Reducing agents (along with complementary oxidants) constitute the most fundamental class of reagents available to synthetic chemists for the conversion of organic substrates to desirable organic products. As a consequence of their central importance, the search for novel reducing agents remains the focus of intense exploration. Especially valued are those reagents that not only transform a broad range of diverse functional groups, but do so with a high degree of selectivity. Since the early 1980s, samarium(II) iodide (SmI2) has been increasingly recognized as a reducing agent capable of meeting the intensifying demands of synthetic organic chemistry. Although the compound itself has been known for many years, it was not until Kagan and co-workers developed a convenient synthesis of SmI2 and outlined its general reactivity with common organic functional groups that SmI2 became of general interest and importance to synthetic organic chemists.
Samarium(II) iodide is a powerful one-electron reducing agent that can be prepared in moderate concentrations (0.1 M) in tetrahydrofuran (THF) by one of several different reactions from samarium metal.
Deep blue solutions of SmI2 are generated in virtually quantitative yields by these procedures, but the preparations using diiodomethane or diiodoethane as the oxidants would appear to be the easiest and most reliable methods. The reducing agent can be stored as a solution in THF for reasonably long periods of time, particularly when it is stabilized by a small amount of samarium metal. Alternatively, the solvent may be removed, providing SmI2(THF)n powder. For synthetic purposes, SmI2 is typically generated and utilized in situ, although THF solutions of SmI2 are commercially available.
The reduction potential of Sm+2/Sm+3 as measured in water is −1.55 V. However, the reduction potential as well as the ability of SmI2 to promote the reduction of diverse organic substrates varies widely according to the solvent and the presence of various additives.
Kagan’s pioneering studies on the reaction of SmI2 with organic substrates not only provided a general outline for its reactivity, but also inspired an extraordinary number of subsequent studies with important ramifications for selective organic synthesis. As a consequence of these extensive studies, SmI2 has emerged as one of the more useful reducing agents in synthetic organic chemistry. The complementary reactivity of SmI2 as compared to the vast inventory of other available reducing agents constitutes one reason for its appeal. Another attraction of SmI2 is its ready accessibility, either from commercial sources or by in situ preparation via one of the convenient methods outlined above. The high chemoselectivity exhibited by SmI2 and the ability to change its reactivity (selectivity) rather dramatically based upon solvent effects further enhance its attractiveness. Finally, SmI2 is easily handled by standard techniques for the manipulation of air-sensitive materials.
This chapter outlines some of the practical aspects of reactions employing SmI2, but is strictly limited to functional group reductions. Thus those reactions promoted by SmI2 that result in the formation of new carbon–carbon bonds are not included. The chapter is organized according to the type of organic functional groups involved, and topics covered include the reduction of organic halides, sulfonates, and sulfones, reductive elimination/fragmentation processes; reduction of aldehydes and ketones; reduction of carboxylic acids and their derivatives; reduction of conjugated carbonyl substrates; reductive cleavage of α-heterosubstituted carbonyl substrates; reduction of cyclopropyl ketones; various deoxygenation reactions; reduction of nitrogen-based functional groups; and finally an outline of the reduction of miscellaneous functional groups that cannot be appropriately assigned to the general classifications outlined above. In covering these topics, the literature has been surveyed through 1992.