Pushing the Upper Limit of Nucleophilicity Scales by Mesoionic <i>N</i>‐Heterocyclic Olefins

TL;DR: Mesoionic N-heterocyclic olefins are exceptionally strong nucleophiles, exceeding the reactivity of conventional NHOs and even mono- and diacceptor-substituted carbanions. Their reactivity parameters allow accurate prediction of reaction rates and scope.

Abstract: Abstract A series of mesoionic, 1,2,3‐triazole‐derived N ‐heterocyclic olefins (mNHOs), which have an extraordinarily electron‐rich exocyclic CC‐double bond, was synthesized and spectroscopically characterized, in selected cases by X‐ray crystallography. The kinetics of their reactions with arylidene malonates, ArCH=C(CO 2 Et) 2 , which gave zwitterionic adducts, were investigated photometrically in THF at 20 °C. The resulting second‐order rate constants k 2 (20 °C) correlate linearly with the reported electrophilicity parameters E of the arylidene malonates (reference electrophiles), thus providing the nucleophile‐specific N and s N parameters of the mNHOs according to the correlation lg k 2 (20 °C)= s N ( N + E ). With 21&lt; N &lt;32, the mNHOs are much stronger nucleophiles than conventional NHOs. Some mNHOs even excel the reactivity of mono‐ and diacceptor‐substituted carbanions. It is exemplarily shown that the reactivity parameters thus obtained allow to calculate the rate constants for mNHO reactions with further Michael acceptors and predict the scope of reactions with other electrophilic reaction partners including carbon dioxide, which gives zwitterionic mNHO‐carboxylates. The nucleophilicity parameters N correlate linearly with a linear combination of the quantum‐chemically calculated methyl cation affinities and buried volumes of mNHOs, which offers a valuable tool to tailor the reactivities of strong carbon nucleophiles.