L-Proline-Grafted Mesoporous Silica with Alternating Hydrophobic and Hydrophilic Blocks to Promote Direct Asymmetric Aldol and Knoevenagel-Michael Cascade Reactions

Promotion of heterogeneous asymmetric catalysis is of major interest in the asymmetric catalysis field. In this work, a novel strategy for the synthesis of L-proline-grafted mesoporous silica with alternating hydrophobic and hydrophilic blocks to promote the heterogeneous asymmetric catalysis was reported. The surface synergies in the neat environment and the interface acceleration in aqueous medium thereby fostered high catalytic activities and enantioselectivity in the direct aldol reaction and the Knoevenagel Michael cascade reaction. The L-proline loading could be reduced to as low as 0.63 mol %, giving 95% ee for anti-isomers and 81% ee for syn-isomers in the catalytic asymmetric aldol reaction of nitrobenzaldehyde and cyclohexanone, which was hard to accomplish on the homogeneous counterpart. In the direct asymmetric aldol reaction of ethyl-2-oxoacetate and cyclohexanone, 82% yield in 24 h and 90% ee were achieved. More exciting, the catalysts were applied to more exigent reactions. As an example, in the Knoevenagel-Michael cascade reaction, 8596 yield in 10 h and up to 91% ee was achieved.