Polymer-Supported Chiral-at-Metal Lewis Acid Catalysts

Abstract

The covalent immobilization of a chiral-at-metal bis-cyclometalated iridium(III) catalyst on a solid support is reported, and its catalytic activity has been investigated. As a catalyst immobilization strategy, a catalyst precursor was tethered to polystyrene macrobeads through an ester or amide linkage and subsequently converted to the immobilized active chiral Lewis acid by treatment with a Brønsted acid. The amide-linked catalyst displays high robustness and can be recycled multiple times without deterioration of enantioselectivity and only a gradual loss of catalytic activity. Chiral Lewis acid activity was demonstrated as an example for the enantioselective Friedel–Crafts alkylation of indole with an α,β-unsaturated 2-acyl imidazole and for the enantioselective Diels–Alder reactions of an α,β-unsaturated 2-acyl imidazole with 2,3-dihydrofuran or isoprene.