Permanent porous hydrogen-bonded frameworks with two types of Br\xf8nsted acid sites for heterogeneous asymmetric catalysis

Wei Gong,Dandan Chu,Yan Liu,Hong Jiang,Yong Cui,Xu Chen

doi:10.1038/s41467-019-08416-6

Abstract

The search for porous materials with strong Brønsted acid sites for challenging reactions has long been of significant interest, but it remains a formidable synthetic challenge. Here we demonstrate a cage extension strategy to construct chiral permanent porous hydrogen-bonded frameworks with strong Brønsted acid groups for heterogeneous asymmetric catalysis. We report the synthesis of two octahedral coordination cages using enantiopure 4,4’,6,6’-tetra(benzoate) ligand of 1,1’-spirobiindane-7,7’-phosphoric acid and Ni4/Co4-p-tert-butylsulfonylcalix[4]arene clusters. Intercage hydrogen-bonds and hydrophobic interactions between tert-butyl groups direct the hierarchical assembly of the cages into a permanent porous material. The chiral phosphoric acid-containing frameworks can be high efficient and recyclable heterogeneous Brønsted acid catalysts for asymmetric [3+2] coupling of indoles with quinone monoimine and Friedel-Crafts alkylations of indole with aryl aldimines. The afforded enantioselectivities (up to 99.9% ee) surpass those of the homogeneous counterparts and compare favorably with those of the most enantioselective homogeneous phosphoric acid catalysts reported to date.

Highlights

The search for porous materials with strong Brønsted acid sites for challenging reactions has long been of significant interest, but it remains a formidable synthetic challenge
We demonstrated that strong chiral Brønsted acid sites (BASs) can be directly built in hydrogenbonded organic frameworks (HOFs) through a cage extension strategy for heterogeneous asymmetric catalysis
We report how to address this issue through careful design of coordination cages covering with two types of acid groups capable of forming strong hydrogen bonds to another, as exemplified in the context of hierarchical construction of chiral permanent porous HOFs based on octahedral cages

Summary

Introduction

The search for porous materials with strong Brønsted acid sites for challenging reactions has long been of significant interest, but it remains a formidable synthetic challenge. We demonstrate a cage extension strategy to construct chiral permanent porous hydrogenbonded frameworks with strong Brønsted acid groups for heterogeneous asymmetric catalysis. Metal-organic cages are molecular containers[27,28,29,30,31] and are attractive building blocks for construction of supramolecular porous materials as they provide high chemical and structural diversity and possess inherent porosity and functions[27,32,33,34] Their structures are intrinsically porous, the cage solids generally collapse upon solvent removal due to the lack of strong bonding between cages[27,28,29,30,31,32,33,34]. We report how to address this issue through careful design of coordination cages covering with two types of acid groups capable of forming strong hydrogen bonds to another, as exemplified in the context of hierarchical construction of chiral permanent porous HOFs based on octahedral cages. The phosphoric acid-containing solids can be highly enantioselective heterogeneous catalysts for [3+2]

Methods

Results

Conclusion