Polymer-supported phase-transfer preparation

Pyridine-bound resins were also prepared and successfully employed as polymer-supported phase transfer catalysts in bromide displacement from 1-bromoalkanes by salt phenoxide or naphthoxide, even though the controlling factor (diffusivity or preferential sorption) for the observed substrate selectivity effects was difficult to determined.[133]... 

The preparation of various polymer supported phase transfer catalysts is described in literature. ... 

The title compounds known for their anti-inflammatory activity were prepared earlier in low yields/ Polymer supported phase transfer catalysts have also been used (for details see sec 7.4.5) for various reactions. These have now been prepared" by the condensation of 2-aminophenols with phenacyl bromide in presence of a PTC in aq. K2CO3 (Scheme 26). 

Although phase transfer agents have been attached to clays, silica and alumina, the vast majority of studies have used organic polymers, especially polystyrene, as the support. The earliest of these triphase catalysts was prepared from 12% chloromethylated polystyrene crosslinked with 2% divinylbenzene by reaction with a tertiary amine. A wide range of triphase catalysts has since been reported, some examples of which are shown in Figure 5.16. 

Aside from the use of polymers as supports for phase transfer catalyst centers, much excellent work has been reported on the use of PTC in polymer chemistry for pol)rmerization methods (28), for the chemical modification of already formed polymers(29). for the modification of polymer surfaces without change of the bulk polvmerOO). and for the preparation and purification of monomers(31). 

Dispersed phase polymerisation of HIPEs has also been used to prepare polymer-supported quaternary onium phase transfer catalysts [162]. One strategy involved the polymerisation of a concentrated emulsion of vinyl benzyl chloride (VBC) in water and subsequent quaternisation of the polymer resin with tertiary amines and phosphines (Fig. 22). 

Self-supported MIP membranes can be seen as an alternative format to the traditional MIP particles for applications in separation and sensor technology, avoiding the limitations of mass transfer across conventional MIP materials. Two main approaches have been used for the preparation of membranes composed of an MIP in situ polymerisation and polymer solution phase inversion. 

The multifaceted applications of phase-transfer catalysts (PTC) in organic synthesis contributed decisively to the establishment of organic catalysts as useful preparative tools. Polymer-supported PTC was examined extensively but it was noted that the catalytic activity of the insoluble polystyrene-supported catalysts was strongly reduced in com-... 

The asymmetric alkylation of glycine derivatives is one of the most simple methods by which to obtain optically active a-amino acids [31]. The enantioselective alkylation of glycine Schiff base 52 under phase-transfer catalysis (PTC) conditions and catalyzed by a quaternary cinchona alkaloid, as pioneered by O Donnell [32], allowed impressive degrees of enantioselection to be achieved using only a very simple procedure. Some examples of polymer-supported cinchona alkaloids are shown in Scheme 3.14. Polymer-supported chiral quaternary ammonium salts 48 have been easily prepared from crosslinked chloromethylated polystyrene (Merrifield resin) with an excess of cinchona alkaloid in refluxing toluene [33]. The use of these polymer-supported quaternary ammonium salts allowed high enantioselectivities (up to 90% ee) to be obtained. 

A few years ago Cahard reported a series of studies on the use of immobilized cinchona alkaloid derivatives in asymmetric reactions with phase-transfer catalysts [17[. Two types of polymer-supported ammonium salts of cinchona alkaloids (types A and B in Scheme 8.4) were prepared from PS, and their activity was evaluated. The enantioselectivity was found to depend heavily on the alkaloid immobilized, with the type B catalysts usually giving better results than the type A catalysts. By performing the reaction in toluene at -50 °C in the presence of an excess of solid cesium hydroxide and 0.1 mol equiv of catalyst 10, benzylation of the tert-butyl glycinate-derived benzophenone imine afforded the expected (S)-product in 67% yield with 94% ee, a value very close to that observed with the nonsupported catalyst. (Scheme 8.4, Equation b) Unfortunately-and again, inexplicably-the pseudoenantiomer of 10 proved to be much less stereoselective, affording the R)-product in only 23% ee. No mention of catalyst recycling was reported [18]. 

Phase transfer catalysis is a significant advance in preparative organic chemistry, but there is a practical limitation—phase transfer agents sometimes stabilize emulsions, which make product recovery difficult. A variation of the technique, called triphase catalysis, involves bonding the phase transfer agent to a support, such as a gel-form polymer (see 14.2.4.1) . If the polymer has an affinity for boA liquid phases, the phase... 

As a development in the field of solution phase polymer-supported organic synthesis, xanthate transfer technology has been used to prepare polymer-bound protected sugar derivatives which are soluble in organic solvents. For example, protected D-c/iiro-inositol 159 was elaborated into polystyrene bound 160 by radical pol3mierisation with 15 equivalents of styrene in the presence of 12 mol% lauroyl peroxide. ... 

The alkyldiphenylphosphine is used in the preparation of phase-transfer catalysts and as a ligand in polymer-supported organometallic compounds. What is compound X ... 

Polymer-supported Reagents. Polymer-supported toluene-sulfonyl azide I3I was prepared by reaction of a macroreticular p-toluenesulfonyl chloride resin (prepared, in turn, by chlorosul-fonation of Amberlite XE 305) with excess sodium azide. Resin 13, a solid-phase equivalent of />-toluenesulfonyl azide, can be used for diazo transfer to /3-dicarbonyl compounds (eq 14). Unlike tosyl azide, resin 13 does not detonate on shock treatment and is stable at room temperature. Recently, an analogous benzenesul-fonyl azide resin has been prepared from PS-TsCl. ... 

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