Hydroperoxide-dependent epoxidation

Labeque, R. and Marnett, L.J. (1988). Reaction of hematin with allylic fatty acid hydroperoxides identification of products and implications for pathways of hydroperoxide-dependent epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo [a]pyrene. Biochemistry 27, 7060-7070. 

Hamberg, M. and Hamberg, G. Hydroperoxide-dependent epoxidation of unsaturated fatty acids in the broad bean (Vicia faba L.), Arch. Biochem. Biophys. 283(2) (1990), 409-416... 

Hydroperoxide-dependent peroxygenase (epoxygenase). This enzyme, detected in soybeans [7] and broad beans [8], controls the fatty acid hydroperoxide-dependent epoxidation of unsaturated fatty acids. Hydroperoxide molecule plays a role of oxygen donor for epoxidation. This pathway was proposed as the source of natural epoxides of linoleate, vemolic and coronaric acids, occurring in different amounts in many seed oils. 

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. 

Thus, there may be an unlimited increase in selectivity by the elimination of free-radical intermediates. For example, selectivity of liquid-phase epoxidation with organic hydroperoxides depends on two factors [3] ... 

Hughes ME, Chamulitrat W, Mason RP, Eling TE. Epoxidation of 7,8-dihydroxy7,8-dihydrobenzo [a] pyrene via a hydroperoxide-dependent mechanism catalyzed by lipoxygenases. Carcinogenesis 1989 10 2075-2080. 

The epoxidation of olefmic hydrocarbons without other coordinating groups is 10 times slower in the presence of vanadium complexes than with molybdenum catalysts. Nonetheless, the reaction of tert-bnXyX hydroperoxide with an olefin such as cyclohexene in the presence of [VO(acac)2], [V(acac)3], [V(oct)3] or [VO( -BuO)3], is nearly quantitative at 84 C [408, 386]. Rate laws are consistent with reaction via rate determining attack of olefin on a vanadium (V)-hydroperoxide complex. Epoxidations were first order each in olefin and in catalyst but exhibited a Michaelis-like dependency on hydroperoxide, equation (258), where is a limiting specific rate (at very high ratios of hydroperoxide to catalyst), [Vq] is the total concentration of added vanadium, and Kp is the association constant for the vanadium(V) complex presumed to be the active intermediate. 

It has been shown that the course of Mo(VI)-catalyzed epoxidation of olefins by hydroperoxides depends upon the oxygen source. With H2O2 or PhjCOOH, stable reactive peroxo complexes (8) have been isolated these react as shown in Scheme 7 (path a, [Mo] = MoL2Cl( 0), where L = DMF or HMPT). In the case of other alkyl peroxides 0-labeling experiments rule out such intermediates and an alkyl-peroxides species (9) is believed to be formed (Scheme 7, path b). ... 

Blee E and Schuber F. Efficient epoxidation of unsaturated fatty acids by a hydroperoxide-dependent oxygenase. J. Biol. Chem. 1990 265 12887-12894. 

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 dependence of the epoxidation rate on the concentrations of olefin and hydroperoxide is described by the Michaelis-Menten equation... 

The idea of double asymmetric induction is also applicable to asymmetric epoxidation (see Chapter 1 for double asymmetric induction). In the case of asymmetric epoxidation involving double asymmetric induction, the enantiose-lectivity depends on whether the configurations of the substrate and the chiral ligand are matched or mismatched. For example, treating 7 with titanium tet-raisopropoxide and t-butyl hydroperoxide without (+)- or ( )-diethyl tartrate yields a mixture of epoxy alcohols 8 and 9 in a ratio of 2.3 1 (Scheme 4 3). In a... 

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... 

The oxygen that is transferred to the allylic alcohol to form epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity of the reaction results from a titanium complex among the reagents that includes the enantiomerically pure tartrate ester as one of the ligands. The choice whether to use (+) or (-) tartrate ester for stereochemical control depends on which enantiomer of epoxide is desired. 

The photosensitized oxygenation of (+)-carvomenthene, 19, was also carried out by Kenney and Fisher,182 who did not find alcohol 20 but, in addition to the other alcohols, a carvomenthene epoxide. Probably because of these findings, the authors concluded that this photosensitized oxygenation reaction occurs by a nonconcerted, nonstereoselective reaction mechanism. According to all our experiences, epoxidation is not a primary reaction in photosensitized oxygenation reactions of olefins. It may occur as an after-reaction of the primarily formed hydroperoxides its occurrence depends on the hydroperoxide and olefin concentration as well as on the reaction temperature.184... 

Asymmetric epoxidation of ailylic alcohols.1 Epoxidation of allylic alcohols with r-bulyl hydroperoxide in the presence of titanium(lV) isopropoxide as the metal catalyst and either diethyl D- or diethyl L-tartrate as the chiral ligand proceeds in > 90% stereoselectivity, which is independent of the substitution pattern of the allylic alcohol but dependent on the chirality of the tartrate. Suggested standard conditions are 2 equivalents of anhydrous r-butyl hydroperoxide with 1 equivalent each of the alcohol, the tartrate, and the titanium catalyst. Lesser amounts of the last two components can be used for epoxidation of reactive allylic alcohols, but it is important to use equivalent amounts of these two components. Chemical yields are in the range of 70-85%. 

Temperature Effect. The rate of hydroperoxide conversion depends on the reaction temperature. It is slow at temperatures below 90°C. but increases rapidly with increasing temperature. However, the epoxide yield at constant hydroperoxide conversion tends to decrease at temperatures greater than optimum (Table III). 

Effect of Olefin Structure. The reaction rate of the epoxidation depends on olefin structure. In general, the more alkyl substituents bonded to the carbon atoms of the double bond, the faster the reaction rate. This was shown by a reaction of 2-methyl-2-pentene, cyclohexene, and 2-octene with cumene hydroperoxide under the same conditions (Table V). The yield of epoxide was quantitative. The results indicate that 2-methyl-2-pentene reacts faster than cyclohexene and 2-octene. 

The molybdenum-hydroperoxide complex (Step 3) reacts with the olefin in the rate-determining step to give the epoxide, alcohol, and molybdenum catalyst. This mechanism explains the first-order kinetic dependence on olefin, hydroperoxide, and catalyst, the enhanced reaction rate with increasing substitution of electron-donating groups around the double bond, and the stereochemistry of the reaction. 

The fact that the epoxide yield decreases at higher temperatures, longer reaction time, higher catalyst concentration, and lower olefin concentration may be caused by two possible side reactions—decomposition of the hydroperoxide and addition of the alcohol to the epoxide. Initial kinetic studies of the decomposition of tert-butyl hydroperoxide in the presence of molybdenum hexacarbonyl showed second-order dependence on hydroperoxide and first-order dependence on catalyst concentration. These results indicate that the decomposition of hydroperoxide is caused by the reaction between the hydroperoxide-metal complex and another molecule of hydroperoxide. With higher temperature, higher... 

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. 

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