What is a hindered base

Non nucleophilic base - Non-nucleophilic base

As the name suggests, is one non-nucleophilic base a hindered organic base that is a poor nucleophile. Normal bases are nucleophiles, too, but chemists often seek the proton removal ability of a base with no other function. Typical non-nucleophilic bases are bulky so that protons can bind to the basic center, but alkylation and complexation are inhibited.

Not nucleophilic bases

A variety of amines and nitrogen heterocycles are useful medium strength bases (pK a conjugate acid around 10-13)

  • N , N- Diisopropylethylamine (DIPEA, also called Hünig base), pK a = 10,75
  • 1,8-diazabicycloundec-7-ene (DBU) - useful for E2 elimination reactions, pK a = 13,5
  • 1,5-diazabicyclo (4.3.0) non-5-en (DBN) - comparable to DBU
  • 2,6-di-tert-butylpyridine, a weak non-nucleophilic base pK a = 3,58
  • Phosphazene bases such as t-Bu-P 4

High strength non-nucleophilic bases are usually anions. For this species the pK a s of the conjugate acids at about 35-40.

Other strong non-nucleophilic bases are sodium hydride and potassium hydride. These compounds are dense, salt-like materials that are insoluble and act through surface reactions.

Some reagents are of high basicity (pK a of the conjugate acid around 17), but of modest, but not negligible, nucleophilicity. Examples include sodium tert- butoxide and potassium tert- butoxide.

example

The diagram below shows how the hindered base, lithium diisopropylamide, is used to deprotonate an ester to give the enolate in the Claisenester condensation, rather than making a nucleophilic substitution.

This reaction (deprotonation with LDA) is commonly used to generate enolates.

References

  1. ^ KL Sorgi, "Diisopropylethylamine", Encyclopedia of Reagents for Organic Synthesis, 2001. doi: 10.1002 / 047084289X.rd254
  2. ^ Rafael R. Kostikov, Sánchez-Sancho Francisco, Maria Garranzo and M. Carmen Murcia "2,6-Di-t-butylpyridine" Encyclopedia of Reagents for Organic Synthesis 2010. doi: 10.1002 / 047084289X.rd068.pub2
  3. ^ 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