(621ck) Hyperbranched Polyethylene-Supported L-Proline: A Highly Selective and Recyclable Organocatalyst for Asymmetricaldol Reactions

L-proline and L-proline derivatives have been widely used as efficient organocatalysts in asymmetric C–C bond formation reactions. However, it is still a challenge to develop recyclable L-proline catalysts with high activity and selectivity. In this work, a novel hyperbranched polyethylene (HBPE)-supported L-proline was prepared via a bottom-up copolymerization strategy. The copolymerization of ethylene and protected proline acrylate comonomer with cationic Pd-diimine catalyst was conducted, followed by de-protection of the proline groups. Well-defined HBPE copolymers having molecular weights of 10.3-50.3 kDa containing 3.2-15.6 L-proline molecules per HBPE polymer chain were synthesized. The effects of the L-proline amount, the molecular weight of HBPE, and the type of solvents on the catalyst performance were investigated. The HBPE catalysts were found to be efficient in asymmetric aldol reactions of p-nitrobenzaldehyde (p-NBA) or benzaldehyde derivatives with cyclohexanone. High p-NBA conversions of up to 99% and excellent product selectivities with anti/syn = 98/2 and ee>99% were achieved. Good catalytic performance with different substrates demonstrates the universality of this catalyst in asymmetric Aldol reaction. Moreover, the HBPE catalysts could be easily reclaimed by simply adding water into the product. The reclaimed catalysts could then be reused for multiple times with only a slight decline in reactivity and selectivity.