Multifunctional Magnetic Chiral HKUST MOF Decorated by Triazine, Fe₃O₄, and Cu(<scp>l</scp>-Proline)₂ Complex for Green and Mild Asymmetric Catalysis

Abstract

A multifunctional magnetic chiral metal–organic framework (MOF) was developed for asymmetric applications by utilizing strategies of chiralization and multifunctionalization. Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 was employed as a chiral secondary agent to synthesize a chiral hybrid nanocomposite within a MOF. The use of a chiral secondary agent efficiently induces chirality in an achiral MOF structure that cannot be directly chiralized. The HKUST-1@Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 nanocomposite was afforded by first anchoring chiral Cu( l -proline) 2 on the Triazine/Fe 3 O 4 @SiO 2 –NH 2 surface and then encapsulating the Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 nanoparticles with HKUST-1 via in situ ultrasonication synthesis. In the synthesis of the HKUST-1@Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 nanocomposite, Cu( l -proline) 2 was used as a chiral complex due to its Lewis acidic/basic hydroxyl groups, carboxylate carbonyl functional groups acting as Lewis bases, an active Cu site functioning as a Lewis acid center, and azine groups of TCT acting as Lewis bases, all synergistically interacting with the Lewis acidity of the Cu centers in HKUST-1. To assess these synergic effects, HKUST-1@Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 was used in the formation of an asymmetric C–C bond in nitroaldol condensation and asymmetric cycloaddition of CO 2 to epoxides. The findings demonstrated that under mild and green conditions, in both the asymmetric nitroalcohol condensation and the asymmetric cycloaddition of CO 2, HKUST-1@Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 had better enantioselectivity than the Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 nanoparticles and a higher selectivity toward β-nitroalcohol and cyclic carbonate over the pure HKUST-1. Despite its simple and easy synthesis, HKUST-1@Cu( l -proline) 2 -Triazine/Fe 3 O 4 @SiO 2 –NH 2 exhibited exceptional performance in the asymmetric nitroaldol condensation and asymmetric cycloaddition of CO 2 to epoxides. Additionally, the mechanism of the reactions was depicted with reference to the total energy of the reactants, intermediates, and products.

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