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Updated: Feb 20, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
Are There Carbenes in N-Heterocyclic Carbene Organocatalysis?
Sascha Gehrke1,2, Oldamur Hollóczki1
1Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstrasse 4+6, 53115, Bonn, Germany.
Abstract:
Azolium cations are widely employed in organocatalysis to catalyse highly valuable synthetic processes in the presence of a base. These reactions are called "N-heterocyclic carbene catalysis", based on the assumption that they are initiated by the formation of a free carbene through deprotonation, which can then react with the substrates and thereby affect their reactivity to obtain the desired products. However, we herein provide evidence that an electrophilic aromatic substitution mechanism is energetically more favourable, in which the azolium cation reacts directly with the substrate, avoiding the formation of the free carbene in solution.
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