Peroxyacid reaction with alkenes

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. 

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. 

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. 

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... 

As a rule, alkenes do not react with 78-80 unless there is another reagent present—specifically, a transition metal. This reaction will not be discussed further. In sharp contrast, peroxycarboxylic acids such as 81 react directly with alkenes. Peroxycarboxylic acids 81 are named by adding the term peroxy to the name of the carboxylic acid (see Chapter 5, Section 5.9.3 and Chapter 16, Section 16.4). Using the common names, the peroxy analog of formic acid is peroxyformic acid (82), and others include peroxyacetic acid (83), peroxytrifiuo-roacetic acid (84), peroxybenzoic acid (85), and me a-chloroperoxybenzoic acid (abbreviated mCPBA, 86). Peroxycarboxylic acid 85 is a derivative of the aromatic carboxylic acid benzoic acid (PhCOOH), and the carboxylic acid precursor to 86 is clearly another aromatic carboxylic acid. (The nomenclature and structural features of benzoic acid and other aromatic carboxylic acid derivatives will be discussed in detail in Chapter 21, Section 21.2.) The salient feature of peroxyacids 82-86 is the presence of the electrophihc oxygen atom mentioned previously, which will react with an alkene. 

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). 

In recent years, peroxyacids have been less available than they once were. In addition, high-purity hydrogen peroxide is rather dangerous to handle and difficult to obtain. This fact has forced manufacturers to use dilute aqueous solutions of hydrogen peroxide to prepare peroxides this gives lower purity peroxide products and lower yields of epoxides when they react with alkenes. The by-product of such reactions is a carboxylic acid, which can sometimes react with the epoxides product unless a buffer is used. Remember also that terminal alkenes are difficult to epoxidize using peroxyacids. 

An alkene can be converted to an epoxide by a peroxyacid. An epoxide is an ether in which the oxygen is incorporated into a three-membered ring, and a peroxyacid is a carboxylic acid with an extra oxygen. The overall reaction amounts to the transfer of an oxygen from the peroxyacid to the alkene. It is an oxidation reaction because it increases the number of C—O bonds. 

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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