Kalluri V. S. Ranganath

Superparamagnetic nanoparticles for asymmetric catalysis—a perfect match

The aim of this perspective is to highlight the potential application of (superpara)magnetic nanoparticles in asymmetric catalysis. The unique combination of high enantioselectivity and enhanced reactivity combined with its recyclability and ease of separation makes this chiral nanotechnology one of the most promising strategies for the formation of enantiomerically enriched compounds on an industrial scale. This perspective focuses on the most representative examples of this young and emerging research field and highlights recent achievements and future prospects of magnetic nanoparticles in asymmetric catalysis.

Comparison of Superparamagnetic Fe3O4-Supported N-Heterocyclic Carbene-Based Catalysts for Enantioselective Allylation

Functional magnetic nanoparticles (MNPs) are becoming increasingly important for a wide range of applications, which include nanoelectronics, chemical sensing, information storage, and medical diagnostics. Because of their chemical activity, robustness, and recyclability, MNPs have become increasingly popular for applications in (asymmetric) catalysis, either as simple supports or as active components. Transition-metal catalysts and organocatalysts have been immobilized on MNPs not only to facilitate catalyst recovery and separation but also to achieve new levels of reactivity and selectivity. 5] N-Heterocyclic carbenes (NHCs) are especially versatile and attractive ligands and organocatalysts, and molecular Pd–NHC complexes immobilized on the surface of magnetite nanoparticles (NPs) were reported to catalyze Suzuki, Sonogashira, and Heck coupling reactions. Recently, we reported the use of NHCs as chiral modifiers for Pd/Fe3O4 NPs and their application in asymmetric a-arylations. The combination of NHCs and MNPs represents an attractive platform for the design of novel, active NP catalysts. Herein, we report on the immobilization of an enantiomerically pure imidazolium salt on the surface of magnetite (Fe3O4) and the formation of three catalysts derived thereof: an organocatalyst (1), a molecular Pd complex (2), and Pd NPs (3). All these catalysts were successfully applied in the allylation of 4nitrobenzaldehyde. The supported chiral NHC 1 was prepared by the concise route outlined in Scheme 1. As 1,1’-binaphthyl units are frequently used chiral elements in catalyst design, we developed an (R)-binol-derived imidazolium salt. An organic silane was utilized for the immobilization of the chiral imidazolium salt on the surface of the magnetite NPs because silanes are known to have a large affinity for coordinatively unsaturated surface sites of metal oxide particles. 11] Specifically, modified MNPs were prepared by the treatment of Fe3O4 with 3-chloropropyltriethoxysilane followed by reaction with imidazole 4, which provides functionalized NPs 1. Besides other methods, successful coupling was indicated by elemental analysis, which showed 1 to have a nitrogen content of 0.58 %. This material was intended for use as an organocatalyst (see below). In addition, 1 was converted to 2, which bears Pd–NHC complexes on the surface of magnetite NPs (Scheme 1), by following the literature procedure reported by Gao et al. The catalyst was characterized by performing scanning electron microscopy–energy-dispersive Xray spectroscopy (SEM–EDX) analysis, and the Pd content of the catalyst was found to be 6.2 %. The catalyst was also characterized by using X-ray photoelectron spectroscopy (XPS) to establish the oxidation state of Pd in the catalyst. The binding energy of Pd 3d5/2 in the catalyst was found to be 337.5 eV, which is lower than that obtained for [Pd(OAc)2] (338. 2 eV) and which indicated the formation of a Pd complex. Finally, Pd NPs on the surface of NHC-modified MNPs (3) were prepared according to the literature procedure reported by Alper et al. for platinum NPs supported on Fe3O4 NPs. [12] Magnetite-supported Pd NPs were prepared according to this procedure by mixing 1, the magnetite NPs modified by imidazole 4, with K2PdCl4, and then reducing the Pd salt with an excess of hydrazine. An ion exchange of chloride anions with Pd salts took place before the Pd was directly reduced on the surface of magnetite NPs (Scheme 1). The supported Pd NPs Scheme 1. Preparation of three different Fe3O4 NP catalysts: a) an immobilized organocatalyst, b) a molecular Pd catalyst, and c) a Pd NP catalyst.

Selective Acetylation of 5-Numbered Aromatic Heterocycle Compounds Using Metal-Exchanged Clay Catalysts

Acetylation of 5-numbered aromatic heterocycle compounds under very mild conditions to 2-acetyl derivatives selectively in good yields using metal-exchanged clays as catalysts is described for the first time.