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
cyclization 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)2BF4 and [Rh­(nbd)­Cl]2/AgSbF6, the MOF precatalysts I·Rh­(BF4) and I·Rh­(SbF6) efficiently catalyzed highly enantioselective (up to 99%
ee) reductive cyclization and Alder-ene cycloisomerization of 1,6-enynes,
respectively. I·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 MOF which prevents bimolecular
catalyst deactivation pathways. However, I·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·Rh,
effectively catalyzed Pauson–Khand cyclization reactions between
1,6-enynes and carbon monoxide at 10 times higher activity than the
homogeneous control. II·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 MOF-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 metallocycle transition states/intermediates in asymmetric
intramolecular cyclization reactions.