The Ambivalent Chemistry of a Free Anionic N-Heterocyclic Carbene Decorated with a Malonate Backbone: The Plus of a Negative Charge

The anionic heterocycle [maloNHC](-), ([1](-)), is the archetype of a growing family of N-heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [1]Li) is presented, and rationalized using DFT calculations at the B3LYP/6-31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [1]Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [1]Li was also seen to react with the nucleophilic tert-butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [1]Li revealed its unexpected reactivity toward p-chlorobenzaldehyde, resulting in a formal CH activation and the first structurally characterized keto-tautomer of the Breslow intermediate. Finally, [1]Li remarkably activates polar EH bonds, including NH bonds from ammonia and amines, SiH bonds, and BH bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the LiO distance between the remote malonate group and the counterion.