On the Role of Pre‐ and Post‐Electron‐Transfer Steps in the SmI2/Amine/H2O‐Mediated Reduction of Esters: New Mechanistic Insights and Kinetic Studies

The mechanism of the SmI₂‐mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI₂, amine, H₂O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl‐containing radical clocks demonstrates that the reaction occurs via fast, reversible first electron transfer, and that the electron transfer from simple Sm(II) complexes to aliphatic esters is rapid. Notably, the mechanistic details presented herein indicate that complexation between SmI₂, H₂O and amines affords a new class of structurally diverse, thermodynamically powerful reductants for efficient electron transfer to carboxylic acid derivatives as an attractive alternative to the classical hydride‐mediated reductions and as a source of acyl‐radical equivalents for C?C bond forming processes.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^?), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^? or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C? C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^?, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN⁻), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN⁻ or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN⁻, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

The Novel Preparation of Hydrocinnamonitriles via the Reduction of α-Phenylsulfonyl Cinnamonitriles with SmI2/CH2OH/THF System

Hydrocinnamonitriles were readily obtained via the reduction of α-phenylsulfonyl cinnamonitriles by SmI₂/CH₃OH/THF system.     

A concise route to the C2-symmetric tricyclic skeleton of ryanodine

Ryanodine is a potent calcium channel modulator. In this Letter, we report the 10-step synthesis of the highly substituted tricyclic ring system of ryanodine. Diels-Alder reaction via dearomatization of 2,5-dimethylbenzene-1,4-diol and subsequent SmI₂-mediated reductive coupling of eight-membered 1,5-diketone efficiently introduced the four consecutive fully substituted carbons of the tricyclo[3.3.2.0^2,6]decane system.     

Synthesis of 6,6-dimethyltricyclo[5.4.0.02,8]undecane-2,9-diol for (ent-)longipinane-type sesquiterpenoids using two types of radical cyclization reactions

A synthetic route to 6,6-dimethyltricyclo[5.4.0.0^2,8]undecane-2,9-diol, a key precursor to (ent-)longipinane-type sesquiterpenoids, is described. This unique core common to (ent-)longipinanes was constructed using two types of intramolecular radical cyclization reactions, namely, intramolecular coupling of an acid chloride and an alkyl iodide mediated by SmI₂, TBAI and HMPA, and the coupling of a ketone and an epoxide mediated by Cp₂TiI₂ and SmI₂.     

Regioselective radical ring-opening reaction of bicyclo[4.2.0]octan-2-ones promoted by samarium(II) iodide

Radical ring-opening reactions of bicyclo[4.2.0]octanones, its C6 alkyl derivatives, and tricyclic ketones promoted by SmI₂ gave cyclohexanones via fission of the external cyclobutane bond. The CO₂Me, CN, and phenyl derivatives led to the production of the eight-membered ring compounds through cleavage of the central cyclobutane bond. Using this regioselective reaction, the synthesis of (±)-acorenone was achieved.     

Synthese und Kristallstrukturen der Samarium-Komplexe [SmI2(DME)3] und [Sm2I(NPPh3)5(DME)]

Synthesis and Crystal Structures of the Samarium Complexes [SmI₂(DME)₃] and [Sm₂I(NPPh₃)₅(DME)] When treated with ultrasound, the reaction of samarium metal with N-iodine-triphenylphosphaneimine in 1,2-dimethoxyethane (DME) leads to the two samarium complexes [SmI₂(DME)₃] ( 1 ) and [Sm₂I(NPPh₃)₅(DME)] ( 2 ), which are separated from each other by fractional crystallization. 1 could be isolated in two different crystallographic forms, namely as brownish black crystals ( 1 a ) and as violet-black crystals ( 1 b ), both of them are characterized by crystal structure analyses. 1 a : Space group P2₁/c, Z = 4, lattice dimensions at –80 °C: a = 1459.4(1), b = 1314.4(1), c = 2293.6(2) pm, β = 99.245(8)°, R = 0.0344. The structure of 1 a holds two crystallographically independent molecules [SmI₂(DME)₃], in which the samarium atoms have coordination number eight. The two individuals differ from each other particularly in their I–Sm–I bond angles, which are 157.94 and 178.45°. 1 b : Space group P2₁, Z = 2, lattice dimensions at –80 °C: a = 849.4(3), b = 1060.1(3), c = 1235.1(6) pm, b = 93.86(5)°, R = 0.0251. 1 b has a molecular structure similar to that of 1a with a bond angle I–Sm–I of 158.40°. The phosphoraneiminato complex [Sm₂I(NPPh₃)₅(DME)] ( 2 ) forms colourless, moisture sensitive crystals which contain two molecules DME per formula unit. 2 · 2 DME: Space group P1, Z = 2, lattice dimensions at –80 °C: a = 1405.0(4), b = 1656.5(3), c = 2208.3(7) pm, α = 89.60(3)°, β = 72.96(4)°, γ = 78.70(3)°, R = 0.0408. In 2 the two samarium atoms are linked via the μ-N atoms of two phosphoraneiminato ligands to form a planar Sm₂N₂ four-membered ring. One of the Sm atoms is terminally coordinated by the N atoms of two (NPPh₃)^– groups, thus achieving a distorted tetrahedral surrounding. The second Sm atom is coordinated by the N atom of one (NPPh₃)^– group, by the terminally bonded iodine atom, and by the O atoms of the DME chelate, thus achieving a distorted octahedral surrounding.     

In situ formation of Sm(III) initiator from SmI2/MMA for ring opening polymerization of cyclic esters

Sm(III) complex was generated in situ by one-electron transfer reaction of SmI₂ with MMA. The resulting Sm(III) complex was tried as ring opening polymerization (ROP) initiator for cyclic esters like ε-caprolactone (CL). The polymers were obtained in high yield with low polydispersity index (1.1-1.2). The system was also used for the synthesis of block copolymers. Structural characterization of the resulting polymers was done using 1D (¹H, ¹³C, ¹⁹F) and 2D (¹H-¹³C heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple-bond correlation (HMBC)) NMR spectroscopic techniques.     

SmI2/H2O/amine promoted reductive cleavage of benzyl-heteroatom bonds: optimization and mechanism

The SmI₂/H₂O/pyrrolidine mediated cleavage of benzylic alcohols and benzyl groups was studied and found to be a viable alternative to the Birch reduction yielding the corresponding deoxygenated product in excellent yield. The reaction has been investigated by kinetic methods, and a mechanism involving a pre-complexation of the alcohol to SmI₂ followed by an amine mediated electron transfer and subsequent bond cleavage and transfer of a second electron and proton to yield the toluene product has been proposed. The reaction is strongly inhibited at higher concentrations of water, indicating that it proceeds via an inner-sphere electron transfer from samarium(II) to the benzyl group, and excess of water prevents coordination of benzyl alcohol to samarium.     

Synthesis of cis-3-hydroxypipecolic acid via SmI2-mediated cyclization of aldehydo β-aminovinyl sulfoxides

Stereoselective synthesis of cis-3-hydroxypipecolic acid was achieved via chirality transfer in the SmI₂-mediated cyclization reactions of aldehydo β-aminovinyl sulfoxides.     

Samarium Diiodide Mediated Reactions in Total Synthesis

Introduced by Henri Kagan more than three decades ago, samarium diiodide (SmI₂) has found increasing application in chemical synthesis. This single‐electron reducing agent has been particularly useful in C? C bond formations, including those found in total synthesis endeavors. This Review highlights selected applications of SmI₂ in total synthesis, with special emphasis on novel transformations and mechanistic considerations. The examples discussed are both illustrative of the power of this reagent in the construction of complex molecules and inspirational for the design of synthetic strategies toward such targets, both natural and designed.     

A Novel Method for the Synthesis of α,β‐Unsaturated Amides Mediated by Samarium Diiodide

Promoted by Samarium diiodide (SmI₂), α,β‐unsaturated amides were formed from nitrogen anions (formed in situ by the reduction of nitro compounds) and α,β‐unsaturated esters. This reaction contrasts with the conjugate addition between amines and α,β‐unsaturated esters promoted by samarium triiodide (SmI₃) and provides an alternative attractive way to obtain α,β‐unsaturated amides using SmI₂.     

Novel reducing properties of a series of lanthanoid metals in the presence of SmI2

The reduction of dodecyl iodide, as model compound, with a series of lanthanoid (Ln) metals in the presence of catalytic amounts of samarium diiodide (SmI₂) has been investigated in detail. The reducing activity of SmI₂/Ln and the kinds of reduced products were found to be dependent on the individual Ln, but not on SmI₂.     

A Facile Synthetic Route to α-Selenoketones Promoted by SmI2 OR SmI3

α—Arylseleno or α—alkylselenoketones were synthesized by reactions of α—haloketones with RSeBr mediated by SmI₂ or SmI₃     

Samarium diiodide-promoted sequential coupling-aldol-reduction reactions of ferrocene-substituted enones

On treatment with SmI₂ in THF, 1-ferrocenyl-3-phenyl-2-propen-1-one and its related ferrocene-substituted enones underwent cyclodimerizations to give aldols (2a–g) with 3,4-trans configuration. Further reduction by using increased amounts of SmI₂ produced the corresponding diols. The stereoselectivity in this study was comparable with that in the SmI₂-promoted cyclodimerization of chalcones, but in contrast to that in the SmI₂-promoted cyclization-aldol reaction of 1,1’-dicinnamoylferrocenes. The thienyl- and furyl-substituted enones 5a and 5b could be visualized as an extended system of conjugated ketones, so that the SmI₂-mediated coupling reactions occurred preferably by linkages of β-carbons with thiophene or furan rings, giving compounds 8a,b and 9a,b.

     

Synthesis of α-Benzotriazole Sulfides Promoted by Samarium Diiodide

1-(Benzotriazol-1-yl) unsymmetrical diorganyl sulfides were synthesized via replacement of chlorine atom in 1-(benzotriazol-1-yl)-1-chloromethane with thiolate anions promoted by SmI₂.     

Synthesis of α-Benzotriazole Selenides and Tellurides Promoted by Samarium Diiodide

1-(Benzotriazol-1-yl) unsymmetrical diorganyl selenides and tellurides were synthesized via replacement of chlorine atom in 1-(benzotriazo-l-yl)-l-chloromethane with selenolate and tellurolate anions promoted by SmI₂.     

Sm2O2I – A New Mixed‐Valence Samarium(II,III) Oxide Halide

Dark red single crystals of Sm₂O₂I were obtained from a reaction of SmI₂ (in the presence of SmOI) and Na in a sealed tantalum ampoule at 650 °C. The title compound crystallizes in the monoclinic system (C2/m, Z = 4, a = 12.639(2), b = 4.100(1), c = 9.762(3) Å, β = 117.97(2)°). The structure consists of corrugated [Sm²⁺Sm³⁺(O^2?)₂]⁺ layers of edge and vertex‐connected Sm₄O tetrahedral units with I^? anions separating the layers.     

Synthesis of the C1-C12 acid fragment of amphidinolide T marine macrolides via SmI2-mediated enantioselective reductive coupling of aldehydes with a chiral crotonate

A new strategy for enantioselective assembly of the trisubstituted tetrahydrofuran ring has been established for synthesis of the C1-C12 acid fragment of amphidinolide T series marine macrolides. The key steps involve the SmI₂-mediated highly enantioselective reductive coupling of an aldehyde with the (1S,2R)-N-methylephedrine-derived crotonate to form the cis-3,4-disubstituted γ-butyrolactone and the subsequent BF₃-mediated 1,3-anti-selective allylation of the five-membered-ring oxocarbenium ion with allyltrimethylsilane. The desired C1-C12 acid fragment was obtained in >25% overall yield via a 9-step sequence.