title: The ambivalent chemistry of a free anionic N-heterocyclic carbene decorated with a malonate backbone: the plus of a negative charge.
abstract: 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 C-H activation and the first structurally characterized keto-tautomer of the Breslow intermediate. Finally, [1]·Li remarkably activates polar E-H bonds, including N-H bonds from ammonia and amines, Si-H bonds, and B-H 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 Li-O distance between the remote malonate group and the counterion.
authors.affiliation: CNRS, LCC (laboratoire de chimie de coordination), 205 route de Narbonne, BP44099, 31077 Toulouse Cedex 4 (France); Université de Toulouse, UPS, INPT, 31077 Toulouse (France). vincent.cesar@lcc-toulouse.fr.
authors.forename: Vincent
authors.initials: V
authors.lastname: César
authors.forename: Stéphane
authors.initials: S
authors.lastname: Labat
authors.forename: Karinne
authors.initials: K
authors.lastname: Miqueu
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Sotiropoulos
authors.forename: Rémy
authors.initials: R
authors.lastname: Brousses
authors.forename: Noël
authors.initials: N
authors.lastname: Lugan
authors.forename: Guy
authors.initials: G
authors.lastname: Lavigne
keywords: carbenes; density-functional calculations; heterocyclic compounds; ion pairs; lithium