Cavity-induced enantioselectivity reversal in a chiral metal-organic framework Bronsted acid catalyst

A pair of highly porous chiral metal-organic frameworks (CMOFs 1 and 2) were constructed from [Cu-2(carboxylate)(4)] secondary building units (SBUs) and chiral 3,3',6,6'-or 4,4',6,6'-tetra(benzoate) ligands derived from 1,1'-binaphthyl-2,2'-phosphoric acid. Both 1 and 2 were active catalysts for Friedel-Crafts reactions between indole and imines. Interestingly, the 1-catalyzed asymmetric reactions yielded the major enantiomers of the opposite chirality to those afforded by the corresponding homogeneous catalyst. Structural analyses and QM/MM calculations revealed that the flip of product handedness results from the chiral environment of CMOF-1 cavity, similar to enzymatic catalysis in which the product stereoselectivity is determined by the enzyme pocket.