Broadening the Scope of Photostimulated SmI2Reductions

Quantitation of the Interactions of Alcohols and Amines with SmI2: Pros and Cons of VIS and NMR Spectroscopies

While additives play an important role in the reactions of samarium iodide, ligand-SmI₂ complexation constants are scarce. Here, VIS spectroscopy was harnessed along with NMR to determine the first complexation constant for most of the alcohols and amines used in SmI₂ reactions. The second equilibrium constant was determined for selected ligands. In cases where both methods could be applied, in general, a good correlation between the equilibrium constants was obtained.

Samarium Iodide Showcase: Unraveling the Mechanistic Puzzle

SmI₂ 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 SmI_2, 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.

Contrasting Effect of Additives on Photoinduced Reactions of SmI2

The work described herein compares the effect of additives (HMPA, methanol, ethylene glycol, pinacol, N-methylethanolamine) on thermal and photochemical reactions of samarium diiodide (SmI₂ ). In thermal reactions, additives that coordinate to SmI₂ induce a significant increase in reaction rate. In photochemical reactions, the presence of an electronegative atom with a highly localized negative charge on the substrate leads to a rate deceleration. In order to benefit from the columbic interaction with the positively charged samarium cation, these substrates react preferentially by an inner sphere reduction mechanism. The addition of ligands prevents this close interaction causing rate retardation. Furthermore, studies demonstrate that excited state quenching of Sm^II by ethylene glycol and other additives indicate that it is unlikely to be the major cause for the observed rate retardation. This effect provides a simple diagnostic tool to distinguish between an inner and an outer sphere reduction mechanism.

Task-Dependent Coordination Levels of SmI2

Ligation plays a multifaceted role in the chemistry of SmI₂. Depending on the ligand, two of its major effects are increasing the reduction potential of SmI₂, and in the case of a ligand, which is also a proton donor, it may also enhance the reaction by protonation of the radical anion generated in the preceding step. It turns out that the number of ligand molecules that are needed to maximize the reduction potential of SmI₂ is significantly smaller than the number of ligand molecules needed for a maximal enhancement of the protonation rate. In addition to the economical use of the ligand, this information can also be utilized as a diagnostic tool for the reaction mechanism in differentiating between single and multistep processes. The possible pitfalls in applying this diagnostic tool to PCET and cyclization reactions are discussed.

Aqueous Lanthanide Chemistry in Asymmetric Catalysis and Magnetic Resonance Imaging

This account describes lanthanide coordination chemistry with a focus on the similarities between lanthanide complexes used in catalysis and those used as contrast agents in magnetic resonance imaging.