Epoxidation

Epoxidation
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Epoxidation".

The chemical structure of the epoxide glycidol, a common chemical intermediate

An epoxide is a cyclic ether with only three ring atoms. This ring approximately is an equilateral triangle, i.e. its bond angles are about 60°, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers, especially towards nucleophiles. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in chloromethyloxirane. As a functional group epoxides obtain the epoxy prefix such as in the compound 1,2-epoxycycloheptane which can also be called cycloheptene epoxide.

A polymer containing unreacted epoxide units is called a polyepoxide or an epoxy. Epoxy resins are used as adhesives and structural materials. Polymerization of an epoxide gives a polyether, for example ethylene oxide polymerizes to give polyethylene glycol, also known as polyethylene oxide.

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1 Synthesis
2 Reactions
3 See also
4 References

Synthesis

Epoxides are usually created by one of the following reactions:

Olefin peroxidation

Olefin peroxidation, also known as the Prilezhaev reaction ^[1]^[2] involves the oxidation of an alkene with a peroxide, usually a peroxyacid like m-CPBA or with a dioxirane like DMDO. An example is the epoxidation of styrene with perbenzoic acid to styrene oxide:^[3]

The reaction proceeds via what is commonly known as the Butterfly Mechanism.^[4] It is easiest to consider the oxygen to be an electrophile, and the alkene a nucleophile, although they both operate in that capacity, and the reaction is considered to be concerted (the numbers in the mechanism below are for simplification).

Related processes include some catalytic enantioselective reactions, such as the:

Intramolecular S_N2 substitution

This method is a variant of the Williamson ether synthesis. In this case, the alkoxide ion and the halide are right next to each other in the same molecule (such compounds are generically called halohydrins), which makes this a simple ring closure reaction. For example, with 2-chloropropanol:^[5]

Johnson-Corey-Chaykovsky reaction

In the Johnson-Corey-Chaykovsky reaction epoxides are generated from carbonyl groups and sulfonium ylides.

Reactions

Typical epoxide reactions are listed below.

Nucleophilic addition to an epoxide can be base or acid catalyzed.

Under acidic conditions, the nucleophile attacks the carbon that will form the most stable carbocation, i.e. the most substituted carbon (similar to a halonium ion). Under basic conditions, the nucleophile attacks the least substituted carbon, in accordance with standard S_N2 nuclephilic addition reaction process.

Hydrolysis of an epoxide in presence of an acid catalyst generates a glycol. The hydrolysis process of epoxides can be considered to be the nucleophilic addition of water to the epoxide under acidic conditions.
Reduction of an epoxide with lithium aluminium hydride and water generates an alcohol. This reduction process can be considered to be the nucleophilic addition of hydride (H-) to the epoxide under basic conditions.

Reduction with tungsten hexachloride and n-butyllithium generates the alkene. This reaction in effect is a de-epoxidation:^[6]

De-epoxidation with tungsten hexachloride / n-butyllithium

References

^ March, Jerry (1985). Advanced Organic Chemistry, Reactions, Mechanisms and Structure, third Edition, John Wiley & Sons. ISBN 0-471-85472-7.
^ Nikolaus Prileschajew (1909). "Oxydation ungesättigter Verbindungen mittels organischer Superoxyde". Berichte der deutschen chemischen Gesellschaft 42 (4): 4811–4815. doi:10.1002/cber.190904204100.
^ Harold Hibbert and Pauline Burt (1941). "Styrene Oxide". Org. Synth.; Coll. Vol. 1: 494.
^ Bartlett Rec. Chem. Prog 1950, 11 47.
^ Koppenhoefer, B.; Schurig, V. (1993). "(R)-Alkyloxiranes of High Enantiomeric Purity from (S)-2-Chloroalkanoic Acids via (S)-2-Chloro-1-Alkanols: (R)-Methyloxirane". Org. Synth.; Coll. Vol. 8: 434.
^ K. Barry Sharpless, Martha A. Umbreit, Marjorie T. Nieh, Thomas C. Flood (1972). "Lower valent tungsten halides. New class of reagents for deoxygenation of organic molecules". J. Am. Chem. Soc. 94 (18): 6538-6540. doi:10.1021/ja00773a045.

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