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

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.

Chiral covalent organic frameworks: design, synthesis and property

“…Functional porous materials such as MOFs, [137][138][139][140] inorganic zeolites 141,142 and functional cages 15,[143][144][145] used for heterogeneous catalysis have been developed for decades. As emerging porous materials, COFs exhibit great potential to be functionalized as effective and robust heterogeneous catalysts for organo-, electro-, or photocatalytic applications.…”

Enantioselective three-component aminomethylation of α-diazo ketones with alcohols and 1,3,5-triazines

“…1a, Eq 1). On the other hand, while the activation of stable imines by chiral Brønsted acid catalysts has been extensively investigated in a variety of enantioselective Mannich reactions [25][26][27][28][29][30][31][32][33][34][35][36] , this electrophile-based activating protocol has not been applied to aminomethylation reactions with the in situ-generated formaldimines, probably owing to their generally unstable nature and the lack of steric environment on the carbon atom for efficient stereocontrols (Fig. 1a, Eq 2).…”

A Pseudo Five-Fold Symmetrical Ligand Drives Geometric Frustration in Porous Metal-Organic and Hydrogen Bonded Frameworks

“…One example is the work by Gong et al 33 where a highly distorted tetrahedral tetracarboxylic acid was combined with a tetrapod calixarene type metal node. The tetrapod nature of the metal node leads to difficulty in translating this motif into three dimensions, due to the acute angles between adjacent linkers in combination with the incompatible geometry of the organic 4-c linker.…”