Ti sites grafted on MCM-41, pretreated at different temperatures, have been modified with chiral tartrates, leading to different species. Those solids are active catalyst for the epoxidation of styrene with tert-butyl hydroperoxide, although their efficiency is not optimal, mainly due to the subsequent epoxide rearrangement to phenyl acetaldehyde and further oxidation of it. Styrene oxide is obtained with moderate enantioselectivity, in the range of 55-62% e.e., and the best overall results correspond to the use of MCM-41 calcined at the lowest temperature (550ºC). These results are much better with regard to styrene oxide selectivity than those obtained in homogeneous phase under the same conditions, whereas enantioselectivity is similar. The heterogeneous catalysts are recoverable, but only partially due to a loss in activity, although enantioselectivity is kept at similar levels, demonstrating the stability of the supported Ti-tartrate species.

Please cite this article as: Marwa Fadhli, Ilyes Khedher, José M.Fraile, Modified Ta/MCM-41 catalysts for enantioselective oxidation of thioanisole, Journal of Molecular Catalysis A: Chemical http://dx.Abstract 3 Highlights-Ta has been grafted on MCM41 and modified with chiral tartrates.-Ta catalyst are active in the oxidation of thioanisole.-They are more efficient with H 2 O 2 than with alkyl hydroperoxides.-Best results are obtained with diisopropyl tartrate (up to 25% ee).-Catalysts are recyclable with respect to activity but without enantioselectivity. 4 Abstract Ta-MCM41 catalysts have been prepared by grafting of Ta(OEt) 5 on MCM41, pre-calcined at three different temperatures (550, 650 and 750°C). These solids have been modified with two chiral ligands: R-(+)-diethyl L-tartrate (DET) and R-(+)-diisopropyl L-tartrate (DIPT). The formation of the chiral tantalum species and their influence on the structure of MCM41 have been studied by several characterization techniques, such as XRD, FTIR, N 2 adsorption isotherms and MAS-NMR. The grafted tantalum species are always much less active than the homogeneous analogues in the oxidation of thioanisole (methyl phenyl sulfide) with either aqueous H 2 O 2 or alkyl (tert-butyl, TBHP, or cumyl, CHP) hydroperoxides. The enantioselectivities obtained with the heterogeneous catalysts are also always lower than those obtained with the homogeneous ones under the same conditions, and the best results are obtained with the DIPT-modified solids, up to 25% ee. The calcination temperature of the support and the nature of the oxidant are also parameters that significantly affect the enantioselectivity. The recovered catalysts show an increase in the catalytic activity and a decrease in the enantioselectivity, in agreement with a loss of chiral tartrate, whereas the Si-O-Ta bond remains stable and recoverable.

Chiral TieMCM-41 and TaeMCM-41 catalysts have been prepared by grafting of Ti(O i Pr) 4 and Ta(OEt) 5 and the modification withIn general, the solid catalysts are more active and selective than their homogeneous counterparts in the epoxidation of styrene with tert-butyl hydroperoxide. The enantioselectivities depend on both the nature of the chiral ligand and the calcination temperature of the support, as it is supposed this controls the type of surface species that are formed. The best result of 71% ee is obtained with DIPTeTaeMCM 550 and is the first example of the use of a Ta catalyst for the enantioselective epoxidation of unfunctionalized alkenes. Nonetheless, the recovered Ta catalysts are less active and selective.

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