Imidazo[1,5-a]pyridines – A Versatile Platform for Structurally Distinct N-Heterocyclic Olefins and π-Extended Heterocycles

The synthesis of polarized N-heterocyclic olefins (NHOs) based on an imidazo[1,5-a]pyridine scaffold is presented. In contrast to regular NHOs the unique bicyclic heterocyclic core allows the incorporation of various substituents in close proximity to the highly polarized exocyclic C–C bond. The donor properties of the new carbon-based ligand class were quantified and the coordination chemistry explored. Alkenyl or alkynyl groups at the C5-position lead to spontaneous cyclization to yield imidazo[2,1,5-de]quinolizines. The unique cyclization strategy was compatible with a wide range of substitution patterns and yields highly electron-rich π-delocalized heterocycles. The electronic structure of the novel partially antiaromatic heterocycle was thoroughly investigated. One-electron oxidation occurs at low potentials and lead to a stable monomeric radical-cation confirmed by XRD, which showed solution phase fluorescence expanding into the field of open-shell organic materials.