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.[1][2]
Synthesis
Trimethylsilyl enol ethers can be prepared from ketones in presence of a strong base and trimethylsilyl chloride or a weak base and trimethylsilyl triflate. [3] Alternatively, enolate salts react with silyl electrophiles.
A rather exotic way to generate silyl enol ethers is via the Brook rearrangement of appropriate substrates.
Reactions
Silyl enol ethers react as nucleophiles in:
- Mukaiyama aldol addition
- Michael reactions
- Alkylations
- Haloketone formation with halogens[4]
- Acyloin formation by organic oxidation with an electrophilic source of oxygen such as an oxaziridine or mCPBA[5]
Saegusa–Ito oxidation
In the Saegusa–Ito oxidation certain silyl enol ethers are oxidized to enones with palladium(II) acetate. In the original publication[6] equal amounts of palladium and 1,4-benzoquinone are used to achieve the reaction with the benzoquinone acting as a co-oxidant. The intermediate is an oxo-allylpalladium complex.
In one application, a dienone is synthesized in two steps from a cyclohexanone:[7][8]
Ring contraction
Cyclic silyl enol ethers have been demonstrated to be viable substrates for regiocontrolled one-carbon ring contractions.[9][10] These reactions employ electron-deficient sulfonyl azides, which undergo chemoselective, uncatalyzed [3+2] cycloaddition 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.
Ketene silyl acetals
Ketene silyl acetals are related compounds formally derived from ketenes and acetals with general structure R-C=C(OSiR3)(OR').
References
- ^ Peter Brownbridge (1983). "Silyl Enol Ethers in Synthesis - Part I". Synthesis: 1–28. doi:10.1055/s-1983-30204.
- ^ Ian Fleming (2007). A Primer on Organosilicon Chemistry. Wiley. doi:10.1002/9780470513323.ch7.
- ^ "Acetone Trimethylsilyl Enol Ether". Org. Synth. 65: 1. 1987. doi:10.15227/orgsyn.065.0001.
{{cite journal}}: Unknown parameter|authors=ignored (help) - ^ Organic Syntheses, Coll. Vol. 8, p.286 (1993); Vol. 69, p.129 (1990) Article
- ^ Organic Syntheses, Coll. Vol. 7, p.282 (1990); Vol. 64, p.118 (1986) Article.
- ^ Ito, Yoshihiko; Hirao, Toshikazu; Saegusa, Takeo (1978). "Synthesis of alpha, beta-unsaturated carbonyl compounds by palladium(II)-catalyzed dehydrosilylation of silyl enol ethers". J. Org. Chem. 43 (5): 1011–1013. doi:10.1021/jo00399a052.
{{cite journal}}: Unknown parameter|lastauthoramp=ignored (|name-list-style=suggested) (help) - ^ Clive, Derrick L. J.; Sunasee, Rajesh (2007). "Formation of Benzo-Fused Carbocycles by Formal Radical Cyclization onto an Aromatic Ring". Org. Lett. 9 (14): 2677–2680. doi:10.1021/ol070849l. PMID 17559217.
{{cite journal}}: Unknown parameter|lastauthoramp=ignored (|name-list-style=suggested) (help) - ^ reagents in step 1 are trimethylsilyl triflate and 2,6-lutidine
- ^ (a) Wohl, R. Helv. Chim. Acta 1973, 56, 1826. (b) Xu, Y. Xu, G.; Zhu, G.; Jia, Y.; Huang, Q. J. Fluorine Chem. 1999, 96, 79.
- ^ Mitcheltree, M. J.; Konst, Z. A.; Herzon, S. B. Tetrahedron 2013, 69, 5634.