Epoxidation, by peroxy acids

Various computational models agree that the reaction occurs by a concerted mechanism.91 Comparison between epoxidation by peroxy acids and dioxiranes suggests that they have similar transition structures. 

A modest increase in the rate of alkene epoxidation by peroxy acid in benzene occurs on addition of TCA or TFA.183 Calculations predict a much lower activation barrier for epoxidation of ethene with fully protonated peroxyfonnic acid (A/v = 6.4kcalmof ) ... 

The following section describes the preparation of epoxides by the base promoted ring closure of vicinal halohydrms Because vicinal halohydrms are customarily prepared from alkenes (Section 6 17) both methods—epoxidation using peroxy acids and ring closure of halohydrms—are based on alkenes as the starting materials for preparing epoxides... 

Excellent reviews by Swem167 M7f have already been cited in connexion with epoxide synthesis by peroxy acid oxidation of oIMuim (see Motiiat 1111,4.). These papers are equally useful in connexion with the present topic, and should be consulted for many details that cannot be discussed here. 

The tertiary amine bridge of 9 is preferentially eliminated by peroxy acid oxidation to give 1,2,3,4-tetrafluoronaphthalenc (10) 22° no epoxidation of the C = C bond in the starting compound is observed. Almost the same yields are achieved when using 30% hydrogen peroxide in refluxing methanol.220... 

Molecular-orbital calculations indicate that the stereoselective epoxidation of the alkene 192 by peroxy acids arises from stereoelectronic control exerted by a CF3— C bond orientated anti to the alkene plane, in contrast to the previously proposed model for epoxidation of allylic fluoride in which the F—C bond and alkene bonds are in a syn arrangement305. 

Of special interest are olefm epoxidation with peroxy acids at the moment of their formation from acid anhydrides and hydrogen peroxide. Among the catalysts of olefin oxide formation from olefins interacting with H202 in the presence of metal (Ti, W, V, Ta, Ge, Mo, U, Ru) oxides or acids are tungstic acid and its salts [47], Epoxidation is described by the following mechanism ... 

The pattern you saw for epoxidation with peroxy-acids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is the bromination reaction itself. The chart shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1.1- S... 

Sulfides are readily oxidiz by peroxy acids as well as TBHP/Mo. Hence the chemoselective epoxidation of the unsaturated sulfide (183) has been effected by an indirect method the alkene is first trans-... 

Oxidation of Alkenes to Oxirans by Peroxy-acids. An improved procedure for epoxidation using aromatic peroxy-acids has been reported. After a normal epoxidation with 3-chloroperoxybenzoic acid (mCPBA) in CH2CI2, activated KF is added to the crude mixture, and this results in the precipitation of both wCPBA and the aromatic acid by-product, leaving an acid-free reaction mixture for normal work-up. As an alternative, the insoluble mCPBA-KF complex itself may be used for the epoxidation of alkenes overnight at room temperature. After filtration and treatment of the CH2CI2 solution with more KF (to ensure removal of any residual peroxy-acid), normal work-up leads to yields in excess of 95% for cyclohexene and styrene oxides. 

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