Peroxyacids, reaction with alkenes

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. 

Perhaps the most common method for the epoxidation of simple alkenes is their reaction with peroxyacids, which has occasionally been referred to as the Prilezhaev (Prileschajew) reaction. O This reaction does not require transition metal catalysis and the yield of epoxide is often high. Peroxyacids such as 156 are prepared by reaction of carboxylic acids with hydrogen peroxide. An equilibrium is established during the reaction that favors the peroxyacid, although several alternative methods are available.279 in general, strong... 

You can think of ds-fused rings as looking like a butterfly or an open book. The key to stereoselectivity in their reactions is that everything happens on the outside (on the cover of the book—the exo face). Nucleophiles add to carbonyl groups from the outside, enolates react with alkyl halides or Michael acceptors on the outside, and alkenes react with peroxyacids on the... 

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. 

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. 

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. 

Conjugated dienes can be epoxidized (1,2-addition), although the reaction is slower than for corresponding alkenes, but a,p-unsaturated ketones do not generally give epoxides when treated with peroxyacids.The epoxidation of a,p-unsaturated ketones with hydrogen peroxide under basic conditions is known as the Waits-Schejfer epoxidation, discovered in 1921. This fundamental reaction has been extended to a,()-unsaturated ketones (including quinones), aldehydes, and... 

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. 

Neighboring group effects are important in reactions of peroxyacids, as they were with reactions of hydroperoxides. Reexamination of the epoxidation of 171 using MCPBA rather than TBHP reveals that the peroxyacid gave a 95 5 mixture of 172 and 173. Using MCPBA rather than TBHP led to a slight increase in diastereoselectivity for the reaction, although this depends on the substitution pattern of the alkene. Sharpless... 

The reaction of peroxyacids with alkenes is illustrated by the experiment that reacts cyclopentene with peroxyacetic acid (83). There are two isolated products cyclopentene oxide (88) in 57% yield and acetic acid (89). The yield is only 57% because the remainder of the 100% is unreacted alkene. Labeling experiments (replacing the natural isotope with the less abundant isotope 1 0) show that the oxygen atom of the epoxide in 88 comes from the OH oxygen in the peroxyacid (labeled in red in 83 and 88). 

Therefore, a tetrasubstituted alkene (R2C=CR2) should react faster with peroxyacids than a trisubstituted alkene (R2C=CHR), which reacts faster than a disubstituted alkene. Monosubstituted alkenes such as RHC=CH2 react slowest in this reaction. In fact, simple alkenes such as 1-pentene can be very difficult to epoxidize under normal conditions because the reaction is very slow. In order to simplify this reaction, assume that all alkenes given as problems in this book can be epoxidized. However, in reality, there are vast differences in the reaction rate among various alkenes. 

What stereoisomers would be obtained from the reaction of the alkenes in Problem 29 with a peroxyacid followed by reaction with hydroxide ion ... 

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... 

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