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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.

Heads or tails? Chemo‐, regio‐, and stereoselective access to cross‐ and linear conjugated poly‐ynene scaffolds from alkynyl‐derived group six metal carbenes is reported. The metal carbenes shown (M=Cr, W; TIPS=triisopropylsilyl) readily undergo low‐temperature, tail‐to‐tail dimerization in the presence of potassium tert‐butoxide by reaction at either of the Cβ atoms to provide E‐polyalkynylethenes.