Silyl Enol Ether

Silyl enol ethers in organic chemistry are a class of organic compounds that share a common functional group composed of an enolate bonded through its oxygen end to an organosilicon group. They are important intermediates in organic synthesis.

Synthesis

Silyl enol ethers are generally prepared by reacting an enolizable carbonyl compound with a silyl electrophile and a base, or just reacting an enolate with a silyl electrophile.Clayden, J., Greeves, N., & Warren, S. (2012). Silyl enol ethers. In ''Organic chemistry'' (Second ed., pp. 466-467). Oxford University Press. Since silyl electrophiles are hard and silicon-oxygen bonds are very strong, the oxygen (of the carbonyl compound or enolate) acts as the nucleophile to form a Si-O single bond.

The most commonly used silyl electrophile is trimethylsilyl chloride. To increase the rate of reaction, trimethylsilyl triflate may also be used in the place of trimethylsilyl chloride as a more electrophilic substrate.

When using an unsymmetrical enolizable carbonyl compound as a substrate, the choice of reaction conditions can help control whether the kinetic or thermodynamic silyl enol ether is preferentially formed.Chan, T.-H. (1991). Formation and Addition Reactions of Enol Ethers. In ''Comprehensive Organic Synthesis'' (pp. 595–628). Elsevier. doi:10.1016/B978-0-08-052349-1.00042-1 For instance, when using lithium diisopropylamide (LDA), a strong and sterically hindered base, at low temperature (e.g., -78℃), the kinetic silyl enol ether (with a less substituted double bond) preferentially forms due to sterics.Clayden, J., Greeves, N., & Warren, S. (2012). Kinetically controlled enolate formation. In ''Organic chemistry'' (Second ed., pp. 600-601). Oxford University Press. When using triethylamine, a weak base, the thermodynamic silyl enol ether (with a more substituted double bond) is preferred.

Alternatively, a rather exotic way of generating silyl enol ethers is via the Brook rearrangement of appropriate substrates.

Reactions

General reaction profile

Silyl enol ethers are neutral, mild nucleophiles (milder than enamines) that react with good electrophiles such as aldehydes (with Lewis acid catalysis) and carbocations.Clayden, J., Greeves, N., & Warren, S. (2012). Silyl enol ethers in aldol reactions. In ''Organic chemistry'' (Second ed., pp. 626-627). Oxford University Press.Clayden, J., Greeves, N., & Warren, S. (2012). Silyl enol ethers are alkylated by S_N1-reactive electrophiles in the presence of Lewis acid. In ''Organic chemistry'' (Second ed., p. 595). Oxford University Press.Clayden, J., Greeves, N., & Warren, S. (2012). Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product. In ''Organic chemistry'' (Second ed., pp. 608-609). Oxford University Press. Silyl enol ethers are stable enough to be isolated, but are usually used immediately after synthesis.

Generation of lithium enolate

Lithium enolates, one of the precursors to silyl enol ethers, can also be generated from silyl enol ethers using methyllithium.House, H. O., Gall, M., & Olmstead, H. D. (1971). Chemistry of carbanions. XIX. Alkylation of enolates from unsymmetrical ketones. ''The Journal of Organic Chemistry'', ''36''(16), 2361–2371. doi:10.1021/jo00815a037 The reaction occurs via nucleophilic substitution at the silicon of the silyl enol ether, producing the lithium enolate and tetramethylsilane.

C–C bond formation

Silyl enol ethers are used in many reactions resulting in alkylation, e.g., Mukaiyama aldol addition, Michael reactions, and Lewis-acid-catalyzed reactions with S_N1-reactive electrophiles (e.g., tertiary, allylic, or benzylic alkyl halides). Alkylation of silyl enol ethers is especially efficient with tertiary alkyl halides, which form stable carbocations in the presence of Lewis acids like TiCl₄ or SnCl₄.

Halogenation and oxidations

Halogenation of silyl enol ethers gives haloketones.Clayden, J., Greeves, N., & Warren, S. (2012). Reactions of silyl enol ethers with halogen and sulfur electrophiles. In ''Organic chemistry'' (Second ed., pp. 469-470). Oxford University Press.

Acyloins form upon organic oxidation with an electrophilic source of oxygen such as an oxaziridine or mCPBA.

In the Saegusa–Ito oxidation, certain silyl enol ethers are oxidized to enones with palladium(II) acetate.
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Sulfenylation

Reacting a silyl enol ether with PhSCl, a good and soft electrophile, provides a carbonyl compound sulfenylated at an alpha carbon. In this reaction, the trimethylsilyl group of the silyl enol ether is removed by the chloride ion released from the PhSCl upon attack of its electrophilic sulfur atom.

Hydrolysis

Hydrolysis of a silyl enol ether results in the formation of a carbonyl compound and a disiloxane.Clayden, J., Greeves, N., & Warren, S. (2012). Hydrolysis of enol ethers. In ''Organic chemistry'' (Second ed., pp. 468-469). Oxford University Press. In this reaction, water acts as an oxygen nucleophile and attacks the silicon of the silyl enol ether. This leads to the formation of the carbonyl compound and a trimethylsilanol intermediate that undergoes nucleophilic substitution at silicon (by another trimethylsilanol) to give the disiloxane.

Ring contraction

Cyclic silyl enol ethers undergo regiocontrolled one-carbon ring contractions.Mitcheltree, M. J.; Konst, Z. A.; Herzon, S. B. ''Tetrahedron'' 2013, ''69'', 5634. These reactions employ electron-deficient sulfonyl azides, which undergo chemoselective, uncatalyzed +2cycloaddition to the silyl enol ether, followed by loss of dinitrogen, and alkyl migration to give ring-contracted products in good yield. These reactions may be directed by substrate stereochemistry, giving rise to stereoselective ring-contracted product formation.

Silyl ketene acetals

Silyl enol ethers of esters or carboxylic acids are called silyl ketene acetals. These compounds are more nucleophilic than the silyl enol ethers of ketones.

References

Functional groups
Organosilicon compounds
Ethers
Alkene derivatives
Silanes