CO2-Switchable Single-Chain Polymeric Nanoparticles Enable Gas-Controllable Reaction Separation for Asymmetric Catalysis in Water

Abstract

Water-soluble single-chain polymeric nanoparticles (SCPNs), which isolated catalytic sites within a hydrophobic interior, made water-incompatible organometallic catalysis highly efficient in water. However, it is still a great challenge to conveniently control their hydrophilicity, so as to combine reactivity and recovery of the catalytic SCPNs in aqueous systems. Herein, we have developed a series of catalytic SCPNs, which possessed CO2-switchable hydrophilic/hydrophobic behavior, to realize the gas-controlled reaction and separation for asymmetric sulfa-Michael addition (SMA) in water. A novel series of CO2-switchable random copolymers were thus synthesized by copolymerization of CO2-responsive amidine derivatives with hydrophobic chiral salen FeIII monomers via reversible addition-fragmentation chain transfer polymerization. Characterization suggested their CO2-controlled self-collapse behavior in water due to CO2-switched change in hydrophilicity/hydrophobicity of the amidine moiety. The resultant CO2-switchable SCPNs provided hydrophobic, catalytic compartments for asymmetric SMA in water upon CO2 addition, giving various chiral β-keto sulfides with almost quantitative yields (90–98%) and high enantioselectivities (93–99%). When CO2 is removed by N2 bubbling, they were collapsed and spontaneously precipitated from the aqueous system for steady reuse. The gas-controlled reaction-separation approach provides an energy-efficient way to combine reactivity and recovery of catalytic SCPNs in aqueous systems, which should be quite practical in large-scale industrial applications.