Phase-transfer catalysis reactions

In phase transfer catalysis of the solid/liquid type, the organic phase (containing dissolved organic reactant and a small amount of the crown) is mixed directly with the solid inorganic salt. Such a procedure enables the reaction to proceed under anhydrous conditions this is a distinct advantage, for example, when hydrolysis is a possible competing reaction. Because of their open structure, crown ethers are readily able to abstract cations from a crystalline solid and are often the catalysts of choice for many solid/liquid phase transfer reactions. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

BINAP, 127, 171, 191, 194, 196 olefin reaction, 126, 167, 169, 191 organic halides, 191 Pancreatic lipase inhibitors, 357 Pantoyl lactone, 56, 59 para-hydrogen, 53 Peptides, matrix structure, 350 Perhydrotriphenylene, crystal lattice, 347 Pericyclic reactions, 212 chiral metal complexes, 212 Claisen rearrangement, 222 Diels-Alder, 212, 291 ene reaction, 222, 291 olefin dihydroxylation, 150 Phase-transfer reactions asymmetric catalysis, 333... 

The aim of this book is to provide a concise and comprehensive treatment of this continuously growing field of catalysis, focusing not only on the design of the various types of chiral phase-transfer catalyst but also on the synthetic aspects of this chemistry. In addition, the aim is to promote the synthetic applications of these asymmetric phase-transfer reactions by giving solid synthetic evidence. Clearly, despite recent spectacular advances in this area, there is still plenty of room for further continuous development in asymmetric phase-transfer catalysis. 

For reviews of phase transfer reactions, see for example (a) Keller, W. E. Phase Transfer Reactions. Fluka Compendium-, Thieme Stuttgart, 1986 Vols. 1, 2. (b) Dehmiow, E. V. Phase Transfer Catalysis-, Verlag Chemie Deerfield Beach, 1980. (c) Starks, C. M. Liotta, C. Phase Transfer Catalysis, Principles and Techniques-, Academic Press New York, 1978. (d) Dockx, J. Acta Chem. Scand. 1973, 441. (e) Dehmiow, E. V. Angew. Chem., Int. Ed. Engl. 1974, 13, 170. For a mechanistic review of hydroxide mediated reactions under PTC conditions, see (0 Rabinovitz, M. Cohen, Y Halpern, M. Angew. Chem., Int. Ed. Engl. 1986, 25,960. 

H. Alper, Phase transfer reactions catalyzed by metal complexes, in Phase Transfer Catalysis New Chemistry, Catalysts, and Applications (Ed. Ch. M. Starks), ACS Symposium Ser. No. 326, American Chemical Society, Washington, DC, USA, 1987, Chapter 2, p. 8. 

Phase-transfer catalysis. A Polish group reported that the Wittig-Horner reaction with a-phosphoryl sulfoxides, sulfones, and sulfides could be conducted in a two-phase system (aqueous NaOH-methylene chloride) with benzyltriethyl-ammonium chloride as catalyst. Later work showed that a catalyst was not necessary because these sulfur compounds themselves can function as catalysts for phase-transfer reactions. Thus (1) is an effective catalyst for alkylation of ketones by alkyl halides in the presence of 50% aqueous NaOH. Related, but somewhat less active, catalysts are sulfones such as (2), a-disulfoxides (3), and bisphosphonates (4). 

Because of the success experienced in organic synthesis by using phase transfer reaction conditions IS), it was expected that this would be an excellent method for hydrogenating organic soluble dienes by using the water soluble catalyst K3[Co(CN)5H]. Others recently have carried out homogeneous catalysis processes under phase-transfer reaction conditions 14,15,16). 

Phase transfer catalysis is a valuable tool in organic synthesis. The process is exemplified by the convenient synthesis of 2-chlorophenyl phosphoro-dichloridothioate. Using this phase transfer reaction, a number of dichloridothioates of substituted phenyl, benzyl, thiophenyl, and thiobenzyl alcohols are accessible. The phosphorodichloridothioate reacts with various coupling reagents to form activated species that are useful in the synthesis of oligonucleotide phosphorothioates via the phosphotriester approach as illustrated below.5.6... 

Presumably, this will lead to the ability to stabilize a series of less stable inorganic and organic cationic, anionic and neutral convex compounds by molecular encapsulation. We can foresee phase transfer reactions and phase transfer catalysis with masked uncharged organic molecules modified in this way in their solubility and reactivity. Approved reagents may be modified with regard to reactivity and selectivity of the reaction. 

Many studies of asymmetric chemical conversions through the catalysis of Cinchona-derived PTC catalysts have been performed to expand the application of phase-transfer catalysis to various organic reactions. In addition to the reactions classified above, some selected examples of asymmetric phase-transfer reactions are shown below. 

Comparative study of the kinetics of substitution of the mesyl group in 1-methylheptyl methanesulfonate by halogen under phase-transfer catalysis in liquid-liquid and liquid-solid two-phase systems revealed an initial jump on the kinetic curves for the liquid-solid system. Such phenomena were known previously for some complex enzymic reactions, but not for phase-transfer reactions. 

Phase transfer processes rely on the catalytic effect of quaternary onium or crown type compounds to solubilize in organic solutions otherwise insoluble anionic nucleophiles and bases. The solubility of the ion pairs depends on lipophilic solvation of the ammonium or phosphonium cations or crown ether complexes and the associated anions (except for small amounts of water) are relatively less solvated. Because the anions are remote from the cationic charge and are relatively solvation free they are quite reactive. Their increased reactivity and solubility in nonpolar media allows numerous reactions to be conducted in organic solvents at or near room temperature. Both liquid-liquid and solid-liquid phase transfer processes are known the former ordinarily utilize quaternary ion catalysts whereas the latter have ordinarily utilized crowns or cryptates. Crowns and cryptates can be used in liquid-liquid processes, but fewer successful examples of quaternary ion catalysis of solid-liquid processes are available. In most of the cases where amines are reported to catalyze phase transfer reactions, in situ quat formation has either been demonstrated or can be presumed. 

Displacement of Halogen by Nucleophiles Phase-transfer Methods, Catalysis of the Su reactions of alkyl halides [equation (17)] by phase-transfer methods has... 

Phase-transfer catalysis, also often referred to as ion pair partition" is a novel synthetic technique which has been the subject of much interest in recent years not only in the field of organic synthesis but also in polymer chemistry. The term "phase-transfer catalysis" was first introduced in 1971 by Stark > who studied kinetics in detail and the mechanism of reactions which are catalyzed by small amounts of onium salts such as quaternary ammonium or phosphonium compounds. Brandstrbm and Makosza also made major Initial contributions in the understanding of such reactions and the application thereof in various synthetic reactions. A generally accepted phase-transfer reaction scheme is shown in... 

The use of solid-liquid phase transfer catalysis in the conjunction with bis(carbonylimidazolides) (i) bis(p-nitrophenylcarbonates) 10) as developed by Fre-chet for the synthesis of novel tertiary copolycarbonates. The instability of tertiary chloroformates renders tertiary polycarbonates inaccessible through conventional chloroformate monomers or intermediates. The bis(carbonylmiidazoiide) monomer was shown to polymerize with both tertiary (3) and secondary alcohols (lljy demonstrating the utility of the method in forming polycarbonates fi-om less reactive steri-cally hindered monomers. Most of the examples reported involved benzene dimethanol derivatives, or 1,4-butynediol, indicating that an adjacent site of unsaturation may activate the alcohol in the solid-liquid phase transfer reaction scheme. 

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