Polyesters Containing Chiral Imidazolidinone Salts in Polymer Main Chain: Heterogeneous Organocatalysts for the Asymmetric Diels-Alder Reaction

摘要

Novel main-chain polyesters functionalized with chiral imidazolidinone salts were successfully synthesized. Polycondensation of a chiral imidazolidinone dimer bearing two hydroxyphenyl groups with selected achiral dicarboxylic acid chlorides followed by the addition of an acid afforded polyesters with chiral imidazolidinone salts incorporated into the polymer main chain. Main-chain ionic polyesters functionalized with chiral imidazolidinone salts in the polymer main chain were synthesized by the neutralization polymerization of a chiral imidazolidinone dimer with selected aromatic disulfonic acids. These polyesters were used as heterogeneous organocatalysts in the asymmetric Diels-Alder reaction of trans-cinnamaldehyde and 1,3-cyclopentadiene. By optimizing the polymer main-chain structure, enantioselectivity of up to 97% was achieved, which is higher than that attained using the corresponding monomeric and dimeric chiral imidazolidinone salts. These heterogeneous organocatalysts were easily recovered from the reaction mixture and reused several times without significant loss of the enantioselectivity.