Intramolecular C−H Insertion Reactions of (η5-Cyclopentadienyl)dicarbonyliron Carbene Complexes:  Scope of the Reactions and Application to the Synthesis of (±)-Sterpurene and (±)-Pentalenene

(η5-Cyclopentadienyl)dicarbonyliron carbene complexes, [(η5-C5H5)(CO)2FeCHR]+BF4 -, are generated as reactive intermediates from thioether derivatives, (η5-C5H5)(CO)2FeCH(R)SPh, by S-alkylation with trimethyloxonium tetrafluoroborate and loss of thioanisole. The carbene complexes undergo intramolecular C−H insertion into appropriately situated side chains to form cyclopentane derivatives. The reaction has been developed into a general procedure employing cycloalkanones as scaffolds bearing the iron carbene moieties and the side chains at C(2) and C(3), respectively. The products of the intramolecular insertion reactions are substituted bicyclo[n.3.0]alkanones. The scope and limitations of the reaction are described. The reaction is applied to a total synthesis of sterpurene and to a formal synthesis of pentalenene. Overall, this approach to cyclopentane annulation complements the related metal-catalyzed insertion reactions of diazocarbonyl compounds, which are also believed to occur via metal carbene complexes.