Identification of Reaction Sites on Metal-Organic Framework-Based Asymmetric Catalysts for Carbonyl-Ene Reactions

The characteristics of catalytic sites in metal-organic framework (MOF)-based catalysts could be approximately classified by their location, i.e., the inside of the pore and/or on the surface of the crystal. This classification of catalytic sites in a single-MOF crystal has been widely overlooked. In particular, in a chiral MOF, the environment of any specific reaction site will vary depending on its location. Thus, pin-pointing the reaction site for a MOF-based heterogeneous catalyst is an intriguing issue. In this study, the active site of a MOF-based catalyst is revealed after a thorough investigation comparing substrate size versus reaction rate for two distinct mechanisms of carbonyl-ene reactions. Both Zn-mediated stoichiometric carbonyl-ene reactions and Ti-catalyzed carbonyl-ene reactions were performed separately and compared using homogeneous and heterogeneous media. These results could provide a clear answer to the question of locating the reaction sites within the MOF. Through this work, it became evident that the entire MOF crystal is effective; however, the inside of the pore is an important contributor to having chirality control in the stoichiometric reaction. In addition, for a catalytic reaction, our findings suggest that the substrate size is mostly irrelevant as catalysis can simply take place on the surface of the crystals. Thus, comparison of the reaction rate and substrate size may not be a valid method to ascertain whether a reaction occurs inside the MOF or on its surface. This conclusion is further supported by the effect of particle size on the reaction efficiency and the enantioselectivity along with visualization of the guest-accessible space using two-photon fluorescence microscopy.