8.9. Reaction: Epoxidation

Instead of adding two groups across the alkene, it is possible to add a single group connected to both carbons of the alkene. This forms a three membered ring. While many three-membered rings are unstable, epoxides (a form of cyclic ether) are not. It is possible to add oxygen (O) to both sides of an alkene, forming an epoxide (Scheme 8.29).

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Scheme 8.29 – Generalized Reaction Equation for Epoxidation of an Alkene.

The most common reagents used to form epoxides are peroxyacids, with the overwhelming majority of reactions using meta-chloroperoxybenzoic acid (mCPBA). This is only because this compound is easily available; there is nothing special about this particular peroxyacid.

8.9.1. “Mechanism”

The currently accepted mechanism for epoxidation is complex, with 14 electrons flowing in a single step. This can be simplified/approximated by showing the movement of only 10 electrons (Scheme 8.30). Typically, having this many electrons flowing in a single reaction step would be highly unlikely. However, empirical evidence suggests this is the most accurate representation of what is occurring.

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Scheme 8.30 – Approximated Reaction Mechanism for Epoxidation of the Alkene of (E)-But-2-ene.

8.9.2. Regioselectivity

Because a single group is being added to both positions, there are no regioisomers and regioselectivity is not a concern.

8.9.3. “Stereoselectivity”

Technically, epoxidations of alkenes are stereospecific (diastereospecific). The reaction forms a three-membered ring, so both C-O bonds must be above or below the previous plane of the alkene (Scheme 8.31).

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Scheme 8.31 – Diastereospecific Epoxidation of the Alkene of Methylcyclohexene.

It is very important to remember that this is a form of diastereoselectivity (by being diastereospecific). There is no enantioselectivity.

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