Rhodium-catalyzed C-C bondactivation of strained spiropentanes leads to a cyclopentenones. In terms of mechanism, the reaction proceeds by apparent oxidative addition of the 4-5 carbon-carbon bond, leading to a rhodacyclobutane intermediate. In the presence of carbon monoxide, migratory insertion of CO into one of the carbon-rhodium bonds gives a rhodacyclopentanone intermediate. Beta-carbon elimination to form an alkene from the other carbon-rhodium bond leads to a rhodacyclohexanone intermediate with an exocyclic double bond. Reductive elimination of the two carbon-rhodium bonds followed by isomerization of the exocyclic double bond leads to the desired beta-substituted cyclopentenone product. This reaction was applied to the total synthesis of (±)-β-cuparenone.
Using the same rhodium(I) catalyst and C-C bond activation strategy one can access compounds with fused rings. Once again the reaction involves oxidative addition to give a rhodacyclobutane eventually affording a rhodacycloheptene intermediate. Insertion of carbon monoxide into one of the carbon-rhodium bonds form a rhodacyclooctenone intermediate that can reductively eliminate to yield a 6,7-fused ring system. The authors propose that the regioselectivity of the initial oxidative addition is controlled by coordination of the endocyclic double bond to the rhodium catalyst.