HETEROGENEOUS ASYMMETRIC CATALYSTS: Strategies for Achieving High Enantioselection

Heterogeneous asymmetric catalysis remains a topic of intense research interest because control of enantioselectivity represents one of the major synthetic challenges today. Successful strategies to achieve high enantioselection have been developed using three key approaches: (a) adsorption of chiral modifiers onto an active metal surface, which is found to be particularly effective for enantioselective hydrogenation; (b) covalent tethering of homogeneous catalysts; and (c) electrostatic interaction between a negatively charged framework and a cation. Other approaches have been advocated, e.g., encapsulation, but in general these lead to lower enantioselectivity for these heterogeneous asymmetric systems when compared with the equivalent homogeneous catalyst. This review covers the strategies for achieving high enantioselection as well as recent interesting features concerning confinement effects, the inversion of enantioselectivity, and specific cases in which enantioselection increases with conversion, all being factors that can guide researchers to identifying improved systems.