77Se NMR Research Articles

Copper(I)‐Catalyzed Conjugate Addition/Enantioselective Protonation with Selenols and α‐Substituted α,β‐Unsaturated Thioamides

AbstractHerein, a copper(I)‐catalyzed asymmetric conjugate addition/protonation with selenols and α‐substituted α,β‐unsaturated thioamides is disclosed, which affords a series of chiral selenides in high to excellent enantioselectivity. As for both selenols and α‐substituted α,β‐unsaturated thioamides, the reaction enjoys broad substrate scopes. The present catalytic system is also successfully applied to asymmetric selenation of β‐substituted α,β‐unsaturated thioamides. A [Cu‐(R,RP)‐TANIAPHOS]‐SePh species is characterized by its 77Se NMR spectra, which gives a chemical shift at δ 462 ppm. Moreover, a {[Cu‐(R)‐TOL‐BINAP]‐SePh}2 species is characterized by X‐ray analysis, which confirms the formation of Cu−Se bond in the reaction. Finally, the transformations of the thioamide group to amine and thioester are demonstrated to be straightforward.

Copper(I)-Catalyzed Conjugate Addition/Enantioselective Protonation with Selenols and α-Substituted α,β-Unsaturated Thioamides.

Herein, a copper(I)-catalyzed asymmetric conjugate addition/protonation with selenols and α-substituted α,β-unsaturated thioamides is disclosed, which affords a series of chiral selenides in high to excellent enantioselectivity. As for both selenols and α-substituted α,β-unsaturated thioamides, the reaction enjoys broad substrate scopes. The present catalytic system is also successfully applied to asymmetric selenation of β-substituted α,β-unsaturated thioamides. A [Cu-(R,RP )-TANIAPHOS]-SePh species is characterized by its 77 Se NMR spectra, which gives a chemical shift at δ 462 ppm. Moreover, a {[Cu-(R)-TOL-BINAP]-SePh}2 species is characterized by X-ray analysis, which confirms the formation of Cu-Se bond in the reaction. Finally, the transformations of the thioamide group to amine and thioester are demonstrated to be straightforward.

A molecular descriptor of intramolecular noncovalent interaction for regulating optoelectronic properties of organic semiconductors

In recent years, intramolecular noncovalent interaction has become an important means to modulate the optoelectronic performances of organic/polymeric semiconductors. However, it lacks a deep understanding and a direct quantitative relationship among the molecular geometric structure, strength of noncovalent interaction, and optoelectronic properties in organic/polymeric semiconductors. Herein, upon systematical theoretical calculations on 56 molecules with and without noncovalent interactions (X···Y, X = O, S, Se, Te; Y = C, F, O, S, Cl), we reveal the essence of the interactions and the dependence of its strength on the molecular geometry. Importantly, a descriptor S is established as a function of several basic geometric parameters to well characterize the noncovalent interaction energy, which exhibits a good inverse correlation with the reorganization energies of the photo-excited states or electron-pumped charged states in organic/polymeric semiconductors. In particular, the experimental 1H, 77Se, and 125Te NMR, the optical absorption and emission spectra, and single crystal structures of eight compounds fully confirm the theoretical predictions. This work provides a simple descriptor to characterize the strength of noncovalent intramolecular interactions, which is significant for molecular design and property prediction.

An efficient one-pot synthesis of highly functionalized selenophenes

The reaction between active alkenes such as 2-pyridin-2-yl-methylene-malononitrile or ethyl-2-cyano-3-pyridin-2-yl-acrylate and acetylenic esters in the presence of KSeCN at room temperature provided a simple and efficient one-pot route for the synthesis of highly functionalized selenophenes. The reaction is characterized by mild conditions, short reaction time and tolerance to various functional groups and the products were fully characterized by multinuclear (1H, 13C, and 77Se) NMR spectroscopy.

Processing Gray Selenium in Phosphonium-Based Ionic Liquids.

The dissolution of gray selenium in tetraalkylphosphonium acetate ionic liquids was investigated by UV-vis, NMR, and Raman spectroscopy as well as quantum chemical calculations and electrochemical methods. Acetate anions and tetraalkylphosphonium cations facilitate the formation and stabilization of oligoselenides Sen2- and cationic Se species in the ionic liquid phase. Chemical exchange of selenium atoms was demonstrated by variable-temperature 77Se NMR experiments. Additionally, uncharged cycloselenium molecules exist at high selenium concentrations. Upon dilution with ethanol, amorphous red selenium precipitates from the solution. Moreover, crystalline Se1-xTex solid solutions precipitate when elemental tellurium is added to the mixture as confirmed by powder X-ray diffraction and Raman spectroscopy.

Electronic properties and supramolecular study of selenoureas with fluorinated-NHC ligands derived from imidazo[1,5-a]pyridines

Twelve selenourea having fluorinated fragments were synthesized and fully characterized and their fluorine content influence studied by 77Se NMR.

Chlorinated polyhedral selenaboranes revisited by joint experimental/computational efforts: the formation of closo-1-SeB9Cl9 and the crystal structure of closo-SeB11Cl11.

The recent success in the formation of chlorinated telluraboranes and the reactivities of pnictogenaboranes prompted us to re-examine the vacuum co-pyrolysis of B2Cl4 with Se2Cl2 at various molar ratios and temperatures in order to search for the generation of other polyhedral selenaboranes than closo-SeB5Cl5 (1a) and closo-SeB11Cl11 (1b), the latter being observed earlier. Interestingly, a new compound with the elemental composition SeB9Cl9 (2) was detected, this time by high- and low-resolution mass spectrometry. Further characterization by 1- and 2-D 11B-NMR spectroscopy suggests that 2 should adopt a closed bicapped square-antiprismatic geometry with selenium at the apical position. Moreover, vacuum sublimation gave suitable crystals of 1b, which were subjected to single-crystal X-ray structure determination. Crystallographic data analysis confirmed that 1b, consistent with its 26 skeletal electron count, adopts a distorted icosahedral structure close to the symmetry of C5v. Computations at the DFT-D3 level have revealed that 33% of the total computed binding motifs in the grown 1b crystals are due to the very strong chalcogen bonding. Moreover, SAPT decomposition has shown that the bonding motifs in the crystals are stabilized mainly by dispersion and electrostatic terms. Homodecoupling and high resolution 11B NMR and 77Se NMR experiments have resolved both coupling constants 1J(11B11B) and 1J(77Se11B) as well as the 77Se chemical shift of 1a and 1b, which are in reasonable agreement with the corresponding computed values. The computed 11B chemical shifts of 2 were determined by the well-established DFT/GIAO/NMR structural tool based on its B3LYP/6-311+G** internal coordinates. They agree well with the experimental values and provide a good representation of the molecular structure of 2 in solution. The extraordinary downfield 11B NMR chemical shift of B(10) in 2 has been ascribed to the intensive paramagnetic contribution to the shielding tensor in this bicapped square-antiprismatic motif. Calculations of the synproportionation free energies of smaller (n - 1) closo-selenaboranes with larger-sized (n + 1) ones support the extraordinary stability of octahedral, bicapped square-antiprismatic and icosahedral closo motifs in the SeBnCln family (n = 4-12).

Metal-promoted synthesis of steroidal ethynyl selenides having anticancer activity

In this study, we have described simple and efficient methodology for the metal-promoted (Cu2I2) preparation of steroidal ethynyl selenides. The compounds were characterized using 1H, 13C and 77Se NMR, FT IR spectroscopy, and MS analysis. A proposed mechanism of the metal-promoted reaction involves the formation of a σ-bound copper acetylide. Due to the fact that organoselenium-based compounds possess a pleiotropic properties and associated with their promising biological activities, in the next step of the study biocompatibility and anticancer activity of the synthesized compounds was evaluated. Steroidal selenides were tested in vitro against estrogen-depend breast cancer cells MCF-7 using spectrophotometric, fluorometric and luminometric methods. Designed selenides showed high hemocompatibility, lack of toxicity against cardiomyocytes cell and great anti-cancer activity in vitro against estrogen-depend breast cancer cells upon 24 h of treatment. We revealed that selenides decrease the viability and proliferation ability of MCF-7 cells by induction of cell apoptosis. It has been noted that the overproduction of reactive oxygen species (ROS) and associated with its activation of Caspase 3/7 are a major mechanism that is responsible of selenides-caused cell death. These data indicate that organoselenium based compounds have great antineoplastic potential and might be developed as novel class of agents dedicated to the breast-cancer therapies.

Improving the Configurational Stability of Chiral-at-Iron Catalysts Containing Two N-(2-Pyridyl)-Substituted N-Heterocyclic Carbene Ligands.

Recently, we introduced the first example of chiral-at-iron catalysts in which two achiral N-(2-pyridyl)-substituted N-heterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong Y.Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc.2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The π-acceptor properties of the altered NHCs were investigated by Ganter's 77Se NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.

Dissolution Dynamic Nuclear Polarization of the 77Se Nucleus

Abstract77Se is a spin , low sensitivity nucleus with a natural abundance of 7.6 %. Although 77Se NMR is very useful in the characterization of selenium containing molecules including seleno‐proteins, the detection of 77Se is challenging in biological samples without enrichment. Therefore, the goal of this work was to establish whether the 77Se signal could be enhanced in the liquid state by dissolution dynamic nuclear polarization (DNP) NMR without the need of enrichment. The dominant spin‐lattice relaxation mechanism for 77Se is via chemical shift anisotropy, which is highly dependent on the molecular symmetry. Here we tested three selenium compounds (sodium selenate, sodium selenite and selenocystine) with different molecular symmetries in dissolution DNP experiments and demonstrated that 77Se DNP using commercially available hardware is feasible but the achieved NMR signal enhancements (1368‐fold for selenate, 125‐fold for selenite and no enhancement for selenocystine at 9.4 T) were strongly dependent on molecular symmetry.

Synthesis and characterization of unsymmetrical Selenoethers: Facile cleavage of the Se-C bond

Installation of in situ generated arylselenide (ArSe-) on 2‑methyl napthalene afforded the following selenoethers: 2-C10H7CH2Se(C6H5), 2-C10H7CH2Se(p-CH3C6H5), 2-C10H7CH2Se(1-C10H7) and 2-C10H7CH2Se(2,4,6-Me3C6H2), as yellow crystalline solids. These molecules can be prepared by the treatment of in situ generated arylselenide with 2-C10H7CH2Br in ethanol solvent at 0 °C in excellent yield. These molecules are characterized by 1H, 13C, 77Se NMR and elemental analysis. Among these, 2-C10H7CH2Se(C6H5) and 2-C10H7CH2Se(2,4,6-Me3C6H2) are also characterized by single-crystal X-ray studies. Phenyl mercury acetate was isolated in good yield through the treatment of 2-C10H7CH2Se(C6H5) with mercuric acetate. In this reaction cleavage of the C(aryl)–Se bond takes place. Similarly, 2-C10H7CH2Se(C6H5) and 2-C10H7CH2Se(p-CH3C6H5) also exhibited cleavage of the C(aryl)–Se bond with HgCl2 and afforded [2-C10H7CH2Se(C6H5)(C6H5HgCl)] and [2-C10H7CH2Se(p-MeC6H4)(p-MeC6H4HgCl)] complexes in good yield respectively. A organopalladium(II) complex, [3-C10H6PdCl(2-CH2SeC6H5)]2, was also developed through the reaction of 2-C10H7CH2Se(C6H5) with Na2PdCl4.

Trichloroisocyanuric Acid (TCCA): A Suitable Reagent for the Synthesis of Selanyl‐benzo[b]chalcogenophenes

AbstractA new approach for the synthesis of 3‐organoselanyl‐benzo[b]chalcogenophenes promoted by trichloroisocyanuric acid is reported. The reaction was carried out with electrophilic selenium species which were generated in situ by the reaction between trichloroisocyanuric acid and diorganyl diselenides using ethanol as solvent. The developed method uses mild reaction conditions and was efficient for the synthesis of different 3‐organoselanyl‐benzo[b]chalcogenophenes in a good to excellent yields. Analysis of 77Se NMR spectroscopy indicates that the electrophilic PhSeCl was formed in situ and a plausible reaction mechanism was proposed.

Revisiting the anticancer properties of phosphane(9-ribosylpurine-6-thiolato)gold(I) complexes and their 9H-purine precursors

New mono- and di-nuclear thio-purine and thio-purine nucleoside gold(I) complexes were synthesized, characterized, and evaluated in vitro for biological activities in comparison to related known purine complexes. By combining known anti-tumoral thio-purines with R3PAu moieties as present in auranofin, complexes with enhanced effects and selectivities were obtained, which not only act as cytostatics, but also disrupt tumor-specific processes. Their IC50 values in cytotoxicity test with tumor cell lines ranged from three-digit nanomolar to single-digit micromolar, revealing a tentative structure–activity relationship (SAR). Both the residues R2 of the phosphane ligand and R1 at C2 of the pyrimidine ring had a significant impact on the cytotoxicity. In most cases, the introduction of a ribo-furanosyl group at N9 of the purine led to a distinctly more cytotoxic complex. Most complexes were more active against multi-drug-resistant tumor cells or such lacking functional p53 when compared to the respective untreated wild type cell lines. Some nucleoside complexes displayed an interesting dose-dependent dual mode of action regarding cell cycle arrest and DNA repair mechanism. Some phosphane(purine-6-thiolato)gold (I) complexes had a stronger inhibitory effect on the thioredoxin reductase (TrxR) and on the reactive oxygen species (ROS) generation in cancer cells than is typical of other gold complexes. They also led to DNA fragmentation and showed anti-angiogenic effects. Their stability under test conditions was demonstrated by 77Se NMR monitoring of an exemplary selenopurine complex.Graphical abstract

Facile Access to 2-Selenoxo-1,2,3,4-tetrahydro-4-quinazolinone Scaffolds and Corresponding Diselenides via Cyclization between Methyl Anthranilate and Isoselenocyanates: Synthesis and Structural Features.

A practical method for the synthesis of 2-selenoxo-1,2,3,4-tetrahydro-4-quinazolinone was reported. The latter compounds were found to undergo facile oxidation with H2O2 into corresponding diselenides. Novel organoselenium derivatives were characterized by the 1H, 77Se, and 13C NMR spectroscopies, high-resolution electrospray ionization mass spectrometry, IR, elemental analyses (C, H, N), and X-ray diffraction analysis for several of them. Novel heterocycles exhibited multiple remarkable chalcogen bonding (ChB) interactions in the solid state, which were studied theoretically.

Lipase-catalyzed ring-opening copolymerization of macrocycles for diselenide-functionalized long-chain polycarbonate: Synthesis, kinetic process and ROS responsiveness

Herein, the lipase-catalyzed ring-opening copolymerization of the diethylene diselenide carbonate dimer (SeC) with its analogue macrocycle (1,3,12,14-Tetraoxacyclodocosane-2,13-dione, TDC) was explored, and the small-ring carbonate (TMC) was used as control. The structure and composition of the diselenide-functionalized long-chain polycarbonates were confirmed by NMR and FTIR. The 13C NMR and 77Se NMR showed that the copolymers observed random sequence. Moreover, the kinetic process of TDC + SeC copolymerization demonstrated a faster polymerization rate and higher conversion of SeC in comparison with TMC + SeC. Besides, the copolymers exhibited ROS-responsive behaviors when exposed to the oxidizing environment owing to the diselenide groups, which was confirmed by UV–vis, DLS and TEM. Therefore, the ROS responsiveness facilitated potential application for the diselenide-functionalized copolymers.

Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H2O2 to H2O and O2 and Activation of I2 and NBS.

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis(2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H2O and O2 with first-order kinetics with respect to H2O2 for the first time by any organoselenium molecules as monitored by 1H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 103 μM min-1 with a rate constant of 8.57 ± 0.50 × 10-3 s-1, whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H2O2 at rates of 7.9 ± 0.3 × 103 and 2.9 ± 0.3 × 103 μM min-1 with rate constants of 1.16 ± 0.02 × 10-3 and 0.325 ± 0.025 × 10-3 s-1, respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H2O2 has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O2 probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H2O2 by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I2 toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by 77Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H2O2, I2, and NBS. The reaction of H2O2, I2, and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N-donor atom.

Transition Metal Complexes of NHC Ligands Functionalized with the Cationic (η5‐Cyclopentadienyl)(η6‐phenyl)iron(II) Motif

AbstractStarting from [(η5‐cyclopentadienyl)(η6‐phenyl)iron(II)]imidazole, dicationic imidazolium salts were prepared by N‐alkylation. Reaction of these compounds with basic metal precursors such as mesityl copper(I) or palladium(II) acetate led to mono respectively dicationic transition metal NHC complexes (NHC=N‐heterocyclic carbene). Transmetalation using the copper(I) complexes opened up the access to NHC gold(I) compounds. PEPPSI‐type NHC complexes of palladium(II) and platinum(II) were prepared by offering a neutral pyridine ligand to the transition metal center. A rhodium(I) NHC complex was accessible by deprotonation of the dicationic imidazolium precursor and subsequent treatment with [(COD)Rh(μ2‐Cl)]2 (COD=1,5‐cyclooctadiene). The new NHC complexes were investigated by means of NMR spectroscopy, mass spectrometry as well as single crystal X‐ray structure analysis. Both, the palladium(II) containing PEPPSI‐type and the gold(I) complex, were investigated for their catalytic properties in typical model reactions such as cyclization reactions, Suzuki coupling and cyanation. In addition, a selenium adduct was synthesized in order to study the electronic properties of the underlying ligand backbone. Based on the chemical shift in the 77Se NMR spectrum, it is evident that these NHC ligands possess rather poor π‐acidity.

Pyrazoles in the Intersection of Mesomeric Betaines and N-Heterocyclic Carbenes: Formation of NHC Selenium Adducts of Pyrazolium-4-aminides

AbstractStarting from 4-nitropyrazole, eight mesoionic pyrazolium-4-aminides were prepared by a six-step reaction sequence. The deprotonation of 1,2-disubstituted 4-amido-1H-pyrazolium salts by an anion exchange resin in its hydroxide form is the final step of the synthesis. A tautomeric equilibrium between the mesoionic compounds (pyrazolium-4-aminides) and N-heterocyclic carbenes (pyrazol-3-ylidenes) can be formulated; however, the NHC tautomers were not detected by means of NMR spectroscopy in polar aprotic solvents such as DMSO-d 6 or MeCN-d 3. Apart from tautomerism, anionic N-heterocyclic carbenes can be formulated as a result of a deprotonation of the mesoionic compounds­. Trapping reactions were performed with selenium, which resulted in the formation of pyrazole-3-selenones. Methylation at the selenium atom gave the corresponding 3-(methylselanyl)-4-amido-1H-pyrazolium salts, which were deprotonated to give new mesomeric betaines, 3-(methylselanyl)-1H-pyrazolium-4-aminides as unique compounds­. DFT-calculations as well as 77Se NMR spectroscopic measurements were carried out.

Three-component reaction between ethyl carbazate, potassium selenocyanate and aroyl chlorides: Synthesis of 1,2,4-triazole-3-selones

We reported a three-component, simple, one-pot, and novel method for the synthesis of 1,2,4-triazole-3-selones from the condensation of potassium selenocyanate and aroyl chlorides with ethyl carbazate at room temperature. The structures of the compounds were deduced from elemental analysis and their IR, 1H,13C, and 77Se NMR NMR spectra. This new protocol offers advantages such as mild reaction conditions, easy work-up, and high yields.

Antioxidant Activity of a Selenopeptide Modelling the Thioredoxin Reductase Active Site is Enhanced by NH···Se Hydrogen Bond in the Mixed Selenosulfide Intermediate

Background: Thioredoxin reductase (TrxR), one of the representative selenoenzymes, is an important antioxidant enzyme suppressing oxidative stress in living organisms. At the active site of human TrxR, the presence of a Sec•••His•••Glu catalytic triad was previously suggested. Method. In this study, a short selenopeptide mimicking this plausible triad, i.e., H-CUGHGE-OH (1), was designed, synthesized, and evaluated for the TrxR-like catalytic activity. Method: In this study, a short selenopeptide mimicking this plausible triad, i.e., H-CUGHGE-OH (1), was designed, synthesized, and evaluated for the TrxR-like catalytic activity Results: The molecular simulation in advance by REMC/SAAP3D predicted the preferential formation of Sec•••His•••Glu hydrogen bonding networks in the aqueous solution. Indeed, a significant antioxidant activity was observed for 1 in the activity assay using NADPH as a reductant and H2O2 as a substrate. Tracking the reaction between 1 and GSH by 77Se NMR revealed a reductive cleavage of the selenosulfide (Se-S) bond to generate the diselenide species. The observation suggested that in the transiently formed mixed Se-S intermediate, the NH•••Se hydrogen bond between the Sec and His residues leads a nucleophilic attack of the second thiol molecule not to the intrinsically more electrophilic Se atom but to the less electrophilic S atom of the Se-S bond. Ab initio calculations for the complex between MeSeSMe and an imidazolium ion at the MP2/6-31++G(d,p) level demonstrated that NH•••Se and NH•••S hydrogen bonds are equally favorable as the interaction modes. Thus, importance of the relative spatial arrangement of the Se-S bond with respect to the imidazole ring was suggested for the exertion of the TrxR-like catalytic activity. Conclusion: The proposed umpolung effect of NH•••Se hydrogen bond on the reactivity of a Se-S bond will be a useful tool for developing efficient TrxR models with high redox catalytic activity.