Oxiranes single-step epoxidation

Oxiranes (or epoxides) are conveniently formed by delivery of electrophilic oxygen to one of the faces of an alkene. This is achieved with a peroxy acid (peracid) in a single-step reaction (Scheme 4.2) that necessarily delivers oxygen syn to the double bond. Isotope labelling experiments have shown that the oxygen atom that is transferred to the alkene is the one that is further from the carbonyl group of the peroxy acid. 

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. 

It is important to be dear why we are so sure that epoxidation occurs in a single step. Why not just form the carbocation and then close up to the oxirane (Rg. 10.26) ... 

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