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Keywords:

  • chirality;
  • copper;
  • density functional calculations;
  • ionic liquids;
  • zeolites

Abstract

Emerging trends in recyclable homogeneous chiral catalysts and their application in asymmetric synthesis are prompting a renewed interest in the field of catalysis as well as in industry. However, owing to fatal disadvantages of the homogeneous catalyst, they are not easily recoverable and recyclable. Herein, a comparison is made of the recyclability and stability of a homogeneous chiral CuII Schiff base complex of general formula CuL, in which L=C22H24N2O4, with that of a zeolite-Y-confined heterogeneous catalyst. On the basis of our experimental evidence, we demonstrate how self-decomposition and bimetallic formation disrupt the catalytic activity of a homogeneous catalyst. By exploiting the special structure of faujasite zeolite, self-decomposition of the chiral catalyst is fully controlled and efficiently used for the asymmetric nitroaldol reaction. The use of ionic liquid results in catalytic enhancement and provides a convenient way to recycle the homogeneous catalyst up to three cycles. When encapsulated in a cavity of faujasite by means of the “ship-in-a-bottle” synthesis method, it shows much better catalytic activity with enhanced enantioselectivity, recyclability, and stability. The heterogeneous catalyst is recycled up to nine cycles and retains the catalytic activity for a longer period of time relative to its homogeneous counterpart. The synthesized heterogeneous CuII complex provides the nitroaldol product with a maximum of 84 % yield and 87 % enantiomeric excess (ee; R isomer). Structural and reactivity differences between the homogeneous and heterogeneous chiral complexes are substantiated by density functional theory (DFT) calculations.