2- 2/TBHP catalyzed epoxidation

VO(acac)2/TBHP catalyzed epoxidation of 2-(2-alkenyl)phenols. highly... 

VO(acac)2/TBHP CATALYZED EPOXIDATION OF 2-(2-ALKENYL)PHENOLS. HIGHLY REGIO- AND DIASTEREOSELECTIVE OXIDATIVE CYCLIZATION TO 2,3-DIHYDROBENZOFURANOLS AND 3-CHROMANOLS... 

VO(acac)2/TBHP CATALYZED EPOXIDATION OE 2-(3,7-DIMETHYL-0CTA-2,6-DIENYL)-PHEN0L ... 

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. 

A possible mechanism for this transformation, similar to the proposed enzymatic cleavage of carotenoids (Fig. 3), involves 0 = Ru = 0 porphyrin 21 catalyzed epoxidation of 17 to 22, followed by nucleophilic attack of TBHP and ring opening with assistance of 23. Subsequent fragmentation yields the aldehydes (Fig. 8). 

One molybdenum-catalyzed epoxidation, which is of indnstrial importance, needs special mention the Halcon process , which is the molybdennm-catalyzed epoxidation of propylene with TBHP or 1-phenylethyl hydroperoxide on a large scale (Scheme 74), and has been developed and patented by researchers from Halcon and Atlantic Richfield . 

SCHEME 79. Asymmetric epoxidation of olefins by TBHP catalyzed by a molybdenum catalyst in the presence of chiral diols... 

Table I. Epoxidation of alkenes by TBHP catalyzed by V2O5...

The formation of methylperoxy intermediates—i.e., the product of a formal insertion of O2 into the metal-methyl bond—was substantiated by the observation of epoxidation of allylic alkoxides (Scheme 6), in analogy to the proposed mechanism for the Sharpless epoxidation utilizing tert-butylhydroperoxide (TBHP). A similar oxygen atom transfer from a coordinated alkylperoxide to olefin was also postulated for the epoxidation of olefins with TBHP catalyzed by Cp Mo(0)2Cl [31]. The use of organomolybdenum oxides in olefin epoxidafion cafalysis (albeit not with O2) has recently been reviewed [32]. 

Epoxidation of allylic alcohols (5, 75-76). Corrected values for the erythro threo ratios obtained on epoxidation of allylic alcohols with TBHP catalyzed with VO(acac)2 or Mo(CO)6 have been reported. In general greater selectivity is observed with the vanadium catalyst. In simple systems formation of erythro-epoxy alcohols is markedly favored over the threoAsomers, but substitution of a c -alkyl group on the double bond favors formation of threo-epoxy alcohols. 

Bergman [88] reported on the Mo 0x0 complex-catalyzed epoxidation of olefins by alkyl hydroperoxides (e. g., r-butyl hydroperoxide, TBHP) (eq. (10)). The active Cp Mo02Cl catalyst is generated by irradiation of Cp Mo(CO)3Cl in the presence of dioxygen. 

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