Peroxy acids, stability

The reason is shown in the transition state the OH group can hydrogen bond, through the H of the alcohol, to the peroxy-acid, stabilizing the transition state when the epoxidation is occurring syn. This hydrogen bond means that peroxy-acid epoxidations of alkenes with adjacent hydroxyl groups are much faster than epoxidations of simple alkenes, even when no stereochemistry is involved. 

Functional groups that stabilize radicals would be expected to increase susceptibility to autoxidation. This is illustrated by two cases that have been relatively well studied. Aldehydes, in which abstraction of the aldehyde hydrogen is fecile, are easily autoxidized. The autoxidation initially forms a peroxycarboxylic acid, but usually the corresponding carboxylic acid is isolated because the peroxy acid oxidizes additional aldehyde in a... 

Hydroxy172 and amino173 groups favor syn stereoselectivity. This is similar to the substituent effects observed for peroxy acids and suggests that the substituents may stabilize the TS by hydrogen bonding. 

Vacuum evaporation of the product of untreated conversion of the pyridine to its N--oxide with 5% excess of the peroxy acid in chloroform gave a residue which decomposed violently [1], This was attributed to the relative stability of the peroxy acid in the cold pure state, which when concentrated and finally heated with other materials underwent accelerating decomposition [2],... 

Ethyl peracetate was the first ester of a peroxy acid, and was characterized by Baeyer and Villiger in 1901. Kinetic studies of perester decomposition were reported by Blomquist and Ferris in 1951, and in 1958 Bartlett and Hiatt proposed that concerted multiple bond scission of peresters could occur when stabilized radicals were formed (equation 46). As noted below (equation 57), polar effects in perester decomposition are also significant. 

Ionization of peroxy acids such as peroxyacetic acid yields an anion that cannot be stabilized by resonance in the same way that acetate can. 

The question of iron(III) tetraphenylporphyrin mimic stabilization [44-46] is resolved by replacing all phenyl hydrogen atoms by halides, which significantly extends the lifetime of the catalyst. It is also proven that oxidants (H202, ROOH, peroxy acids and iodosylben-zene) in hydroxide solvents (diluters) dissociate heterolytically [30, 31],... 

It is all true. Evidence that a silyl group stabilizes the S 2 transition state comes from the reactions of the epoxides of vinyl silanes. These compounds can be made stereospecifically with one equivalent of a buffered peroxy-acid such as m-CPBA. Epoxidation is as easy as the epoxidation of simple alkenes. You will see in a moment why acid must be avoided. 

A three-stage synthesis of allylic alcohols has been devised (Scheme 32)," which consists of (i) alkylation of a sulfone-stabilized allylic carbanion (ii) peroxy acid oxidation of the allylic sulfone to give a 2,3-epoxy sulfone and (iii) reductive elimination of the 2,3-epoxy sulfone to give the allylic alcohol. The overall strategy is similar to that of the Evans-Mislow allylic alcohol synthesis based on the 2,3-sig-matropic rearrangement of allylic sulfoxides. However, there are regiochemical advantages to the sul-... 

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." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

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