Facile and Versatile Annulation of the Imidazole Ring: Single and Sequential Cyclization Reactions of Fischer Carbene Complexes with 1,4‐Diazafulvenes

We examined the reactivity of dimethylaminodiazafulvene 1 toward Fischer alkenylcarbene 2 and alkynylcarbene 3 complexes. Diazafulvene 1 reacts with alkenylcarbenes 2 through a formal [6+3] heterocyclization in a regio‐ and stereoselective manner to afford dihydroimidazo[1,2‐ a ]pyridines 4 . Acid‐promoted dimethylamine elimination in compound 4 c gives rise to the aromatic imidazo pyridine 5 . A likely mechanism for this reaction is a 1,2‐nucleophilic addition/[1,2]‐shift metal‐promoted cyclization sequence. On the other hand, diazafulvene 1 and alkynyl carbenes 3 undergo a [6+2] cyclization to afford pyrrolo[1,2‐ a ]imidazole carbene complex 6 that can be readily oxidized to the corresponding esters 7 . When enynylcarbenes 3 e – i are treated with diazafulvene 1 , consecutive and diastereoselective [6+2]/cyclopentannulation cyclization reactions take place affording new polycyclic complex systems 8 , 9 , and 12 that can be appropriately demetallated to the corresponding imidazole‐based polyfused systems 10 , 11 , and 13 respectively. Finally, enynylcarbenes 3 d , f undergo consecutive [6+2]/[5+1] cyclization reactions with diazafulvene 1 and t BuNC, respectively, to yield tetracyclic adducts 14 and 15 . All these processes result in high yields and provide a route to the preparation of imidazopyridines and pyrroloimidazoles as well as other polycyclic molecules that contain imidazole groups, which are interesting from a pharmacological and biological point of view.