Thermo-responsive Self-assembled Nanoreactors Accelerate Catalytic Reaction In Water

How to efficiently perform organic catalytic reaction in water is a challenging task, since most of substrates and catalysts are insoluble in water. In this paper, we designed and prepared a series of surfactant-type catalysts that have thermos-responsive self-assembled function in water. The catalysts were self-assembly orderly through hydrophobic interactions and salvation in water, forming micelles with a hydrophilic shell and a hydrophobic core. The hydrophobic core created a hydrophobic cavity in aqueous phase, providing confined hydrophobic microenvironment for aqueous organic reactions. Thus, the self-assembled micelles behaved as “nanoreactors” to solve limited mass transfer associated with aqueous organic reaction, making the traditional organic reaction efficient in water. In addition, the catalysts had thermos-responsive water-soluble characteristics. After completion of the reaction, the surfactant-type catalysts could be precipitated from the aqueous reaction system for reused by heating above their LCST.Thermo-responsive PNIPAAm has been covalently grafted onto hydrophobic Salen CoIII complex, providing the “thermo-responsive self-assembled” Salen CoⅢ complexes. At room temperature, the catalysts could be efficiently catalyzed asymmetric Baeyer-Villiger oxidation of cyclobutanone with H2O2 as oxidant, and the conversion rate was over 90%. After the reaction, the catalysts could be facilely recovered from the aqueous reaction system by heating above their LCST, and were reused for six times without loss of activities and selelctivities.A novel L-proline catalyst with thermo-responsive surfactant properties was prepared by covalently appending a pristine L-proline on the hydrophobic block of thermo-responsive amphiphilic copolymers of poly(N-isopropylacrylamide-co-N,N-dimethyl acrylamide)(poly(NIPAAM-co-DMAAM)). Characterization results suggested self-assembly behavior of the catalysts in water at room temperature. The self-assembled micelle effectively accelerated the direct asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde in water, leading to almost quantitative conversion of substrate. Moreover, the catalysts could be easily recovered by thermo-controlled separation and be efficiently reused for several cycles.