Samarium(II) iodide

Samarium(II) iodide (SmI₂) is a green solid composed of samarium and iodine, with a melting point of 520 °C.^[1] It can be formed by high temperature decomposition of SmI₃ (the more stable iodide), but a convenient lab preparation is to react Sm powder with 1,2-diiodoethane in anhydrous THF,^[2] or CH₂I₂ may also be used. Samarium(II) iodide is a powerful reducing agent - for example it rapidly reduces water to hydrogen. It is available commercially as a dark blue 0.1 M solution in THF.

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Reactions

Samarium(II)iodide has become a popular reagent for carbon-carbon bond formation, for example in a Barbier reaction (similar to the Grignard reaction) between a ketone and an alkyl iodide to form a tertiary alcohol:^[3]

RI + R'COR → R(R')C(OH)R

Typical reaction conditions use SmI₂ in THF in the presence of catalytic NiI₂.

Esters react similarly (adding two R groups), but aldehydes give by-products. The reaction is convenient in that it is often very rapid (5 minutes or less in the cold). Although samarium (II) iodide is considered a powerful single-electron reducing agent, it does display remarkable chemoselectivity among functional groups. For example, sulfones and sulfoxides can be reduced to the corresponding sulfide in the presence of a variety of carbonyl-containing functionalities (such as esters, ketones, amides, aldehydes, etc). This is presumably due to the considerably slower reaction with carbonyls as compared to sulfones and sulfoxides. Furthermore, hydrodehalogenation of halogenated hydrocarbons to the corresponding hydrocarbon compound can be achieved using samarium (II) iodide. Also, it can be monitored by the color change that occurs as the dark blue color of SmI₂ in THF discharges to a light yellow once the reaction has occurred. The picture shows the dark colour disappearing immediately upon contact with the Barbier reaction mixture.

Work-up is with dilute hydrochloric acid, and the samarium is removed as aqueous Sm³⁺.

Carbonyl compounds can also be coupled with simple alkenes to form five, six or eight membered rings.^[4]

The applications of SmI₂ have been reviewed.^[5]

References

1. ^ Chemistry of the Elements, NN Greenwood & A Earnshaw, Pergamon Press.
2. ^ P. Girard, J. L. Namy and H. B. Kagan (1980). "Divalent lanthanide derivatives in organic synthesis. 1. Mild preparation of samarium iodide and ytterbium iodide and their use as reducing or coupling agents". J. Am. Chem. Soc. 102 (8): 2693-2698. doi:10.1021/ja00528a029.
3. ^ Synlett, 1996, 633-4.
4. ^ Molander, G. A.; McKiie, J. A. J. Org. Chem. 1992, 57, 3132-3139.
5. ^ (a) J. Chem. Soc., Perkin Trans. I, 2001, 2727-2751. (b) Molander, G. A.; Harris, C. R. Chemical Reviews 1996, 96, 307.