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Levens, Alison; An, Feng; Breugst, Martin; Mayr, Herbert; Lupton, David W. (2016): Influence of the N-Substituents on the Nucleophilicity and Lewis Basicity of N-Heterocyclic Carbenes. In: Organic Letters, Vol. 18, No. 15: pp. 3566-3569
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Abstract

The ability to modulate the nucleophilicity and Lewis basicity of N-heterocyclic carbenes is pivotal to their application as organocatalysts. Herein we examine the impact of the N-substituent on the nucleophilicity and Lewis basicity. Four N-substituents popular in NHC organocatalysis, namely, the N-2,6-(CH3O)(2)C6H3, N-Mes, N-4-CH3OC6H4, and N-tertbutyl groups, have been examined and found to strongly affect the nucleophilicity. Thus, the N-2,6-(CH3O)(2)C6H3 group provides the most nucleophilic irnidazolylidene NHC reported and the N-tert-butyl group one of the least. This difference in nucleophilicity is reflected in the catalyst efficiency, as observed with a recently reported trienyl ester rearrangement.