Heterogenization of chiral mono oxazoline ligands by grafting onto mesoporous silica MCM-41 and their application in copper-catalyzed asymmetric allylic oxidation of cyclic olefins

Abstract

A series of chiral 4-oxazolinylaniline ligands 8 were conveniently synthesized on a gram scale from inexpensive and commercially available 4-aminobenzoic acid in four steps. The obtained organic chiral ligands have been covalently grafted onto ordered mesoporous silicas MCM-41 and the resulting inorganic–organic hybrid materials have been characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), powder X-ray diffraction, BET and BJH nitrogen adsorption–desorption methods, energy-dispersive X-ray spectroscopy (EDX), CHN analysis, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The catalytic and induced asymmetric effects of the chiral copper (I) complexes of these new chiral supported heterogeneous catalysts on the asymmetric allylic oxidation of cycloolefins were investigated under different conditions. Reactions using the catalyst exhibited moderate to good enantioselectivities, up to 80%, and good yields, up to 95% better than the corresponding homogeneous reaction. The catalyst could be recovered easily and reused five times without remarkable loss of reactivity, yield, or enantioselectivity. This is, to the best of our knowledge, the first heterogenization of chiral 4-oxazolinylaniline ligands on an inorganic (silica) surface and their application as a heterogeneous catalyst in the asymmetric Kharash–Sosnovsky reaction.