Electrophilic addition peroxy acids

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

Electrophilic addition to double bonds gives three-membered ring intermediates with Br2, with Hg2+, and with peroxy-acids (in which case the three-membered rings are stable and are called epoxides). All three classes of three-membered rings react with nucleophiles to give 1,2-difunctionalized products with control over (1) regioselectivity and (2) stereoselectivity. Protonation of a double bond gives a cation, which also traps nucleophiles, and this reaction can be used to make alkyl halides. Some of the sorts of compounds you can make by the methods of this chapter are shown below. 

Simple alkenes, including cyclohexene, react rapidly with electrophiles such as bromine or peroxy-acids (Chapter 20). Bromine gives a product of tram addition, peracids give epoxides by cis addition. Under the same conditions benzene does not react with either reagent. 

Other Addition Reactions.—A quantitative study of the epoxidation of 3-substituted cholest-5-enes with peroxy-acid shows that both the rate and the epimer ratio vary according to the C(3)-substituent. " The epoxidation clearly has some electrophilic character. o-Sulpho-perbenzoic acid, which may be used in aqueous-organic solvents, converted cholesterol efficiently into the a-epoxide (89%). The A -olefinic bond in cholestan-5,16-dien-3 -ol is sufficiently reactive, perhaps as a consequence of ring strain, to permit selective 16a, 17a-epoxidation. " ... 

Stereoselectivity. Epoxidation involves an electrophilic yyn-addition of the oxygen moiety of the peroxy acid to the double bond. The concerted formation of two new C-0 bonds ensures that the reaction is stereospecific cA-alkenes furnish the corresponding cA-epoxides and trans-alkenes the corresponding trans-isomers (racemic). 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

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