# The use of silyl enol ethers over aza-enolates

I do not quite understand how alkylations using tertiary halides would work better with silyl enol ethers, compared with using the aza-enolate nucleophile. The following was mentioned in Clayden, Greeves & Warren (2012) on p. 594:

Aza-enolates are the best general solution for alkylating aldehydes with most electrophiles. With very $$\ce {S_N2}$$-reactive alkylating agents, enamines can be used, and with very $$\ce {S_N1}$$-reactive alkylating agents, silyl enol ethers must be used.

I would think that it does not matter what nucleophile is used when alkylating with tertiary halides since $$\ce {S_N1}$$ processes do not depend on the nucleophilicity of the nucleophile. Thus, whether the nucleophile is very strong and basic, in the case of an aza-enolate, or relatively weaker, in the case of a silyl enol ether, the alkylation would still proceed at around the same pace.

Reference

Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.

• Silyl enol ethers (neutral) are weaker nucleophiles than aza-enolates (anions). The former needs a reactive electrophile, i.e., the tertiary cation (SN1). The aza-enolate can tolerate a poorer electrophile. If the aza-enolate sees a tertiary halide, E2 elimination will occur. – user55119 Feb 25 at 14:44
• @user55119 Isn't the substitution an SN2? If a tertiary halide were used, the mechanism would become SN1 due to the steric hindrance around the carbon of the tertiary halide. Or as you said, elimination may take place. So is the reason why silyl enol ether is used preferentially because of the possibility of elimination occurring when using aza-enolates? – Tan Yong Boon Feb 25 at 23:06
• Didn't you say it was a tertiary halide? The silyl enol ether is not as nucleophilic as the aza-enolate. If the aza-enolate causes elimination with a tertiary halide and not an SN2 reaction, why would a weaker nucleophile effect SN2 substitution? As a weaker nucleophile, wouldn't it require a reactive electrophile, i.e., a cation. – user55119 Feb 26 at 0:47
• @user55119 So it is because of the fact that aza-enolates cause elimination to occur... Maybe you could write a response based on that? – Tan Yong Boon Feb 26 at 7:46
• See this URL for examples of rxns of t-halides with silyl enol ethers. chem.wisc.edu/areas/reich/OrgMet/silicon%7B09%7D.htm they use a Lewis acid, which you didn't mention. SN1 through the cation. – user55119 Feb 26 at 13:15