Hydroperoxide-dependent epoxidation oxidation

A potentially powerful probe for sorting out the contribution of hydroperoxide-dependent and mixed-function oxidase-dependent polycyclic hydrocarbon oxidation is stereochemistry. Figure 9 summarizes the stereochemical differences in epoxidation of ( )-BP-7,8-dihydrodiol by hydroperoxide-dependent and mixed-function oxidase-dependent pathways (31,55,56). The (-)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (+)-anti-diol epoxide by both pathways whereas the (+)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (-)-anti-diol epoxide by hydroperoxide-dependent oxidation and to the (+)-syn-diol epoxide by mixed-function oxidases. The stereochemical course of oxidation by cytochrome P-450 isoenzymes was first elucidated for the methycholanthrene-inducible form but we have detected the same stereochemical profile using rat liver microsomes from control, phenobarbital-, or methyl-cholanthrene-induced animals (32). The only difference between the microsomal preparations is the rate of oxidation. 

In 2010, Bryliakov reported on the use of a series of [(L)Mn(03SCF3)2] complexes (Scheme 13) for the epoxidatiOTi of olefins using peracetic acid, OT-chlorobenzoic acid, teri-butyl hydroperoxide, cumyl hydroperoxide, PhIO, and MesIO [59]. The enantioselectivities of the epoxidations varied depending on oxidant. Based on this finding, as well as other observations, iodosylarene [(L)Mn=0(OIAr)] complexes were proposed to be the active oxidizing species, similar to the intermediates proposed earlier by Goldberg [52]. 

The first practical method for asymmetric epoxidation of primary and secondary allylic alcohols was developed by K.B. Sharpless in 1980 (T. Katsuki, 1980 K.B. Sharpless, 1983 A, B, 1986 see also D. Hoppe, 1982). Tartaric esters, e.g., DET and DIPT" ( = diethyl and diisopropyl ( + )- or (— )-tartrates), are applied as chiral auxiliaries, titanium tetrakis(2-pro-panolate) as a catalyst and tert-butyl hydroperoxide (= TBHP, Bu OOH) as the oxidant. If the reaction mixture is kept absolutely dry, catalytic amounts of the dialkyl tartrate-titanium(IV) complex are suflicient, which largely facilitates work-up procedures (Y. Gao, 1987). Depending on the tartrate enantiomer used, either one of the 2,3-epoxy alcohols may be obtained with high enantioselectivity. The titanium probably binds to the diol grouping of one tartrate molecule and to the hydroxy groups of the bulky hydroperoxide and of the allylic alcohol... 

Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

The epoxidation of propylene to propylene oxide is a high-volume process, using about 10% of the propylene produced in the world via one of two processes [127]. The oldest technology is called the chlorohydrin process and uses propylene, chlorine and water as its feedstocks. Due to the environmental costs of chlorine and the development of the more-efficient direct epoxidation over Ti02/Si02 catalysts, new plants all use the hydroperoxide route. The disadvantage here is the co-production of stoichiometric amounts of styrene or butyl alcohol, which means that the process economics are dependent on finding markets not only for the product of interest, but also for the co-product The hydroperoxide route has been practiced commercially since 1979 to co-produce propylene oxide and styrene [128], so when TS-1 was developed, epoxidation was looked at extensively [129]. 

Besides the chiral, secondary hydroperoxides employed by Adam and coworkers and the tertiary hydroperoxide used by Seebach, the optically active carbohydrate hydroperoxides 72, 93 and 94 have been tested by Taylor and coworkers in epoxidation reactions of the quinones 95 under basic conditions (Scheme 41). The yields of the corresponding epoxides 96 that were obtained with this type of oxidant varied from 33 to 83% and the enantioselectivities were moderate and in some cases good (23 to 82%), depending... 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

Cobalt- or manganese-substituted PW12O40 and SiWiiOj9Ru(OH2)5 catalyze the oxidation of paraffins such as cyclohexane and adamantane (320, 321) as well as the epoxidation of cyclohexene with ter/-butyl hydroperoxide, iodosylbenzene potassium persulfate, and sodium periodate (321, 322). The reactivity depends on the transition metals. In the case of epoxidation of cyclohexene with iodosylbenzene, the order of catalytic activity of PW] i(M)03 is M = Co > Mn > Cu > Fe, Cr. 

Mechanistically, it has been proposed that the reaction proceeds predominantly via epoxidation of the aromatic species 28, which leads to unstable arene-oxides 29-31 (equation 47). Rearrangement of the arene-oxides 29-31 involving the migration of a hydride anion (NIH-shift) forms the phenolic product 32 or 33. Alternative flavin-dependent oxidases have been proposed to involve a hydroperoxide intermediate. ... 

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