Synthesis of Imidazopyridinium-Fused Metallacycloallene via One-Pot Reaction of η²‑Alkynol-Coordinated Osmacycle with 2‑Aminopyridine

Metallacycloallenes are metallacyclic derivatives of cyclic allenes, in which a CH2 (type A) or CH (type B) segment is formally replaced by an isolobal transition-metal fragment. In constrast to the well-developed chemistry of metallacycloallenes of type A, the synthesis of metallacycloallenes with the structural features of type B has met with limited success. In this study, we present the reaction of η2-alkynol-coordinated osmacycle 1 with 2-aminopyridine in the presence of hypervalent iodine reagent, leading to the formation of imidazopyridinium-fused 4-osmacyclohexa-2,3,5-trienone 2 and 4-osmacyclohexa-2,5-dienone 3. Two key intermediates, η2-ethynyl ketone coordinated osmacycle 4 and 4-osmacyclohexa-2,5-dienone 5, were isolated and fully characterized, which suggest the hypervalent iodine reagent plays an important role in the formation of the fused metallacycloallene 2.