Reusable chiral salen Mn(III) complexes with phase transfer capability efficiently catalyze the asymmetric epoxidation of unfunctionalized olefins with NaClO

• Chiral salen Mn(III) polymers interleaved with PEG-IL moiety were first prepared.
• Amphipathic PEG moiety endowed the catalysts with built-in phase transfer capability.
• Excellent conversion (91%) and ee (93%) was achieved over PICP-3 with chromene-based substrates.
• Phase-transfer catalysts can be easily recovered for reuse by solvent precipitation.

A series of chiral salen Mn(III) polymers with build-in phase transfer capability was prepared by bridging the chiral salen Mn(III) units with polyethylene glycol (PEG)-based di-imidazolium ionic liquid (IL) side by side. Technologies of characterization suggested the alternation of intact chiral salen Mn(III) unit with PEG-based dicationic imidazolium IL moiety in the rigid one-dimension polymers. Amphipathic nature of PEG-based di-imidazolium IL moiety allowed the obtained catalysts to undergo inherent phase transfer catalysis in asymmetric epoxidation of unfunctionalized olefins with NaClO, which in turn increased the reaction rate of epoxidation in water–dichloromethane biphasic system. Decreasing total length of polyether chain leads to an increase in built-in phase transfer capability of corresponding complex, which further enhances the catalytic performance. 91–97% of conversion was obtained in the enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complex where number of ethylene oxide unit is 3 within 60 min, which is significant higher than that observed for the neat complex (56–74%) and the ICP (62–85%). High enantiomeric excess (ee) for the epoxides (in the range of 67–93%) was also achieved, except for styrene (ee, 34%) and α-methylstyrene (ee, 41%). Furthermore, the efficient phase transfer catalysts could be easily recovered by solvent precipitation and be recycled for seven times without significant loss of the activity and enantioselectivity.

Polymeric chiral salen Mn(III) complexes with built-in phase-transfer capability were first prepared and proved highly efficient and reusable in asymmetric epoxidation of unfunctionalized olefins with aqueous NaClO. Almost quantitative conversion (91%) with excellent enantioselectivity (93% ee value) was achieved in asymmetric epoxidation of chromene-based substrates with PICP-3.Figure optionsDownload high-quality image (259 K)Download as PowerPoint slide