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 terminus to an organosilicon group.

Silyl enol ethers are important intermediates in organic synthesis.

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.^[1][2]
• A rather exotic way to generate silyl enol ethers is via the Brook rearrangement of appropriate substrates.^[3]

Organic 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 dienenone is synthesized in two steps from a cyclohexanone:^[7][8]

Ketene silyl acetals

Ketene silyl acetals are related compounds formally derived from ketenes and acetals with general structure R-C=C(OSiR₃)(OR').

References

1. ^ Organic Syntheses, Coll. Vol. 9, p.564 (1998); Vol. 73, p.123 (1996) Article
2. ^ Organic Syntheses, Coll. Vol. 8, p.277 (1993); Vol. 66, p. 43 (1988) Article.
3. ^ Tong, R.; McDonald, F. E. (2008). "Mimicking Biosynthesis: Total Synthesis of the Triterpene Natural Product Abudinol B from a Squalene-like Precursor". Angewandte Chemie 47 (23): 4377–4379. doi:10.1002/anie.200800749. 
4. ^ Organic Syntheses, Coll. Vol. 8, p.286 (1993); Vol. 69, p.129 (1990) Article
5. ^ Organic Syntheses, Coll. Vol. 7, p.282 (1990); Vol. 64, p.118 (1986) Article.
6. ^ 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. 
7. ^ 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. 
8. ^ reagents in step 1 are trimethylsilyl triflate and 2,6-lutidine