A Zincke-Inspired Cycloreversion/Cyclization Sequence with Arrested Rearomatization: Synthesis of 2‑Aminodihydropyridinium Complexes

The Zincke reaction combines a pyridinium salt bearing
an N-withdrawing group and a primary aliphatic amine
to form
an alkylated pyridinium salt through a ring-opening/ring-closing sequence.
Herein, we explore the analogous reaction sequence for a pyridinium
salt η2-bound to a transition metal. We find that
the N-sulfonylated pyridinium ligand (pyR1, where R1 = mesyl or tosyl) of [WTp(NO)(PMe3)(η2-pyR1)]OTf selectively reacts with
a primary amine, and the resulting 2-aminodihydropyridine complex
then undergoes a tungsten-stabilized ring-scission to form the corresponding
η2-azatriene complex. Subsequent ring-closure between
the newly installed amine and the sulfonylated imine results in a
new aminodihydropyridinium species. This dihydropyridinium resists
rearomatization due to a stabilizing influence of the tungsten fragment.
Subsequent displacement of the sulfonamide by pendent heteroatoms
leads to the formation of new heterocyclic frameworks. Herein the
syntheses of 30 heterocyclic complexes are described (3 characterized
by SC-XRD) including 7 examples of multicyclic systems.