Confinement of acyclic amino acids inside metal-organic frameworks with topology-varied asymmetric catalysis performances.

With cyclic proline as the sole exception, almost all acyclic amino acids suffer from poor catalytic performances owing to conformational flexibility and weak substrate affinity. In this study, polymorphic zirconium-based metal-organic frameworks (PCN-777 and UMCM-309) featuring distinct topological architectures are employed as catalytic platforms for the confinement of acyclic amino acids. PCN-777-confined catalysts not only exhibit 158% increased catalytic turnover frequencies, but crucially achieve 2.7-fold enhancements in product enantiomeric excess for the asymmetric aldol reaction. However, fewer performance improvements are observed for the polymorphic UMCM-309-confined catalysts. Electrostatic potential calculations and probe experiment results consistently exclude divergency of charge distribution onto the metal nodes. In return, molecular docking simulations identify divergent steric constraints depending on the topologies of the MOFs as the primary factor dominating performance differences. The proposed simple yet effective strategy not only significantly improves the limited catalytic performance of raw amino acids, but also elucidates a previously overlooked steric mechanism fundamental to small-molecule asymmetric catalysis.