Immobilization of reagents

The immobilization of reagents onto sorbents often results in increase of their sensitivity and, in some cases, selectivity, allows to simplify the analysis and to avoid necessity of use of toxic organic solvents. At the same time silicas are characterized by absence of swelling, thenual and chemical stability, rapid achievement of heterogeneous equilibrium. 

CM has been reported to provide a synthetic tool for immobilization of reagents. Polymer-supported synthesis with an allylsilyl unit as a linker was developed. Divinylbenzene cross-linked allyldimethylsilylpolystyrene has been reported to undergo highly efficient ruthenium-catalyzed CM with functionalized terminal alkenes (Eq. 45) [78]. Products have been liberated by proto-desilylation with trifluoroacetic acid. 

Except for the synthesis of peptides and ohgonucleic acids, Uttle attention has been spent on the question of how synthesis can be carried out in an environment of sophisticated technologies which includes improved hardware. While peptides and oligonucleotides are conveniently prepared by Merrifield s solid phase technique, solution phase synthesis of most other synthetic targets have not been substantially replaced by this solid phase approach. Without discussing this aspect in detail it is obvious that today a renaissance of sophisticated solution phase synthesis can be noted. Immobilization of reagents and particularly catalysts, an old concept indeed, recently returned back onto the stage and this is addressed in this volume of Topics in Current Chemistry in a broader sense. 

An important breakthrough in that respect was the use of solid-phase organic synthesis (SPOS) where the attachment of the substrate to an insoluble support allowed for easy workup (filtration) and for rapid generation of products via spHt-mix procedures [1,2]. An important subsequent development consisted of the immobilization of reagents, scavengers and catalysts. This technique, coined polymer-assisted solution phase chemistry (PASP), allowed solution phase synthesis of compounds, yet still enjoying the bene-... 

Solid-phase chemistry associated with spectrophotometric detection in flow systems is also a good alternative for the determination of aspartame in food items. In this topic, one must consider not only the utilization of solid phase exchangers or adsorbents to build flow-through optosensors [36,37,40] but also the immobilization of reagents on a solid matrix in packed reactor format [41,42], This approach has been successfully explored mainly for the analysis of analyte mixtures through the use of microcolumns... 

Immobilization of reagents on particles has several advantages It reduces the natural loss of biological activity, allows for preconcentration of the analyte, and thus inaeases the sensitivity of the assay. ... 

Immobilization of reagents for optical sensors can be achieved in a number of ways including entrapping of the reagent within polymer matrices. In that respect, a number of polymer membrane films have been employed in optical sensors. 

It is also important to prevent the reaction of dye developers with one another. An unoxidized dye developer migrating through overlying layers of the negative could reduce and release an oxidized, immobilized dye developer, thus effecting an exchange. Reaction with quaternary salts included in the reagent aids in the immobilization of the oxidized species (66). 

In recent years the solid-phase hydrosilylation reaction was successfully employed for synthesis of hydrolytically stable surface chemical compounds with Si-C bonds. Of special interest is application of this method for attachment of functional olefins, in particular of acrolein and some chiral ligands. Such matrices can be used for subsequent immobilization of a wide range of amine-containing organic reagents and in chiral chromatography. 

Immobilization of hazardous reagents and catalysts by attaching active groups to polymeric or immobile backbones. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

Recyclability can be achieved by heterogenization of the reaction mixture, by binding the catalyst and products to different phases. This can be achieved by (i) immobilization of the catalyst on a solid inorganic or polymeric support (solid-liquid protocols) or (ii) partitioning the catalyst and reagents/products in different liquid phases (liquid-liquid protocols) (see Chapter 9.9 for more details on supported catalysts). 

Kirkbright G.F., Narayanaswamy R., Welti N.A., Studies with immobilized chemical reagents using a flow-cell for the development of chemically sensitive fiber-optic devices. Analyst 1984 109 15. 

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