Epoxidation with TBHP

The treatment leads to a significant improvement in alkene conversion in cyclohexene epoxidation in the case of Ti-MCM-41 and Ti-MCM-48 (273). Although epoxide selectivity improved in the former case, there was a decrease in the latter. In the case of hexane oxidation, silylation did not improve the conversion. An enhancement in the number of turnovers and selectivity for the epoxide on silylation was also observed in the cyclohexene epoxidation with TBHP catalyzed by Ti-SBA-15 (Table LII) (274). Ti-SBA-15 was claimed to be thermally more stable than Ti-MCM-41. Ti leaching was absent. 

For such reasons, the following section considers in more detail some of the most significant results obtained by our team on the epoxidation with TBHP of unsaturated FAMEs over mesoporous titanium-grafted silicates. In these examples, the epoxidation tests were carried out either in ethyl acetate, which could be even obtained, in principle, from renewable sources and which is relatively less harmful than other polar non-protic solvents, or under solvent-free conditions. 

The ability of titanium-grafted silicas in catalyzing the epoxidation with TBHP of fatty compounds was first tested on two pure Qg monounsaturated FAMEs methyl oleate (ds-9-octadecenoate Scheme 12.1) and methyl elaidate (trans-9-octadecenoate) [49]. In both cases, selectivity to 9,10-epoxystearate was very high (>95%) and the reaction was fully stereospecific, confirming that epoxidation with titanium catalysts and TBHP proceeds via a non-radical mechanism with retention of configuration at the C=C bond. Ti-MCM-41 was more active than Ti-SiC>2 (Fig. 12.1). Actually, methyl oleate was almost completely converted after... 

Table 1 Cyclooctene epoxidation with TBHP/MCM-supported Mo02X2(THF)2...

Table 2 Catalytic cyclooctene epoxidation with TBHP/zeolite-supported MoCp(CO)3Cl...

Table 3 Allylic alcohol epoxidation with TBHP/dioxomolybdenum Mo02(acac)(L ) catalysts, where the catalysts were homogeneous or zeolite-Y-supported... 

Table4 Cyclooctene epoxidation with TBHP/MCM-spacer ligand-tethered Cp Mo(CO)3R complexes... 

The results for cyclooctene epoxidation with TBHP show that the method of heterogenisation plays a decisive role in the activity (Table 6). Indeed, the materials obtained from the complex grafted onto a fimctionahsed matrix (odd entries. Table 6) are less active (seen by comparing TOP values) than those synthesised via the tethering of fimctionahsed complexes on a mesoporous material (even entries. Table 6). 

The MCM samples show about 90-95% cyclooctene epoxidation with TBHP, with nearly 100% selectivity after a reaction time of 24 h (entries 2 and 3, Table 7). Conversion is lower with the SBA-15 catalysts (entry 4, Table 7). The TOPS are about one order of magnitude lower than the respective homo-... 

Materials synthesised by autocatalysis provide high activity (100% conversion in 3 h) and selectivity (100%) for cyclooctene epoxidation with TBHP, as well as excellent recycling ability (the catalytic properties are almost fully recovered after three rims). 

Table 9 Cyclooctene epoxidation with TBHP/complexes M0X2O2L2 (X = Cl, Me L = Lewis base) 1-3 and CpMo(CO)3Me, CpMo(CO)3Cl, Me5CpMo(CO)3Me in different RTlLs...

NaHCOi) gives a mixture of 2 and the isomeric a-epoxide in the ratio 1 1. Epoxidation with TBHP/VO(acac)2 gives the two epoxides in the ratio 1.5 1.This reaction becomes more stereoselective if the 6,7-c/s-diol unit is protected as the benzylidene derivative (1). In this reaction the two epoxides arc formed in the ratio 3 1, presumably because the 6,7-diol can no longer form a complex with the metal catalyst.1... 

In the synthesis of an epoxide from an alkene with TBHP/Mo, the stereochemistry of the alkene is retained in the epoxide. It has been suggested that the reaction proceeds through the transition state (124). ° 1-Deccne and the alkene (115) (see equation 42) have been epoxidized with TBHP/Mo (equations 45 and 46). The epoxidation of (115) is regio- as well as stereo-selective the reagent approaches the electron-rich double bond from the less hindered face. 

However, the catalytic activity observed with molybdenum porphyrins [27] in epoxidations with TBHP favors the mechanisms involving direct attack of the olefin on the electrophilic oxygen of the alkylperoxometal complex (see earlier). Because of the steric hindrance of the macrocyclic ligand it was considered... 

Similarly the alkali metal-exchanged titanium beta was shown to be an effective catalyst for olefin epoxidations with TBHP [47]. Titanium has also been incorporated into mesoporous molecular sieves, such as MCM-41, by framework substitution [48] or by grafting Ti(IV) species to the internal surface by reaction... 

In yet another approach, titanium(IV) silsesquioxanes were shown to be excellent homogeneous catalysts for epoxidations with TBHP [55]. A heterogeneous variant was prepared by adsorbing the Ti(IV) silsesquioxane in the pores of MCM-41 that had been silylated with Ph2SiCl2 to passivate the external surface. The resulting material was reported to be a stable, recyclable catalyst for epoxidations with TBHP [56]. 

Table 14.1 also presents the turnover frequencies (TOF) of the CVD-prepared Ti/Si02 catalysts. For 1-octene epoxidation with TBHP in a batch reactor at 110 °C... 

A borderline case is V(V) which has an intermediate oxidation potential [3] and can react via both mechanisms depending on the substrate used. It catalyzes epoxidations with TBHP via the peroxometal mechanism and alcohol oxidations via the oxometal pathway [7]. 

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