Epoxidation, by peroxy

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 principal possibility of olefm epoxidation by peroxy radicals is indicated. The simplest representative of this class can be obtained from H202. 

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... 

Epoxidation (Section 6 18) Conversion of an alkene to an epoxide by treatment with a peroxy acid... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

Conversion of alkenes to epoxides The simplest epoxide, ethylene dioxide, is prepared by catalytic oxidation of ethylene, and alkenes are also oxidized to other epoxides by peracid or peroxy acid (see Section 5.7.2). 

Viewed in this way, reactions yielding cyclic ethers should be thought of as two-step processes, epoxides probably arising from peroxy radical additions to olefins followed by 1,3-displacements, and larger rings arising via intramolecular hydrogen abstraction by peroxy radicals followed by 1,4- or 1,5-displacements. In all such reactions it is probably the first step which is slow and which determines the yield of product observed. 

Mechanism. The following types of evidence ere pertinent in selecting on acceptable mechanism for olefin epoxidation by means of peroxy acids (1) the nature of the peroxy acid and the electronic effect of eubBtituents on its reactivity (2) the electronic effect of substituents on the reactivity of the olefin component (3) stereochemical factors affecting the reactivity of the olefin (4) the possibility of acid dialysis (5) solvent effects and (6) neighboring group effects. 

A particularly significant aspect of hypohalous arid addition is tlu-fact that it can lead to epoxides isomeric in configuration to thcompare Tables 6 and 0 fo illustrations of this principle). 

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. 

An important method of locating double bonds in uneaturatod fteequitBipenei is to convert these into epoxides by oxidation with a peroxy acid, and to condense the resulting epoxide derivatives with methyhxmgnesiiun iodide. Further degradations then yield additional structural information. Three illustrations ( q0. 800-867) of this... 

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. 

Under controlled conditions, the KMnCVCuSCU mixture oxidizes cholesterol 361 to furnish the /J-epoxide 362 rather than the w-epoxide, which is normally formed by peroxy... 

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 ... 

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