Percarboxylic acids, epoxidation with

Another one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of alkenes with percarboxylic acids (Figure 3.19). Most often, meta-chloroperbenzoic acid (MCPBA) is used for epoxidations. Magnesium monoperoxyphthalate (MMPP) has become an alternative. Imidopercarboxylic acids are used to epoxidize olefins as well. Their use (for this purpose) is mandatory when the substrate contains a ketonic C=0 double bond in addition to the C=C double bond. In compounds of this type, percarboxylic acids preferentially cause a Baeyer-Villiger oxidation of the ketone (see Section 14.4.2), whereas imidopercarboxylic acids selectively effect epoxidations (for an example see Figure 14.35). 

Fig. 3.19. Stereospecific cis-epoxidations of alkenes with percarboxylic acids.

In the transition state of the epoxidation of alkenes with a percarboxylic acid the C=C axis of the alkene is rotated out of the plane of the percarboxylic acid group by 90° ( spiro transition state ). In this process, four electron pairs are shifted simultaneously shifted. This very special transition state geometry make peracid oxidations of C=C double bonds largely insensitive to steric hindrance. The epoxidation given in Figure 3.20 provides an impressive example. 

Fig. 3.20. Due to the transition state geometry shown there is hardly any steric hindrance in cis-epoxidations with percarboxylic acids—as is impressively demonstrated by the example given here.

Examination of this mechanism suggests that the nature of the R group should not make much difference in the reaction. In fact, a number of different percarboxylic acids can be used to epoxidize alkenes, as illustrated in the following examples. As expected, the additions occur with syn stereochemistry. 

A third one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of olefins with percarboxylic acids (Figure 3.14). Suitable percarboxylic acids must, however, not be (too) explosive. Thus, aromatic percarboxylic adds are preferable. Until recently one epoxidized almost exclusively with mefa-chloroper-benzoic acid (MCPBA). An alternative has become magnesium monoperoxyphthalate (MMPP). In the transition state of this type of epoxidation, four electron pairs are shifted simultaneously (which is a record in this book except for the Corey-Winter elimination in Figure 4.42). 

Performic acid is an unstable, hazardous percarboxylic acid, and must always be generated in situ. Epoxidation with in situ performic and peracetic acid are well established commercial processes. They find application in the epoxidation of alkenes, particularly those of high molecular weight. Many such epoxides are produced on a large scale, and can be classified as vegetable oils, unsaturated esters, unsaturated acids, a-alkenes, natural polymers and synthetic polymers. The most important vegetable oil which is epoxidized commercially is soyabean oil. World production of epoxidized soyabean oil (ESBO) exceeds 150000 metric tons per annum. Epoxidized linseed oil is also important, but produced at a lower rate than ESBO. Both products are formed by usual in situ performic and peracetic acid techniques.23,24 Typical procedures are outlined in Table 3.1.25... 

The peracid methods invariably open the epoxide with reversion of configuration, i.e. trans-diol formation. Aryl substituents, however, are converted to the cw-diols with retention of configuration.118-120 Olefins which have been hydroxylated by means of in situ percarboxylic acid techniques include cyclohexene (65-73%),121 dodecane (91 %)122 and oleic acid (99%).123 Chlorestrol has been frans-hydroxylated with performic acid in high yield (91%).124... 

A percarboxylic acid, prepared by adding 2-cyanoethyltriethoxysilane and tetraethyl orthosilicate to aqueous ethanol containing 1-dodecylamine to give a cyanated silica which is hydrolyzed and treated with HjO and MsOH, is a useful epoxidizing agent."... 

Epoxidation is an easy and extensively used method for creating reactive intermediates and is the focus of this section. The versatile reactivity of epoxy groups makes epoxidation of PBD a preferred first step for subsequent nucleophilic addition reactions. The epoxidation procedure used most frequently relies on percarboxylic acids, which are added as reagent or are prepared in situ. Epoxi-dations are also carried out with transition metal oxides as well as organic precursors such as dimethyl dioxirane. 

Larger scale epoxidations are frequently performed in aromatic solvents such as toluene [104] even though it is known that the Jt-donor aromatic system of toluene competes with the olefin for coordination to the positively charged carbonyl carbon [105]. The epoxidation reaction is generally faster in more polar solvents. Laboratory scale epoxidations of PBD with peracids can be performed in chlorinated solvents such as dichloromethane or chloroform. Ethers such as dioxane are known to decrease the reactivity of peroxides in the epoxidation of PBD because their oxygen atoms can interact with the percarboxylic acid, thereby decreasing the reactivity (Fig. 2) [106]. Epoxidations in cyclohexane show lower conversion than epoxidations in toluene [107]. 

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