Alkenes epoxidation with peracids

The introduction of oxygen atoms into unsaturated organic molecules via dihydroxylation reactions leads to 1,2-diols. 1,2-Diols can be synthesized by the reaction of alkenes either with peracids via corresponding epoxides and subsequent hydrolysis or with OSO4, KMn04, RUO4 and Cr(VI) compounds. 

Less regularly used reagents are rerf-butyl hydroperoxide/terr-butyliithium,600,601 ozone.602 dioxiranes,603,604 fluorine/ water/acetonitrile,605 or A, A -diethylhydroxylaminc.606 Alkenes carrying a donor substituent can also be epoxidized with peracids.607-609 Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclectivity.610... 

The Sharpless epoxidation provides good examples of both directed and asymmetric catalytic reactions. It has long been known that alkenes can be epoxidized with peracids, which deliver an electrophilic oxygen atom, as shown... 

Epoxidations with peracids can exhibit high degree of chemoselectivity and these generally display preferences for reaction with more nucleophilic alkenes. This phenomenon is illustrated in Vandewalle s work directed towards the total synthesis of the sesquiterpene estafiatin (6, Equation 3) [52]. The final step included selective epoxidation of the trisubstituted olefin from its more accessible convex face (dr =97 3). 

As a, (3-unsaturated ketones are electron-poor alkenes, they do not generally give epoxides when treated with peracids. They can be epoxidized with hydrogen peroxide which involves nucleophilic attack by HOO- to give the epoxy ketone (Figure 4.1). 

Since the epoxidation of alkenes with peracids was discovered by Prilezajew in 1909 [29], epoxides have played a major role in organic chemistry and industry, providing important intermediates for the synthesis of more complex molecules. Metal-catalyzed epoxidation reactions have received much attention in recent decades since the discovery of the Sharpless epoxidation [30, 31], but most epoxides were prepared from alkenes primarily by their interaction with peracids. 

VI. THE EPOXIDATION OF ALKENES WITH PERACIDS A. Early Mechanistic Studies... 

Alkenes are also oxidized to epoxides hy peracid or peroxyacid (RCO3H), e.g. peroxyhenzoic acid (C6H5CO3H). A peroxyacid contains an extra oxygen atom compared with carboxylic acid, and this extra oxygen is added to the double bond of an alkene to give an epoxide. For example, cyclohexene reacts with peroxyhenzoic acid to produce cyclohexane oxide. 

The epoxidation of an alkene with peracid to give an oxirane. The commercial available mCPBA is a widely used reagent for this conversion, while magnesium mono-perphthalate and peracetic acid are also employed. 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

Ab initio molecular orbital studies have been carried out on the mechanism of epoxidation of alkenes with peracids. Numerous examples have been reported of the formation of products where the stereochemistry differs from the known general regularities some characteristic instances of these will be presented below. 

Epoxidation. With this peracid alkenes are epoxidized in dichloromethane, often in quantitative yield. The carboxylic acid by-product is very insoluble in the reaction medium and easily removed. 

Prilezhaev (Prileschajew) reaction. Formation of epoxides by the reaction of alkenes with peracids. 

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