Ionic, Core-Corona Polymer Microsphere-Immobilized MacMillan Catalyst for Asymmetric Diels-Alder Reaction

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.

Recent Development of Recoverable MacMillan Catalyst in Asymmetric Organic Transformations

“…Recently, Ullah reported an ionic core‐corona polymer microsphere immobilized chiral MacMillan organocatalyst 91 for the asymmetric Diels‐Alder reaction [76] . The authors synthesized the ionic core‐corona polymer microsphere immobilized chiral MacMillan organocatalyst 91 by a neutralization reaction of core‐corona microsphere with sulfonic acid (CCM−SO 3 H) and the imidazolidinone catalyst.…”

Synthesis of homo- and copolymer containing sulfonic acid via atom transfer radical polymerization

“…p -Phenyl styrenesulfonate (SS) was synthesized according to the previously reported procedure [ 60 , 61 ]. Yield: 3.082 g, 65%, R f = 0.54.…”

Asymmetric Diels‐Alder Reaction Catalyzed by Facile Recoverable Ionically Core‐Corona Polymer Microsphere‐Immobilized MacMillan Catalyst

“…The degree of immobilization was slightly increased when low crosslinked polymer microsphere 4d 10 sB 20 was used instead of that with 20 mol% of crosslinking 4d 20 sC 20 (entry 1 vs 7). [32] A significant improvement was observed when the reaction solvent was changed from THF/ MeOH to CH 2 Cl 2 /MeOH mixed solvent (entry 7 vs 8). The catalyst contents in 6 determined from the recovered catalyst were in the range of 0.462 and 1.14 mmol g À 1 .…”

“…In communication, we have successfully synthesized ionically core-corona polymer microsphere-immobilized MacMillan catalyst using styrene comonomer in the core by the neutralization reaction of core-corona polymer microsphere having sulfonic acid moiety with the precursor of MacMillan catalyst. [32] The effect of the solvent and the length of corona was evaluated in the asymmetric Diels-Alder reaction of trans-cinnamaldehyde and 1,3-cyclopentadiene.…”