A Crystalline Annelated Pyridin-1-ylidene and Its Isomerization into a Pyridin-3-ylidene

An isolable ring-fused pyridinylidene (a so-called Hammick intermediate) was synthesized from a benzo[h]isoquinolinium salt, and its structure in the solid state was determined. The isolation of this first-of-its-kind pyridine-derived aromatic N-heterocyclic carbene was made possible due to the presence of an adamantyl group on nitrogen, forcing the nitrogen to be planar and enhancing both π-donation and steric protection, and to its unique polycyclic structure, which displays an intramolecular C–H···C_carbene interaction. Additionally, an example of isomerization of a carbene into another carbene, namely, a benzo[h]isoquinolin-1-ylidene into a benzo[h]isoquinolin-3-ylidene, is reported.