Enantioselective Conjugate Addition Reactions
via Phase-transfer Catalysis
The formation of a chiral ion pair between a nucleophile of anionic nature and a chiral cation (typically an ammonium salt) can be used as the vehicle to asymmetric catalysis for carrying out enantioselective Michael reactions under phase-transfer conditions. In this chapter, the methods developed for carrying out Michael-type reactions using catalysts operating through this kind of mechanism will be presented, showing that this possibility is a good alternative to other organocatalytic reactions, especially with regard to the use of pronucleophiles which are not susceptible to be employed in enamine/iminium catalysis or under H-bonding activation. Applying this concept, a variety of methodologies for carrying out Michael reactions using enolizable carbonyl compounds as Michael donors and a variety of different activated olefins as Michael acceptors have been developed by several research groups, achieving excellent levels of stereocontrol in many cases. As it has been done in the previous chapters, all the examples reported will be organized according to the nature of the nucleophile and the Michael acceptor employed.