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.