Polymer-supported catalysts and reagents

Soluble polymers that have been used in hquid-phase methodologies are listed in Fig. 5.1 [3, 7, 8, 34, 35]. Polyethylene glycol and non-cross-linked polystyrene are some of the most often used polymeric carriers for organic synthesis and have found frequent use in the preparation of soluble polymer-supported catalysts and reagents consequently, a brief discussion of these polymers is warranted. 

Chiral aminoalcohols have been widely used for the preparation of polymer-supported catalysts and reagents [8, 9]. Polymer-bound aminoalcohols can be easily obtained from simple... 

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. 

New Directions in Academic Research. The future of academic research on polymer-supported catalysts and reagents may depend on the success of currently available species in industrial processes. Large scale successes would encourage many more chemists to employ polymer-supported species routinely in the laboratory and to invent new polymeric catalysts and reagents. 

Many books and reviews cover the preparation, characterization4 and use of homogeneous5 and heterogeneous6 catalysts. Others describe supported metal catalysts,7 other supported catalysts and reagents,8 and their use in preparative chemistry,9 the use of polymers as supports,10 trends in industrial catalysis,11 and the environmentally friendly nature of solid catalysts and reagents.12... 

The microwave-assisted one-pot three-step synthesis of thiohydantoins has been studied together with a polymer-supported catalyst and/or reagent [55]. Cyclization of a number of AT-substituted amino acids of type 50 and thioiso-cyanates 51 has given satisfactory yields within 5 min at 170 to 180 °C (see Scheme 29). Polymer-supported dimethylaminopyridine (PS-DMAP) 52 has been utilized as a supported base herein to catalyse the reaction however, in most cases the yield was comparatively lower to the use of triethylamine (TEA). PS-DMAP also assisted in simplifying the purification. 

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

For the use of soluble polymers in synthesis, see Liquid-Phase Chemistry Recent Advances in Soluble Polymer-Supported Catalysts, Reagents, and Synthesis, P. Wentworth, Jr., K. D. Janda, Chem. Commun. 1999, 1917-1924 and references cited therein. 

Note 3 Use of the term resin to describe the polymer beads used in solid-phase synthesis and as polymer supports, catalysts, reagents, and scavengers is also discouraged. 

Metal complexes bound to soluble polymers act as homogeneous catalysts they can be selectively precipitated and separated from products, and thus behave as immobilized homogeneous catalysts because of the ease with which they can be separated from reaction products. Alternatively, complexes can be physically trapped within the pores of swellable polymers. In this maimer, they can be effectively immobilized without a direct bond to the support. Polymers may be imprinted to give additional selectivity in the reactions of the supported metal catalysts and reagents. ... 

Wentworth, P, Janda, K D, Liquid phase chemistry recent advances in soluble polymer-supported catalysts, reagents and synthesis, Chem. Commun., 1917-1924, 1999. 

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