Phase-Transfer Catalysis: Chiral Phase-Transfer-Catalyzed Formation of Carbon-Carbon Bonds

Abstract

It is only within the last two decades that heterogeneous enantioselective syntheses involving chiral phase-transfer catalysts (quaternary ammonium and phosphonium salts, macrocyclic miltidentate ligands, polypeptides, polymer-bound quaternary salts and macrocyclic multidentate ligands, etc.) were initiated. The technique has been applied to a wide variety of organic reactions. These include reductions of carbonyls with sodium borohydride [1], epoxidations of α, β-unsaturated carbonyls [2], oxidation of ketones to α-hydroxyketone [3], alkylations [4] involving carbon and nitrogen acids, Michael reactions on α, β-unsatuarated carbonyls [5], Darzens reactions [6], Cory-Chaykovshy reactions [6], chloroform addition reactions to carbonyls [6], and dichlorocarbene additions to alkenes [6]. The asymmetric induction accompanying each of the above reactions has been reported in terms of optical rotation, optical purity, and enantiomeric excess. In some cases only chemical yields were reported with no indication of the degree of asymmetric induction. The reported enantiomeric excesses have varied from just a few percent to the mid to upper nineties. In several cases, the reported enantiomeric excesses were found to be incorrect due to contamination by degradation products of the chiral catalyst [40]. Since each of the above reaction types has been extensively reviewed [6], this chapter will not attempt to cover the broad topic of chiral phase transfer catalysis (PTC).