a cryptate complex, the cation is enclosed wholly or partially in a hydrophobic sheath, so that not only are salts of this complexed cation soluble in nonpolar organic solvents but also extractable from aqueous solutions into organic solvents immiscible with water (144). Specific cryptands may be used to selectively complex metals from crude materials or wastes, particularly if they are immobilized on a polymer support (101, 114, 145). 

Covalent attachment of ligands to polymer supports retains their complexing properties156 and widens their applications. For instance, immobilized crown poly-thers and cryptands used as phase-transfer catalysts can be recycled55. Chiral ligands have been used for a chromatographical separation of D- and L-amino acids75. ... 

So far, much research has gone into finding new synthetic routes, new products and novel selective syntheses, and in the analysis of important factors affecting yield and in some cases selectivity. However, other practical constraints relevant to process development for industrial-scale synthesis have to be tackled. For example, new insights are needed to develop cost-effective, stable, and selective PT catalysts (especially effective immobilized triphase catalysts). Other relevant factors include the recovery and recycle of the PT catalyst, catalyst decomposition, environmental issues such as catalyst toxicity, and ease of product recovery. Catalyst costs are not very high when quats are used, as against the more expensive crown ethers or cryptands. In most cases, the overall process is more than cost-effective since PTC allows the use of cheap alternative raw materials, prevents the use of costly dipolar solvents, is less energy intensive (due to lower temperatures) than alternative methods, alleviates the need... 

A rational approach to developing a fluorescent chemosensor for potassium (K ) is presented. In this approach, J-M. Lehn s [222] cryptand, which selectively binds K, is covalently attached to coumarin at positions 6 and 7. In this hybrid system, coumarin plays the role of a transducer translating the free energy of supramolecular interaction between the cryptand and into measurable enhancement of its fluorescence. By Immobilizing this system on an optical fiber, the continuous monitoring of in patients undergoing open-heart surgery can be achieved. 

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)]. 

Membranes containing macrocyclic compounds can be very efficient in the separation and identification of ions and, therefore, in the preparation of ion-selective sensors (201). The immobilization of crown ethers and cryptands by intCTcalation on 2 1 phyllosilicates (45) has been used in the preparation of composite membranes made up of sandwich-like materials encapsulated into thin polybutadiene coatings to improve their mechanical properties (202). The entrapped macrocyclic compounds modulate the transport properties of cations in aqueous solution, and this can be used for their individual discrimination and recognition. Although the mechanism controlling the ion selectivity of these composite membranes is still not clear, development of ion-selective sensors based on the coating of electrodes by this class of membranes can be envisaged. 

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. ... 

When the cryptand is immobilized, we assume that the backbone or the framework of the polymer hinders the anion to react at the brominated carbon and a greater proportion of the glycoside is obtained. When the reaction is carried out in CH2CI2... 

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