Iron complexes ferracyclopentadiene

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

Both iron- and nickel-mediated cyclooligomerization processes have been used in direct routes to pyrrole derivatives. Preformed ferracyclopentadiene complexes can be converted in moderate yields into 2,3,4,5-tetraphenyl-... 

Thiophenes are desulfurized by chromium and iron atoms. The role of complex intermediates is uncertain but with iron the final product appears to be a ferracyclopentadiene derivative (18) ... 

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