Convenient immobilization of chiral imidazolidinone to hollow polystyrene nanospheres (HPNs) for efficient heterogeneous enantioselective Friedel-Crafts reaction

Abstract

The current anchoring of chiral imidazolidinones to solid supports to achieve heterogeneous catalysis usually involves the 5–8 steps for the synthesis of ready-to-anchor precursors bearing anchoring points and following immobilization to solid supports, leading to a high-cost for large-scale application. In this study, the anchoring steps of ready-to-anchor chiral imidazolidinone, (2S,5S)-5-benzyl-2-(tert‑ butyl)-3-methylimidazolidin-4-one (MacM), is shortened, including the four-step synthesis of MacM bearing bromine anchoring point in 18.3 % overall yield using phenylalanine as a raw material and covalent immobilization to well-shaped hollow polystyrene nanospheres (HPNs) bearing –B(OH)2 groups via Suzuki coupling in 1.2 mmol g-1 loading of MacM with an utilization of 96 %. The as-fabricated HPNs-supported chiral imidazolidinone (MacM@HPNs) possesses the morphology of hollow interior, mesoporous shell and thin shell thickness, providing fast mass transfer for reactants to quickly access to the active sites of anchored MacM. In the heterogeneous Friedel-Crafts reaction of acroleins and N-methylindoles, MacM@HPNs shows the similar moderate to good yields (79–88 %) in good to excellent selectivities (83–97 % ee) as homogeneous MacM. Furthermore, the catalyst maintains its catalytic performances without a significant decline in yield and enantioselectivity during the reuses. Overall, this study presents a more economical strategy for anchoring chiral imidazolinones to well-shaped polystyrene with the architectural structures suitable for achieving fast mass transfer of reactants, offering a reference way to anchor other chiral imidazolidinones and achieve the efficient synthesis of optically active molecules.