Immobilization systems polyethers

Typical systems. A considerable number of immobilized polyether systems have been synthesized both for phase transfer catalysis as just discussed and for use in a number of analytical applications. Such immobilized systems are generally synthesized by either copolymerization of suitably functionalized macrocycles in the presence of cross-linking agents or by appending functionalized macrocycles to existing polymeric substrates. Structures (184)-(186) give examples of different... 

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)]. 

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

Matson et al. ( ) designed a flow system for measurements of HjS permeability through ILMs at temperatures of 363-403 K and total feed pressures of 2.1 10 kPa. The ILM consisted of a 30 wt. < KjCO, aqueous solution immobilized in microporous cellulose acetate and polyether sulfone films. The feed gas, a mixture of H S, COx, and N2 was humidified and sent to a temperature controlled membrane cell. A helium sweep gas was similarly humidified and served to carry the permeating gases to analysis by gas chromatography. 

The earliest reported examples of a system binding cations and neutral molecules simultaneously employed traditional inacrocyclic host molecules, such as calixarenes 1, which are described more fully below, and cyclic polyethers. Incorporating pyridyl or Schiff base groups into crown ethers leads to macrocycles with two possible binding modes. "" in the complex 2,6-pyrido[27]crown-9.LiC104.urea, both the Li" cation and urea are encapsulated within the crown ether cavity with interactions between them. In contrast, the complex of a Schiff-base-derived crown ether 2 has the Ni immobilized at the Schiff-base binding sites in a square-planar geometry. 

Portnoy and Goren immobilized M-alkylated imidazoles on polystyrene-bound polyether dendrons via three different synthetic routes (Scheme 15.43). All these systems catalyzed the Baylis-Hillman addition of methyl vinyl ketones to aromatic aldehydes, displaying a very strong positive dendritic effect and a significant enhancement effect of water as a cosolvent. Remarkably, substrates that did not undergo the reaction with the nondendritic immobilized or soluble M-alkyl imidazole catalysts underwent a smooth reaction with one of the catalytic second-generation dendrons. 

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