Phase transfer catalysis polymer based

Although the hydrolysis of alkyl halides to alcohols has been extensively investigated, an alternative two-step sequence involving substitution with carboxylate ion is more practical for the preparation of alcohols. Activation of the carboxylate anion prepared by the reaction of the acid with a base can be achieved (i) by use of a polar aprotic solvent and (ii) by use of aprotic apolar solvents under phase transfer catalysis, polymer conditions, or with crown ethers. 

In contrast, liquidiliquid phase-transfer catalysis is virtually ineffective for the conversion of a-bromoacetamides into aziridones (a-lactams). Maximum yields of only 17-23% have been reported [31, 32], using tetra-n-butylammonium hydrogen sulphate or benzyltriethylammonium bromide over a reaction time of 4-6 days. It is significant that a solidiliquid two-phase system, using solid potassium hydroxide in the presence of 18-crown-6 produces the aziridones in 50-94% yield [33], but there are no reports of the corresponding quaternary ammonium ion catalysed reaction. Under the liquidiliquid two-phase conditions, the major product of the reaction is the piperazine-2,5-dione, resulting from dimerization of the bromoacetamide [34, 38]. However, only moderate yields are isolated and a polymer-supported catalyst appears to provide the best results [34, 38], Significant side reactions result from nucleophilic displacement by the aqueous base to produce hydroxyamides and ethers. 

A series of polyphosphites, polyphosphates, polythiophosphates, and other polymers containing sulfone functions, based on 1, have also been described [17,119]. An efficient synthesis of polyethers from 1 and 1,8-dibromo or dimesyl octane by microwave-assisted phase transfer catalysis has been reported [120]. 

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. 

Although the reaction mechanism is not at the moment fully clarified, some points seem to be well established the radical character of the reaction and the contemporary need of an anion activator as PEG is (maybe the function of the anion activator is to increase the strength of the base as they do in Phase-Transfer Catalysis) (14). In place of PEG other anion activators may be used, as the condensation compounds between ethylene oxide and propylene oxide such latter polymers, being viscous liquids, offer some advantage in particular applications such as the decontamination of surfaces. 

Phase transfer catalyzed reactions in which ylides are formed from allylic and ben-zylic phosphonium ions on cross-linked polystyrenes in heterogeneous mixtures, such as aqueous NaOH and dichloromethane or solid potassium carbonate and THF, are particularly easy to perform. Ketones fail to react under phase transfer catalysis conditions. A phase transfer catalyst is not needed with soluble phosphonium ion polymers. The cations of the successful catalysts, cetyltrimethylammonium bromide and tetra-n-butylammonium iodide, are excluded from the cross-linked phosphonium ion polymers by electrostatic repulsion. Their catalytic action must involve transfer of hydroxide ion to the polymer surface rather than transport of the anionic base into the polymer. Dicyclohexyl-18-crown-6 ether was used as the catalyst for ylide formation with solid potassium carbonate in refluxing THF. Potassium carbonate is insoluble in THF. Earlier work on other solid-solid-liquid phase transfer catalyzed reactions indicated that a trace of water in the THF is necessary (40). so the active base for ylide formation is likely hydrated, even though no water is included deliberately in the reaction mixture. 

Finally, it is appropriate to say a few words on the choice of solvent for the chemical modification of polymers under phase transfer catalysis. As was mentioned earlier, numerous reactions which do not proceed in non-polar solvents such as toluene or dichloromethane in the absence of a phase transfer catalyst do proceed satisfactorily In DMF. Thus, many research groups, including ours, have used DMF extensively in polymer modifications with or without added catalyst, with increases in reaction rates and conversions being observed in the former case. As DMF is often a solvent for both the polymer and, in many instances, at least some of the reagent, it is debatable whether or not the term "phase transfer catalysis" applies (Ref. 50). More important perhaps is the fact that considerable amounts of dimethylamine can be produced through decomposition of DMF when the solvent is treated with concentrated aqueous base in the presence of a phase transfer catalyst. Obviously this may lead to undesirable side-reactions with incorporation of dimethylamine moieties into the modified polymers (Ref. 50). 

Percec V, Nava H, Jonsson H (1987) Functional polymers and sequential copolymers by phase transfer catalysis. 24. The influence of molecular weight on the thermotropic properties of a random copolyether based on 1,5-dibromopentane, 1,7-dibromoheptane, and 4,4 -dihydroxy-a-methylstilbene. J Polym Sci A Polym Chem 25 1943-1965... 

Ohtani et al. used polystyrene-supported ammonium fluoride as a phase transfer catalyst (triphase catalysis) for several base-catalyzed reactions, such as cyanoethylation, Knoevenage reaction, Claisen condensation and Michael addition. The catalytic activity of the polystyrene-supported ammonium fluid was comparable to that of tetrabutylammonium fluoride (TBAF). The ionic loading and the ammonium structure of the fluoride polymers hardly affected the catalytic efficiency. The reaction was fast in a non-polar solvent (e.g., octane or toluene) from which the rate-determining step of the base-catalyzed reaction is very similar to that of the nucleophilic substitution reactions. 

Very early experiments revealed that the amine was absolutely critical in the first step of the reaction, hydrolysis. In the absence of any amine catalyst, chloroformate could be recovered virtually unreacted from the interfacial mixture even when pH s approached 14. If base catalysis were the dominant means of chloroformate hydrolysis, then typical phase-transfer catalysts in the presence of sodium hydroxide should at least promote hydrolysis of chloroformate to phenols. However, a variety of phase-transfer catalysts, including n-Bu4N" OH, produced little or no reaction of the bischloroformate during the time frame of a normal cyclization reaction. Under homogeneous conditions or very long reaction times, the chloroformate can be consumed to produce primarily linears and polymer with negligible levels of cyclics. 

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