Silyl enol ether

[4][5] 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.

[6][7] When using triethylamine, a weak base, the thermodynamic silyl enol ether (with a more substituted double bond) is preferred.

[10] Silyl enol ethers are neutral, mild nucleophiles (milder than enamines) that react with good electrophiles such as aldehydes (with Lewis acid catalysis) and carbocations.

Reacting a silyl enol ether with PhSCl, a good and soft electrophile, provides a carbonyl compound sulfenylated at an alpha carbon.

[22][20] In this reaction, the trimethylsilyl group of the silyl enol ether is removed by the chloride ion released from the PhSCl upon attack of its electrophilic sulfur atom.

[25][26] 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.

The general structure of a silyl enol ether
Example synthesis of a kinetic silyl enol ether by reacting an unsymmetrical ketone with trimethylsilyl chloride and LDA at low temperature.
Example synthesis of a thermodynamic silyl enol ether by reacting an unsymmetrical ketone with trimethylsilyl chloride and triethylamine. Two possible mechanisms are shown.
Generation of a lithium enolate from a silyl enol ether, using methyllithium.
Example alkylation of a silyl enol ether using a tertiary alkyl halide in the presence of the Lewis acid TiCl 4 .
Example Michael reaction using a disubstituted enone and the silyl enol ether of acetophenone, catalyzed by the Lewis acid TiCl 4 at low temperature.
More example reactions of silyl enol ethers.
Example halogenation of a silyl enol ether.
Example sulfenylation of a silyl enol ether.
Example hydrolysis of a silyl enol ether to give a carbonyl compound and hexamethyldisiloxane .
General structure of a silyl ketene acetal.