Enantioselective Henry Reaction Catalyzed by “Ship in a Bottle” Complexes

Abstract

Two chiral Schiff-base complexes of copper(II) have been successfully encapsulated inside the cavity of zeolite-NaY via a "ship in a bottle" synthesis method. The presence of the two complexes inside the cages of zeolite-Y has been confirmed based on various spectrochemical and physicochemical techniques, viz. FTIR, UV-vis/DRS, ESR, XPS, CV, EDX, SEM, and TGA. Zeolite-encapsulated chiral copper(II) Schiff-base complexes are found to give a high-enantioselective (84% ee, R conformation) nitro-aldol product at -20 °C. The encapsulated copper complexes are found to show higher catalytic efficiency than their homogeneous counterparts under identical conditions. Density functional theory (DFT) calculation has been implemented to understand the effect of the zeolite matrix on structural, electronic, and reactivity properties of the synthesized complexes. Theoretical calculation predicts that upon encapsulation into the zeolite matrix the Cu center becomes more susceptible to nucleophilic attack, favoring a nitro-aldol reaction. A plausible mechanism is suggested based on the experimental and theoretical results. The structures of reaction intermediates and transition state(s) involved in the catalytic cycle are derived using DFT.

Non-Hydrothermal Synthesis of Cu(I)-Microleaves from Cu(II)-Nanorods

“…Taking 4‐NB and nitromehane (CH 3 NO 2 ) as reactant the nitro‐aldol reaction was performed with 10 mg of Cu(II)‐SB‐NR at room temperature in ethanol as solvent. From our previous experience we observed that nitro‐aldol reaction proceeds well at low temperature . With the nanocatalyst, Cu(II)‐SB‐NR nitro‐aldol product was achieved in good yield (65 %) after 12 h at room temperature.…”

“…Bands at 886–887 cm −1 were due to CH 2 ‐vibration which appeared very sharp and as doublet in case of Cu(I)‐SB‐NR@Si, Figure e. Peaks between 1000–1200 cm −1 in Cu(I)‐SB‐NR@Si were attributed to Si−O linkage, Figure e . Comparison of Raman intensity of nanorod structure with that of the Si‐modified Cu‐Schiff base having lamellar morphology; it was observed that Raman signals were greatly enhanced due to surface enhancement.…”

“…In recent times Cu‐Schiff base metal complexes has been well explored as catalyst for nitro‐aldol reaction . Cu‐Schiff base complexes either in homogeneous or in heterogeneous phase acts as excellent catalyst for such organic transformation.…”

“…Cu‐Schiff base complex are important class of metal complex which has gained lot of application in the field of catalysis . From our group we have reported for supported Cu‐Schiff base complexes encapsulated in zeolite‐Y . Those catalysts were also used as recyclable chiral catalyst for asymmetric Henry reaction .…”

“…From our group we have reported for supported Cu‐Schiff base complexes encapsulated in zeolite‐Y . Those catalysts were also used as recyclable chiral catalyst for asymmetric Henry reaction . However, to best of our knowledge Cu(I)/Cu(II)‐Schiff base metal complexes in nano or micro dimensions are not explored for such reaction .…”