Harpoon base

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A harpoon base is an organic base that is a very strong base but at the same time a poor nucleophile. In thermodynamic reaction control an electron donor molecule acts as a nucleophile, in kinetic reaction control the electron donor abstracts a proton (or rather the proton gets harpooned). For this reason harpoon bases are said to be involved in kinetic deprotonation.

Harpoon bases are:

• Lithium diisopropylamide
• Silicon-based amides such as potassium hexamethyldisilazide KHMDS or ((CH₃)₃Si)₂NK, with a pKa of 26 or sodium bis(trimethylsilyl)amide
• Lithium tetramethylpiperidide
• Sodium tert-Butoxide

The following diagram shows how the hindered base, lithium diisopropylamide, is used to form to deprotonate an ester to give the enolate in the Claisen ester condensation, instead of undergoing a nucleophilic substitution.