Selective reduction of barbituric acids using SmI2/H2O: synthesis, reactivity, and structural analysis of tetrahedral adducts

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.

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.

An Experimental and Theoretical Study of the 1,3-Dipolar Cyclo­addition of Alloxan-Derived Azomethine Ylides to Cyclopropenes

A diastereoselective synthesis of biologically interesting spirobarbiturates has been achieved via [3+2] cycloaddition of alloxan-derived azomethine ylides to 3-R-1,2-diphenylcyclopropenes. With this approach, a range of spirobarbiturate-3-azabicyclo[3.1.0]hexanes and spirobarbiturate-cyclopropa[a]pyrrolizines were obtained in moderate to good yields with excellent diastereoselectivities. DFT calculations (M11 density functional theory) were carried out to shed light on the molecular mechanism of 1,3-dipolar cycloaddition of alloxan-derived azomethine ylides to cyclopropenes. The cytotoxic activity of some obtained compounds against human erythroleukemia (K562) cell line was evaluated in vitro by MTS-assay.

Reduced Molecular Flavins as Single-Electron Reductants after Photoexcitation

Flavoenzymes mediate a multitude of chemical reactions and are catalytically active both in different oxidation states and in covalent adducts with reagents. The transfer of such reactivity to the organic laboratory using simplified molecular flavins is highly desirable, and such applications in (photo)oxidation reactions are already established. However, molecular flavins have not been used for the reduction of organic substrates yet, although this activity is known and well-studied for DNA photolyase enzymes. We report a catalytic method using reduced molecular flavins as photoreductants and γ-terpinene as a sacrificial reductant. Additionally, we present our design for air-stable, reduced flavin catalysts, which is based on a conformational bias strategy and circumvents the otherwise rapid reduction of O₂ from air. Using our catalytic strategy, we were able to replace superstoichiometric amounts of the rare-earth reductant SmI₂ in a 5-exo-trig cyclization of substituted barbituric acid derivatives. Such flavin-catalyzed reductions are anticipated to be beneficial for other transformations as well and their straightforward synthesis indicates future use in stereo- as well as site-selective transformations.

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.

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...

Cobalt and Iron Stabilized Ketyl, Ketiminyl and Aldiminyl Radical Anions

Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.

Chemoselective synthesis of 5,4'-imidazolinyl spirobarbiturates via NBS-promoted cyclization of unsaturated barbiturates and amidines

A selective cyclization of unsaturated barbiturates and amidines promoted by N-bromosuccinimide has been successfully developed to afford a vast variety of 5,4'-imidazolinyl spirobarbiturates in moderate to good yields. The present protocol features broad substrate scope, facile work-up procedure and mild reaction conditions, providing a novel strategy for the highly selective and efficient construction of structurally diverse spiroimidazolines.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion

Methyl benzoate, N,N-dimethylbenzamide, and benzophenone were reduced by U^III [N(SiMe₃ )₂ ]₃ resulting in uranium(IV) products. Reduction of benzophenone lead to U^IV [OC⋅Ph₂ )][N(SiMe₃ )₂ ]₃ , (1.1) which forms the dinuclear complex, [N(SiMe₃ )₂ ]₃ U^IV (OCPhPh-CPh₂ O)U^IV [N(SiMe₃ )₂ ]₃ (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with U^III [N(SiMe₃ )₂ ]₃ resulted in U^IV [OC⋅(Ph)(NMe₂ )][N(SiMe₃ )₂ ]₃ (2), a uranium(IV) compound and the first example of a charge-separated amide radical. In the case of methyl benzoate, the reduction resulted in U^IV (OMe)[N(SiMe₃ )₂ ]₃ (3) and benzaldehyde as the reduced organic fragment. Compound 2 showed the ability to act as a uranium(III) synthon in its reactivity with trimethylsilyl azide, a reaction that yielded U^V (=NSiMe₃ )[N(SiMe₃ )₂ ]₃ . Additionally, 2 was reduced with potassium graphite resulting in [U(μ-O)[O=C(NMe₂ )(Ph)][N(SiMe₃ )₂ ]₂ ]₂ (4), a dinuclear uranium compound bridged by oxo ligands. Reduction of 2 in the presence of 15-crown-5 afforded isolation of the mono-oxo compound, [(15-crown-5)₂ K][UO[N(SiMe₃ )₂ ]₃ ] (5). The results expand the reduction capabilities of U^III complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

Synthesis of Nitrogen Heterocycles Using Samarium(II) Iodide

Nitrogen heterocycles represent vital structural motifs in biologically-active natural products and pharmaceuticals. As a result, the development of new, convenient and more efficient processes to N-heterocycles is of great interest to synthetic chemists. Samarium(II) iodide (SmI₂, Kagan’s reagent) has been widely used to forge challenging C–C bonds through reductive coupling reactions. Historically, the use of SmI₂ in organic synthesis has been focused on the construction of carbocycles and oxygen-containing motifs. Recently, significant advances have taken place in the use of SmI₂ for the synthesis of nitrogen heterocycles, enabled in large part by the unique combination of high reducing power of this reagent (E_1/2 of up to −2.8 V) with excellent chemoselectivity of the reductive umpolung cyclizations mediated by SmI₂. In particular, radical cross-coupling reactions exploiting SmI₂-induced selective generation of aminoketyl radicals have emerged as concise and efficient methods for constructing 2-azabicycles, pyrrolidines and complex polycyclic barbiturates. Moreover, a broad range of novel processes involving SmI₂-promoted formation of aminyl radicals have been leveraged for the synthesis of complex nitrogen-containing molecular architectures by direct and tethered pathways. Applications to the synthesis of natural products have highlighted the generality of processes and the intermediates accessible with SmI₂. In this review, recent advances involving the synthesis of nitrogen heterocycles using SmI₂ are summarized, with a major focus on reductive coupling reactions that enable one-step construction of nitrogen-containing motifs in a highly efficient manner, while taking advantage of the spectacular selectivity of the venerable Kagan’s reagent.

Titanocene-Catalyzed Radical Opening of N-Acylated Aziridines

Aziridines activated by N-acylation are opened to the higher substituted radical through electron transfer from titanocene(III) complexes in a novel catalytic reaction. This reaction is applicable in conjugate additions, reductions, and cyclizations and suited for the construction of quaternary carbon centers. The concerted mechanism of the ring opening is indicated by DFT calculations.

Selective construction of quaternary stereocentres in radical cyclisation cascades triggered by electron-transfer reduction of amide-type carbonyls

Radical cyclisation cascades triggered by electron-transfer to amide-type carbonyls using SmI₂–H₂O–LiBr, result in the selective construction of quaternary carbon stereocentres.

Selective Synthesis of Cyclooctanoids by Radical Cyclization of Seven-Membered Lactones: Neutron Diffraction Study of the Stereoselective Deuteration of a Chiral Organosamarium Intermediate

Seven-membered lactones undergo selective SmI2 -H2 O-promoted radical cyclization to form substituted cyclooctanols. The products arise from an exo-mode of cyclization rather than the usual endo-attack employed in the few radical syntheses of cyclooctanes. The process is terminated by the quenching of a chiral benzylic samarium. A labeling experiment and neutron diffraction study have been used for the first time to probe the configuration and highly diastereoselective deuteration of a chiral organosamarium intermediate.

The role of H2O in the electron transfer-activation of substrates using SmI2: insights from DFT

The first detailed theoretical study on the synthetically important electron transfer (ET) reductant SmI2-H2O has been conducted in the context of the activation of important alkyliodide, ketone, lactone and ester substrates, processes of importance in cross-coupling. Our studies give major insights into the nature of the reagent and suggest that; (i) H2O has a high affinity for Sm(ii) and displaces iodine from the metal center; (ii) SmI2-H2O has 6-7 molecules of H2O directly bound to the metal center; (iii) binding of H2O to Sm(II) promotes coordination of the substrate to Sm(II) and subsequent ET; (iv) resultant ketyl radicals are stabilized by hydrogen-bonding to H2O. The findings add greatly to the understanding of SmI2-H2O and the role of H2O in ET processes, and will facilitate the design of new processes initiated by reductive ET.

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.

Palladium-catalyzed Suzuki–Miyaura coupling of amides by carbon–nitrogen cleavage: general strategy for amide N–C bond activation

A unified strategy for the palladium-catalyzed Suzuki–Miyaura cross-coupling of amides with boronic acids for the synthesis of ketones by N–C bond activation is reported.

Sm(II)-Mediated Electron Transfer to Carboxylic Acid Derivatives: Development of Complexity-Generating Cascades

Reductive electron transfer (ET) to organic compounds is a powerful method for the activation of substrates via the formation of radicals, radical anions, anions, and dianions that can be exploited in bond-cleaving and bond-forming processes. Since its introduction to the synthetic community in 1977 by Kagan, SmI2 has become one of the most important reducing agents available in the laboratory. Despite its widespread application in aldehyde and ketone reduction, it was widely accepted that carboxylic acid derivatives could not be reduced by SmI2; only recently has our work led to this dogma being overturned, and the reduction of carboxylic acid derivatives using SmI2 can now take its place alongside aldehyde/ketone reduction as a powerful activation mode for synthesis. In this Account, we set out our studies of the reduction of carboxylic acid derivatives using SmI2, SmI2-H2O, and SmI2-H2O-NR3 and the exploitation of the unusual radical anions that are now accessible in unprecedented carbon-carbon bond-forming processes. The Account begins with our serendipitous discovery that SmI2 mixed with H2O is able to reduce six-membered lactones to diols, a transformation previously thought to be impossible. After the successful development of selective monoreductions of Meldrum's acid and barbituric acid heterocyclic feedstocks, we then identified the SmI2-H2O-NR3 reagent system for the efficient reduction of a range of acyclic carboxylic acid derivatives that typically present a significant challenge for ET reductants. Mechanistic studies have led us to propose a common mechanism for the reduction of carboxylic acid derivatives using Sm(II), with only subtle changes observed as the carboxylic acid derivative and Sm(II) reagent system are varied. At the center of our postulated mechanism is the proposed reversibility of the first ET to the carbonyl of carboxylic acid derivatives, and this led us to devise several strategies that allow the radical anion intermediates to be exploited productively in efficient new processes. First, we have used internal directing groups in substrates to "switch on" productive ET to esters and amides and have exploited such an approach in tag-removal cyclization processes that deliver molecular scaffolds of significance in biology and materials science. Second, we have exploited external ligands to facilitate ET to carboxylic acid derivatives and have applied the strategy in telescoped reaction sequences. Finally, we have employed follow-up cyclizations with alkenes, alkynes, and allenes to intercept radical anion intermediates formed along the reaction path and have employed this strategy in complexity-generating cascade approaches to biologically significant molecular architectures. From our studies, it is now clear that Sm(II)-mediated ET to carboxylic acid derivatives constitutes a general strategy for inverting the polarity of the carbonyl, allowing nucleophilic carbon-centered radicals to be formed and exploited in novel chemical processes.

Total Syntheses of (-)-Mersicarpine, (-)-Scholarisine G, (+)-Melodinine E, (-)-Leuconoxine, (-)-Leuconolam, (-)-Leuconodine A, (+)-Leuconodine F, and (-)-Leuconodine C: Self-Induced Diastereomeric Anisochronism (SIDA) Phenomenon for Scholarisine G and Leuconodines A and C

Enantioselective total syntheses of title natural products from a common cyclohexenone derivative (S)-18 were reported. Ozonolysis of (S)-18 afforded a stable diketo ester (R)-17 that was subsequently converted to two skeletally different natural products, i.e., (-)-mersicarpine (8) with a [6.5.6.7] fused tetracyclic ring system and (-)-scholarisine G (9) with a [6.5.6.6.5] fused pentacyclic skeleton, respectively. The postcyclization diversification was realized by taking advantage of the facile conversion of (+)-melodinine E (6) to N-acyliminium ion 7, from which a hydroxy group was selectively introduced to the C6, C7, C10 and the central C21 position of diazafenestrane system, leading to (-)-leuconodine A (11), (+)-leuconodine F (12), (-)-scholarisine G (9), (-)-leuconodine C (13), and skeletally different (-)-leuconolam (5). Furthermore, an unprecedented non-natural oxabridged oxadiazafenestrane 68 was formed by oxidation of (+)-melodinine E (6). During the course of this study, a strong self-induced diastereomeric anisochronism (SIDA) phenomenon was observed for scholarisine G (9), leuconodines A (11) and C (13). X-ray structures of both the racemic and the enantiopure natural products 9, 11, and 13 were obtained. The different crystal packing of these two forms nicely explained the chemical shift differences observed in the (1)H NMR spectra of the racemic and the enantio-enriched compounds in an achiral environment.

Allylsamarium Bromide-Mediated Cascade Cyclization of Homoallylic Esters. Synthesis of 2-(2-Hydroxyalkyl)cyclopropanols and 2-(2-Hydroxyethyl)bicyclo[2.1.1]hexan-1-ols

In continuation of our previous study on the intramolecular reductive coupling of simple homoallylic esters promoted by allylSmBr/HMPA/H2O, which afforded a facile synthesis of 2-(2-hydroxyalkyl)cyclopropanols, here we report the reductive cascade cyclization of but-3-enyl but-3-enoates mediated by allylSmBr/HMPA/CuCl2·2H2O, in which the two C═C bonds were successively coupled to allow the construction of the structurally interesting bridged bicyclic tertiary alcohols. Thus, the 2-(2-hydroxyethyl)bicyclo[2.1.1]hexan-1-ols were prepared in moderate to good yields with excellent diastereoselectivity.

SmI2-H2O-mediated 5-exo/6-exo lactone radical cyclisation cascades

The SmI2-H2O reagent system mediates challenging 5-exo/6-exo lactone radical cascade cyclisations that deliver carbo[5.4.0]bicyclic motifs in a diastereoselective, one-pot process that establish two new carbocyclic rings and four stereocentres.