Supported reagents macroporous

The first polymer-supported reagents were derived from ion-exchange resins by immobilizing ionic reagents on macroporous polystyrene resins [5], This approach enables easy access to many reagents. For preparation, a... 

Macroporous resins are generally highly (>5%, typically 20-25%) crosslinked polystyrene microbeads [9,10,21]. The term macroporous refers to their inner skeleton, which is made of a permanent porous structure even in the dry state (cf Scheme Ic). Historically, functionalised macroporous resins have mainly been used for ion exchange and separation. Nowadays, many new applications, especially in the field of polymer-supported reagents [31, 32] and catalysts [5,7], have been developed. 

Keshavarz and Albadi used an azide-supported reagent in a macroporous polymer for developing click chemistry [91]. The reaction involves the nucleophihc addition of the supported azide to a-bromo carbonyl compounds (ketones, amides, or esters) followed by the 1,3-dipolar cycloaddition of the preformed azide to an alkyne catalyzed by a polymer-supported... 

For quantitative analysis of solid supported thiol residues on free macroporous or PEG grafts, Ellman s reagent has been used [5,5 -dithio-hfr-(2-nitrobenzoic acid]. However only qualitative information can be gained using lightly crosslinked polystyrene resins [Badyal et al. Tetrahedron Lett 42 8531 2007]. 

Note 3 Macroporous polymers are used, for example, as precursors for ion-exchange polymers, as adsorbents, as supports for catalysts or reagents, and as stationary phases in size-exclusion chromatography columns. 

The fact that macroporous, highly cross-linked polystyrene does not swell makes this support particularly interesting for continuous-flow synthesis in columns. This support has also been successfully used as an alternative to CPG for the solid-phase synthesis of oligonucleotides [90,91]. Furthermore, because reagents do not need to penetrate into the polystyrene network, enzyme-mediated reactions should also proceed smoothly on macroporous polystyrene [85]. 

Different forms of silicon dioxide have been used as supports for solid-phase organic synthesis. Silica gel is a rigid, insoluble material, which does not swell in organic solvents. Commercially available silica gel differs in particle size, pore size (typically 2-10 nm), and surface area (typically 200-800 m2/g). Like macroporous, highly cross-linked polystyrene, silica gel enables efficient and rapid transfer of solvents and reagents to its entire surface. Because the synthetic intermediates are only located on the surface of the support, enzyme-mediated reactions can be realized on silica [189,190], Silica gel is particularly well suited for continuous-flow synthesis because its volume stays constant and diffusion rates are high. 

An early example for the direct coupling of a tin reagent to Amberlite XE 305, a macroporous polystyrene, was reported by Crosby and coworkers157. The synthetic route is shown in Scheme 35. Reaction of the polymer-supported diorganotin dihydride 70 with iodooctane to give octane indicated a minimum content of 2 mmol tin hydride per gram of the polymer. 

Hi. The monomer polymerization route. Compared with the resin-functionalization route, the homo- and copolymerization of organotin-containing monomers permits one to influence the polymer resin structure to a greater extent. In principle, it is possible to prepare gel-type, macroporous, microporous or nonporous polymers. The pore structure, tin loading, solubility and other factors which influence the reactivity of the polymer-supported organotin reagents can be controlled by appropriate... 

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