Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

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.

Quantification of the Interaction of SmI2 with Substrates and Ligands

The method developed and introduced here enables for the first time (to the authors' knowledge), a quantitative assessment of the interaction of SmI₂ with substrates prior to the electron transfer stage. As a proof of concept, equilibrium constants for some model substrates including carbonyl compounds and aromatic nuclei are reported here. In addition, the first equilibrium constants with some common ligands were also determined. The equilibrium constants range from approximately 0.07 m^-1 for diisopropyl ketone to 2500 m^-1 for hexamethylphosphoramide (HMPA). It is shown that the data acquired by this method, which is based on the concept of shift reagents, can shed light on the most intimate details of the reaction mechanism, and this method is a useful tool for planning a synthetic process.

Coordination of tridentate ligands to SmI2: cooperativity and incremental effect on reduction potential and on reactivity

The effect of coordination of a series of tridentate ligands (TDLs) on various features of SmI2 was determined. The TDLs used in this study were diethylene glycol (OOO), diethanolamine (ONO), 2-(2-Aminoethoxy) ethanol (OON), N-(2-Hydroxyethyl) ethylene diamine (ONN) and glycerol (GLY). Of special interest is the effect of these additives on the reduction potential of SmI2. The cyclic voltammograms of the TDLs with nitrogen at the binding sites display simultaneously several peaks, each corresponding to a different coordination level of SmI2, enabling determination of three equilibrium constants. The results are in concert with electronic spectra of SmI2 complexes with these ligands. The second and third equilibrium constants were found to be larger than the first, demonstrating the cooperativity effect. Moreover, the incremental effect of each moiety on the reduction potential of SmI2 was determined. Regarding reactivity of SmI2, excessive coordination of some ligands is shown to have an adverse effect.

Proton-coupled electron transfer in the reduction of carbonyls using SmI2–H2O: implications for the reductive coupling of acyl-type ketyl radicals with SmI2–H2O

The feasibility of concerted PCET in the reduction of carbonyl groups using SmI₂–H₂O is quantitatively assessed.