A series of isoreticular chiral metal-organic frameworks as a tunable platform for asymmetric catalysis

Abstract

Metal-organic frameworks, built by bridging metal ions with organic linkers, represent a new class of porous hybrid materials with attractive tunability in compositions, structures and functions. In particular, the mild conditions typically employed for their synthesis allow for the functionalization of their building blocks, and thus the rational design of novel materials. Here we demonstrate the systematic design of eight mesoporous chiral metal-organic frameworks, with the framework formula [LCu2(solvent)2] (where L is a chiral tetracarboxylate ligand derived from 1,1'-bi-2-naphthol), that have the same structures but channels of different sizes. Chiral Lewis acid catalysts were generated by postsynthesis functionalization with Ti(OiPr)4, and the resulting materials proved to be highly active asymmetric catalysts for diethylzinc and alkynylzinc additions, which converted aromatic aldehydes into chiral secondary alcohols. The enantioselectivities of these reactions can be modified by tuning the size of the channels, which alters the diffusion rates of the organic substrates.