Synthetic Approach to Iminyl Radicals Stabilized by N-Heterocyclic Carbenes

Abstract

N-heterocyclic carbenes (NHCs) are heterocyclic aromatic compounds containing a carbene carbon and at least one nitrogen atom within the ring structure. The remarkable stability of the carbene carbon is derived from the structural features of the ring, the electronic effects of nitrogen heteroatoms, and the steric effects of nitrogen-substituted substituents. NHCs possess sp2-hybridized lone pairs in the highest occupied molecular orbital (HOMO) and vacant p orbitals in the lowest unoccupied molecular orbital (LUMO). The sp 2-hybridized lone pairs enable NHCs to exhibit nucleophilic behavior, serving as σ-donor ligands in coordinating with transition metals or as organic catalysts. Furthermore, the π-accepting ability and steric bulk of NHCs play a crucial role in stabilizing reactive species. In this study, the focus is on utilizing N -heterocyclic carbenes to synthesize iminyl radicals, nitrogen-centered radicals known for their potential to introduce nitrogen into various organic compounds. Nitrogen-containing compounds find extensive applications in biology, medicine, and other fields due to the significant impact of nitrogen atoms on chemical parameters. While iminyl radicals have been synthesized as intermediates in various C–N bond-forming reactions, their high reactivity has hindered their isolation and detailed characterization. Successful isolation of iminyl radicals would provide easy access to previously challenging synthetic pathways and allow for a more in-depth analysis of theoretically proven mechanisms. Moreover, NHCs, known for their role as σ-donor ligands coordinating with transition metals, find applications in catalysis and luminescent materials. Controlling the electronic and steric properties of NHCs through backbone substitutions allows for fine-tuning their characteristics. The introduction of nitrile substituents, which convert carbene to an electrophilic species, enhances the π- accepting ability, potentially lowering the LUMO energy level. Herein, we confirmed the introduction of cyanide into NHCs (IPr and IMes) and the synthesis of a new radical stabilized by NHC (IPr) through a 1-electron reduction process, as verified by Electron Paramagnetic Resonance (EPR) analysis. And for IMes, the confirmation of dimerization involving two radicals was achieved through crystallographic analysis, followed by mechanism analysis through Density Functional Theory (DFT) calculations. Furthermore, by incorporating cyanide into the backbone via a 2-electron transfer pathway, we successfully synthesized an electrophilic carbene and isolated it. By DFT calculations, it is anticipated that this carbene will act as a better π-acceptor ligand due to the backbone's nitrile moiety.