Comparison of Ta–MCM-41 and Ti–MCM-41 as catalysts for the enantioselective epoxidation of styrene with TBHP

Abstract

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.

Bulky olefin epoxidation under mild conditions over Mo-based oxide catalysts

“…These mechanistic considerations may also apply for W-catalysed olefin epoxidation, 89–92 and other metals ( e.g. , active oxidizing species possessing the moiety {M-OOR} were reported for Ta-containing catalysts in olefin epoxidation 93 ). Hence, one cannot exclude the possible roles of Mo and M sites of the Mo,M oxides, as discussed below.…”

“…[85][86][87][88] This leads to the formation of a moiety of the type {Mo-OOR} responsible for the oxygen atom transfer step to the olefin, giving the epoxide product (plus the Catalysis Science & Technology Paper coproduct of TBHP, namely tert-butanol). These mechanistic considerations may also apply for W-catalysed olefin epoxidation, [89][90][91][92] and other metals (e.g., active oxidizing species possessing the moiety {M-OOR} were reported for Ta-containing catalysts in olefin epoxidation 93 ). Hence, one cannot exclude the possible roles of Mo and M sites of the Mo,M oxides, as discussed below.…”

The Control of the Coordination Chemistry for the Genesis of Heterogeneous Catalytically Active Sites in Oxidation Reactions**

“…A solid equivalent of the Sharpless chiral epoxidation of alkenes was reported over a mesoporous MCM-41 silica support. [81] Two enantioselective Ti-MCM-41 and Ta-MCM-41 catalysts were prepared by grafting the alkoxide precursors, e.g. Ti(OiPr) 4 and Ta-(OEt) 5 , and modifying the obtained solids with R-(+)-diethyl L-tartrate or R-(+)-diisopropyl L-tartrate.…”

The Control of the Coordination Chemistry for the Genesis of Heterogeneous Catalytically Active Sites in Oxidation Reactions**

“…In these cases, the careful and robust control of the chemical and steric environment that surrounds the catalytically active metal centre is essential. A solid equivalent of the Sharpless chiral epoxidation of alkenes was reported over a mesoporous MCM‐41 silica support [81] . Two enantioselective Ti‐MCM‐41 and Ta‐MCM‐41 catalysts were prepared by grafting the alkoxide precursors, e.g.…”