New ternary immobilization of chiral sulfonato-(salen)manganese(III) complex for aqueous asymmetric oxidation reactions

This article reports our study on asymmetric epoxidation of unfunctionalized alkenes and asymmetric oxidation of sulfides catalyzed by a new triply immobilized (salen)Mn(III) system, when commercial sodium hypochlorite and hydrogen peroxide were loaded as terminal oxidant, respectively. A chiral sulfonato-(salen)Mn(III) complex was grafted onto a novel polysiloxane, then axially coordinated by 3-aminopropyl functionalized silica gel (40–63 μm, 1 mmol g−1 NH2 loading) as a secondary anchoring mode, and eventually dispersed into ionic liquid BMImX (BMIm+ = 1-n-butyl-3-methylimidazolium; X− = PF6−, BF4−, NO3−) for purpose of catalyst recycling. Catalysts were fully characterized by 1H NMR, FT-IR, molar conductivity, elemental analysis, nitrogen adsorption, aqueous particle size and zeta potential analysis, along with XRD. Interestingly, lateral immobilization led to a novel catalyst structure featuring parallel stacking of one-dimensional planers, whereas the lateral together with axial immobilization made supported complex nano-sized crystallite particles (diameter 24.2 nm). After stepwise screening, doubly-immobilized catalyst and BMImBF4, in combination, showed best enantiomeric excess (e.e.) values and most stable recycling behavior in two kinds of catalysis, but generally sulfoxidation had better turnover frequencies than epoxidation reaction.

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► Lateral, axial and package immobilization of (salen)Mn(III) in oxidation.
► Lateral and axial immobilization made catalyst nano-sized particles.
► Imidazolium ionic liquid BMImBF4 showed best enantioselectivities.
► Styrene, 6-cyano-2,2-dimethylchromene and thioanisole were suitable substrates.