Carbene enzyme-inspired confined catalysis in metal-organic framework for enhanced selectivity

Abstract

Developing catalysts with precise micro-environments to regulate catalytic performance has been a continuous endeavor. A carbene enzyme-inspired crystalline porous metal–organic framework (MOF) material incorporating dense and ordered imidazolium cations was synthesized and named IMI-1. In a proper alkaline environment, IMI-1 was found to have dynamic N-heterocyclic carbene (NHC) on its inner pore walls. This catalytic material exhibits potential for various NHC-catalyzed reactions. After applied to annulation reactions between chalcone and 3-aryl acrolein, IMI-1 has shown a notable selectivity with a high diastereomeric ratio up to 139:1 (trans/cis) for p-alkoxylphenyl acrolein, a top-level performance compared to existing literature. Critical transition states that determine reaction selectivity were assessed with semi-empirical computations, and initial findings suggested that a unique micro-environment near the NHC catalytic center is the primary determinant of this enhancement. Similar to natural enzymes, in IMI-1 pores, the Breslow intermediate, a key molecule attached to the pore wall, was uniquely constrained to a conformation that significantly differentiated the energies required for generating different geometric isomers. This regulative capability of IMI-1 has similarities to natural enzymes and could shed some light on the future design of catalysts.