Epoxide selectivity

When the OAc group was a hydroxyl, the epoxidation selectivity was not very good, presumably because of the known directing effect of hydroxyl groups in peracid epoxidations. 

The AE reactions on 2,5,5-trisubstituted allyl alcohols have received little attention, due in part the limited utility of the product epoxides. Selective ring opening of tetrasubstituted epoxides are difficult to achieve. Epoxide 39 was prepared using stoichiometric AE conditions and were subsequently elaborated to Darvon alcohol. Epoxides 40 and 41 were both prepared in good selectivity and subsequently utilized in the preparation of (-)-cuparene and the polyfunctoinal carotenoid peridinin, respectively. Scheme 1.6.12... 

By complexation of MnNaY with 1,4,7-trimethyltriazacyclononane, a new heterogeneous catalyst was obtained for olefin epoxidation with H202. Excellent epoxide selectivities were obtained, with limited epoxide solvolysis. The oxygenation appears to go through a radical intermediate. The manganese trizacyclononane epoxidation catalyst was also heterogenized via surface gly-cidylation.103... 

The more hindered alkoxide Ti(OiPr)4 was used as the precursor complex with surface silanols of an amorphous silica support this reaction is reported to lead to the same environment of Ti as in TS-1, but only when the reaction is carried in cyclohexanol as the solvent. Epoxidation of octene, cyclohexenol, and norbornene with FI202 in phenylethanol leads to 95-98% epoxide selectivity.147... 

The yields for reactions of unsubstituted terminal alkenes were lower than for substituted alkenes but they were still reasonable and could be increased further by increasing the aldehyde alkene ratio. Total conversions of substrate were reported with epoxide selectivity as high as 95% in some cases. The FBC system allows for a much higher substratexatalyst ratio (1000 1) than the non-fluorous epoxidation reported (20 1) previously. Recycling the fluorous layer once showed no reduction in conversion or selectivity. 

The greater activity of Ti-beta (vs. TS-1) in the oxidation of the bulky cyclohexane was noted in the previous section. Table IX provides a comparison of the conversion and epoxide selectivity in the reaction catalyzed by TS-1 and three large-pore/mesoporous Ti-silicates in the epoxidation of a single, linear allyl alcohol (pentenol). 

Catalyst Temperature (K) Pentenol conversion (%) Epoxide selectivity (%)a Reference... 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

Catalyst Ti02 (wt%) Solvent Hex-l-ene conversion Epoxide selectivity (%) h2o2 selectivity (%) TONb... 

Catalyst Ti02 wt % Acid conversiona Epoxide selectivity H202 selectivity... 

Although the mesoporous materials, such as Ti-MCM-41, have lower intrinsic epoxidation selectivity than TS-1 and Ti-beta, they must nevertheless be used as catalysts for reactions of large molecules typical in the fine chemicals industry. It is, therefore, interesting to elucidate how these ordered mesoporous materials compare with the earlier generation of amorphous titania-silica catalysts. Guidotti et al (189) recently compared Ti-MCM-41 with a series of amorphous titania-silica catalysts for the epoxidation of six terpene molecules of interest in the perfumery industry (Scheme 6). Anhydrous TBHP was used as the oxidant because the catalytic materials are unstable in water. The physical characteristics of these catalysts are compared in Table XII. 

It was observed that no leaching of Ti occurs during the catalytic reaction in the anhydrous medium. The acidity of the catalysts (which gave rise to many side products) was evaluated by a comparison of their reaction rates in the acid-catalyzed conversion of citronellol into isopulegol (Scheme 7). The acidity of the catalysts decreased in the following order A>C>D>B = E. The catalytic activity and epoxidation selectivities are compared in Table XIII. 

The epoxide selectivity did not depend noticeably on the gross structural features of the catalyst. For instance, the selectivity in the epoxidation of 4 is about 85% on all solids (Table XIII). 

Even as large a molecule as cholesterol was epoxidized in the presence of Ti-MCM-41 catalyst (198). An epoxide selectivity of 53% at 48% conversion was achieved. The oxidation of the OH group and allylic oxidations were important side reactions. 

Run no. Catalyst Reactant Initial/ before H202 addition pH After H202 addition At die end of die reaction TOF Olefin conversion (mol%) h2o2 efficiency Epoxide selectivity (mol%)... 

Reactant Catalyst T (K) Conversion (%) Epoxide selectivity"1 (%) Productivity13 (mmol/g/h)... 

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. 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

Titanosilicates molecular sieves, especially TS-1, have been widely studied for the selective oxidation of a variety of organic substrates, using aqueous H202. ° Recently, there have been attempts to substitute aqueous H2O2 by a mixture of H2 and O2 in the presence of metals such as Pd, Pt, Au, etc. Selectivities of 99% for propylene oxide formation from propylene were observed by Haruta and co-workers over Au-containing catalysts. We had also found that the epoxide selectivity in the epoxidation... 

Catalyst Oxidant TOP H2O2 Efficiency (%) Conversion (wt.%) Epoxide Selectivity (wt.%)... 

Pinene eo-oxidation with IBA gave selectively a-pinene epoxide and isobutyrie aeid. It is noteworthy that the aetivity of the supported Co-POM catalysts was comparable to the activity of the homogeneous Co-POM, while the epoxide selectivity could increase after the Co-POM immobilization. 

In contrast to a-pinene autoxidation, for which no effect of the support on the reaetion seleetivity was observed, the seleetivity of the eo-oxidation process could be altered by varying the amount of NH2-groups on the support. Thus for Co-POM supported on NH2-MCF and NH2-SBA-I5 (NH2/C0-POM = 30 and 16 mol/mol, respectively), the a-pinene epoxide selectivity attained 94 and 76%, respeetively, at 96% of substrate conversion. 

This may be rationalized if we take into account that the excessive surfaee NH2-groups may partially neutralize carboxylic acid formed during the co-oxidation process and thus improve the epoxide selectivity. 

Catalysts tor epoxidation" Ti (wt%) (pretreatment) Cyclohexene conversion (%)" Epoxide selectivity Epoxide TON... 

Fripiat et al. studied the oxidation of propylene dissolved in benzene at 150°C under a total pressure of 45 atm [20]. On Mo-X, 70% epoxide selectivity is obtained at propylene conversions of 7.5%. Side products are formed by further oxidation of the epoxide, by C-C cleavage in the olefin with formation of methanol, formic and acetic acid, and by fast esterification of the epoxide with these acids. 

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