Non-Classical Anionic Naked N-Heterocyclic Carbenes: Fundamental Properties and Emerging Applications in Synthesis and Catalysis

Abstract

Ongoing research exploring the chemistry of N-heterocyclic carbenes (NHCs) has led to the development and discovery of new NHC subclasses that deviate beyond Arduengo’s prototypical N,N′-disubstituted imidazol-2-ylidene-based structures. These systems continue to enable and extend the fundamental role of NHC ligands in synthesis and catalysis. In this regard, the advent of protic NHCs has garnered particular interest. This derives in part from their applications to the selective preparation of unique molecular scaffolds and their unprecedented bifunctional reactivity, which can be exploited in transition metal-catalyzed processes. In comparison, the synthetic applications of closely related anionic naked NHCs remain rather underexplored. With this in mind, this review highlights the interesting fundamental properties of non-classical anionic naked NHCs, and focuses on their emerging applications in synthesis and catalysis.

Cationic Charge-Appended Abnormal Carbenes: Synthesis and Study of Electronically Modified Abnormal N-Heterocyclic Carbenes

“…This is largely attributed to the greater anionic character and increased electron donating capability of the aNHC carbon . Such a unique characteristic has led to their inclusion in a variety of ligand scaffolds and even their emergence in the relatively new field of protic NHC ligands . In contrast to nNHCs, which may be represented in a neutral resonance form, aNHCs are notable for their mesoionic character that cannot be represented in a reasonable neutral resonance form…”

Monosubstituted, Anionic Imidazolyl Ligands from N−H NHC Precursors and Their Activity in Pd‐Catalyzed Cross‐Coupling Reactions

“…[2] In addition, protic NHCs can be deprotonated to generate anionic (X-type) 2imidazolyl ligands, where the Lewis basic nitrogen can then participate in substrate binding or transmetallation. [3] Recent results from the Lindsay laboratory suggest that the basic nitrogen on a benzimidazolyl ligand can coordinate a boronic ester substrate to facilitate faster transmetallation in Suzuki-Miyaura cross-coupling reactions (Figure 1a). [4] Despite these potential advantages, reports on the development and use of imidazolyl ligands in transition metal catalysis are sparse.…”

“…[4] Despite these potential advantages, reports on the development and use of imidazolyl ligands in transition metal catalysis are sparse. [3] Traditional methods for the preparation of metal-NHC complexes involve the deprotonation of the acidic imidazolium hydrogen to form the free carbene species, followed by addition of the metal salt. [1] This preparation method limits the types of carbene complexes that can be formed due to rapid dimerization of unhindered carbene intermediates.…”

“…One intriguing structural characteristic of these new Pd-NHC complexes is the potential to remove the N-H of the imidazole and create an anionic NHC or imidazolyl-type ligand. [3] Grotjahn and coworkers have previously demonstrated that deprotonation of the N-H on a monosubstituted Ru-NHC complex led to formation of an anionic NHC ligand with high nucleophilicity at the imidazole nitrogen. [10] We wondered whether deprotonation in this manner would lead to a more donating NHC ligand that could enhance the reactivity of the transition metal in catalysis.…”

Recent Advances of Imidazolin-2-iminato in Transition Metal Chemistry

“…[28][29] NHCs received considerable attention due to the high thermal stability, ease of preparation, and nontoxic character. [30][31][32] Although NHCs were introduced as potential alternatives for phosphines in organometallic chemistry, now they applied widely in diverse fields of chemistry. One of such is the application of NHCs in the synthesis of silylenes, a class of divalent silicon compound.…”