Non-nucleophilic base

Lore & Background

Strong non-nucleophilic bases are usually anions, with conjugate acid pKas around 35–40. Examples include lithium diisopropylamide (LDA, pKa 36), silicon-based amides such as sodium and potassium bis(trimethylsilyl)amide (NaHMDS and KHMDS), and lithium tetramethylpiperidide (LiTMP, also called harpoon base). Other strong non-nucleophilic bases are sodium hydride and potassium hydride, which are dense, salt-like materials that are insoluble and operate by surface reactions. Some reagents, like sodium tert-butoxide and potassium tert-butoxide, have high basicity (pKa around 17) but modest, not negligible, nucleophilicity.

Reader's Guide

Non-nucleophilic bases are essential in organic synthesis for selective deprotonation without competing nucleophilic attack. Their significance is illustrated by the use of lithium diisopropylamide (LDA) to deprotonate an ester to give the enolate in the Claisen ester condensation, instead of undergoing a nucleophilic substitution. This reaction is commonly used to generate enolates. The development of these bases has expanded the toolkit for chemists, allowing controlled formation of reactive intermediates. Their legacy lies in enabling transformations that would otherwise be complicated by side reactions, particularly in the synthesis of complex molecules. The variety of bases—from weak hindered pyridines to strong amides and hydrides—provides a range of basicity and steric profiles to suit different substrates and conditions.

Did You Know?

• N,N-Diisopropylethylamine is also called Hünig's Base and has a pKa of 10.75.
• DBU (1,8-Diazabicycloundec-7-ene) is useful for E2 elimination reactions and has a pKa of 13.5.
• Lithium diisopropylamide (LDA) has a pKa of 36 and is used to generate enolates in the Claisen ester condensation.
• Sodium hydride and potassium hydride are dense, salt-like materials that operate by surface reactions.