Vinylcarbene complexes, Dotz

A particularly interesting piece of evidence concerning the nature of this sequence has been presented by Barluenga et al.21 When the vinylcarbene complex 14 was heated, decarbonylation afforded the alkene-stabilized complex 15. Upon treatment with dimethyl acetylenedicarboxylate, the alkyne-insertion product 16 was isolated. This complex proved to be unstable in solution at room temperature and decomposed readily to 17, the expected product of a Dotz reaction with an aminovinylcarbene. 

A new chromahexatriene (118) route has been proposed for the mechanism of the Dotz benzannulation reaction between vinylcarbene complexes (117) and ethynes (Scheme 46). ... 

Analysis of product distributions has often been used to evince the presence of 774-vinylketenes in the Dotz reaction. Wulff has studied the reactions of a wide variety of substituted aryl chromiumcarbenes and drawn conclusions from the data that point directly toward i74-vinylketene complexes as key intermediates.13 He has also isolated40 bicyclic lactone products (e.g., 23) characteristic of a van Halban-White cyclization41-44 from the reaction of vinylcarbenes and ketoalkynes. These bicyclic lactones are not generally... 

The mechanism of the Dotz benzannulation reaction has not been fully elucidated. The first step is the ratedetermining dissociation of one carbonyl ligand from the Fischer carbene complex, which is cis to the carbene moiety. Subsequently, the alkyne component coordinates to the coordinatively unsaturated carbene complex, and then it inserts into the metal-carbon bond. After the alkyne insertion, a vinylcarbene is formed that can lead to the product by two different pathways (Path A or Path b). ... 

Fischer-type chromium carbene complexes with 1-ethoxycyclopropylalkynyl substituents at the carbene carbon, e.g. 25, on reaction with dimethylamine and subsequent conversion of the resulting vinylcarbene with alkynes surprisingly did not give phenol derivatives, as would be expected from the known Dotz reaction, but gave cyclopenta[ )]pyrans, e.g. 26. The reaction is interpreted as a double alkyne-insertion/CO-insertion sequence with formation of a trienylketene intermediate, which undergoes intramolecular hetero-Diels-Alder cycloaddition and dimethylamine elimination. ... 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

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