Epoxidizing agents

Three peroxyacids are produced commercially for the merchant market peroxyacetic acid as a 40 wt % solution in acetic acid, y -chloroperoxybenzoic acid, and magnesium monoperoxyphthalate hexahydrate. Other peroxyacids are produced for captive use, eg, peroxyformic acid generated in situ as an epoxidizing agent. 

Epoxidation by Dioxirane Derivatives. Another useful epoxidizing agent is dimethyldioxirane (DMDO),86 which is generated by in situ reaction of acetone and peroxymonosulfate in buffered aqueous solution. Distillation gives about aO.lM solution of DMDO in acetone.87... 

In the case of the hematin-catalyzed reaction we have proposed that peroxyl radicals are the epoxidizing agents (40). The mechanism is illustrated in Figure 8. Hematin reduces the hydroperoxide to an alkoxyl radical that cyclizes to the adjacent double bond. 

The use of alkylhydroperoxides as epoxidizing agents for allylic alcohols under catalytic conditions was soon expanded into enantioselective epoxidation with use of the more mild titanium alkoxides in the presence of chiral tartaric esters116. As concerns the epoxidation of functionalized dienes, these now so-called Sharpless conditions [Ti(OPr )4, dialkyl tartrate, TBHP] have been utilized to enantioselectively epoxidize 1,4-pentadiene-... 

The cyclic carbonate could also be synthesized directly from the alkenes in the same reactor by reacting the alkenes in the presence of Ti-MCM-41 with a mixture of an epoxidizing agent (such as H202 or tert-butyl hydroperoxide) and... 

Continued investigation revealed that the principal epoxidizing agents for combined oxidation of unsaturated compounds and aldehydes are not the corresponding peracids, but the radicals of acyl peroxides. 

Kinetic resolution.l33 Since enantiomers react with chiral compounds at different rates, it is sometimes possible to effect a partial separation by stopping the reaction before completion. This method is very similar to the asymmetric syntheses discussed on p. 102. An important application of this method is the resolution of racemic alkenes by treatment with optically active diisopinocampheylborane,134 since alkenes do not easily lend themselves to conversion to diastereomers if no other functional groups are present. Another example is the resolution of allylic alcohols such as 45 with one enantiomer of a chiral epoxidizing agent (see 5-36).135 In the case of 45 the discrimination was extreme. One enantiomer was converted to the epoxide and the other was not, the rate ratio (hence the selectivity factor)... 

In cooxidation of alkenes and aldehydes, and in photosensitized epoxidations acyl peroxy radicals are the epoxidizing agents. The mechanism of the former oxidation, on the basis of kinetic measurements and the nonstereospecificity of the reaction, involves alkyl peroxy radicals formed through the cr-bonded radical 29 as the intermediate ... 

In work reported elsewhere (31) we have shown that the oxidation of styrene under mild conditions is promoted by many group VIII metal complexes. The product profile depends on the nature of the metal center and often differs from that observed when radical initiators are used (Table IX). Substantial quantities of styrene oxide are found in some cases but not in others (31). The epoxide which is formed, however, seems to arise via the co-oxidation of styrene and formaldehyde which is formed by oxidative cleavage of the double bond. Formaldehyde may be oxidized to performic acid or formylperoxy radicals which are efficient epoxidizing agents. Reactions of styrene with oxygen in the presence of group VIII complexes exhibit induction periods and are severely retarded by radical inhibitors (31). Thus, the initial step in the oxidation of styrene in the presence of the Ir(I),Rh(I), Ru(II), and Os(II) com-... 

Optically active diisopinocamphenylborane can be used to resolve racemic olefins. The reagent adds to one enantiomer, and the other is unchanged. Optical purities on the order of 37-65% are possible. Chiral ally lie alcohols can be resolved with chiral epoxidizing agents derived from tartrate complexes of titanium. One enantiomer is epoxidized and the other is not. Thus, die two alcohol enantiomers can be separated, one as the unsaturated alcohol and one as the epoxy alcohol. Use of die other tartrate isomer reverses die stereoselectivity. Selectivities on die order of >100 are possible with this method. As in any kinetic resolution, however, only one enantiomer can be recovered. The other is converted to a different chiral product. 

Notwithstanding this safety aspect, mCPBA continues to be used as an epoxidizing agent. In one such reaction, a diepoxidation (equation 1) was brought about as a key step in the synthesis of the spirocyclic core of aranorosin15, which is a novel antibiotic. A radical inhibitor was added in order to achieve an acceptable yield of 46%. 

Dimethyldioxirane has also been used as the epoxidizing agent in a key step in the synthesis of A-norsteroids69,70. The reaction occurs in dichloromethane-acetone and is highly regio- and stereoselective as shown in equation 9. Dioxiranes may also be generated in situ, by reaction of potassium monoperoxysulfate (sold commercially as OXONE) and cyclohexanones. In this case, cyclohexene derivatives may be smoothly epoxidized in 40-100% yields (equation 10)71. 

This behavior parallels that of alkyl peroxides with electron-withdrawing substituents, compounds that are powerful epoxidizing agents. 

An abbreviation for mefa-chloroperoxybenzoic acid, a common epoxidizing agent, (p. 646) An abbreviation for magnesium monoperoxyphthalate, a relatively stable peroxyacid often used in large-scale epoxidations. (p. 646)... 

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