Iron complexes, cyclopentadienyl carbene cyclopropanation

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

Carbene)iron complexes are often only labile intermediates that undergo followup reactions as usually observed in carbene chemistry such as C-H insertion or cyclopropanation. Intramolecular C-H insertion reaction of (rj -cyclopentadienyl)-dicarbonyliron-carbene complexes provides bicyclic cyclopentanes (Scheme 4-60). The reaction has been applied for the synthesis of the fungal metabolite ( )-sterpurene and ( )-pentalenene. ... 

Dicarbonyl(ri5-cyclopentadienyl)iron-alky] complexes represent useful precursors for iron-carbene complexes [47]. For example, iron-carbene complexes are intermediates in the acid-promoted reaction of Fp-alkyl ether derivatives with alkenes to provide cyclopropanes via a [2 + l]-cycloaddition (Scheme 1.16). 

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