Denitrogenative Ring-Contraction of Pyridines to a Cyclopentadienyl Skeleton at a Dititanium Hydride Framework

Selective removal of the nitrogen atom from an aromatic <i>N</i>-heterocycle, such as pyridine, is of significant interest and importance, yet it remains highly challenging. Here, we report an unprecedented denitrogenative ring-contraction reaction of pyridines at a dititanium hydride framework, yielding cyclopentadienyl and nitride species under mild conditions. The reaction of pyridine with a dititanium tetrahydride complex (<b>1</b>) bearing rigid acridane-based PNP-pincer ligands at room temperature produced a cyclopentadienyl/nitride complex (<b>2</b>), in which the two Ti atoms are bridged by a nitride atom and one Ti atom is bonded to a cyclopentadienyl group formed by pyridine denitrogenation and ring-contraction. The reactions of 2-, 3-, and 4-methylpyridines with <b>1</b> under similar conditions yielded the same product (<b>3</b>), a methylcyclopentadienyl-ligated analog of <b>2</b>. When 2,4- or 3,5-dimethylpyridine reacted with <b>1</b> at 60 °C, the 1,3-dimethylcyclopentadienyl-ligated analog (<b>5</b>) formed almost quantitatively. The mechanistic details have been elucidated by isolation of key intermediates and density functional theory calculations. It was revealed that the reaction proceeded via coordination of the N atom of a pyridine unit to a Ti atom in <b>1</b> followed by H₂ release, C═N reduction, two C-N bond cleavage (ring-opening and denitrogenation), and C-C coupling (ring closing). The involvement of C-H activation in an isopropyl group of a PNP ligand at the later stages of the reaction significantly contributed to the stabilization of the denitrogenative ring-contraction product. This work not only provides unprecedented mechanistic insights into denitrogenation of aromatic <i>N</i>-heterocycles but also represents a novel example of skeletal editing of aromatic <i>N</i>-heterocycles.