Introduction

1.1.1 Aims of the Book

Since its introduction to the synthetic chemistry community in 1977 by Kagan, samarium diiodide (SmI2) has captured the imagination of organic chemists and has become one of the most important reducing agents available in the laboratory. The main chapters of this practical guide deal with the remarkable ability of SmI2 to transform functional groups selectively and to orchestrate carbon–carbon bond formation. Other chapters deal with our understanding of mechanism and additive effects in reactions mediated by SmI2 – an area that should still be considered as very much a work in progress. The final chapter of the book deals with selected emerging areas in the use of SmI2 in synthesis and reflects the authors' research interests.

The book aims to steer a difficult course by providing a sufficient level of detail and new developments without burying the basics. Representative procedures have been included to encourage the reader to take their first steps into the fascinating organic chemistry of SmI2.

1.1.2 Further Reading

The many excellent reviews on the use of SmI2 in organic chemistry are a rich source of additional information. The major reviews and their authors are categorised according to their coverage below in Figure 1.1.

1.2 Introducing the Reagent

1.2.1 Working with SmI2

Samarium(II) iodide (SmI2) is commercially available as a solution in THF or can be prepared readily using one of several straightforward methods that have been described (see Chapter 2, Section 2.1). SmI2 is air sensitive, but is tolerant of water and can be handled using standard syringe techniques. Reactions are typically carried out in THF although the use of other solvents has been investigated. Part of the reagent's popularity arises from its ability to mediate both radical and anionic processes and sequences involving both. As a result, it has been utilised in a wide range of synthetic transformations ranging from functional group interconversions to carbon–carbon bond-forming reactions. In addition, the reagent is often highly chemoselective and transformations instigated by SmI2 tend to proceed with high degrees of stereoselectivity. Further adding to its appeal, the reactivity, chemoselectivity and stereo-selectivity of SmI2 can be manipulated and fine-tuned by the addition of various salts and cosolvents to the reaction mixture (see Chapter 2, Section 2.2).

1.2.2 Electronic Configuration of Sm(II)

Samarium, like all lanthanide elements, preferentially exists in the + 3 oxidation state. The loss of the three outermost electrons, namely the 5d1, 6s2 electrons, results in enhanced thermodynamic stability in which a closed-shell Xe-like electronic configuration is adopted. The + 2 oxidation state is most relevant for samarium (f6, near half-filled), europium (f7, half-filled), thulium (f13, nearly filled) and ytterbium (f14, filled). In order to attain the more stable + 3 oxidation state, SmI2 readily gives up its final outer-shell electron, in a thermodynamically driven process, making it a very powerful and synthetically useful single-electron transfer reagent.

1.2.3 Reduction Potential

The redox potential of the SmI2-SmI2+ couple has been established through the use of linear sweep and cyclic voltammetry and was found to be approximately -1.41V, determined for a solution of SmI2 in THF. Although the preparation of SmI2 in a variety of different solvents is known, THF is by far the most common solvent associated with its use. It would be expected, however, that the observed reduction potential of the reagent will vary greatly depending on the choice of solvent in which the measurement is carried out owing to the differences in the strength and number of solvent interactions.

It is possible to manipulate the reduction potential of SmI2 through the use of various additives; most commonly these are found to be molecules containing neutral or Lewis basic oxygen functionalities. The most common example of this is the use of HMPA (hexamethylphosphoramide) as an additive, for which it was determined that, upon addition of 4 equiv, the reduction potential of SmI2 in THF is increased to approximately -1.79 V, thereby significantly increasing its potency as a single-electron transfer reagent. Similar effects are also observed in the presence of alcohols and even water, for which it has been reported that upon the inclusion of 500 equiv, the reduction potential can be increased as far as -1.9 V. The use and mechanistic role of such additives will be discussed in more detail in Chapter 2, Section 2.2.

1.2.4 Coordination Chemistry

Lanthanides typically adopt coordination numbers greater than six, depending largely on the size of the lanthanide ion and on the size of the ligands. SmI2 is an oxophilic reagent and, as such, much of the coordination chemistry observed involves the close association of oxygenated molecules, including solvents and substrates, to the metal centre.

It has been established that in a solution of THF, SmI2 exists in a hepta-coordinate geometry in which five THF molecules are equatorially bound to the central Sm(II) ion through their oxygen lone pairs, with iodide ligands axial. The coordination number and geometry of SmI2 is variable depending on the nature of the ligands involved. For example, in the [SmI2(HMPA)4] complex alluded to previously, the more sterically demanding HMPA ligands are positioned equatorially around the Sm(II) ion, but this time only four ligands are involved and an octahedral (hexacoordinate) geometry results. Complexes in which the coordination number is as high as eight ([FORMULA OMITTED]) and nine ([FORMULA OMITTED]) have also been identified.

The oxophilic nature of SmI2 is in many cases a highly beneficial quality: The coordination of two or more oxygenated reactive centres to samarium in both radical and ionic processes mediated by the reagent can often lead to high levels of diastereoselectivity in the formation of products. The coordination of Sm(II) and Sm(III) to oxygen donors on the substrate, solvent or cosolvent is a common theme that runs through each of the subsequent chapters.

The Reagent and the Effect of Additives

2.1 Preparing SmI2

Although SmI2 is commercially available as a 0.1 M solution in THF from several suppliers, it is also easy to prepare using one of several known procedures.

During Kagan's early work with SmI2, he developed a convenient method to prepare the reagent from samarium metal using 1,2-diiodoethane in THF (Scheme 2.1). Stirring this mixture under an inert atmosphere for several...