SmI2

Samarium(II) iodide (SmI2) is a versatile single-electron transfer reagent used prominently in organic synthesis. In dry tetrahydrofuran (THF) it forms deep blue solutions and acts as a powerful, chemoselective reductant capable of delivering one electron to a broad range of substrates, including carbonyl compounds, halides, and related species. The resulting ketyl or alkyl radical intermediates enable diverse transformations such as cyclizations, radical couplings, and deoxygenations.

Preparation and handling typically involve generating SmI2 in situ in dry THF. Classic routes include reducing samarium(III) iodide with samarium metal or zinc, often by combining Sm metal with iodine to give SmI2 in THF. Commercial solutions of SmI2 in THF are also available at varying concentrations. The reagent is air- and moisture-sensitive and is usually employed under inert atmosphere. Its reducing power can be modulated by additives: complexation with donors such as hexamethylphosphoramide (HMPA) or glymes increases reactivity, though HMPA is toxic and alternative ligands are frequently used. Proton sources like water or tert-butanol can further tune selectivity and reaction rate.

Mechanistically, SmI2 operates via single-electron transfer to the substrate, generating a ketyl radical or alkyl radical that undergoes intramolecular cyclization, inter- or intramolecular coupling, or protonation to furnish the product after workup. Notable applications include reductive cyclizations of enones and aldehydes, pinacol couplings to give vicinal diols, and reductions of certain carbon–halogen bonds.

Applications include method development in total synthesis and complex molecule construction. Limitations include the need for strictly anhydrous conditions, stoichiometric consumption of SmI2, and sensitivity to additives or competing pathways depending on substrate and solvent system.