Abstract
The improvement of the catalytic activity of a heterogeneous chiral catalyst is one of the most critical issues, as are its recovery and reuse. The design of a heterogeneous chiral catalyst, including the immobilization method and the support polymer, is of significance for the catalytic activity in asymmetric reactions. An ionic, core-corona polymer microsphere-immobilized MacMillan catalyst (ICCC) was successfully synthesized by the neutralization reaction of sulfonic acid functionalized core-corona polymer microsphere (CCM–SO3H) with a chiral imidazolidinone precursor. We selected the core-corona polymer microsphere as the polymer support for the improvement of catalytic activity and recovery. The MacMillan catalyst was immobilized onto the pendant position of the corona with ionic bonding. ICCC exhibited excellent enantioselectivity up to 92% enantiomeric excess (ee) (exo) and >99% ee (endo) in the asymmetric Diels-Alder (DA) reaction of (E)-cinnamaldehyde and 1,3-cyclopentadiene. ICCC was quantitatively recovered by centrifugation because of the microsphere structure. The recovered ICCC was reused without significant loss of the enantioselectivity.
Highlights
Chiral organocatalysis is one of the most attractive methods for the production of optically active medical and pharmaceutical products due to advantages such as being metal-free, exhibiting higher stability against moisture and oxygen, and facilitating experimental operation [1,2,3]
We describe the synthesis of an ionic, core-corona polymer microsphere-immobilized MacMillan catalyst (ICCC) by the neutralization reaction of the core-corona polymer microsphere having a sulfonic acid moiety (CCM–SO3 H) with the precursor of the MacMillan catalyst
Benzyl chloride-functionalized polymer microsphere having phenylsulfonate 1 was prepared by the precipitation polymerization of styrene (St), divinylbenzene (DVB), and 4-vinylbenzyl chloride (VBC) [53]
Summary
Chiral organocatalysis is one of the most attractive methods for the production of optically active medical and pharmaceutical products due to advantages such as being metal-free, exhibiting higher stability against moisture and oxygen, and facilitating experimental operation [1,2,3]. Chiral imidazolidinones and their salts, so-called MacMillan catalysts, have been applied to various asymmetric reactions via HOMO or LUMO activation mechanisms, such as the Diels-Alder (DA) reaction [4], 1,3-dipolar cycloaddition [5], Friedel-Crafts alkylation [6], indole alkylation [7], the α-halogenation of aldehydes [8,9], direct aldol reaction [10], intramolecular Michael reaction [11], and the epoxidation [12] of aldehydes. The covalent immobilization of MacMillan catalyst onto a heterogeneous support represents an attractive approach to solving the issues, in regard to its sustainability.
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