Asymmetric phase-transfer catalysis carbanions

The representative reaction system applied in asymmetric phase-transfer catalysis is the biphasic system composed of an organic phase containing an acidic methylene or methine compound and an electrophile, and an aqueous or solid phase of inorganic base such as alkaline metal (Na, K, Cs) hydroxide or carbonate. The key reactive intermediate in this type of reaction is the onium carbanion species, mostly onium enolate or nitronate, which reacts with the electrophile in the organic phase to afford the product. 

The fate of the onium carbanion Q+R incorporated into the organic phase depends on the electrophilic reaction partner. The most studied area in the asymmetric phase-transfer catalysis is that of asymmetric alkylation of active methylene or methine compounds with alkyl halides, in an irreversible manner. The reaction mechanism illustrated above is exemplified by the asymmetric alkylation of glycine Schiff base (Scheme 1.5) [8]. 

One class of application that readily highlights the enormous potential of asymmetric phase-transfer catalysis is the stereoselective ot-alkylation of different carbanion nucleophiles, in particular enolates. Although these types of transformations are most important in organic chemistry, there are stiU only a limited number of stereoselective catalytic methods available and the use of chiral PTCs represents one of the most versatile strategies to achieve such transformations. One example of special interest is the asymmetric a-alkylation of glycine Schiffbase 374 (Scheme 85)... 

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