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Proton Affinities of N-Heterocyclic Carbene Super Bases

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Hao Chen, Dina R. Justes, and R. Graham Cooks*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907

Org. Lett., 2005, 7 (18), pp 3949–3952

DOI: 10.1021/ol0514247

Publication Date (Web): August 4, 2005

Section:

Physical Organic Chemistry

Abstract

The gas-phase proton affinity of the N-heterocyclic carbene, 1-ethyl-3-methylimidazol-2-ylidene, was determined to be 251.3 ± 4 kcal/mol using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far. Density functional theory calculations have been carried out at the B3LYP/6-31+G(d) level to compare the high experimental value with that estimated theoretically. Experimental results also show that two other N-heterocyclic carbenes with larger substituents have even higher proton affinities.

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