| 404.16 g/mol|
Samarium(II) iodide (SmI2, also known as "Kagan's reagent") is a green solid composed of samarium and iodine, with a melting point of 520 °C where the samarium atom has a coordination number of seven in a capped octahedral configuration. It can be formed by high temperature decomposition of samarium(III) iodide (SmI3), the more stable iodide. A convenient lab preparation is to react Sm powder with diiodoethane (ICH2CH2I) in anhydrous THF, or diiodomethane (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.
Sm + ICH2
I → SmI2
Samarium(II) iodide Wikipedia
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:
I + R2
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.
Tosyl groups can be removed from N-tosylamides almost instantaneously, using SmI2 in conjunction with a base. The reaction is even effective for the synthesis of sensitive amines such as aziridines:
In the Markó-Lam deoxygenation, an alcohol could be almost instantaneously deoxygenated by reducing their toluate ester in presence of SmI2.
The applications of SmI2 have been reviewed. The book "Organic Synthesis Using Samarium Diiodie" published in 2009 gives a detailed overview of reactions mediated by SmI2.