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CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 13 - Professor J. Michael McBride

Addition to Form Three-Membered Rings: Carbenoids and Epoxidation

After drill on the mechanism of the pinacol rearrangement, this lecture applies molecular-orbital analysis to simultaneous electrophilic/nucleophilic attack by a single atom to form a three-membered ring from an alkene. These reactions provide drill in consistent use of the curved-arrow formalism for describing electron-pair shifts. Two alternative mechanisms for formation of cyclopropanes by the alkylzinc Simmons-Smith "carbenoid" reagent are proposed, and the one-step mechanism is supported by theory. Epoxidation of alkenes by peroxycarboxylic acids also seems to go by way of a concerted electrophilic/nucleophilic process involving a single transition state. The stereochemistry and scale of various paths to epoxides is discussed in the context of their commercial utility.

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