Silyl enol ether

1=Class of organosilicon compounds of the form R3Si–O–CR=CR2

In organosilicon chemistry, silyl enol ethers are a class of organic compounds that share the common functional group |R3Si\sO\sCR\dCR2, composed of an enolate (|R3C\sO\sR) bonded to a silane (|SiR4) through its oxygen end and an ethene group (|R2C\dCR2) as its carbon end. They are important intermediates in organic 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. 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. 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. For instance, when using lithium diisopropylamide (LDA), a strong and sterically hindered base, at low temperature (e. g., 78°C), the kinetic silyl enol ether (with a less substituted double bond) preferentially forms due to sterics. 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.

Silyl enol ethers are neutral, mild nucleophiles (milder than enamines) that react with good electrophiles such as aldehydes (with Lewis acid catalysis) and carbocations. Silyl enol ethers are stable enough to be isolated, but are usually used immediately after synthesis. 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.

Reacting a silyl enol ether with PhSCl, a good and soft electrophile, provides a carbonyl compound sulfenylated at an alpha carbon. In this reaction, water acts as an oxygen nucleophile and attacks the silicon of the silyl enol ether. 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.

Silyl enol ethers of esters (|\sOR) or carboxylic acids (|\sCOOH) are called silyl ketene acetals and have the general structure |R3Si\sO\sC(OR)\dCR2. These compounds are more nucleophilic than the silyl enol ethers of ketones (|>C\dO).