Thermal and magnetic dual-responsive l-proline nanohybrids for aqueous asymmetric aldol reaction

Thermal and magnetic dual-responsive l-proline nanohybrids (l-Pro-based-TMNHs) were prepared by grafting thermal-responsive thiol-terminated P(NIPAM-co-l-ProlA)-b-POEGA-SH (PNLPO-SH) to vinyl-functionalized Fe3O4@SiO2-MPS through the thiol-ene click chemistry. The catalytic activity, selectivity and recyclability of l-Pro-based-TMNHs were evaluated in a model asymmetric aldol reaction of cyclohexanone and 4-nitrobenzaldehyde in water. Thermal responsiveness of the l-Pro-based-TMNHs induced self-assembly to produce a hydrophobic microenvironment in aqueous medium for catalyzing asymmetric aldol reaction. The catalytic performance can be tuned by the molar ratio of PNLPO-SH to Fe3O4@SiO2-MPS, and l-Pro-based-TMNHs-2 with a ratio of 1:6 exhibited best catalytic activity and selectivity. At 50 °C and 5 mol% catalyst loading, 94% conv., 81/19 anti/syn and 95% ee were obtained. On the other hand, magnetic responsiveness of the l-Pro-based-TMNHs facilitates catalyst separation and reuse in the presence of an external magnetic field, and these results indicated no loss of both catalytic activity and selectivity associated with the course of recycling.