|Acids and bases|
As the name suggests, a non-nucleophilic base is an organic base that is a poor nucleophile. Normal bases are also nucleophiles, but often chemists seek the proton-removing ability of a base without any other functions. Characteristic of non-nucleophilic bases is their steric bulk, which allows protons to attach to the basic atom but prevents alkyl groups from doing so.
A variety of amines and nitrogen heterocycles are useful bases of moderate strength (pKa of conjugate acid around 10-13)
- N,N-Diisopropylethylamine, or DIPEA (also called Hünig's Base)
- 1,8-Diazabicycloundec-7-ene, or DBU—a favorite for the E2 elimination reaction
- 2,6-Di-tert-butylpyridine, a weak non-nucleophilic base
- Phosphazene bases, such as t-Bu-P4
Non-nucleophilic bases of high strength are usually anions. For these species the pKa's of the conjugate acid is around 35-40.
- Lithium diisopropylamide, or LDA
- Silicon-based amides, such as sodium and potassium bis(trimethylsilyl)amide (NaHMDS and KHMDS, respectively)
- Lithium tetramethylpiperidide, or LiTMP (harpoon base)
The following diagram shows how the hindered base, lithium diisopropylamide, is used to deprotonate an ester to give the enolate in the Claisen ester condensation, instead of undergoing a nucleophilic substitution.
This reaction (deprotonation with LDA) is commonly used to generate enolates.
- K. L. Sorgi, "Diisopropylethylamine," Encyclopedia of Reagents for Organic Synthesis, 2001. doi:10.1002/047084289X.rd254
- Rafael R. Kostikov, Sánchez-Sancho Francisco, María Garranzo and M. Carmen Murcia "2,6-Di-t-butylpyridine" Encyclopedia of Reagents for Organic Synthesis 2010. doi:10.1002/047084289X.rd068.pub2
- Activation in anionic polymerization: Why phosphazene bases are very exciting promoters S. Boileau, N. Illy Prog. Polym. Sci., 2011, 36, 1132-1151, doi:10.1016/j.progpolymsci.2011.05.005
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