Asymmetric Cascade Catalysis with Chiral Polyoxometalate-Based Frameworks: Sequential Direct Aldol and Epoxidation Reactions

Catalytic asymmetric cascade reactions in which the substrates are transferred through well-choreographed consecutive independent steps by a single catalyst have received increasing interest. By incorporating a chiral organocatalyst pyrrolidine, an oxidation catalyst polyoxometalate and a Lewis acid catalyst copper(II) into a single porous coordination network (PCN), two enantiomorph CuW-PYIs (PYI=pyrrolidine-2-yl-imidazole) with novel Kagom 8 lattice were achieved and applied in the conversion of aldehyde with ketone into value-added chiral epoxy ketones in a one-pot procedure. The chiral pyrrolidine organocatalysts interact with the carbonyl group of ketones throughout the reaction, which is beneficial for achieving compatibility between the reaction intermediates and synergy of the multiple catalytic cycles, thus driving the direct aldol/epoxidation cascade reaction in an orderly and asymmetrical way. Moreover, the two-center catalysis principle was well demonstrated in the direct aldol/epoxidation cascade reactions.