Chiral enhancement in the confined space of zeolites for the asymmetric synthesis of β-hydroxy nitroalkanes

(1S,2S)-N1,N2-Bis(3-chlorobenzyl)cyclohexane-1,2-diamine 1a′ and (1S,2S)-N1,N2-bis(4-chlorobenzyl)cyclohexane-1,2-diamine 1b′ were used to prepare chiral Cu(II) complexes Cu-Y-1a, Cu-Y-1b, Cu-mZSM5-1a, and Cu-mZSM5-1b by a flexible ligand method using copper exchanged zeolite Y and mesoporous ZSM-5. The characterization of zeolite supported complexes was performed by microanalysis, IR-, diffuse reflectance spectroscopy (DRS), EPR spectroscopy, specific rotation and thermogravimetric analysis (TGA). The catalytic activity of these supported complexes was explored for the asymmetric nitroaldol reaction of various aldehydes with nitromethane at 0 °C. Excellent yields (up to 99%) of β-hydroxy nitroalkane with an ee of up to 94% were achieved in the case of benzaldehyde as substrate. Significantly, the performance of the supported catalyst was better in terms of enantioselectivity than the complex under homogenous conditions. The supported catalysts were recycled four times with no observable loss in performance and no leaching of the catalytically active complex during the nitroaldol reaction.