Peroxyacids, reaction with alkenes, intermediate

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. 

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. 

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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