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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.

A structurally diverse set of Fischer carbene complexes are reacted with substituted 3-[(trimethylsilyl)oxy]-2-aza-1,3-butadienes 1, yielding 5- to 7-membered nitrogen-containing heterocycles in a selective manner. Aryl and heteroarylmetal carbenes 2 undergo [4 + 1] cycloaddition with 1 leading to pyrrolidone derivatives 3 − 5 or 6 − 9, depending on the C1-substituent of 1. The [(trimethylsilyl)ethynyl]carbene 10a gives rise to metal-containing and metal-free [4 + 2] cycloadducts 11 and 12, respectively, whereas the (phenylethynyl)carbene 10b furnishes azafluorenones 13 by a tandem [4 + 2] cycloaddition/pentaannulation process. In the case of alkenyl carbene complexes 14 the regioselective [4 + 3] cycloaddition is the only observed transformation. Thus, their reaction with the phenyl-substituted azadiene 1d resulted in the formation of a ≈1:1 mixture of diastereoisomers 15 and 16, whereas in the case of the tert-butyl-substituted azadiene 1c the cis-diastereoisomers 15 are selectively formed. This heptaannulation is proposed to occur by a cyclopropanation/aza-Cope rearrangement.

The reactivity of alkynyl and enynyl Fischer carbene complexes towards 8-azaheptafulvenes is examined. Alkynyl carbenes 1 a-f undergo regioselective [8+2] heterocyclization with 8-aryl-8-azaheptafulvenes 2 a, b providing cycloheptapyrroles 3 and 4 with metal carbene or ester functionality at C3. Moreover, consecutive cyclization reactions are involved when enynyl carbenes are used. Thus, the cyclopenta[b]pyrrole framework 7 is formed by the consecutive [8+2] cyclization and cyclopentannulation reactions. The initially formed cyclopentannulation adduct can be intercepted through a Diels-Alder reaction with classic dienophiles to afford increasing structural complexity (compounds 8 and 9). More importantly, the construction of the indole skeleton is accomplished with a high degree of substitution and functionalization (compounds 11-15) by a one-pot sequence that involves [8+2] cyclization, R--NC or CO insertion, and ring closure.

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Pentafulvenes are regioselectively cyclopropanated with group 6 Fischer carbene complexes leading to the homofulvene ring with complete endo selectivity. The homofulvene adducts undergo in turn a further cyclopropanation with ethyl diazoacetate or cyclopentannulation with a Fischer alkenyl carbene complex to provide substituted cyclopentanones after ozonolysis of the exocyclic carbon=carbon double bond. Fischer alkynyl carbene complexes also produce the corresponding alkynyl homofulvenes, albeit the exo stereoisomer is in this case exclusively or preferentially formed. Under moderate CO pressure, tungsten alkynyl carbene complexes cycloadd to pentafulvenes in a [4 + 3] fashion, giving rise to bicyclo[3.2.1]octadien-2-ones.