Chiral Ni-Schiff Base Complexes inside Zeolite-Y and Their Application in Asymmetric Henry Reaction: Effect of Initial Activation with Microwave Irradiation

Chiral Ni(II)-Schiff base complexes synthesized inside the cavity of zeolite-Y were used as heterogeneous chiral catalyst for asymmetric Henry reaction. Synthesized catalysts were characterized using various spectrochemical and physicochemical techniques. Solid state NMR analysis has been used to confirm the internal location of the metal complexes. To the best of our knowledge MAS NMR has not been used to characterize chiral diamagnetic Ni2+-Schiff base complex inside zeolite-Y. The catalytic activities of the materials were dependent on temperature, solvent, and amount of catalyst. High catalytic transformation of aldehydes with nitromethane to nitro-aldol product (92% yield and 83% ee, S-isomer) was achieved at -10 degrees C. Initial activation of the reaction with microwave irradiation for 15 min leads to a substantial decrease in the reaction time in comparison to normal stirring. Heterogeneous catalysts were found to be advantageous over the homogeneous counterparts in terms of recyclability of the catalyst. Most importantly, product selectivity, percentage-yield, and enantioselectivity were found to be high with the heterogeneous catalyst. The catalytic activities of the metal complexes were influenced by the structural modification of the Schiff base ligands. Calculation of the energy barrier using density functional theory (DFT) suggests that the activation barrier is less in the case of the encapsulated complexes.