Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص608-609
2025-06-29
662
The best alternatives to enamines for conjugate addition of enols of aldehyde, ketone, and carboxylic acid derivatives are silyl enol ethers. These stable neutral nucleophiles react very well with Michael acceptors either spontaneously or with Lewis acid catalysts such as TiCl4 at low temperature. If the 1,5-dicarbonyl compound is required, then an aqueous work-up with either acid or base cleaves the silicon–oxygen bond in the product.
Addition of the silyl enol ether derived from acetophenone (PhCOMe) to a disubstituted enone promoted by titanium tetrachloride is very rapid and gives the diketone product in good yield even though a quaternary carbon atom is created in the conjugate addition. This is a typical example of this very powerful class of conjugate addition reactions.
It is even possible to use a silyl enol ether to create a new C–C bond that joins two new quaternary centres. Silyl ketene acetals (the silyl enol ethers of esters) are more nucleophilic than ordinary silyl enol ethers, and in this example the silyl ketene acetal undergoes conju gate addition to an unsaturated ketone catalysed by the usual Lewis acid (TiCl4) for such reactions.
Ketene acetals
Because enol ethers of esters have two identical OR groups joined to the same end of the same double bond, you will see them called ‘ketene acetals’ or, here, ‘silyl ketene acetals’. This is a reasonable description as you can imagine the carbonyl group of a ketene forming an acetal in the same way as an aldehyde. In fact, they cannot be made this way.
In these reactions, the electrophile coordinates to the TiCl4 Lewis acid first, producing an activated enone that is attacked by the silylated nucleophile. It is difficult to determine at what stage the trimethylsilyl group moves from its original position and whether it is trans ferred intramolecularly to the product. In many cases the anion liberated from the Lewis acid (Cl−, RO−, Br−) is a good nucleophile for silicon so it is reasonable to assume that there is a free trimethylsilyl species (Me3SiX) that captures the titanium enolate:
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