Silyl enol ether
Silyl enol ethers are important intermediates in organic 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.
- Silyl enol ether can form by capturing any enolate formed in a nucleophilic conjugate addition.
- A rather exotic way to generate silyl enol ethers is via the Brook rearrangement of appropriate substrates.
Silyl enol ethers react as nucleophiles in:
- Mukaiyama aldol addition
- Michael reactions
- Haloketone formation with halogens
- Acyloin formation by 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. In the original publication 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.
Cyclic silyl enol ethers have been demonstrated to be viable substrates for regiocontrolled one-carbon ring contractions. 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
- Organic Syntheses, Coll. Vol. 9, p.564 (1998); Vol. 73, p.123 (1996) Article
- Organic Syntheses, Coll. Vol. 8, p.277 (1993); Vol. 66, p. 43 (1988) Article.
- Tong, R.; McDonald, F. E. (2008). "Mimicking Biosynthesis: Total Synthesis of the Triterpene Natural Product Abudinol B from a Squalene-like Precursor". Angewandte Chemie (Int. ed.) 47 (23): 4377–4379. doi:10.1002/anie.200800749.
- 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.
- 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.
- 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.