Helical Channel Mesoporous Materials with Embedded Magnetic Iron Nanoparticles: Chiral Recognition and Implications in Asymmetric Olefin Epoxidation

Abstract

AbstractIn this work a mesoporous nanocomposite material comprising helical chiral channels and with embedded magnetic iron oxide nanoparticles in its Si‐based MCM‐41 type framework was synthesized. Afterwards, a bipyridine derivative was grafted to the inner surface used as ligand to coordinate a molybdenum(II) precursor complex. The Mo loading was found to be 2.42 wt% Mo, corresponding to 0.26 mmolMo g−1. The successful preparation of this helical mesoporous material was evidenced by an extensive characterization process using powder XRD, SEM/TEM analysis, FT‐IR and Raman spectroscopies, TGA analysis and also by SQUID measurements. Selective adsorption of enantiopure D‐ and L‐phenylalanine shows that the material has a preference for the D‐ over the L‐ enantiomer. This provides evidence that the channels are chiral (although the particles are not) and that it is useful for chiral recognition applications. It also sets the explanation to the good results achieved in the selective epoxidation of olefins. The resulting material was tested as catalytic precursor in the epoxidation of cis‐cyclooctene, styrene, R‐(+)‐limonene and trans‐hex‐2‐en‐1‐ol, using tert‐butyl hydroperoxide (tbhp) as oxygen source. The catalytic studies show that the synthesized material yields selectively the desired epoxides of the tested substrates with very good results, especially at higher temperatures and using toluene as solvent. The major achievement of the catalyst was an outstanding stereocontrol of the reaction products imposed by the confined space of the helical chiral channels. Therefore these materials offer an important contribution to asymmetric catalysis and to fields where chiral recognition is a relevant concept.magnified image