New Mechanistic Insights into Reductions of Halides and Radicals with Samarium(II) Iodide

This account summarizes the current mechanistic understanding of the reductions of halides and radicals with samarium(II) iodide. Special emphasis is placed on the samarium Barbier reaction (reductive coupling of a halide and a ketone with samarium(II) iodide). Previous evidence that has been interpreted in support of ketyl-radical coupling mechanisms or in support of free anions is critically evaluated in the light of new mechanistic evidence. It is suggested that bimolecular samarium Barbier reactions often proceed by an organometallic addition mechanism with three key steps: (a) the halide is reduced by samarium(II) iodide to form a free radical, (b) the radical is reduced by samarium(II) iodide to form an alkyl samarium reagent, and (c) the alkylsamarium reagent adds to the carbonyl group. Past results are re-evaluated in this new mechanistic light, and new synthetic possibilities are suggested for both radical and organometallic reactions. 1. Introduction 2. Kagan's Mechanisms 3. Critical Analysis of Existing Mechanistic Evidence 4. New Mechanistic Evidence: The Samarium Grignard Reaction 5. New Mechanistic Evidence: The Samarium Barbier Reaction 6. Mechanistic Summary 7. Mechanistic Consequences 8. Unanswered Questions 9. Samarium Diiodiode as a Reagent for Radical Reactions 10. Conclusions