Alkenes peroxyacid reactions

Alkenes undergo reaction with peroxycarboxylic acids (RCO3H) to give three-membered-ring cyclic ethers called epoxides. For example, 4-octene reacts with a peroxyacid to yield 4,5-epoxyoctane ... 

Another synthetic example illustrates both the epoxidation capabilities of this reaction as well as the stereochemical influence of neighboring alcohol unit noted above with the transformation 163 164. In a synthesis of (-)-malyngolide by Ogasawara and co-workers, allylic alcohol 176 was treated with VO(acac)2 and t-BuOOH, in the presence of 2,6-lutidine, and a 66% yield of 177 was obtained.266 xhe hydroxyl unit directed the epoxidation to the exo face of the bicyclic system, as shown. Also notice that the more substituted alkene was epoxidized rather than the less substituted one, which is also typical of epoxidation reactions of this type, and will be discussed in more detail below, in connection with peroxyacid reactions. 

The three most common alkene oxidation reactions are epoxidation, dihydrox-ylation, and ozonolysis. Epoxides are formed when an alkene is treated with a peroxyacid, such as mCPBA. Since both C-O bonds are formed in the same step (described as a concerted mechanism), the stereochemistry of the starting alkene is preserved in the product. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Treatment of the following alkene with a peroxyacid yields an epoxide different from that obtained by reaction with aqueous Br2 followed by base treatment. Propose structures for the two epoxides, and explain the result. 

These reagents may be considered to be one of the elusive aza-analogues of peroxyacids, and there are significant mechanistic similarities between the Rees-Atkinson reaction and the Bartlett epoxidation. Chiral Q-reagents have been used to effect highly stereoselective aziridination of alkenes (Scheme 4.13) [1],... 

Alkenes can be epoxidized with any of a number of peroxyacids, " of which m-chloroperoxybenzoic has been the most often used. The reaction, called the... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

A process that is effective for epoxidation and avoids acidic conditions involves reaction of an alkene, a nitrile, and hydrogen peroxide.82 The nitrile and hydrogen peroxide react, forming a peroxyimidic acid, which epoxidizes the alkene, by a mechanism similar to that for peroxyacids. An important contribution to the reactivity of the peroxyimidic acid comes from the formation of the stable amide carbonyl group. 

Figure 3-12 gives an example of this process. The reaction of the alkene with a peroxyacid can lead to the trans-glycol (anti addition). 

Alkenes react with peroxyacids (RCO3H) followed by hydrolysis to give trans-1,2-diols. The products are always anft-diols, since the reaction occurs with anti addition. 

The epoxide 6 is naturally electrophilic, but where does the epoxide come from By far the most important method of epoxide synthesis is the treatment of alkenes 19 with peroxy acids RCO3H 21. Alkenes are naturally nucleophilic 2 they react with bromine to give dibromides 20 and with electrophilic peroxyacids 21 to give epoxides. Again, these reactions convert nucleophilic alkenes into electrophilic derivatives. A very popular reagent for epoxidation is mCPBA (meta-chloro-perbenzoic acid) 21 R = 3-chlorophenyl but many other compounds are used. 

When an alkene is treated with a peroxyacid (RC03H) it forms a epoxide (Following fig.). m-Chloroperoxybenzoic acid is one of the most commonly used peroxyacids for this reaction. The reaction is unusual because there is no carbocation intermediate, and it involves a one-step process without intermediates. 

A peroxyacid epoxidizes an alkene by a concerted electrophilic reaction where several bonds are broken and several are formed at the same time. Starting with the alkene and the peroxyacid, a one-step reaction gives the epoxide and the acid directly, without any intermediates. 

Epoxides (Oxiranes) We have already encountered some of the chemistry of epoxides in Section 8-12. Epoxides are three-membered cyclic ethers, usually formed by peroxyacid oxidation of the corresponding alkenes. The common name of an epoxide is formed by adding oxide to the name of the alkene that is oxidized. The following reactions show the synthesis and common names of two simple epoxides. 

Peroxyacids (sometimes calledperacids) are used to convert alkenes to epoxides. If the reaction takes place in aqueous acid, the epoxide opens to a glycol. Therefore, to make an epoxide, we avoid strong acids. Because of its desirable solubility properties, meta-chloroperoxybenzoic acid (MCPBA) is often used for these epoxidations. MCPBA is a weakly acidic peroxyacid that is soluble in aprotic solvents such as CH2C12. 

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