Polymer-supported phase transfer catalyst stability

The utility of polymer-supported phase transfer catalysts depends upon their ease of synthesis and their chemical and physical stability. The advantages of the heterogeneous catalysts are the ease of separation of the catalyst from reaction mixtures and reuse. Although there may occasionally be cases of higher activity of heterogeneous... 

Apart from reactions in which anionic counterparts of phosphonium cations are essentially implicated in a phase-transfer catalysis process (polymer-supported or soluble catalysts see above), some kinds of chemical transformations in which the anion s reactivity is involved have been studied. There are two major advantages, one being experimental and the other the regenerating capability of the reagent, in monomer- or polymer-supported form. The anionic counterparts of phosphonium salts can have an influence on their own stability or structure (the formation of betaines163 and allyl-phosphonium-vinylphosphonium isomerization, for example275,278). 

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

An early report by Konig deals with rapid parallel Suzuki reactions in water with phase-transfer catalysts. The solid support used in these reactions was PEG, and a variety of aryl palladium precursors were evaluated aryl halides as well as aryl tri-flates and nonaflates (Scheme 15.8). The inclusion of PEG is appealing, because it not only helps to solubilize the reagents but is also suggested to stabilize the palladium catalyst in the absence of phosphine ligands. Both the polymers and the esters were reported to withstand 10 min of 900 W multimode MW irradiation whereas the thermal conditions induced substantial ester cleavage (up to 45%). Nonaflates were found to be associated with lower yields and diminished product purity [32]. 

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