Phase chiral onium

The reaction conditions are rather mild, so that the possibility of side reactions, such as catalyst decomposition, is considerably reduced. The major challenge associated with these reactions is an absence of prochirality in the anionic species. Namely, the chiral onium anion must discriminate the enantiotopic face of the distant electronically neutral reaction partner in the organic phase. 

Chiral Induction.—As with many synthetic techniques in organic chemistry, PTC methods have been under investigation to determine whether asymmetric induction (chiral selection) can be achieved. In this case chiral onium salts are necessary, and the systems investigated to date have been quaternary derivatives (23) of A -methyl ephedrine. These salts have been used to catalyse two-phase metal boro-hydride reductions of ketones, and the chiral tetra-alkylammonium borohydride ion pair does accelerate this reaction " in organic solvents. However, either no asymmetric reduction is observed, " or low optical purity of products is achieved (less than 15% enantiomer excess). - A low chiral selectivity is also observed in... 

Chiral Onium Salts (Phase-Transfer Reactions)... 

I 74 Chiral Onium Salts (Phase-Transfer Reactions) (a) Chiral Ammonium Fluorides... 

Considerable efforts made for the synthesis of biologically relevant molecules by means of asymmetric phase-transfer catalysis are summarized in this chapter. Because the phase-transfer reaction is usually insensitive to the contamination of air, moisture, and even acidic or inorganic-salt impurities, and it is set up with simple and user-friendly protocols. It is recognized as one of the easiest methods for large-scale, stereoselective production of functional molecules as exemplified by the studies reported from pharmaceutical companies. In addition, ready accessibility of chiral onium salts as a catalyst facilitates an initial trial in... 

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. 

Asymmetric phase-transfer catalysis using chiral nonra-cemic onium salts or crown ethers has now grown into a practical method whereby a large number of reactions can be performed and some optically pure compounds can be produced effectively on a large scale. 

Recent advances in asymmetric phase-transfer catalysis using chiral nonracemic onium salts and crown ethers 07AG(E)4222. 

Over the past decades, quaternary ammonium- and phosphonium salts have been widely employed as effective phase-transfer catalysts in reactions between snb-stances located in different immiscible phases. Recently, efforts have been made to unlock the full potential of chiral non-racemic onium salts as versatile catalysts for asymmetric carbon-carbon bond formation. These reactions can be condncted nnder mild biphasic conditions and the phase-transfer catalysts can often be derived from readily available naturally occurring alkaloids. The reaction proceeds since the catalyst forms a well-defined chiral ion pair with the electrophile. As a result one enantiotopic face is shielded and enantioselective carbon-carbon bond formation can be realized. 

In the present paper we wish to report on the most significant examples of chiral polymer-supported onium salts and polymeric amines which have been used in heterophase reactions. In the first part attention will be paid to describe the Michael-type reactions that have been rather extensively studied by several research groups. Even though they constitute an example of application placed right at the borderline of base catalyzed reactions performed under conventional and phase transfer conditions, they appear worthy of comment by virtue of the achieved valuable and reproducible optical yields in the chemical transformation of several prochiral substrates. 

Recently in the search for new and more versatile chiral phase transfer catalysts, onium salts 14 and 15 derived from L-methionine... 

The rather exciting and stimulating "phase transfer" synthetic procedure based on the use of polymer supported active functional moieties, such as onium groups, crown ethers, cryptands and poly-glymes, becomes even more naive when the active sites are bound to chiral matrices, whose prevalent chirality can be either intrinsi-... 

Based on the design of chiral quaternary onium bromides and chlorides as chiral phase-transfer catalysts, chiral quaternary onium fluorides [5] and phenoxides [6] have been developed as chiral base catalysts for homogeneous reactions. 

Although quaternary onium bromides and chlorides as phase-transfer catalysts are generally beUeved to require base additives for phase-transfer reactions of active methylene and methine compounds, which were discussed above, we have recently discovered an hitherto unknown base-free neutral phase-transfer reaction system in asymmetric conjugate additions (Scheme 14.5) [23]. The reactions were efficiently promoted by chiral bifunctional ammonium bromide (S)-7 under neutral conditions with water-rich biphasic solvent. The role of hydroxy groups in the bifunctional catalyst was clearly shown in the transition-state model of the reaction based on the single-crystal X-ray structure of ammonium amide [23b] and nitro-nate [23c]. 

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