Construction of hierarchically chiral metal–organic frameworks for fast and mild asymmetric catalysis

Abstract

The production of hierarchically porous metal–organic frameworks (HP-MOFs) while retaining their crystallinity utilizing the mixed-linker approach is still challenging due to the perturbation of the MOF during its formation, which can lead to partial amorphization and even collapse of the framework. Herein, a green method was applied for the synthesis of the first chiral Zr-fumarate framework, CMOF(ASP), through the replacement of fumaric acid in MOF-801 with L-aspartic acid. The one-pot insertion of Zr sites as Lewis acid sites, –OH and NH₃⁺ sites as Brønsted acid sites and –NH₂ as Lewis base sites enabled the CMOF-801(ASP) framework to promote asymmetric catalysis for the production of valuable products with maximum efficiency within only a few minutes. It was found that the superior catalytic activity of CMOF-801(ASP) originated from its abundant active sites, which were created via the missing-cluster defect strategy, significantly facilitating the diffusion and activation of the substrates. The results showed that CMOF-801(ASP) could achieve the maximum conversion and enantiomeric excess (ee%) in ring-opening and nitroaldol reactions within only 5 and 15 min, respectively, at room temperature. Also, chiral oxazolidinone could be synthesized with a conversion of 90% and ee (98%) by applying the heterogeneous catalyst with three functional catalytic sites in solvent-free cycloaddition at ambient pressure without a cocatalyst. Subsequently, CMOF-801(ASP) was reused for 5 consecutive cycles, while retaining its crystallinity. This work clearly showed that the simultaneous insertion of cluster defects and Lewis basic chiral linkers can create multiple catalytic sites in MOFs for fast and green asymmetric catalysis under mild conditions.