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

Abstract

Selective removal of the nitrogen atom from an aromatic N -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 ( 1 ) bearing rigid acridane-based PNP-pincer ligands at room temperature produced a cyclopentadienyl/nitride complex ( 2 ), 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 1 under similar conditions yielded the same product ( 3 ), a methylcyclopentadienyl-ligated analog of 2 . When 2,4- or 3,5-dimethylpyridine reacted with 1 at 60 °C, the 1,3-dimethylcyclopentadienyl-ligated analog ( 5 ) 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 1 followed by H 2 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 N -heterocycles but also represents a novel example of skeletal editing of aromatic N -heterocycles.

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