Epoxidation single-step mechanism

The peroxy acid transfers an oxygen atom to the alkene in a cyclic, single-step mechanism. The result is the syn addition of the oxygen to the alkene, with formation of an epoxide and a carboxylic acid. 

The authors prefer reaction (a) as the main route to ethylene oxide and reaction (d) to combustion products. Nevertheless, they find that selectivity is not a strong function of temperature, suggesting that there is a common rate-determining step. Recently, they studied the effect of dichloro-ethane as a moderator and concluded that such a compound decreases the activity because silver sites are occupied by chloride ions. But the selectivity is higher because the single site mechanism is first order in surface species (epoxidation) and the dual site mechanism second order (combustion). 

Borderline-SN2-Type Mechanism. Some enzymes, such as limonene-1,2-epoxide hydrolase, have been shown to operate via a single-step push-pull mechanism [573]. General acid catalysis by a protonated aspartic acid weakens the oxirane to facilitate a simultaneous nucleophilic attack of hydroxyl ion, which is provided by deprotonation of H2O via an aspartate anion. Due to the borderline-SN2-character of this mechanism, the nucleophile preferentially attacks the higher substituted carbon atom bearing the more stabilized 5 -charge. After liberation of the glycol, proton-transfer between both Asp-residues closes the cycle. 

When the formation of two new bonds occurs in a concerted fashion (i.e., in a single step with a cyclic transition state), then those two bonds will be added to the same face of the substrate, which is called syn addition. Examples of this include dihydroxylation with OSO4, epoxidation, cyclopropanation, and hydroboration. The mechanism of catalytic hydrogenation of an alkene (or of an alkyne with Lindlar s catalyst) is another example of a syn addition, since both hydrogens will be sequentially added to the same face of the pi bond. 

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

Alkene epoxidation is believed to be concerted, occurring by way of a single bimo-lecular elementary step, as shown in Mechanism 6.9. 

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