Epoxide

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A generic epoxide Epoxide generic.svg
A generic epoxide

In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ethers. They are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless and nonpolar, and often volatile. [1]

Contents

Nomenclature

A compound containing the epoxide functional group can be called an epoxy, epoxide, oxirane, and ethoxyline. Simple epoxides are often referred to as oxides. Thus, the epoxide of ethylene (C2H4) is ethylene oxide (C2H4O). Many compounds have trivial names; for instance, ethylene oxide is called "oxirane". Some names emphasize the presence of the epoxide functional group, as in the compound 1,2-epoxyheptane, which can also be called 1,2-heptene oxide.

A polymer formed from epoxide precursors is called an epoxy . However, few if any of the epoxy groups in the resin survive the curing process.

Synthesis

The dominant epoxides industrially are ethylene oxide and propylene oxide, which are produced respectively on the scales of approximately 15 and 3 million tonnes/year. [2]

Aside from ethylene oxide, most epoxides are generated when peroxidized reagents donate a single oxygen atom to an alkene. Safety considerations weigh on these reactions because organic peroxides are prone to spontaneous decomposition or even combustion.

Both t-butyl hydroperoxide and ethylbenzene hydroperoxide can be used as oxygen sources during propylene oxidation (although a catalyst is required as well, and most industrial producers use dehydrochlorination instead). [3]

Ethylene oxidation

The ethylene oxide industry generates its product from reaction of ethylene and oxygen. Modified heterogeneous silver catalysts are typically employed. [4] According to a reaction mechanism suggested in 1974 [5] at least one ethylene molecule is totally oxidized for every six that are converted to ethylene oxide:

Only ethylene produces an epoxide during incomplete combustion. Other alkenes fail to react usefully, even propylene, though TS-1 supported Au catalysts can selectively epoxidize propylene. [6]

Organic peroxides and metal catalysts

Metal complexes are useful catalysts for epoxidations involving hydrogen peroxide and alkyl hydroperoxides. Metal-catalyzed epoxidations were first explored using tert-butyl hydroperoxide (TBHP). [7] Association of TBHP with the metal (M) generates the active metal peroxy complex containing the MOOR group, which then transfers an O center to the alkene. [8]

Simplified mechanism for metal-catalyzed epoxidation of alkenes with peroxide (ROOH) reagents SimpleMOORexpxCyc2.svg
Simplified mechanism for metal-catalyzed epoxidation of alkenes with peroxide (ROOH) reagents

Vanadium(II) oxide catalyzes the epoxidation at specifically less-substituted alkenes. [9]

Nucleophilic epoxidation

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.

Transfer from peroxycarboxylic acids

Peroxycarboxylic acids, which are more electrophilic than other peroxides, convert alkenes to epoxides without the intervention of metal catalysts. In specialized applications, dioxirane reagents (e.g. dimethyldioxirane) perform similarly, but are more explosive.

Typical laboratory operations employ the Prilezhaev reaction. [10] [11] This approach involves the oxidation of the alkene with a peroxyacid such as mCPBA. Illustrative is the epoxidation of styrene with perbenzoic acid to styrene oxide: [12]

PrilezhaevReaction.svg

The stereochemistry of the reaction is quite sensitive. 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.

The reaction proceeds via what is commonly known as the "Butterfly Mechanism". [13] The peroxide is viewed as an electrophile, and the alkene a nucleophile. The reaction is considered to be concerted. The butterfly mechanism allows ideal positioning of the O−O sigma star orbital for C−C π electrons to attack. [14] Because two bonds are broken and formed to the epoxide oxygen, this is formally an example of a coarctate transition state.

The butterfly mechanism for the Prilezhaev epoxidation reaction. Epoxidation butterfly mechanism.svg
The butterfly mechanism for the Prilezhaev epoxidation reaction.

Asymmetric epoxidations

Chiral epoxides can often be derived enantioselectively from prochiral alkenes. Many metal complexes give active catalysts, but the most important involve titanium, vanadium, and molybdenum. [15] [16]

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.

The Sharpless epoxidation reaction is one of the premier enantioselective chemical reactions. It is used to prepare 2,3-epoxyalcohols from primary and secondary allylic alcohols. [17] [18]

Sharpless epoxidation DE.svg

Dehydrohalogenation and other γ eliminations

Epichlorohydrin, is prepared by the chlorohydrin method. It is a precursor in the production of epoxy resins. Epichlorohydryna.svg
Epichlorohydrin, is prepared by the chlorohydrin method. It is a precursor in the production of epoxy resins.

Halohydrins react with base to give epoxides. [20] The reaction is spontaneous because the energetic cost of introducing the ring strain (13 kcal/mol) is offset by the larger bond enthalpy of the newly introduced C-O bond (when compared to that of the cleaved C-halogen bond).

Formation of epoxides from secondary halohydrins is predicted to occur faster than from primary halohydrins due to increased entropic effects in the secondary halohydrin, and tertiary halohydrins react (if at all) extremely slowly due to steric crowding. [21]

Starting with propylene chlorohydrin, most of the world's supply of propylene oxide arises via this route. [3]

Methyloxirane from 2-chloropropionic acid.png

An intramolecular epoxide formation reaction is one of the key steps in the Darzens reaction.

In the Johnson–Corey–Chaykovsky reaction epoxides are generated from carbonyl groups and sulfonium ylides. In this reaction, a sulfonium is the leaving group instead of chloride.

Biosynthesis

Epoxides are uncommon in nature. They arise usually via oxygenation of alkenes by the action of cytochrome P450. [22] (but see also the short-lived epoxyeicosatrienoic acids which act as signalling molecules. [23] and similar epoxydocosapentaenoic acids, and epoxyeicosatetraenoic acids.)

Arene oxides are intermediates in the oxidation of arenes by cytochrome P450. For prochiral arenes (naphthalene, toluene, benzoates, benzopyrene), the epoxides are often obtained in high enantioselectivity.

Reactions

Ring-opening reactions dominate the reactivity of epoxides.

Hydrolysis and addition of nucleophiles

Two pathways for the hydrolysis of an epoxide Epoxide hydrolysis.svg
Two pathways for the hydrolysis of an epoxide

Epoxides react with a broad range of nucleophiles, for example, alcohols, water, amines, thiols, and even halides. With two often-nearly-equivalent sites of attack, epoxides exemplify "ambident substrates". [24] Ring-opening regioselectivity in asymmetric epoxides generally follows the SN2 pattern of attack at the least-substituted carbon, [25] but can be affected by carbocation stability under acidic conditions. [26] This class of reactions is the basis of epoxy glues and the production of glycols. [19]

Lithium aluminium hydride or aluminium hydride both reduce epoxides through a simple nucleophilic addition of hydride (H−); they produce the corresponding alcohol. [27]

Polymerization and oligomerization

Polymerization of epoxides gives polyethers. For example ethylene oxide polymerizes to give polyethylene glycol, also known as polyethylene oxide. The reaction of an alcohol or a phenol with ethylene oxide, ethoxylation, is widely used to produce surfactants: [28]

ROH + n C2H4O → R(OC2H4)nOH

With anhydrides, epoxides give polyesters. [29]

Metallation and deoxygenation

Lithiation cleaves the ring to β-lithioalkoxides. [30]

Epoxides can be deoxygenated using oxophilic reagents, with loss or retention of configuration. [31] The combination of tungsten hexachloride and n-butyllithium gives the alkene. [32] [33]

When treated with thiourea, epoxides convert to the episulfide (thiiranes).

Other reactions

Uses

Ethylene oxide is widely used to generate detergents and surfactants by ethoxylation. Its hydrolysis affords ethylene glycol. It is also used for sterilisation of medical instruments and materials.

The reaction of epoxides with amines is the basis for the formation of epoxy glues and structural materials. A typical amine-hardener is triethylenetetramine (TETA).


Safety

Epoxides are alkylating agents, making many of them highly toxic. [35]

See also

Further reading

Related Research Articles

<span class="mw-page-title-main">Sharpless epoxidation</span> Chemical reaction

The Sharpless epoxidation reaction is an enantioselective chemical reaction to prepare 2,3-epoxyalcohols from primary and secondary allylic alcohols. The oxidizing agent is tert-butyl hydroperoxide. The method relies on a catalyst formed from titanium tetra(isopropoxide) and diethyl tartrate.

Sharpless asymmetric dihydroxylation is the chemical reaction of an alkene with osmium tetroxide in the presence of a chiral quinine ligand to form a vicinal diol. The reaction has been applied to alkenes of virtually every substitution, often high enantioselectivities are realized, with the chiral outcome controlled by the choice of dihydroquinidine (DHQD) vs dihydroquinine (DHQ) as the ligand. Asymmetric dihydroxylation reactions are also highly site selective, providing products derived from reaction of the most electron-rich double bond in the substrate.

The Baeyer–Villiger oxidation is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone, using peroxyacids or peroxides as the oxidant. The reaction is named after Adolf von Baeyer and Victor Villiger who first reported the reaction in 1899.

(<i>E</i>)-Stilbene Chemical compound

(E)-Stilbene, commonly known as trans-stilbene, is an organic compound represented by the condensed structural formula C6H5CH=CHC6H5. Classified as a diarylethene, it features a central ethylene moiety with one phenyl group substituent on each end of the carbon–carbon double bond. It has an (E) stereochemistry, meaning that the phenyl groups are located on opposite sides of the double bond, the opposite of its geometric isomer, cis-stilbene. Trans-stilbene occurs as a white crystalline solid at room temperature and is highly soluble in organic solvents. It can be converted to cis-stilbene photochemically, and further reacted to produce phenanthrene.

<span class="mw-page-title-main">Jacobsen epoxidation</span>

The Jacobsen epoxidation, sometimes also referred to as Jacobsen-Katsuki epoxidation is a chemical reaction which allows enantioselective epoxidation of unfunctionalized alkyl- and aryl- substituted alkenes. It is complementary to the Sharpless epoxidation (used to form epoxides from the double bond in allylic alcohols). The Jacobsen epoxidation gains its stereoselectivity from a C2 symmetric manganese(III) salen-like ligand, which is used in catalytic amounts. The manganese atom transfers an oxygen atom from chlorine bleach or similar oxidant. The reaction takes its name from its inventor, Eric Jacobsen, with Tsutomu Katsuki sometimes being included. Chiral-directing catalysts are useful to organic chemists trying to control the stereochemistry of biologically active compounds and develop enantiopure drugs.

<span class="mw-page-title-main">Hydroperoxide</span> Class of chemical compounds

Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically organic, which contain the hydroperoxy functional group. Hydroperoxide also refers to the hydroperoxide anion and its salts, and the neutral hydroperoxyl radical (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with unsaturated chemical bonds.

<span class="mw-page-title-main">Shi epoxidation</span>

The Shi epoxidation is a chemical reaction described as the asymmetric epoxidation of alkenes with oxone and a fructose-derived catalyst (1). This reaction is thought to proceed via a dioxirane intermediate, generated from the catalyst ketone by oxone. The addition of the sulfate group by the oxone facilitates the formation of the dioxirane by acting as a good leaving group during ring closure. It is notable for its use of a non-metal catalyst and represents an early example of organocatalysis.

Asymmetric catalytic oxidation is a technique of oxidizing various substrates to give an enantio-enriched product using a catalyst. Typically, but not necessarily, asymmetry is induced by the chirality of the catalyst. Typically, but again not necessarily, the methodology applies to organic substrates. Functional groups that can be prochiral and readily susceptible to oxidation include certain alkenes and thioethers. Challenging but pervasive prochiral substrates are C-H bonds of alkanes. Instead of introducing oxygen, some catalysts, biological and otherwise, enantioselectively introduce halogens, another form of oxidation.

<span class="mw-page-title-main">Prilezhaev reaction</span> Chemical reaction

The Prilezhaev reaction, also known as the Prileschajew reaction or Prilezhaev epoxidation, is the chemical reaction of an alkene with a peroxy acid to form epoxides. It is named after Nikolai Prilezhaev, who first reported this reaction in 1909. A widely used peroxy acid for this reaction is meta-chloroperoxybenzoic acid (m-CPBA), due to its stability and good solubility in most organic solvents. The reaction is performed in inert solvents (C6H14, C6H6, CH2Cl2, CHCl3, CCl4) between -10 and 60 °C with the yield of 60-80%.

<span class="mw-page-title-main">Jacobsen's catalyst</span> Chemical compound

Jacobsen's catalyst is the common name for N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane­diaminomanganese(III) chloride, a coordination compound of manganese and a salen-type ligand. It is used as an asymmetric catalyst in the Jacobsen epoxidation, which is renowned for its ability to enantioselectively transform prochiral alkenes into epoxides. Before its development, catalysts for the asymmetric epoxidation of alkenes required the substrate to have a directing functional group, such as an alcohol as seen in the Sharpless epoxidation. This compound has two enantiomers, which give the appropriate epoxide product from the alkene starting material.

Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy.

Oxidation with dioxiranes refers to the introduction of oxygen into organic molecules through the action of a dioxirane. Dioxiranes are well known for their oxidation of alkenes to epoxides; however, they are also able to oxidize other unsaturated functionality, heteroatoms, and alkane C-H bonds.

Epoxidation with dioxiranes refers to the synthesis of epoxides from alkenes using three-membered cyclic peroxides, also known as dioxiranes.

Nucleophilic epoxidation is the formation of epoxides from electron-deficient double bonds through the action of nucleophilic oxidants. Nucleophilic epoxidation methods represent a viable alternative to electrophilic methods, many of which do not epoxidize electron-poor double bonds efficiently.

The Juliá–Colonna epoxidation is an asymmetric poly-leucine catalyzed nucleophilic epoxidation of electron deficient olefins in a triphasic system. The reaction was reported by Sebastian Juliá at the Chemical Institute of Sarriá in 1980, with further elaboration by both Juliá and Stefano Colonna.

<span class="mw-page-title-main">Oxaziridine</span> Chemical compound

An oxaziridine is an organic molecule that features a three-membered heterocycle containing oxygen, nitrogen, and carbon. In their largest application, oxaziridines are intermediates in the industrial production of hydrazine. Oxaziridine derivatives are also used as specialized reagents in organic chemistry for a variety of oxidations, including alpha hydroxylation of enolates, epoxidation and aziridination of olefins, and other heteroatom transfer reactions. Oxaziridines also serve as precursors to nitrones and participate in [3+2] cycloadditions with various heterocumulenes to form substituted five-membered heterocycles. Chiral oxaziridine derivatives effect asymmetric oxygen transfer to prochiral enolates as well as other substrates. Some oxaziridines also have the property of a high barrier to inversion of the nitrogen, allowing for the possibility of chirality at the nitrogen center.

<span class="mw-page-title-main">Metal salen complex</span> Coordination complex

A metal salen complex is a coordination compound between a metal cation and a ligand derived from N,N′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt Co2+, usually denoted as Co(salen). These complexes are widely investigated as catalysts and enzyme mimics.

<span class="mw-page-title-main">Allyl glycidyl ether</span> Chemical compound

Allyl glycidyl ether is an organic compound used in adhesives and sealants and as a monomer for polymerization reactions. It is formally the condensation product of allyl alcohol and glycidol via an ether linkage. Because it contains both an alkene and an epoxide group, either group can be reacted selectively to yield a product where the other functional group remains intact for future reactions.

In chemistry, the Halcon process refers to technology for the production of propylene oxide by oxidation of propylene with tert-butyl hydroperoxide. The reaction requires metal catalysts, which typically contain molybdenum:

<span class="mw-page-title-main">Epoxidation of allylic alcohols</span>

The epoxidation of allylic alcohols is a class of epoxidation reactions in organic chemistry. One implementation of this reaction is the Sharpless epoxidation. Early work showed that allylic alcohols give facial selectivity when using meta-chloroperoxybenzoic acid (m-CPBA) as an oxidant. This selectivity was reversed when the allylic alcohol was acetylated. This finding leads to the conclusion that hydrogen bonding played a key role in selectivity and the following model was proposed.

References

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