Enhanced enantioselectivity in heterogeneous manganese-catalyzed asymmetric epoxidation with nanosheets modified amino acid Schiff bases as ligands by modulating the orientation and the arrangement order

Abstract

Catalytic enantioselective epoxidation of olefins plays an important role in the production of optically-active epoxy. Transition-metal complexes prove efficient for the catalytic epoxidation of un-functionalized olefins by employing privileged chiral ligands with rotational symmetry and bulky substituents, which usually require complicated multistep synthesis and are often very expensive. Here, this work proposes an efficient strategy to promote the enantioselectivity of Mn(III)-catalyzed indene epoxidation, which simply utilizes the nanosheets of layered double hydroxides (LDHs) to modify α-amino acid derivatives as chiral ligands, achieving an enantioselectivity of more than 91.0%, much higher than that (<35%) in the previous reports using chiral ligands without any rotational-symmetry. By virtue of “flexible” interlayer spaces of LDHs, intercalated L-tert-leucine, L-valine, L-leucine, and L-phenylalanine anions have further been derived in situ to L-tert-leucine, L-valine, L-leucine, and L-phenylalanine Schiff base anions. Over the Mn(III) coordinated with L-phenylalanine Schiff base anion intercalated LDHs, an indene conversion of 81.0 ± 1.0%, a chem-selectivity of 89.0 ± 1.0%, and an enantiomeric excesses (ee) value of 94.5 ± 0.5% for (1S,2R)-1,2-epoxyindane have been afforded. The spatial effects of LDH nanosheets on the enantioselectivity have been investigated, focusing on the interlayer orientation and arrangement order of intercalated amino acid derivative anions. The strategy has been proved valid for the epoxidation of functionalized olefins, which is more challenging, including allylic alcohols, chalcone, and chromene derivatives.