Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

The cyclization of an alkene-bearing cyclopentanone to a [2.2.1]-norcamphor ring system is described. The reaction is catalyzed by a combination of rhodium and Brønsted acid. Control experiments indicate that both are needed for acceptable yield. Control experiments with bulky base additives show that rhodium promotes alkene isomerization, likely the first step of this cascade reaction, and that rhodium alone does not promote cyclization. Cyclization is promoted by Brønsted acid in a Prins-type cyclization and carbocation rearrangement process. Trace Brønsted acid present in commercial samples of Rh(cod)₂OTf is likely responsible for the observed reaction. Indeed, the norcamphor product can be obtained simply with strong acid, presumably initiated by acid-promoted alkene isomerization. Since our initial motivation for this work was the development of rhodium catalysts for the activation of C-C bonds adjacent to ketones, this communication serves to identify other, perhaps less obvious, pathways for the reactions of unsaturated ketone compounds by the action of rhodium catalysts.