Mechanism peroxyacid epoxidation

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. 

Epoxidation occurs via the concerted addition of one oxygen atom of the peroxyacid to the k bond as shown in Mechanism 12.4. Epoxidation resembles the formation of the bridged halo-nium ion in Section 10.13, in that two bonds in a three-membered ring are formed in one step. 

The mechanism for the addition of oxygen to a double bond to form an epoxide is analogous to the mechanism described in Section 4.7 for the addition of bromine to a double bond to form a cyclic bromonium ion. In one case the electrophile is oxygen, and in the other it is bromine. So the reaction of an alkene with a peroxyacid, like the reaction of an alkene with Br2, is an electrophilic addition reaction. 

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. 

Anti additions to alkenes typically result in a stepwise mechanism formation of a cationic cyclic intermediate such as a bromonium ion, followed by backside attack by a nucleophile to open up the ring. Such is the mechanism for the anti addition of Br2. The bromination reaction results in trans bromines since the second bromine (as Br ) has to come in from the opposite face as the first bromine (as Br+) in order to do an Sn2 attack on the bromonium ion. Other mechanisms involving the bromonium ion include reaction of an aikene with Br2/H20 (adds -Br and -OH anti) and Br2/ROH (adds -Br and -OR flnfi)-Treatment of an aikene with a peroxyacid in water forms an epoxide that undergoes a ring opening in situ to give a trans diol product. Trans stereoselectivity is also seen in the dissolving metal reduction of alkynes. 

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