Construction Of Light/Thermo-responsive Nanoreactors For Asymmetric Catalysis In Water

Water undoubtedly is an ideal solvent for the asymmetric catalysis from economical and environmental points of view.Yet,successful performing asymmetric reaction in water was restricted by the poor water-solubility of substrates and/or catalysts.In nature,enzymes are efficient biocatalysts that regulate a broad variety of biochemical reactions in aqueous environments in nature.The catalytic properties result from guided three-dimensional folding of a single protein chain,which gives a compartmentalized structure with hydrophobic cores surrounded by hydrophilic shells.Inspired by nature,we have designed a series of light/thermo-responsive random copolymers.The copolymers self-folded in reaction systems through intrachain metal complexation and/orintramolecularhydrophobicinteraction,forming compartmentalized single-chain polymeric nanoparticles?SCPNs?with a hydrophobic,catalytically active interior.The compartmentalized SCPNs acted as catalytic nanoreactors,enriching catalytic motifs and substrates in a confined hydrophobic interior,which thus accelerated the asymmetric catalysis in water.After reaction,they could be facilely recovered for reuse by simple light-or thermo-stimuli.?1?Chiral salen Cr?III?complexes?Cr?salen??were encapsulated in thermo-responsive polymer nanoreactors through folding an amphiphilic random copolymer of poly?N-isopropylacrylamide-co-IL/Cr?salen??(poly?NIPAAM-co-IL/Cr?salen??around Cr?salen?in water.The resulted catalytic nanoreactors exhibited several advantages over the traditional Cr?salen?system for asymmetric epoxidation of alkenes in water.First,they were dispersed in water,behaving as a quasi-homogeneous catalyst for the aqueous asymmetric epoxidation.Second,they effectively sequestered substrates from the surrounding environment,creating a highly concentrated environment for efficient catalysis.Third,water was excluded from the nanoreactor,minimizing the undesired hydrolysis of epoxides.As a result,the compartmentalized catalysts mediated aqueous asymmetric epoxidation with unprecedented yields?92–95%?and enantioselectivities?ee,92–99%?,whereas traditional Cr?salen?catalyst was far less efficient?4-12%of yields and 29-44%of ee?.Thanks to the themo-sensitive of NIPAAm shell,the catalytic nanoreactor could be facilely recovered for reuse by thermo-controlled separation.?2?Metalloenzyme is a source of inspiration for chemists who attempt to create versatile synthetic catalysts for aqueous catalysis.Herein,we imparted metalloenzyme-like characteristics to chiral Fe^II-oxazoline complex by incorporating Fe?II?ion into chiral oxazoline-containing discrete self-folded polymer,to realize the highly enantioselective sulfa-Michael addition?SMA?in water.Intrachain Fe^II-oxazoline complexation together with hydrophobic interaction triggered the self-folding of oxazoline-containing single polymeric chain in water.The formed Fe^II-folded SCPNs significantly accelerated the aqueous asymmetric SMA reaction via self-folded hydrophobic compartment around the catalytic sites,reminiscent of metalloenzymatic catalysis.Only 3.0 mol%of the Fe^II-folded SCPNs was sufficient to give almost quantitative yield?90-96%?of a wide range of chiral?-thioketones with high enantioselectivity?90–99%?in water without the use of any additives.In addition,they could be facilely recovered for reuse by simple thermo-controlled separation due to thermo-responsive properties.?3?Thermocontrolled catalysis often acquires high costs in energy consumption,if operated in a large volume.Herein,we have developed a series of photoresponsive,amphiphilic diblock copolymers of PS_xO_y which composed of a photoresponsive nitrospiropyran block and hydrophobic chiral oxazoline ligand block via RAFT technology.Intrachain Ti^IV-oxazoline complexation together with hydrophobic interaction triggered the self-folding of the PS_xO_y in water.The formed Ti^IV-folded SCPNs exhibited visible-light-controlled water-solubility behavior due to the negative photochromism of nitrospiropyran shell.In the dark,the nitrospiropyran block was in MC?hydrophilic?form.The SCPNs,which composed of hydrophilic MC surfaces and hydrophobic Ti^IV-oxazoline cores,acted as water-soluble catalytic nanoreactor to accelerate the asymmetric sulfoxidation in water.Under visible-light irradiation,the catalytic nanoreactor became hydrophobic due to the re-formation of nitrospiropyran?hydrophobic?.They were thus collapsed and spontaneously precipitated from the aqueous system for recovery.The photo-responsive Ti^IV-folded SCPNs thus typically realized the visible-light-controlled reaction-separation for asymmetric sulfoxidation in water.