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Related Experiment Video

Updated: Oct 14, 2025

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Published on: April 19, 2019

Stable Singlet Carbenes as Organic Superbases.

François Vermersch¹, Sima Yazdani^1,2, Glen P Junor¹

¹UCSD-CNRS Joint Research Laboratory (IRL 3555), Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093-0358, USA.

Angewandte Chemie (International Ed. in English)

|November 3, 2021

Summary

Pyrazol-4-ylidenes, a type of cyclic-bentallene (CBA), exhibit exceptional Brønsted basicity, exceeding that of known superbases. This discovery paves the way for novel carbon-centered superbases with tunable properties.

Keywords:

NHC Verkade base mesoionic carbene organic superbase phosphazene

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Area of Science:

Organic Chemistry
Supramolecular Chemistry

Background:

Mesoionic carbenes, specifically pyrazol-4-ylidenes (cyclic-bentallenes, CBA), represent a class of compounds with significant potential in catalysis and synthesis.
Establishing a reliable method for quantifying and comparing the basicity of novel carbene species is crucial for their application.

Purpose of the Study:

To develop a straightforward experimental protocol for assessing and comparing the Brønsted basicity of carbene compounds.
To evaluate the basicity of pyrazol-4-ylidenes and compare them against established strong bases.

Main Methods:

A simple experimental procedure was employed to determine the Brønsted basicity (pK_aH) of pyrazol-4-ylidenes in acetonitrile.
Comparative analysis was performed against well-known strong organic bases, including Verkade bases and phosphazene superbases.

Main Results:

Pyrazol-4-ylidenes demonstrate exceptionally high Brønsted basicity, with pK_aH values greater than 42.7 in acetonitrile.
These cyclic-bentallenes exhibit stronger basicity than popular proazaphosphatrane Verkade bases and the Schwesinger phosphazene superbase P₄(^tBu).
The basicity of these pyrazol-4-ylidenes can be readily tuned, and they are accessible in multigram quantities.

Conclusions:

Pyrazol-4-ylidenes are identified as potent organic superbases, outperforming existing benchmarks.
The tunable nature and accessibility of these cyclic-bentallenes open new possibilities for the development of novel carbon-centered superbases in organic chemistry.