Silyl Enol Ethers
Whereas enols are seen only as a small proportion of the keto-enol tautomerism, silyl enol ethers are stable and very useful sources of regiochemically pure enolate ions. They are easily prepared – following the route to the enolate ion then treating with a silyl reagent like Tri-Methyl-Silyl-Chloride (‘TMSCl’). For example;
If the ketone is asymmetrical then there are two possibilities for deprotonation, and the proton that is taken will depend on the conditions used.
For kinetic deprotonation, low temperature plus a strong base ensure that equilibria are kept to a minimum and the least hindered proton is taken, giving the least substituted double bond in the enolate/silyl enol ether.
For thermodynamic deprotonation, high temperature plus a weaker base allow the system to equilibrate until the most stable double bond configuration is reached, i.e. the most substituted one. For example;
Reactions of Silyl Enol Ethers
These will react exactly as enolates as soon as the +SiMe3 fragment is removed – there are several ways of doing this, including MeLi (releasing SiMe4) and [tBu4N]+ F– (forms Si-F bond).
Also the double bond itself will react with strong electrophiles because the cation produced will be stabilised if β to the silicon:
This will also work with alkyl halides (especially if a Lewis Acid is present) – NB this is a preferable method for alkylating ketones because simple enolates are basic enough to create competing elimination reactions with alkyl halides.