Polymer supported reagents reuse

Polymer-supported triphenylphosphine ditriflate (37) has been prepared by treatment of polymer bound (polystyrene-2% divinylbenzene copolymer resin) triphenylphosphine oxide (36) with triflic anhydride in dichloromethane, the structure being confirmed by gel-phase 31P NMR [54, 55] (Scheme 7.12). This reagent is effective in various dehydration reactions such as ester (from primary and secondary alcohols) and amide formation in the presence of diisopropylethylamine as base, the polymer-supported triphenylphosphine oxide being recovered after the coupling reaction and reused. Interestingly, with amide formation, the reactive acyloxyphosphonium salt was preformed by addition of the carboxylic acid to 37 prior to addition of the corresponding amine. This order of addition ensured that the amine did not react competitively with 37 to form the unreactive polymer-sup-ported aminophosphonium triflate. 

Kawana et al. used xylofuranose derivates as chiral auxiliaries [7, 13], Through its primary hydroxy function, the auxiliary was loaded to the polystyrene resin. Esterification of the immobilized auxiliary 8 gave a-keto esters. Subsequent nucleophilic additions of Grignard reagents afforded resin-bound a-hydroxy esters. Subsequent saponification afforded the chiral a-hydroxy acids 9 (Scheme 12.5) in 18-84% yields and 36-65% enantiomeric excesses. The recovered polymer supported chiral auxiliaries could be reused without decrease of enantioselectivity. 

Thus, a micro encapsulation technique has been shown to be quite effective for binding catalysts to polymers. Utilizing this technique, unprecedented polymer-supported, microencapsulated rare earth Lewis acids have been prepared. The catalysts thus prepared have been successfully used in many useful carbon-carbon bond-forming reactions. In all cases, the catalysts were recovered quantitatively by simple filtration and reused without loss of activity. This new technique for binding nonpolymer compounds to polymers will be applicable to the preparation of many other polymer-supported catalysts and reagents. 

Oxidative iodination of aromatic compounds by the combination of a hypervalent iodine reagent with iodine is a synthetically important reaction (Section 3.1.4) [34]. Polymer-supported diacetate 4 is a particularly convenient reagent for oxidative iodination since it can be regenerated and reused many times. Reagent 4 gives the best results for the iodination of electron-rich arenes 13, with predominant formation of the para-substituted products 14 (Scheme 5.8) [12,21]. 

A further example used the supported amino alcohols 45,46 and 47 (Scheme 4.76), where the reagents were pumped up from the bottom of the polymer using a pair of long needles connected to peristaltic pumps. The product was collected from the top using another pump and quenched in a solution of dilute hydrochloric acid. For the first run with catalyst 46, the yields and ee were excellent (94% yield in 97% ee), but when 46 was recovered and reused, the yield dropped to 75% and the ee to 50%. This was ascribed to degradation of both the chiral and backbone sites of the polymer by diethyl zinc, again demonstrating that not only do the solid supports need to be mechanically sound but both the backbone support and active site must be also chemically resistant to the reaction conditions [171]. 

Thus, the active functions of the carrier material, which is usuaUy a naturally occurring or synthetically prepared polymer, can be of almost any nature. Introduction of the spacer molecules (functionalisation) is also dte phase determining the activity of the immobilised enzyme preparation the more spacer molecules per unit area, the more enzyme molecules can be attached. Of course, steric hindrance exerts a limit on dtis number. After the carrier has been functionalised, excess reagent is removed by hltration and washing and the enzyme can be attached to the support The immobilised enzyme thus obtained is usually stored in an aqueous medium in ordra to avoid dehydration, which may lead to irreversible deactivation of the enzyme. Just before use, die beads containing the enzyme are collected by filtration, wash, and added to the aqueous solution of substrate. Once tl desired conversion has been ediected, die beads are removed by dltration, washed, and either stored or reused direcdy afterwards. 

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