An epoxide is a cyclic ether with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in chloromethyloxirane. When appearing as a functional group, an epoxide carries the epoxy prefix, as in the compound 1,2-epoxycycloheptane, which can also be called cycloheptene epoxide, or simply cycloheptene oxide. A polymer formed by reacting 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.
- 1 Synthesis
- 1.1 Heterogeneously catalyzed oxidation of alkenes
- 1.2 Olefin oxidation using organic peroxides and metal catalysts
- 1.3 Olefin peroxidation using peroxycarboxylic acids
- 1.4 Homogeneously catalysed asymmetric epoxidations
- 1.5 Intramolecular SN2 substitution
- 1.6 Nucleophilic epoxidation
- 1.7 Biosynthesis
- 2 Reactions
- 3 Uses
- 4 Safety
- 5 See also
- 6 References
Heterogeneously catalyzed oxidation of alkenes
- 7 H2C=CH2 + 6 O2 → 6 C2H4O + 2 CO2 + 2 H2O
The direct reaction of oxygen with alkenes is useful only for this epoxide. It requires a silver catalyst. Other alkenes fail to react usefully, even propylene.
Olefin oxidation using organic peroxides and metal catalysts
Many epoxides are generated by treating alkenes with peroxide-containing reagents, which donate a single oxygen atom. Metal complexes are useful catalysts for these reactions. Typical peroxide reagents include hydrogen peroxide, peroxycarboxylic acids (generated in-situ or preformed), and alkyl hydroperoxides. In specialized applications, other peroxide-containing reagents are employed, such as dimethyldioxirane. Depending on the mechanism of the reaction and the geometry of the alkene starting material, cis and/or trans epoxide diastereomers may be formed. In addition, if there are other stereocenters present in the starting material, they can influence the stereochemistry of the epoxidation relative to them. The metal-catalyzed epoxidation was first explored using tert-butyl hydroperoxide (TBHP) as a source of an O atom. Association of TBHP with the metal generates the active metal catalyst with a peroxy ligand (MOOR), which then transfers an O center to the alkene.
This approach has been used for the production of propylene oxide from propylene using molybdenum-based catalysts. Both t-butyl hydroperoxide or ethylbenzene hydroperoxide can be used as oxygen sources. The process suffers from safety considerations owing to the risks of combustion from the combination of peroxides and alkene substrate, a consideration that weighs on almost all oxidative routes to epoxides.
Olefin peroxidation using peroxycarboxylic acids
More typically for laboratory operations, the Prilezhaev reaction is employed. This approach involves the oxidation of the alkene with a peroxyacid such as m-CPBA. Illustrative is the epoxidation of styrene with perbenzoic acid to styrene oxide:
The reaction proceeds via what is commonly known as the "Butterfly Mechanism." The peroxide is viewed as an electrophile, and the alkene a nucleophile. The reaction is considered to be concerted (the numbers in the mechanism below are for simplification).The butterfly mechanism allows ideal positioning of the O-O sigma star orbital for C-C Pi electrons to attack.
Hydroperoxides are also employed in catalytic enantioselective epoxidations, such as the Sharpless epoxidation and the Jacobsen epoxidation. Together with the Shi epoxidation, these reactions are useful for the enantioselective synthesis of chiral epoxides. Oxaziridine reagents may also be used to generate epoxides from alkenes.
Homogeneously catalysed asymmetric epoxidations
Intramolecular SN2 substitution
This method is a variant of the Williamson ether synthesis. In this case, an alkoxide ion displaces a chloride atom within the same molecule. The precursor compounds are called halohydrins. For example, with 2-chloropropanol: Most of the world's supply of propylene oxide arises via this route.
An intramolecular epoxide formation reaction is one of the key steps in the Darzens reaction.
Electron-deficient olefins, such as enones and acryl derivatives can be epoxidized using nucleophilic oxygen compounds such as peroxides. The reaction is a two-step mechanism. First the oxygen performs a nucleophilic conjugate addition to give a stabilized carbanion. This carbanion then attacks the same oxygen atom, displacing a leaving group from it, to close the epoxide ring.
Important epoxide reactions involve their ring-opening. Alcohols, water, amines, thiols and many other reagents undergo this reaction. This reaction is the basis of the formation of epoxy glues and the production of glycols.
- Under acidic conditions, the position the nucleophile attacks is affected both by steric effects (as normally seen for SN2 reactions) and by carbocationic stability (as normally seen for SN1 reactions). Under basic conditions, the nucleophile attacks the least substituted carbon, in accordance with standard SN2 nucleophilic 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.
- Reductive cleavage of epoxides gives b-lithioalkoxides.
- Reduction with tungsten hexachloride and n-butyllithium generates the alkene:
Ethylene oxide is widely used to generate detergents and surfactants by ethoxylation. Its hydrolysis affords ethylene glycol. Their reaction with amines is the basis of the formation of epoxy glues, e.g., Triethylenetetramine (TETA) as a hardener.
Epoxides are alkylating agents can be highly toxic.
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