Samarium Diiodide Research Articles

Samarium Diiodide/Samarium-Mediated Deoxygenative Hydrosilylation of Ketones.

The first deoxygenative hydrosilylation of ketones promoted by samarium diiodide and samarium has been reported. In this approach, secondary alkyl silanes are synthesized from unactivated ketones and chlorosilanes in one step. The carbonyl group of ketones is activated by SmI2-mediated single-electron transfer process, which follows a deoxygenative elimination of O=Sm. Broad substrate scope and excellent functional group tolerance has been demonstrated.

Reductive samarium (electro)catalysis enabled by SmIII-alkoxide protonolysis.

Samarium diiodide (SmI2) is a privileged, single-electron reductant deployed in diverse synthetic settings. However, generalizable methods for catalytic turnover remain elusive because of the well-known challenge associated with cleaving strong SmIII-O bonds. Prior efforts have focused on the use of highly reactive oxophiles to enable catalyst turnover. However, such approaches give rise to complex catalyst speciation and intrinsically limit the synthetic scope. Herein, we leveraged a mild and selective protonolysis strategy to achieve samarium-catalyzed, intermolecular reductive cross-coupling of ketones and acrylates with broad scope. The modularity of our approach allows rational control of selectivity based on solvent, pKa (where Ka is the acid dissociation constant), and the samarium coordination sphere and provides a basis for future developments in catalytic and electrocatalytic lanthanide chemistry.

Schlenk line protocols for using SmI2 and the impact of THF solvent

Samarium diiodide (SmI2) is a single electron reductant and versatile synthetic tool that can be used to create carbon- carbon bonds or other synthetic transformations. SmI2 is typically used with an Argon atmosphere glovebox and anhydrous solvents from a solvent purification system; however, not all labs have access to these equipment. A more accessible air-free option is Schlenk line glassware. Here in reports an optimized method for the synthesis and use of SmI2 on a Schlenk line. During optimization of the system, it was found that the largest impact on the success of SmI2 was the THF solvent. Anhydrous solvent was achieved though distillation, but after storage of the solvent over two weeks enough moisture/air was accumulated to significantly impact the concentration of the SmI2 formed in solution. The use of molecular sieves further indicates that the impact of water accumulation in the solvent compared to oxygen has the most significant factor on the success of synthesizing SmI2. These observations of THF storage overtime are important to account for when carrying out SmI2 reactions and other air-sensitive reactions on the Schlenk line.

Samarium diiodide/samarium-mediated direct deoxygenative hydroborylation of ketones with hydroborane esters.

A direct deoxygenative hydroborylation of ketones with hydroborane ester promoted by a combination of samarium diiodide, samarium and nickel has been developed. In this method, secondary alkyl borate esters are synthesized from unactivated ketones with hydroborane esters in one step. A broad substrate scope and excellent selectivity toward CO cleavage has been demonstrated. This approach represents a general method for the construction of versatile secondary alkyl borate esters from unactivated ketones.

Chemoselective Reduction of Barbiturates by Photochemically Excited Flavin Catalysts

AbstractPhotocatalytic reductive cyclizations are powerful methods for obtaining structurally complex molecules. Achieving noninherent reactivity in substrates with more than one potential site of reduction is a difficult challenge. We disclose the use of flavin catalysis for the chemoselective reductive cyclization of barbiturates with additional reactive functional groups. Our method provides orthogonal selectivity in comparison to the well-established reductant samarium(II) iodide, which preferentially reduces substrate ketone groups. Flavin catalysis first leads to barbiturate reduction and allows a complete change of chemoselectivity in barbiturates with appended ketones. Additionally, flavin photocatalysis enables the reductive cyclization of substrates with appended oxime ethers in >99% yield, which is not possible with SmI2.

Catalytic Activity of Molybdenum Complexes Bearing PNP-Type Pincer Ligand toward Ammonia Formation.

We newly designed and prepared a novel molybdenum complex bearing a 4-[3,5-bis(trifluoromethyl)phenyl]pyridine-based PNP-type pincer ligand, based on the bond dissociation free energies (BDFEs) of the N-H bonds in molybdenum-imide complexes bearing various substituted pyridine-based PNP-type pincer ligands. The complex worked as an excellent catalyst toward ammonia formation from the reaction of an atmospheric pressure of dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions, where up to 3580 equivalents of ammonia were formed based on the molybdenum atom of the catalyst. The catalytic activity was significantly improved by one order of magnitude larger than that observed when using the complex before modification.

Catalytic Activity of Molybdenum Complexes Bearing PNP‐Type Pincer Ligand toward Ammonia Formation

AbstractWe newly designed and prepared a novel molybdenum complex bearing a 4‐[3,5‐bis(trifluoromethyl)phenyl]pyridine‐based PNP‐type pincer ligand, based on the bond dissociation free energies (BDFEs) of the N−H bonds in molybdenum‐imide complexes bearing various substituted pyridine‐based PNP‐type pincer ligands. The complex worked as an excellent catalyst toward ammonia formation from the reaction of an atmospheric pressure of dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions, where up to 3580 equivalents of ammonia were formed based on the molybdenum atom of the catalyst. The catalytic activity was significantly improved by one order of magnitude larger than that observed when using the complex before modification.

Electrochemical Preparation of Sm(II) Reagent Facilitated by Weakly Coordinating Anions.

Samarium diiodide (SmI2 ) is widely used as a strong one-electron reducing agent and is often employed to form C-C bonds in complex systems. Despite their utility, SmI2 and related salts suffer from several drawbacks that render the use of Sm reducing agents in large-scale synthesis impractical. Here, we report factors influencing the electrochemical reduction of Sm(III) to Sm(II), towards the goal of electrocatalytic Sm(III) reduction. We probe the effect of supporting electrolyte, electrode material, and Sm precursor on Sm(II)/(III) redox and on the reducing power of the Sm species. We find that the coordination strength of the counteranion of the Sm salt affects the reversibility and redox potential of the Sm(II)/(III) couple and establish that the counteranion primarily determines the reducibility of Sm(III). Electrochemically generated SmI2 performs similarly to commercial SmI2 solutions in a proof-of-concept reaction. The results will provide fundamental insight to facilitate the development of Sm-electrocatalytic reactions.

Regioselective SmI2-Mediated Radical ipso-Substitution Cyclization for the Construction of Pyrrolophenanthridinone Skeletons: The Synthesis of Amaryllidaceae Alkaloids.

Here, we report a regioselective, samarium(II) diiodide mediated intramolecular radical ipso-substitution cyclization. Through the use of a methoxy group as a leaving group, it was possible to regulate the regioselectivity of the reaction by changing the temperature and additives. We applied the developed reaction to the synthesis of four Amaryllidaceae alkaloids and have shown that the present reaction successfully overcomes regioselectivity issues encountered with other cyclization methods.

Direct Reaction - One Step Route to Synthesize Lanthanoid-iodide Formamidinates.

This paper describes a novel and simple method - direct reaction of lanthanoid metals with equimolar amounts of iodine and a formamidine under ultrasonication as an effective, metal-based route to lanthanoid(III) diiodide formamidinates, namely I. N,N'-Bis(2,6-diisopropylphenyl)formamidinatodiiodidolanthanoid(III) complexes [Ln(DippForm)I2 (thf)3 ] (Ln=La, 1, Ce, 2, Tb, 3, Ho, 4, Er, 5, Tm, 6); II. N,N'-Bis(2,6-diethylphenyl)formamidinatodiiodidolanthanoid(III) complexes [Ln(EtForm)I2 (thf)3 ] (Ln=Ce, 7, Nd, 8, Gd, 9, Tb, 10, Dy, 11, Ho, 12, Er, 13, Lu, 14). III. N,N'-bis(2,6-dimethylphenyl)formamidinatodiiodidolanthanoid(III) complexes [Ln(XylForm)I2 (thf)3 ] (Ln=Ce, 15, Nd, 16, Gd, 17, Tm, 18, Lu 19); IV. N,N'-bis(phenyl)formamidinatodiiodidolanthanoid complexes [Ln(PhForm)I2 (thf)3 ] (Ln=Nd, 20, Gd, 21, Er, 22). Compound [Ce(XylForm)2 I(thf)2 ] (23) was also synthesized by the same method except the ratio of I2 to XylFormH was 1 : 4. Divalent N,N'-bis(2,6-diisopropylphenyl)formamidinato-iodido-lanthanoid(II) complexes [Eu(DippForm)I(thf)4 ] ⋅ thf (24), [Yb(DippForm)I(thf)3 ] ⋅ 2DippFormH (25), [Sm(DippForm)I(thf)4 ] ⋅ thf (26) have also been synthesized by direct reactions of the free metals, iodine and DippFormH. Interestingly, [Sm(DippForm)I2 (thf)3 ] (27) was obtained by the oxidation of [Sm(DippForm)I(thf)4 ] ⋅ thf (26) on exposure to air. N,N'-Bis(2,6-dimethylphenyl)formamidinatoiodidosamarium(II) [Sm(XylForm)I(thf)3 ]n (28) was also prepared by direct reaction of Sm, iodine and XylFormH (mole ratio of I2 : XylFormH=1 : 2). All products have been identified by X-ray crystallography and all the trivalent complexes [Ln(Form)n I3-n ] (n=1 or 2) are stable to rearrangement.

An Enantiospecific Synthesis of 5-epi-α-Bulnesene.

As a result of its unique fragrance and wider role in traditional medicine, agarwood produced in Aquilaria spp. and certain other trees has been harvested to near extinction as a natural phenomenon. Artificially induced agarwood production in Aquilaria plantations has sated some of the demand although the product quality is variable. Synthetic chemistry may have a role to play in providing sustainable routes to many of the fragrant components identified in agarwood and its smoke when burnt as incense. In this work, we report efforts towards a total synthesis of the guaiane sesquiterpene α-bulnesene, which is found, along with its more fragrant oxidised derivatives, in agarwood. Following the ring-expansion of (R)-carvone using reported procedures, α-butenylation gave a substrate for samarium diiodide mediated reductive cyclisation, the two butenyl epimers of the substrate each leading to a single bicyclic alcohol (24 and 25). Overall homoconjugate hydride reduction of one of these alcohols was achieved by Lewis acid-mediated ionisation and then hydride transfer from triethylsilane to complete an overall seven-step synthesis of 5-epi-α-bulnesene. This new synthesis paves the way for short routes to both α-bulnesene enantiomers and a study of their aerial and enzymatic oxidation products.

Enantioselective Dearomatization of Indoles via SmI2-Mediated Intermolecular Reductive Coupling with Ketones

Samarium diiodide (SmI2) mediated reductive coupling reactions are powerful methods for the construction of carbon-carbon bond in organic synthesis. Despite the extensive development in recent decades, successful examples of the corresponding asymmetric reactions remained scarce, probably due to the involvement of highly reactive radical intermediates. In this Article, we report an enantioselective dearomatization of indoles via SmI2-mediated intermolecular reductive coupling with ketones. The utilization of samarium reductant supported by chiral tridentate aminodiol ligands allows the facile synthesis of indoline molecules bearing two contiguous stereogenic centers in high yields (up to 99%) and stereoselectivity (up to 99:1 er and >20:1 dr). Combined experimental and computational investigations suggested that parallel single-electron transfer to each substrate from the chiral samarium reductant allows the radical-radical recombination in an enantioselective manner, which is a unique mechanistic scenario in SmI2-mediated reductive coupling reactions.

Enabling Molecular-Level Computational Description of Redox and Proton-Coupled Electron Transfer Reactions of Samarium Diiodide.

Samarium diiodide (SmI2, Kagan's reagent) is a one-electron reductant with applications ranging from organic synthesis to nitrogen fixation. Highly inaccurate relative energies of redox and proton-coupled electron transfer (PCET) reactions of Kagan's reagent are predicted by pure and hybrid density functional approximations (DFAs) when only scalar relativistic effects are accounted for. Calculations including spin-orbit coupling (SOC) show that the SOC-induced differential stabilization of the Sm(III) versus the Sm(II) ground state is little affected by ligands and solvent, and a standard SOC correction derived from atomic energy levels is thus included in the reported relative energies. With this correction, selected meta-GGA and hybrid meta-GGA functionals predict Sm(III)/Sm(II) reduction free energies to within 5 kcal/mol of the experiment. Considerable discrepancies remain, however, in particular for the PCET-relevant O-H bond dissociation free energies, for which no regular DFA is within 10 kcal/mol of the experiment or CCSD(T). The main cause behind these discrepancies is the delocalization error, which leads to excess ligand-to-metal electron donation and destabilizes Sm(III) versus Sm(II). Fortunately, static correlation is unimportant for the present systems, and the error may be reduced by including information from virtual orbitals via perturbation theory. Contemporary, parametrized double-hybrid methods offer promise as companions to experimental campaigns in the further development of the chemistry of Kagan's reagent.

Catalytic production of ammonia from dinitrogen employing molybdenum complexes bearing N-heterocyclic carbene-based PCP-type pincer ligands

Mechanistic insight into the catalytic production of ammonia from dinitrogen is needed to improve the synthesis of this vital molecule. Here we study the use of samarium diiodide (SmI2) and water in the presence of molybdenum complexes that bear PCP-type pincer ligands to synthesize ammonia. The proton-coupled electron transfer during the formation of a N–H bond on the molybdenum imide complex was found to be the rate-determining step at high catalyst concentrations. Additionally, the dimerization step of the catalyst became the rate-determining step at low catalyst concentrations. We designed PCP-type pincer ligands with various substituents at the 5- and 6-positions and observed that electron-withdrawing groups promoted the reaction rate, as predicted by density functional theory calculations. A molybdenum trichloride complex that bears a trifluoromethyl group functioned as the most effective catalyst and produced up to 60,000 equiv. ammonia based on the molybdenum atom of the catalyst, with a molybdenum turnover frequency of up to 800 equiv. min−1. The findings reported here can contribute to the development of an environmentally friendly next-generation nitrogen-fixation system.

Coordination Structure of Samarium Diiodide in a Tetrahydrofuran-Water Mixture.

Chemoselective reductive conversion of organic and inorganic compounds has been developed by the combination of samarium(II) diiodide (SmI2) and water. Despite the extensive previous studies to elucidate the role of water in the reactivity of SmI2, the direct structural data of the reactive Sm2+-water complexes, SmI2(H2O)n, in an organic solvent-water mixture have not been reported experimentally so far. Herein, we performed the structure analysis of the Sm2+-water complex in tetrahydrofuran (THF) in the presence of water by in situ X-ray absorption spectroscopy using high-energy X-rays (Sm K-edge, 46.8 keV). The analysis revealed the dissociation of the Sm2+-I bonds in the presence of ≥ eight equivalents of water in the THF-water mixture. The origin of the peak shift in the UV/visible absorption spectra after the addition of water into SmI2/THF solution was proposed based on electron transitions simulated with time-dependent density-functional-theory calculations using optimized structures in THF or water. The obtained structural information provides the fundamental insights for elucidating the reactivity and chemoselectivity in the Sm2+-water complex system.

Design, synthesis and reactivity of dimolybdenum complex bearing quaterphenylene-bridged pyridine-based PNP-type pincer ligand.

Dimolybdenum complexes bearing 3,3'''-(1,1':3',1'':3'',1'''-quaterphenylene)-bridged pyridine-based PNP-type pincer ligand are designed and prepared according to DFT calculations on the cleavage step of dinitrogen-bridged dimolybdenum complexes bearing polyphenylene-bridged pyridine-based PNP-type pincer ligands. The dimolybdenum complexes are found to work as effective catalysts toward ammonia formation from dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions.

Samarium Diiodide Acting on Acetone-Modeling Single Electron Transfer Energetics in Solution.

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA.

Sm(II)-Mediated Proton-Coupled Electron Transfer: Quantifying Very Weak N-H and O-H Homolytic Bond Strengths and Factors Controlling Them.

Coordination of alcohols to the single-electron reductant samarium diiodide (SmI2) results in substantial O-H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI2 in tetrahydrofuran (THF)/alcohol mixtures limits reliable thermodynamic analyses of such systems. Rigorous determination of bond dissociation free energy (BDFE) values in such Sm systems, important to evaluating their reactivity profiles, motivates studies of model Sm systems where contributing factors can be teased apart. Here, a bulky and strongly chelating macrocyclic ligand ((tBu2ArOH)2Me2cyclam) maintains solubility, eliminates dimerization pathways, and facilitates clean electrochemical behavior in a well-defined functional model for the PCET reactivity of SmII with coordinating proton sources. Direct measurement of thermodynamic parameters enables reliable experimental estimation of the BDFEs in 2-pyrrolidone and MeOH complexes of ((tBu2ArO)2Me2cyclam)SmII, thereby revealing exceptionally weak N-H and O-H BDFEs of 27.2 and <24.1 kcal mol-1, respectively. Expanded thermochemical cycles reveal that this bond weakening stems from the very strongly reducing SmII center and the formation of strong SmIII-alkoxide (and -pyrrolidonate) interactions in the PCET products. We provide a detailed analysis comparing these BDFE values with those that have been put forward for SmI2 in THF in the presence of related proton donors. We suggest that BDFE values for the latter systems may in fact be appreciably higher than the system described herein. Finally, protonation and electrochemical reduction steps necessary for the regeneration of the PCET donors from SmIII-alkoxides are demonstrated, pointing to future strategies aimed at achieving (electro)catalytic turnover using SmII-based PCET reagents.

Direct synthesis of cyanate anion from dinitrogen catalysed by molybdenum complexes bearing pincer-type ligand

Dinitrogen is an abundant and promising material for valuable organonitrogen compounds containing carbon–nitrogen bonds. Direct synthetic methods for preparing organonitrogen compounds from dinitrogen as a starting reagent under mild reaction conditions give insight into the sustainable production of valuable organonitrogen compounds with reduced fossil fuel consumption. Here we report the catalytic reaction for the formation of cyanate anion (NCO−) from dinitrogen under ambient reaction conditions. A molybdenum–carbamate complex bearing a pyridine-based 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP)-pincer ligand is synthesized from the reaction of a molybdenum–nitride complex with phenyl chloroformate. The conversion between the molybdenum–carbamate complex and the molybdenum–nitride complex under ambient reaction conditions is achieved. The use of samarium diiodide (SmI2) as a reductant promotes the formation of NCO− from the molybdenum–carbamate complex as a key step. As a result, we demonstrate a synthetic cycle for NCO− from dinitrogen mediated by the molybdenum–PNP complexes in two steps. Based on this synthetic cycle, we achieve the catalytic synthesis of NCO− from dinitrogen under ambient reaction conditions.

Structural Diversity and Multielectron Reduction Reactivity of Samarium(II) Iodido-β-diketiminate Complexes Dependent on Tetrahydrofuran Content.

The molecular structures of complexes [Sm(Nacnac)I(thf)n] (Nacnac = HC(C(Me)Ndipp)2-, dipp = 2,6-diisopropylphenyl, thf = tetrahydrofuran) depending on the number of thf ligands are studied. The complete removal of thf from a known complex [Sm(Nacnac)I(thf)2] leads to a tetranuclear product [Sm(Nacnac)I]4 (4). The partial removal of thf results in mixtures of dinuclear [Sm2(Nacnac)2I2(thf)] (2), trinuclear [Sm3(Nacnac)3I3(thf)] (3), and tetranuclear [Sm4(Nacnac)4I4(thf)2] (4*) complexes and 4, depending on the conditions. The reaction of solvent-free SmI2 with 1 equiv of K(Nacnac) results mainly in [Sm(Nacnac)2] (1), while the interaction of 4 with certain amounts of thf allows obtaining pure 2 and 3 (with the admixture of 4*). Complex 4* is the exact dimer of 2, and both compounds are stable in solutions. Reactions with 3 and 4 as reductants are studied. 4 is oxidized by I2 to stoichiometrically yield two products, mixed-valent tetranuclear [Sm4(Nacnac)4I5] (5) and binuclear [Sm(Nacnac)I2]2 (6) complexes. In the reaction of 4 with nBu3PTe, a trinuclear complex [Sm3(Nacnac)3(μ-I)3(μ3-E)2] (8, E = I or Te) is formed in small amounts, with the formation of 6 as the second product. 3 serves as a two-electron reductant in the reaction with nBu3PTe to yield a trinuclear complex [Sm3(Nacnac)3I3(μ-Te2)] (7). Complexes 2, 4, 4*, 5, 6, and 8 possess a unique flat SmxIy core of heavy atoms, which is assumed to be a consequence of the Nacnac ligand geometry.