Asymmetric Epoxidation of Chromenes Using Manganese(III) Complexes with Novel Chiral Salen-like Schiff Base Ligands

Abstract

Three novel chiral salen-like schiff base ligands and their Mn(III) complexes containing different amino acid unit have been synthesized and characterized. Asymmetric epoxidation reactions show these complexes are effective catalysts for the chromenes with buffer NaOCl as terminal oxidant and pyridine N-oxide as co-catalyst in the presence of ionic liquid. Good-to -excellent enantioselectivity and acceptable yields can be obtained under optimum reaction conditions. Catalyst 4c gives the highest ee (95%) for 6-chloro-2,2-dimethylchromene among these catalytic performances. Furthermore, compared the enantioselectivity of catalyst 4c with the other two catalysts 4a and 4b, the positive experimental results suggest that the steric effect of the ligands plays an important role in the asymmetric catalysis.

Dual stereocontrolled alkylation of aldehydes with polystyrene-supported nickel complexes derived from α-amino amides

“…9 From the different families of chiral ligands used in asymmetric catalysis, nitrogen-containing ligands have received important attention over the last years. 10 In this regard, simple a-amino amides 1, having an amino group and an easily ionisable N-H amide subunit, can be compared to amino alcohols and structures of this class have been recently reported as good ligands for different transformations, such as the Ru(II)-catalyzed asymmetric transfer hydrogenation of acetophenone, 11 epoxidation of chromenes 12 and the conjugate addition of dialkylzinc to chalcones. 13 In previous contributions from our group, Ni(II) complexes of amino amides were shown to be able to efficiently catalyse the addition of dialkylzincs to aldehydes, achieving an excellent dual stereocontrol just by a proper adjustment of the stoichiometry of those Ni complexes.…”