Continuous asymmetric Michael addition of ketones to β-nitroolefins over (1R,2R)-(+)-1,2-DPEN-modified sulfonic acid resin

A trifunctional catalyst was successfully prepared by bonding (1R,2R)-(+)-1,2-DPEN to sulfonic acid resin. The catalyst was characterized by elemental analysis, thermogravimetric (TG) analysis and infrared (IR) spectroscopy. The results indicated the coexistence of sulfonic, sulfonamido and primary amino groups on the surface of the resin. Based on the IR spectroscopy of the catalyst treated with a solution of acetone and beta-nitrostyrene in toluene, the catalytic mechanisms were proposed. It was found that these three groups had a synergistic effect. Subsequently, the continuous Michael addition of acetone to beta-nitrostyrene was achieved in a fixed-bed reactor over this catalyst and the reaction parameters were optimized. Under the optimized conditions moderate beta-nitrostyrene conversion (65.5%) and excellent enantioselectivity (93.0%) were obtained. Finally, the generality of the catalyst was evaluated with the Michael addition of aldehydes or ketones to beta-nitroolefins, and the catalyst exhibited moderate to excellent enantioselectivity (81.6% to 99.0% ee) except for the addition of isobutylaldehyde to beta-nitrostyrene.