Formation of epoxides from alkenes using m-CPBA

by James

Description: meta-chloroperoxybenzoic acid (m-CPBA) or other peroxyacids such as peracetic acids will convert alkenes into epoxides.

Notes: The stereochemistry of this reaction is always “syn” (both bonds to oxygen are formed on the same side of the alkene).

Examples:

Notes: Note that example 2 just shows m-CPBA written out – it’s in no way different from the others. Examples 3 and 4 show that the reaction is stereospecific – that is, trans alkenes give trans products, and cis alkenes give cis products. Also note there is no reaction with alkynes.

Mechanism:

This is a reaction which the arrow pushing formalism isn’t very good at describing. Instead, a transition state is shown here, which shows breaking of the C–C π bond at the same time the C–O bonds are forming. At the same time the O–O bond is breaking and the hydrogen from the OH is being picked up by the carbonyl oxygen.

Notes: Note that the byproduct here is m-chlorobenzoic acid (in grey).

{ 8 comments… read them below or add one }

Chan

Will this same mechanism holds good even for the same reaction with Alkyne?

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james

This reaction doesn’t work with alkynes! Can you think why not? [hint – it has something to do with aromaticity]

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Jeffery

Has it got something to do with anti aromaticity?

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James Ashenhurst

Yes, alkynes do not react here, because they form anti-aromatic oxirenes (very unstable)

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Jeffery

Thanks! ;)

Erik Haensel

What happens if you have a cyclohexane with two double bonds combined with two equivalents of mCPBA? Does it epoxidate both bonds?

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James

It should. 1,4 cyclohexadiene or 1,3 cyclohexadiene? The di=epoxide from 1,4 cyclohexadiene is definitely known.

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Erik Haensel

Thanks!

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