Ionic Liquids in Transition Metal-Catalyzed Enantioselective Reactions

Abstract

Transition metal-catalyzed asymmetric reactions provide a powerful access to the optically active molecules that serve as precursors to pharmaceutically significant compounds. However, separation and recycling of these often expensive chiral catalysts are rather difficult and thus limit their applications in industry. As one of the most promising solutions to these problems, immobilization of a chiral homogeneous catalyst can, in principle, facilitate its separation and recycling, and thus is of considerable interest to both academia and industry. Although many methods have been developed for the immobilization of chiral catalysts via attachment of the catalyst onto a solid support via covalent attachment or noncovalent interactions, obvious decrease in catalytic activity and/or stereoselectivity is often observed due to mass transfer and accessibility of the active sites. Alternatively, chiral catalysts have also been immobilized by the use of aqueous, fluorous, supercritical CO2 (scCO2) and ionic liquid phase systems. In this chapter, we present the recent significant achievements of the transition metal-catalyzed asymmetric reactions in ionic liquids (ILs). Their unique properties render ILs ideal “mobile supports” for the immobilization of chiral transition metal catalysts without laborious catalyst modification, and consequently facilitating catalyst separation and recycling through a biphasic operation. Moreover, chiral transition metal catalysts in ILs often show improved catalytic activities and/or stereoselectivities. Representative examples suggested that asymmetric catalysis in ILs would combine the advantages of both traditional homogeneous and heterogeneous catalysis. The positive “IL effects” together with different strategies and concepts (e.g., catalyst with ionic tag and ionic liquid-supported phase catalysis) of IL use in catalytic applications will also be discussed.