Samarium Iodide Showcase: Unraveling the Mechanistic Puzzle

Abstract

ConspectusSmI2 was introduced to organic chemistry as a single electron transfer agent in 1977. After ca. 15 years of latency, the scientific community has realized the high potential of this reagent, and its chemistry has started blooming. This versatile reagent has mediated a myriad of new bond formations, cyclizations, and other reactions. Its popularity stems largely from the fact that three different intermediates, radical anions, radicals, and anions, depending on the ligand or additive used, could be obtained. Each of these intermediates could in principle lead to a different product. While these options vastly enrich the repertoire of SmI2, they necessitate a thorough mechanistic understanding, especially concerning how appropriate ligands direct the SmI2 to the desired intermediate. Our first paper on this subject dealt with the reduction of an activated double bond. The results were puzzling, especially the H/D isotope effect, which depended on the order of the reagents addition. This seminal paper was fundamental to an understanding of how the SmI2 works and enabled us to later explain various phenomena. For example, it was found that in a given reaction, when MeOH is used as a proton source, a spiro compound is obtained, while a bicyclic product is obtained when t-BuOH is used. Our contribution culminated in formulating guidelines for the rational use of proton donors in SmI2 reactions.The need to understand the complexity of the effect of additives on various processes is nicely demonstrated in photoinduced reactions. For example, hexamethylphosphoramide (HMPA) enhances the reduction of anthracene while hampering the reaction of benzyl chloride. The mechanistic understanding gained enabled us also to broaden the scope of photostimulated reactions from substrates reacting by a dissociative electron transfer mechanism to normal reductions, which are difficult to accomplish at the ground state. Harnessing the classical knowledge of proton transfer mechanisms to our SmI2 research enabled us to decipher an old conundrum: why does the combination of water and amine have such an enhancing effect on the reactivity of SmI2, which is not typical of these two when used separately. In our studies on the affinity of ligands to SmI2, we discovered that, in contradistinction to the accepted dogma, SmI2 is much more azaphilic than it is oxophilic. On the basis of the size difference between Sm3+ and Sm2+, we developed a simple diagnostic tool for the nature of the steps following the electron transfer. The reduction of imines showed that substrate affinity to SmI2 plays also a crucial role. In these reactions, new features such as autocatalysis and catalysis by quantum dots were discovered. Several studies of the ligand effect lead to a clear formulation of when an inner sphere or outer sphere electron transfer should be expected. In addition, several reactions where proton-coupled electron transfer (PCET) is the dominant mechanism were identified. Finally, the surprisingly old tool of NMR “shift reagents” was rediscovered and used to directly derive essential information on the binding constants of ligands and substrates to SmI2.