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 I reacts with alkenylcarbenes 2 through a formal [6+3] heterocyclization in a regio- and stereoselective manner to afford dihydroimidazo[1,2a]pyridines 4. Acid-promoted dimethylamine elimination in compound 4c 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 enynylearbenes 3e-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, envnylcarbenes 3d, f undergo consecutive [6+2]/[5+1] cyclization reactions with diazafulvene 1. and tBuNC, 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.