Enantioselective Addition of Malonates and [beta]-Keto Esters to Nitroalkenes over an Organonickel-Functionalized Periodic Mesoporous Organosilica

A periodic mesoporous organosilica (PMO) with chiral cyclohexyldiamine-based nickel(II) complexes incorporated within the silica framework was prepared through a co-condensation of (1R,2R)-cyclohexyldiamine-derived silane and Ph-bridged silane followed by complexation of nickel(II) bromide in the presence of (1R,2R)-N,N'-dibenzylcyclohexyldiamine. Structural analyses by X-ray powder diffraction, nitrogen sorption and transmission electron microscopy disclosed its orderly mesostructure while characterization by solid-state NMR and X-ray photoelectron spectroscopy demonstrated the well-defined single-site chiral bis(cyclohexyldiamine)-based nickel(II) active centers incorporated within the PMO material. In particular, as a heterogeneous chiral catalyst, this periodic mesoporous organosilica showed high catalytic activity and excellent enantioselectivity in asymmetric Michael addition of 1,3-dicarbonyl compounds to nitroalkenes (more than 92% conversions and up to 99% ee values). More importantly, this heterogeneous catalyst could be recovered easily and reused repeatedly nine times without obviously affecting its ee value, showing good potential for industrial applications.