Samarium diiodide mediated reactions in total synthesis

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.

Photocatalytic Hantzsch Ester-Mediated Pinacol-Type Coupling of Aldehydes and Ketones

We have developed a photocatalyst-free reaction system that uses Hantzsch esters as photoreducing agents to promote the coupling of carbonyl compounds to 1,2-diols. The system fully utilizes the single electron transfer and proton donor roles of Hantzsch esters. The system shows a wide range of substrate application. Aromatic ketones, aliphatic ketones, and aldehydes can be applied to the catalytic system. Both self-coupling and cross-coupling can achieve ideal results.

Visible-Light-Antenna Ligand-Enabled Samarium-Catalyzed Reductive Transformations

Although divalent Sm reagents are some of the most important single-electron transfer reagents for reductive transformations, their catalytic applications are challenging. In this study, a bidentate phosphine oxide ligand substituted with 9,10-diphenylanthracene as a visible-light antenna was designed for Sm-catalyzed reduction reactions under mild reaction conditions. Pinacol coupling of aryl ketones and aldehydes was developed with 1 mol % of Sm catalyst and organic amine (DIPEA) as a sacrificial mild reductant. Mechanistic studies suggest that the visible-light-antenna ligand coordinates to Sm(III) and reduces Sm(III) to Sm(II) under visible-light irradiation. The catalytic system is also applicable for cross-pinacol coupling and other single-electron reductive transformations, including aza-pinacol coupling, flavone dimerization, C-O bond cleavage, C-C ring-opening of cyclopropane, ketyl-olefin coupling, and cross-coupling of the ketyl radical with the α-amino radical.

Total Synthesis of Pallamolides A-E

We have achieved the first total synthesis of pallamolides A-E. Of these compounds, pallamolides B-E possess intriguing tetracyclic skeletons with novel intramolecular transesterifications. Key transformations include highly diastereoselective sequential Michael addition reactions to construct the bicyclo[2.2.2]octane core with the simultaneous generation of two quaternary carbon centers, a one-pot SmI2-mediated intramolecular ketyl-enoate cyclization/ketone reduction to generate the key oxabicyclo[3.3.1]nonane moiety, and an acid-mediated deprotection/oxa-Michael addition/β-hydroxy elimination cascade sequence to assemble the tetracyclic pallamolide skeleton. Kinetic resolution of ketone 14 through Corey-Bakshi-Shibata reduction enabled the asymmetric synthesis of pallamolides A-E.

Reductive Oxy-Nazarov Cyclization toward Expedient Construction of a Cyclopenta[1,2-b]pyrrolo[1,2-a]azepine Ring System: Formal Total Syntheses of Stemonamine and Cephalotaxine

Formal total syntheses of stemonamine and cephalotaxine bearing the core cyclopenta[1,2-b]pyrrolo[1,2-a]azepine ring skeleton were achieved. The general synthetic strategy in the synthesis features the reductive oxy-Nazarov cyclization as key step, leading to the versatile construction of N-substituted spiro quaternary stereogenic centers from readily available starting materials.

A SmI2-mediated reductive cyclisation reaction using the trifluoroacetamide group as the radical precursor

A samarium(II)-mediated reductive cyclisation reaction with the aminoketyl radical from the trifluoroacetamide group for synthesising 2-trifluoromethylindolines was developed. This reaction is the first example of using an acyclic amide group, which is considered difficult to react with SmI2, in a reductive cyclisation. Additionally, the conversion of the obtained product into 2-trifluoromethylindole was achieved.

Interfacial Tuning of Electrocatalytic Ag Surfaces for Fragment-Based Electrophile Coupling

Construction of C‒C bonds in medicinal chemistry frequently draws on the reductive coupling of organic halides with ketones or aldehydes. Catalytic C(sp3)‒C(sp3) bond formation, however, is constrained by the competitive side reactivity of radical intermediates following sp3 organic halide activation. Here, an alternative paradigm deploys catalytic Ag surfaces for reductive fragment-based electrophile coupling compatible with sp3 organic halides. We use in-situ spectroscopy, electrochemical analyses, and simulation to uncover the catalytic interfacial structure and guide reaction development. Specifically, Mg(OAc)2 outcompetes the interaction between Ag and the aldehyde, thereby tuning the Ag surface for selective product formation. Data are consistent with an increased population of Mg-bound aldehyde facilitating the addition of a carbon-centered radical (product of Ag-electrocatalyzed organic halide reduction) to the carbonyl. Electron transfer from Ag to the resultant alkoxy radical yields the desired alcohol. Molecular interfacial tuning at reusable catalytic electrodes will accelerate development of sustainable organic synthetic methods.

Photocatalytic Generation of Divalent Lanthanide Reducing Agents

Divalent lanthanide (Ln) compounds are excellent reducing agents with unique reactivity profiles. These reagents are typically used in superstoichiometric amounts, often in combination with harmful additives. Reactions catalytic in Ln(II) reagents that retain the reactivity and selectivity of the stoichiometric transformations are currently lacking due to the absence of effective and selective methods to form reactive Ln(II) species from stable precursors. Here, active Ln(II) is generated from a Ln(III) precursor through reduction by a photoexcited coumarin or carbostyril chromophore, which, in turn, is regenerated by a sacrificial reductant. The reductant can be metallic (Zn) or organic (amines) and can be used in strictly stoichiometric amounts. A broad range of reactions, including C-halogen, C═C, C═X (X = O, N), P═O, and N═N reductions, as well as C-C, C-X (X = N, S, P), and N-N couplings were readily carried out in yields and selectivities comparable to or better than those afforded by the analogous stoichiometric transformations. The reaction outcomes could be altered by changing the ligand or the lanthanide or through the addition of environmentally benign additives (e.g., water). EPR spectroscopy supported the formation of both Ln(II) and oxidized chromophore intermediates. Taken together, these results establish photochemical Ln(II) generation as a powerful strategy for rendering Ln(II)-mediated reactions catalytic.

Stereo-Controlled Synthesis of Vicinal Tertiary Carbinols: Application in the Synthesis of a Diol Substructure of Zaragozic Acid, Pactamycin and Ryanodol

A novel and flexible approach for the stereo-controlled synthesis of vicinal tertiary carbinols is reported. The developed strategy featured a highly diastereoselective singlet-oxygen (O2 1 ) [4+2] cycloaddition of rationally designed cyclohexadienones (derived from oxidative dearomatization of the corresponding carboxylic-acid appended phenol precursors), followed by programmed "O-O" and "C-C" bond cleavage. In doing so, a highly functionalized and versatile intermediate was identified and prepared in synthetically useful quantity as a plausible precursor to access a variety of designed and naturally occurring vicinal tertiary carbinol containing compounds. Most notably, the developed strategy was successfully applied in the stereo-controlled synthesis of advanced core structures of zaragozic acid, pactamycin and ryanodol.

Reductive Aldol Approach to Natural Products: Bioinspired Synthesis of abeo-11(12 → 13)-Oleanane Triterpenoids

A synthesis of alstoscholarinoid B (1) and 3β-acetoxy-11α-hydroxy-11(12 → 13)abeooleanan-12-al (2) has been accomplished in 7-9 steps and 10%-16% overall yield from oleanolic acid. This synthesis featured a bioinspired SmI2-mediated reductive aldol reaction to establish the abeo-11(12 → 13)-oleanane framework of both 1 and 2 and a retro-aldol/aldol/lactonization cascade to fully construct the skeleton of 1. Moreover, the investigation of the bioinspired aldol reaction also sheds light on the potential biogenesis of natural products.

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.

Total Synthesis and Structure Revision of (+)-Lancilactone C

Lancilactone C is a tricyclic triterpenoid that inhibits human immunodeficiency virus (HIV) replication in H9 lymphocytes with no cytotoxicity. Its tricyclic skeleton comprises trans-dimethylbicyclo[4.3.0]nonane and 7-isopropylenecyclohepta-1,3,5-triene. The latter unique structure, in which all carbon atoms are sp² hybridized, is not found in other triterpenoids and needs to be verified synthetically. Herein, we have accomplished the first total synthesis of lancilactone C (proposed structure) by developing a new domino [4 + 3] cycloaddition reaction involving oxidation, Diels-Alder reaction, elimination, and electrocyclization. We have also revised the structure based on the total synthesis of lancilactone C according to its plausible biosynthetic pathway.

Strategies and Lessons Learned from Total Synthesis of Taxol

Taxol (paclitaxel), the most well-known taxane diterpenoid, is the best-selling natural-source anticancer drug ever produced and one of the most common prescriptions in the treatment of breast, lung, and ovarian cancers, saving countless lives around the world. Structurally, Taxol possesses a highly oxygenated [6-8-6-4] core bearing 11 stereocenters, seven of which are contiguous chiral centers. Moreover, the extremely strained bicyclo[5.3.1] undecane ring system with a bridgehead double bond is a unique structural feature. All these features make Taxol a highly challenging synthetic target. Tremendous synthetic efforts from more than 60 research groups around the world have already culminated in ten total syntheses and three formal syntheses, as well as more than 60 synthetic model studies of Taxol. This review is intended to provide a long-overdue appraisal of the great achievements in the total syntheses of Taxol reported in the last few decades. In doing so, we summarize the development of synthesis toward Taxol from 1994 to 2022, including the evolution of synthetic strategy for accessing this complex molecular scaffold and key lessons learned from such endeavors. Finally, we briefly discuss the future of the research in this area.

Visible-light induced metal-free intramolecular reductive cyclisations of ketones with alkynes and allenes

A visible-light-induced, intramolecular, reductive cyclisation of ketones with an unsaturated hydrocarbon moiety was developed. In contrast to conventional protocols requiring resource precious or hazardous metal sources, this method enables facile access to ketyl radicals under metal-free and mild reaction conditions. By polarity-reversed, ketyl radical hydroalkoxylation of alkynes and allenes, a variety of five-membered (hetero-)cyclic products were generated in good yields with good to excellent stereoselectivities. The embedded homoallylic tertiary alcohol could be transformed into other useful functionalities, highlighting the synthetic utility of this reaction. This efficient and sustainable ketyl-alkyne/allene cross coupling also features broad functional group tolerance and scalability.

Back to the future of organolanthanide chemistry

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp₃ complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

Combining the best of both worlds: radical-based divergent total synthesis

A mature science, combining the art of the total synthesis of complex natural structures and the practicality of delivering highly diverged lead compounds for biological screening, is the constant aim of the organic chemistry community. Delivering natural lead compounds became easier during the last two decades, with the evolution of green chemistry and the concepts of atom economy and protecting-group-free synthesis dominating the field of total synthesis. In this new era, total synthesis is moving towards natural efficacy by utilizing both the biosynthetic knowledge of divergent synthesis and the latest developments in radical chemistry. This contemporary review highlights recent total syntheses that incorporate the best of both worlds.

Strategies to access the [5-8] bicyclic core encountered in the sesquiterpene, diterpene and sesterterpene series

Terpene compounds probably represent the most diversified class of secondary metabolites. Some classes of terpenes, mainly diterpenes (C20) and sesterterpenes (C25) and to a lesser extent sesquiterpenes (C15), share a common bicyclo[3.6.0]undecane core which is characterized by the presence of a cyclooctane ring fused to a cyclopentane ring, i.e., a [5-8] bicyclic ring system. This review focuses on the different strategies elaborated to construct this [5-8] bicyclic ring system and their application in the total synthesis of terpenes over the last two decades. The overall approaches involve the construction of the 8-membered ring from an appropriate cyclopentane precursor. The proposed strategies include metathesis, Nozaki-Hiyama-Kishi (NHK) cyclization, Pd-mediated cyclization, radical cyclization, Pauson-Khand reaction, Lewis acid-promoted cyclization, rearrangement, cycloaddition and biocatalysis.

Metal/Non-Metal Catalyzed Activation of Organic Nitriles

Nitrile activation is a prominent topic in recent developments in chemistry, especially in organic, inorganic, biological chemistry, as well as in the natural synthesis of products and in the pharmaceutical industry. The activation of nitriles using both metal and non-metal precursors has attracted several researchers, who are exploring newer ways to synthesize novel compounds. Nitrile activation can be achieved by combining various catalytic double hydroelementation reactions, such as hydrosilylation, hydroboration, and hydrogenation of organonitriles using silanes, pinacolborane, and other sources of hydrogen. These methodologies have garnered considerable attention since they are effective in the reduction of organonitriles, whose end products are extensively applied in synthetic organic chemistry. In this review, we summarize the development of selective hydroborylation, hydrosilylation, dihydroborysilylation, and hydrogenation of organonitriles, as well as their reaction mechanisms and the role of metal complexes in the catalytic cycles. This review article explains various synthetic methodologies applied toward the reduction of organonitriles into corresponding amines.

General Synthetic Approach to Diverse Taxane Cores

Chemical synthesis of natural products is typically inspired by the structure and function of a target molecule. When both factors are of interest, such as in the case of taxane diterpenoids, a synthesis can both serve as a platform for synthetic strategy development and enable new biological exploration. Guided by this paradigm, we present here a unified enantiospecific approach to diverse taxane cores from the feedstock monoterpenoid (S)-carvone. Key to the success of our approach was the use of a skeletal remodeling strategy which began with the divergent reorganization and convergent coupling of two carvone-derived fragments, facilitated by Pd-catalyzed C-C bond cleavage tactics. This coupling was followed by additional restructuring using a Sm(II)-mediated rearrangement and a bioinspired, visible-light induced, transannular [2 + 2] photocycloaddition. Overall, this divergent monoterpenoid remodeling/convergent fragment coupling approach to complex diterpenoid synthesis provides access to structurally disparate taxane cores which have set the stage for the preparation of a wide range of taxanes.

A symmetry-driven approach toward the total synthesis of dodecahedrane

Herein, we describe our progress toward the total synthesis of dodecahedrane, a complex and highly symmetrical hydrocarbon that bears twelve fused rings arranged in a cage-like architecture. Central to our approach is a late-stage [2+2+2+2+2] polyene cyclization cascade, which is expected to construct five new bonds and ten new rings in a single transformation. Toward this end, we describe efforts to synthesize key monomeric fragments, along with successful dimerization studies using a pinacol coupling approach. Subsequent studies include an attempted olefin metathesis rearrangement cascade in addition to a successful intramolecular photochemical [2+2] reaction. Although attempts to elaborate the photocycloaddition product were unsuccessful, our studies provide access to complex dimeric scaffolds and are expected to help guide our future total synthesis studies.

Total Synthesis of Resiniferatoxin

From both structural and functional perspectives, the large family of daphnane diterpene orthoesters (DDOs) represent a truly remarkable class of natural products. As potent lead compounds for the treatment of pain, neurodegeneration, HIV/AIDS, and cancer, their medicinal potential continues to be heavily investigated, yet synthetic routes to DDO natural products remain rare. Herein we report a distinct approach to this class of complex diterpenes, highlighted by a 15-step total synthesis of the flagship DDO, resiniferatoxin.

De Novo Diastereoselective Synthesis of 1-Hydroxyl Allogibberic Methyl Ester en Route to Diverse Bioactive Molecules

The first de novo synthesis of 1-hydroxyl allogibberic methyl ester, en route to pharbinilic acid and other bioactive molecules, is accomplished in diastereoselective manner. Key reactions of the synthesis include a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, a Lewis acid-catalyzed reductive Prins cyclization reaction, and a SmI₂-mediated transannular pinacol coupling reaction. The synthesis provides a new avenue to access diverse relevant bioactive molecules.

A unified strategy for the total syntheses of eribulin and a macrolactam analogue of halichondrin B

A unified synthetic route for the total syntheses of eribulin and a macrolactam analog of halichondrin B is described. The key to the success of the current synthetic approach includes the employment of our reverse approach for the construction of cyclic ether structural motifs and a modified intramolecular cyclization reaction between alkyl iodide and aldehyde functionalities to establish the all-carbon macrocyclic framework of eribulin. These syntheses, together with our previous work on the total syntheses of halichondrin B and norhalichondrin B, demonstrate and validate the powerful reverse approach in the construction of cyclic ether structural motifs. On the other hand, the unified synthetic strategy for the synthesis of the related macrolactam analog provides inspiration and opportunities in the halichondrin field and related polycyclic ether areas.

Diastereoselective Radical 1,4-Ester Migration: Radical Cyclizations of Acyclic Esters with SmI2

Reductive cyclizations of carbonyl compounds, mediated by samarium(II) diiodide (SmI₂, Kagan’s reagent), represent an invaluable platform to generate molecular complexity in a stereocontrolled manner. In addition to classical ketone and aldehyde substrates, recent advances in radical chemistry allow the cyclization of lactone and lactam-type substrates using SmI₂. In contrast, acyclic esters are considered to be unreactive to SmI₂ and their participation in reductive cyclizations is unprecedented. Here, we report a diastereoselective radical 1,4-ester migration process, mediated by SmI₂, that delivers stereodefined alkene hydrocarboxylation products via radical cyclization of acyclic ester groups in α-carbomethoxy δ-lactones. Isotopic labeling experiments and computational studies have been used to probe the mechanism of the migration. We propose that a switch in conformation redirects single electron transfer from SmI₂ to the acyclic ester group, rather than the “more reactive” lactone carbonyl. Our study paves the way for the use of elusive ketyl radicals, derived from acyclic esters, in SmI₂-mediated reductive cyclizations.

Ketone-Olefin Coupling of Aliphatic and Aromatic Carbonyls Catalyzed by Excited-State Acridine Radicals

Ketone-olefin coupling reactions are common methods for the formation of carbon-carbon bonds. This reaction class typically requires stoichiometric or super stoichiometric quantities of metal reductants, and catalytic variations are limited in application. Photoredox catalysis has offered an alternative method toward ketone-olefin coupling reactions, although most methods are limited in scope to easily reducible aromatic carbonyl compounds. Herein, we describe a mild, metal-free ketone-olefin coupling reaction using an excited-state acridine radical super reductant as a photoredox catalyst. We demonstrate both intramolecular and intermolecular ketone-olefin couplings of aliphatic and aromatic ketones and aldehydes. Mechanistic evidence is also presented supporting an "olefin first" ketone-olefin coupling mechanism.

Ni-Catalyzed Enantioselective Dialkyl Carbinol Synthesis via Decarboxylative Cross-Coupling: Development, Scope, and Applications

The first enantioselective decarboxylative Negishi-type alkylations of α-oxy carboxylic acids are reported via the intermediacy of redox-active esters (RAEs). This transformation enables a radical-based retrosynthesis of seemingly trivial enantiopure dialkyl carbinols. This article includes a discussion of the history of such couplings, the retrosynthetic ramifications of such a coupling, the development of general conditions, and an extensive series of applications that vividly demonstrate how it can simplify synthesis.

Asymmetric total synthesis of prostaglandin C2 TBS ether

We disclose an asymmetric total synthesis of prostaglandin C₂ TBS ether, a derivative of an extremely sensitive natural prostaglandin C₂. The key to the synthesis is a SmI₂-mediated ketyl-enoate reaction that leads to the formation of the functionalized cyclopentane ring with high-level stereochemical control. Access to the crucial alkene system is realized late in the synthesis by the implementation of a Grignard addition/dehydration/metathesis sequence.

Electrocatalytic Ketyl-Olefin Cyclization at a Favorable Applied Bias Enabled by a Concerted Proton-Electron Transfer Mediator

Recent studies showcase reductive concerted proton-electron transfer (CPET) as a powerful strategy for transferring a net hydrogen atom to organic substrates; however, direct application of CPET in the context of C-C bond formation beyond homocoupling is underexplored. We report herein the expansion of electrocatalytic CPET (eCPET) using a Brønsted base-appended cobaltocene mediator ([CpCoCp^NMe₂][OTf]) with keto-olefin substrates that undergo cyclization subsequent to ketyl radical generation via eCPET. Using acetophenone-derived substrates with tethered acrylates as radical acceptors, in the presence of tosylic acid, we demonstrate that ketyl-olefin cyclization is achieved by characterization of cis-lactone and alkene products. Mechanistic analysis of this 2 H⁺/2 e^- process reveals a mixed order in substrate and acid and a Hammett plot with a modest negative slope, highlighting the contribution of sequential CPET and ET/PT steps involved in the overall rate of the reaction and providing support for initial O-H bond formation. The ability to access ketyl radicals at comparatively mild reduction potentials via controlled potential electrolysis enables functional group tolerance across a range of substrates.

Asymmetric Total Synthesis of (-)-Phaeocaulisin A

The therapeutic properties of Curcuma (ginger and turmeric’s family) have long been known in traditional medicine. However, only recently have guaiane-type sesquiterpenes extracted from Curcuma phaeocaulis been submitted to biological testing, and their enhanced bioactivity was highlighted. Among these compounds, phaeocaulisin A has shown remarkable anti-inflammatory and anticancer activity, which appears to be tied to the unique bridged acetal moiety embedded in its tetracyclic framework. Prompted by the promising biological profile of phaeocaulisin A and by the absence of a synthetic route for its provision, we have implemented the first enantioselective total synthesis of phaeocaulisin A in 17 steps with 2% overall yield. Our route design builds on the identification of an enantioenriched lactone intermediate, tailored with both a ketone moiety and a conjugated alkene system. Taking advantage of the umpolung carbonyl-olefin coupling reactivity enabled by the archetypal single-electron transfer (SET) reductant samarium diiodide (SmI₂), the lactone intermediate was submitted to two sequential SmI₂-mediated cyclizations to stereoselectively construct the polycyclic core of the natural product. Crucially, by exploiting the innate inner-sphere nature of carbonyl reduction using SmI₂, we have used a steric blocking strategy to render sites SET-unreceptive and thus achieve chemoselective reduction in a complex substrate. Our asymmetric route enabled elucidation of the naturally occurring isomer of phaeocaulisin A and provides a synthetic platform to access other guaiane-type sesquiterpenes from C. phaeocaulis—as well as their synthetic derivatives—for medicinal chemistry and drug design.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI2, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI2 in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI2-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Deoxygenative Cross-Coupling of Aromatic Amides with Polyfluoroarenes

Considering the ubiquitous nature and ready synthesis of amides, and the great significance of organofluorine-containing species, the cross-coupling of amides and polyfluoroarenes, leading to new carbon-carbon bond-forming methodologies, would find useful applications in synthesis, late-stage functionalization, and rapid generation of molecular diversity. Herein, we present a novel synthesis of α-polyfluoroaryl amines via Sm/SmI₂ -mediated deoxygenative cross-coupling of aromatic amides with polyfluoroarenes through direct C-H functionalization. The structural and functional diversity of these readily available precursors provides a versatile and flexible strategy for the streamlined synthesis of α-polyfluoroaryl amines. Combining experimental and theoretical studies, a novel plausible mechanism of the α-aminocarbene-mediated C-H insertion has been revealed, which may stimulate future work for the development of novel methods in amine synthesis.

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H₂ evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure-reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet-triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp²)-H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H₂ and contra-thermodynamic isomerization.

Nickel-Catalyzed Defluorinative Coupling of Aliphatic Aldehydes with Trifluoromethyl Alkenes

A simple procedure is reported for the nickel-catalyzed defluorinative alkylation of unactivated aliphatic aldehydes. The process involves the catalytic reductive union of trifluoromethyl alkenes with aldehydes using a nickel complex of a 6,6'-disubstituted bipyridine ligand with zinc metal as the terminal reductant. The protocol is distinguished by its broad substrate scope, mild conditions, and simple catalytic setup. Reaction outcomes are consistent with the intermediacy of an α-silyloxy(alkyl)nickel intermediate generated by a low-valent nickel catalyst, silyl electrophile, and the aldehyde substrate. Mechanistic findings with cyclopropanecarboxaldehyde provide insights into nature of the reactive intermediates and illustrate fundamental reactivity differences that are governed by subtle changes in ligand and substrate structure.

SmI2‑Mediated Intermolecular Addition-Elimination of Piperidine and Pyrrolidine N-α-Radicals with Arylacetylene Sulfones

An efficient approach to access α-arylacetylene substituted pyrrolidine and piperidine derivatives has been developed through a samarium diiodide mediated addition-elimination process of pyrrolidine and piperidine N-α-radicals with arylacetylene sulfones.

Samarium(II)-Electrocatalyzed Chemoselective Reductive Alkoxylation of Phthalimides

The unprecedented samarium-eletrocatalyzed reductive alkoxylation of phthalimides in a single step is presented. Under mild conditions, using electrogenerated Sm(II) with TMSCl (trimethyl chlorosilane), N-substituted 3-alkoxyl isoindolin-1-ones are isolated in good...

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

A Stereoselective Synthesis of Fused Carbocycles with a cis-1,2-Diol Moiety by Desymmetrization: SmI2-mediated Pinacol Coupling of meso-Cyclic 1,3-Diones

SmI₂-mediated desymmetrization of a meso-cyclic 1,3-dione pinacol coupling is described. The reaction proceeds with high stereoselectivity to provide fused carbocyclic compounds with three contiguous stereogenic centers featuring an all-carbon quaternary center and a cis-1,2-diol moiety.

Asymmetric Total Synthesis of Taxol

Taxol is one of the most famous natural diterpenoids and an important anticancer medicine. Taxol represents a formidable synthetic challenge and has prompted significant interest from the synthetic community. However, in all the previous syntheses of Taxol, there have been no reports of closing the desired eight-membered ring through C1-C2 bond formation. Furthermore, the existence of Taxol-resistant tumors and side effects of Taxol make the development of new approaches to synthesize Taxol and its derivatives highly desirable. Here, we report the asymmetric total synthesis of Taxol using a concise approach through 19 isolated intermediates. The synthetically challenging eight-membered ring was constructed efficiently by a diastereoselective intramolecular SmI₂-mediated pinacol coupling reaction to form the C1-C2 bond. The unique biomimetic oxygen ene reaction and the newly developed facile tandem C2-benzoate formation and C13 side chain installation improved the efficiency of the synthesis. The mild oxygen ene reaction under light conditions would be an alternative reaction involved in Taxol biosynthesis. This new convergent approach will allow the diverse creation of Taxol derivatives to enable further biological research.

Synthesizing Molecules with Linear Tricyclic 5/5/5 and 6/5/5 Skeletons via [5 + 2 + 1]/Ene Strategy

Report here is the development of a [5 + 2 + 1]/ene strategy for the synthesis of molecules with linear tricyclic 5/5/5 and 6/5/5 skeletons widely found in natural products. The first step of this strategy is applying a Rh-catalyzed [5 + 2 + 1] reaction of ene-vinylcyclopropanes and CO to synthesize 5/8 and 6/8 bicyclic compounds, which can then be transformed to the final target molecules by an InCl₃-catalyzed ene reaction.

Visible light enables catalytic formation of weak chemical bonds with molecular hydrogen

The synthesis of weak chemical bonds at or near thermodynamic potential is a fundamental challenge in chemistry, with applications ranging from catalysis to biology to energy science. Proton-coupled electron transfer using molecular hydrogen is an attractive strategy for synthesizing weak element-hydrogen bonds, but the intrinsic thermodynamics presents a challenge for reactivity. Here we describe the direct photocatalytic synthesis of extremely weak element-hydrogen bonds of metal amido and metal imido complexes, as well as organic compounds with bond dissociation free energies as low as 31 kcal mol^-1. Key to this approach is the bifunctional behaviour of the chromophoric iridium hydride photocatalyst. Activation of molecular hydrogen occurs in the ground state and the resulting iridium hydride harvests visible light to enable spontaneous formation of weak chemical bonds near thermodynamic potential with no by-products. Photophysical and mechanistic studies corroborate radical-based reaction pathways and highlight the uniqueness of this photodriven approach in promoting new catalytic chemistry.

Evolution towards green radical generation in total synthesis

The use of radicals as intermediates in total synthesis has evolved since their initial use in the latter half of the twentieth century. Radical generation from metal hydride methodologies has shifted to "greener" techniques including catalytic metal-mediated systems, electrochemical and photoredox-mediated processes. This review will focus on these classical and contemporary methods for radical generation and their applications in recent total syntheses.

Merging Electron Transfer with 1,2-Metalate Rearrangement: Deoxygenative Arylation of Aromatic Amides with Arylboronic Esters

Amides are essentially inert carboxyl derivatives in many types of chemical transformations. In particular, deoxygenative C-C bond formation of amides to synthetically important amines is a long-standing challenge for synthetic chemists due to the inertness of the resonance-stabilized amide C=O bond. Herein, it is disclosed that by merging electron-transfer-induced activation with 1,2-metalate rearrangement, a wide range of aromatic amides react smoothly with arylboron reagents, affording a series of biologically relevant diarylmethylamines as deoxygenative C-C bond cross-coupling products. With its simplicity and versatility, this reaction shows great promise in the synthesis of amines from amides, which may open up new avenues in retrosynthetic planning and find widespread use in academia and industry.