Samarium(II) iodide (SmI₂), or Kagan’s reagent, is a strong one-electron reducing agent often used in organic chemistry. It is typically used in superstoichiometric amounts. Using SmI₂ in a catalytic manner could potentially reduce both waste and costs. However, existing examples of SmI₂-catalyzed reactions require the use of superstoichiometric amounts of a metal co-reductant to regenerate the Sm(II) catalyst.

David J. Procter, The University of Manchester, UK, and colleagues have developed an SmI₂-catalyzed intermolecular radical coupling of aryl cyclopropyl ketones and alkynes to give cyclopentenes (pictured). The reaction uses loadings of SmI₂ as low as 15 mol% and does not require a superstoichiometric co-reductant to regenerate SmI₂. The team used a range of cyclopropyl phenyl ketones and a wide variety of aryl alkynes as coupling partners, 15–25 mol% of SmI₂ as the catalyst, and tetrahydrofuran (THF) as the solvent. The reaction was performed at 45 °C.

The desired cyclopentene products were obtained in good to excellent yields. The reaction tolerates some functional groups that are typically reduced by non-catalytic amounts of SmI₂. The team proposes a radical-relay mechanism for the intermolecular coupling, starting with a reversible single-electron transfer from SmI₂ to the ketone, which then leads to ring-opening of the cyclopropyl group. Intermolecular coupling with the alkyne and closing of the cyclopentene ring then gives a ketyl radical. This radical can transfer an electron back to Sm(III), regenerate the catalyst, and liberate the product.

• SmI₂-Catalyzed Intermolecular Coupling of Cyclopropyl Ketones and Alkynes: A Link between Ketone Conformation and Reactivity,
  Soumitra Agasti, Nicholas A. Beattie, Joseph J. W. McDouall, David J. Procter,
  J. Am. Chem. Soc. 2021.
  https://doi.org/10.1021/jacs.1c01356