(790d) Hyperbranched Polyethylene-Supported L-Proline As An Efficient and Recyclable Catalyst for Aldol Reactions
L-proline and L-proline derivatives have been widely used as efficient organocatalysts in asymmetric C–C bond formation reactions. However, it is a challenge to develop recyclable L-proline catalysts with high activity and selectivity. In this work, a hyperbranched polyethylene (HBPE)-supported L-proline is developed to serve as an efficient and recyclable catalyst for the asymmetric Aldol reactions. The chain walking copolymerization of ethylene and a protected L-proline acrylate monomer with cationic Pd(II)-diimine catalysts following by a de-protection of the protected L-proline functionalities render successfully the HBPE-supported L-proline catalysts. The effects of the Pd catalysts and the comonomer concentration on the polymerization were investigated. The HBPE catalysts having number-average molecular weights in range of 10 200-27 100 g/mol, polydispertisies of 1.07-1.63, and proline groups of 3.2-14.5 per polymer chain were synthesized. They were used as catalysts in the Aldol reactions of p-nitrobenzaldehyde and cyclohexanone in tetrahydrofuran. High reaction yields and stereoselectivities were achieved with the HBPE catalysts having lower molecular weights. The HBPE catalysts could be easily recovered by adding water into the production solution.