Epoxide

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A generic epoxide.

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.[1] 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.

Synthesis[edit]

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.[1]

Heterogeneously catalyzed oxidation of alkenes[edit]

The epoxidation of ethylene involves its reaction of oxygen according to the following stoichiometry:

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[edit]

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.[2] 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.[3]

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

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.[4] 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[edit]

More typically for laboratory operations, the Prilezhaev reaction is employed.[5][6] 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:[7]

Prilezhaev Reaction

The reaction proceeds via what is commonly known as the "Butterfly Mechanism."[8] 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.[9]

Butterfly Mechanism

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[edit]

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.[10][11]

The Sharpless epoxidation reaction is one of the premier enantioselective chemical reactions. It used to prepare 2,3-epoxyalcohols from primary and secondary allylic alcohols.[12][13]

Intramolecular SN2 substitution[edit]

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:[14] Most of the world's supply of propylene oxide arises via this route.[4]

Methyloxirane from 2-chloroproprionic 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.

Nucleophilic epoxidation[edit]

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.

Biosynthesis[edit]

Epoxides are not common in nature. They arise usually via oxygenation of alkenes by the action of cytochrome P450.[15]

Reactions[edit]

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.

EpoxOpen.png
  • 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.

Niche reactions[edit]

Deoxygenation of an epoxide with tungsten hexachloride/n-butyllithium

Uses[edit]

Illustrative epoxides
Epichlorohydrin is used in the production of epoxy resins
The chemical structure of the epoxide glycidol, a common chemical intermediate. 
Epothilones are naturally-occurring epoxides. 
Epoxidized linolein, a major component of epoxidized soybean oil (ESBO), a commercially important plasticizer
Example 2a

Ethylene oxide is widely used to generate detergents and surfactants by ethoxylation.[1] 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.


Safety[edit]

Epoxides are alkylating agents can be highly toxic.

See also[edit]

References[edit]

  1. ^ a b c Guenter Sienel; Robert Rieth; Kenneth T. Rowbottom (2005), "Epoxides", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a09_531 
  2. ^ Indictor, N.; Brill, W. F. J. Org. Chem., 1965, 30 (6), 2074. (doi: 10.1021/jo01017a520).
  3. ^ W. R. Thiel "Metal catalyzed oxidations. Part 5. Catalytic olefin epoxidation with seven-coordinate oxobisperoxo molybdenum complexes: a mechanistic study" Journal of Molecular Catalysis A: Chemical 1997, Volume 117, pp. 449–454. doi:10.1016/S1381-1169(96)00291-9
  4. ^ a b Dietmar Kahlich, Uwe Wiechern, Jörg Lindner “Propylene Oxide” in Ullmann's Encyclopedia of Industrial Chemistry, 2002 by Wiley-VCH, Weinheim. doi:10.1002/14356007.a22_239 Article Online Posting Date: June 15, 2000
  5. ^ March, Jerry. 1985. Advanced Organic Chemistry, Reactions, Mechanisms and Structure. 3rd ed. John Wiley & Sons. ISBN 0-471-85472-7.
  6. ^ Nikolaus Prileschajew (1909). "Oxydation ungesättigter Verbindungen mittels organischer Superoxyde". Berichte der deutschen chemischen Gesellschaft 42 (4): 4811–4815. doi:10.1002/cber.190904204100. 
  7. ^ Harold Hibbert and Pauline Burt (1941). "Styrene Oxide". Org. Synth. ; Coll. Vol. 1, p. 494 
  8. ^ Bartlett Rec. Chem. Prog 1950, 11 47.
  9. ^ Edwards, J. O. In Perodixe Reaction Mechanism, Ed.; Wiley:New York, 1962, 67-106.
  10. ^ D. J. Berrisford, C. Bolm,. K. B. Sharpless "Ligand-Accelerated Catalysis" Angew. Chem. Int. Ed. Engl. 2003, volume 95, pp. 1059–1070. doi:10.1002/anie.199510591
  11. ^ Sheldon, R. A. Journal of Molecular Catalysis, 1980, 1, 107-206. (doi: 10.1016/0304-5102(80)85010-3).
  12. ^ Katsuki, T.; Sharpless, K. B. (1980). "The first practical method for asymmetric epoxidation". J. Am. Chem. Soc. 102 (18): 5974. doi:10.1021/ja00538a077. 
  13. ^ Hill, J. G.; Sharpless, K. B.; Exon, C. M.; Regenye, R. Org. Syn., Coll. Vol. 7, p. 461 (1990); Vol. 63, p. 66 (1985). (Article)
  14. ^ 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, p. 434 
  15. ^ C. J. Thibodeaux, W.-c. Chang, H.-w. Liu "Enzymatic Chemistry of Cyclopropane, Epoxide, and Aziridine Biosynthesis" Chem. Rev., 2012, vol. 112, pp 1681–1709. doi:10.1021/cr200073d
  16. ^ B. Mudryk, T. Cohen "1,3-Diols From Lithium Β-lithioalkoxides Generated By The Reductive Lithiation Of Epoxides: 2,5-dimethyl-2,4-hexanediol"Org. Synth. 1995, volume 72, 173. doi:10.15227/orgsyn.072.0173
  17. ^ 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.