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

• Published in: Chemistry : a European journal 2006-04, Vol.12 (12), p.3201-3210
• Main Authors: Barluenga, José, García-Rodríguez, Jaime, Martínez, Silvia, Suárez-Sobrino, Angel L., Tomás, Miguel
• Format: Article
• Language: English
• Subjects: azafulvenes; carbenes; chromium; cyclization; imidazole
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200501402

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 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. Se ha investigado la reactividad de dimetilaminodiazafulveno 1 frente a los complejos alquenilcarbeno y alquinilcarbeno de Fischer 2 y 3. El diazafulveno 1 reacciona con los alquenilcarbenos 2 mediante una heterociclación [6+3] dando lugar a dihidroimidazo[1,2‐a]piridinas 4 de forma regio‐ y diastereoselectiva; la eliminación de dimetilamina en medio ácido en 4 c da lugar a la imidazopiridina aromática 5. Un mecanismo plausible para esta reacción implica una secuencia adición nucleófila‐1,2/ciclación inducida por la migración‐[1,2] del fragmento metálico. Por otra parte, el diazafulveno 1 y los alquinilcarbenos 3 dan lugar a una ciclación [6+2] generando los complejos carbeno con estructura de pirrolo[1,2‐a]imidazol 6, los cuales pueden oxidarse fácilmente a los correspondientes ésteres 7. Cuando los eninilcarbenos 3 e–i reaccionan con 1 se obtienen, a través de la secuencia diastereoselectiva ciclación [6+2]/ciclopentanulación, los complejos metálicos policíclicos 8, 9, y 12, cuya desmetalación en condiciones