A Cyclic Diaminocarbene with a Pyramidalized Nitrogen Atom: A Stable N‐Heterocyclic Carbene with Enhanced Electrophilicity

Abstract

Since their discovery,[1,2] thousands of papers have dealt with the preparation and use of stable carbenes. Arguably, among them,[3–6] the N-heterocyclic carbenes (NHCs) A have been the most studied.[7–12] Despite their widespread applications, NHCs still suffer from low modularity, which confines them to a narrow range of strong σ-donor ligands and nucleophiles. In recent years, several strategies have successfully been developed for tuning their σ-donor properties, which are mainly linked to the σ-effect of the carbene substituents.[8] However, the modulation of the π-acceptor properties of NHCs is more challenging since the high-energy LUMO is ruled by the strong π-donation of the two amino groups to the carbene vacant orbital.[13] As shown in Figure 1, the replacement of a π-electron donating and σ-withdrawing amino substituent by a σ-donating alkyl group (B) leads to an increase of electrophilicity but also nucleophilicity.[14,15] Carbonyls can be introduced in the backbone in order to compete with the carbene center for the nitrogen π-donation. The so-called diamidocarbenes C are very electrophilic, but at the expense of their nucleophilicity.[16–19] Here we report the synthesis and single crystal X-ray diffraction study of a cyclic diaminocarbene, which retains the nucleophilicity of classical NHCs, but displays enhanced electrophilicity.