Robust, Chiral, and Porous BINAP-Based Metal-Organic Frameworks for Highly Enantioselective Cyclization Reactions

We report here the design of BINAP-based metal-organic frameworks and their postsynthetic metalation with Rh complexes to afford highly active and enantioselective single-site solid catalysts for the asymmetric cydization reactions of 1,6-enynes. Robust, chiral, and porous Zr-MOFs of UiO topology, BINAP-MOF (I) or BINAP-dMOF (II), were prepared using purely BINAP-derived dicarboxylate linkers or by mixing BINAP-derived linkers with unfunctionalized dicarboxylate linkers, respectively. Upon metalation with Rh(nbd)(2)BF4 and [Rh(nbd)Cl](2)/AgSbF6, the MOP precatalysts I center dot Rh(BF4) and I center dot Rh(SbF6) efficiently catalyzed highly enantioselective (up to 99% cc) reductive cyclization and Alder-ene cycloisomerization of 1,6-enynes, respectively. I center dot Rh catalysts afforded cyclization products at comparable enantiomeric excesses (ee's) and 4-7 times higher catalytic activity than the homogeneous controls, likely a result of catalytic site isolation in the MOP which prevents bimolecular catalyst deactivation pathways. However, I center dot Rh is inactive in the more sterically encumbered Pauson-Khand reactions between 1,6-enynes and carbon monoxide. In contrast, with a more open structure, Rh-functionalized BINAP-dMOF, II center dot Rh, effectively catalyzed Pauson-Khand cyclization reactions between 1,6-enynes and carbon monoxide at 10 times higher activity than the homogeneous control. II center dot Rh was readily recovered and used three times in Pauson-Khand cyclization reactions without deterioration of yields or ee's. Our work has expanded the scope of MOP-catalyzed asymmetric reactions and showed that the mixed linker strategy can effectively enlarge the open space around the catalytic active site to accommodate highly sterically demanding polycyclic metallocyde transition states/intermediates in asymmetric intramolecular cyclization reactions.