Enantioselective Nickel-Catalyzed Denitrogenative Transannulation En Route to N-N Atropisomers.

Nickel-catalyzed transannulation reactions triggered by the extrusion of small gaseous molecules have emerged as a powerful strategy for the efficient construction of heterocyclic compounds. However, it's asymmetric synthesis remains challenging because of the difficulty in controlling stereo- and regioselectivity. Herein, we report the first nickel-catalyzed asymmetric synthesis of N-N atropisomers by the denitrogenative transannulation of benzotriazones with internal alkynes. A broad range of N-N atropisomers was obtained with excellent regio- and enantioselectivity under mild conditions. Moreover, density functional theory (DFT) calculations provided insights into the nickel-catalyzed reaction mechanism and enantioselectivity control.

Palladium-catalyzed asymmetric carbene coupling en route to inherently chiral heptagon-containing polyarenes.

Developing facile and direct synthesis routes for enantioselective construction of cyclic π-conjugated molecules is crucial. However, originate chirality from the distorted structure around heptagon-containing polyarenes is largely overlooked, the enantioselective construction of all-carbon heptagon-containing polyarenes remains a challenge. Herein, we present a highly enantioselective synthesis route for fabricating all carbon heptagon-containing polyarenes via palladium-catalyzed carbene-based cross-coupling of benzyl bromides and N-arylsulfonylhydrazones. A wide range of nonplanar, saddle-shaped tribenzocycloheptene derivatives are efficiently prepared in high yields with excellent enantioselectivities using this approach. In addition, stereochemical stability experiments show that these saddle-shaped tribenzocycloheptene derivatives have high inversion barriers.

Organocatalytic Enantioselective Synthesis of Seven-Membered Ring with Inherent Chirality.

Inherent chirality is used to describe chiral cyclic molecules devoid of central, axial, planar, or helical chirality and has tremendous applications in chiral recognition and enantioselective synthesis. Catalytic and divergent syntheses of inherently chiral molecules have attracted increasing interest from chemists. Herein, we report the enantioselective synthesis of inherently chiral tribenzocycloheptene derivatives via chiral phosphoric acid (CPA)-catalyzed condensation of cyclic ketones and hydroxylamines. This chemistry paves the way to accessing the less stable derivatives of 7-membered rings with inherent chirality. A series of chiral tribenzocycloheptene oxime ethers was synthesized in good yields (up to 97 %) with excellent enantioselectivities (up to 99 % ee).

Fabrication of two novel amino-functionalized and starch-coated CuFe2O4-modified magnetic biochar composites and their application in removing Pb2+ and Cd2+ from wastewater.

Novel magnetic biochar composites (SFeCu@SBCO and FeCu@SBCO-NH2) were fabricated by modifying oxidized sawdust biochar (SBCO) with Fe/Cu loading, starch-coating/amination, characterized (FTIR, XRD, BET, SEM-EDS and XPS) and applied in capturing Pb2+ and Cd2+ from wastewater. Adsorption experiments revealed that SFeCu@SBCO and FeCu@SBCO-NH2 exhibited extraordinary adsorption performance toward Pb2+/Cd2+ with the maximum adsorption capacity reaching 184.26/173.35 mg g-1 and 201.43/190.81 mg g-1, respectively, which were >5 times higher than those of SBC. The great increase in adsorption capacity of the two adsorbents was ascribed to the introduction of CuFe2O4 and starch/amino groups. Pb2+ and Cd2+ adsorption was an endothermic reaction controlled by monolayer chemisorption. Complexation and electrostatic attraction were the two predominant mechanisms. Besides, ion exchange together with physical adsorption also occurred during the adsorption. Additionally, the both adsorbents displayed favorable stability and reusability as well as desirable anti-interfering ability to other metal cations. Taken together, the both adsorbents could be utilized as reusable magnetic adsorbents with promising prospect in the effective remediation of Pb2+/Cd2+ contaminated water. The study not only contributed to the better understanding of biochar modification strategy and the application of modified biochar in heavy metals pollutants removal, but also realized resource utilization of biomass waste.

Catalytic Asymmetric Synthesis of N-N Biaryl Atropisomers.

Atropisomers have emerged as important structural scaffolds in natural products, drug design, and asymmetric synthesis. Recently, N-N biaryl atropisomers have drawn increasing interest due to their unique structure and relatively stable axes. However, its asymmetric synthesis remains scarce compared to its well-developed C-C biaryl analogs. In this concept, we summarize the asymmetric synthesis of N-N biaryl atropisomers including N-N pyrrole-pyrrole, N-N pyrrole-indole, N-N indole-indole, and N-N indole-carbazole, during which a series synthetic strategies are highlighted. Also, a synthetic evolution is briefly reviewed and an outlook of N-N biaryl atropisomers synthesis is offered.

Screening of Deoxynivalenol Cyclic Heptapeptide Mimotope Antigen.

In this study, phage clones that can bind to DON were selected from the phage cyclohepta peptide library by screening through the principle of solid-phase affinity, and mimotope were synthesized to replace the DON toxin standard to establish a green low toxicity detection system. The author conducted four rounds of screening in the phage cyclic heptapeptide library with DON-10a1a monoclonal antibody as the target molecule. Then 38 phage clones were selected and validated, and the results showed that 35 of them could bind to the DON-10a1a monoclonal antibody and were inhibited by DON toxin. Finally, the DNA was extracted and sequenced to obtain 6 different DNA sequences, which were named D1-D6 respectively. The peptides synthesized according to the corresponding amino acid sequences can replace DON toxin to establish a series of green and low toxicity assays.

Atroposelective Synthesis of C-N Vinylindole Atropisomers by Palladium-Catalyzed Asymmetric Hydroarylation of 1-Alkynylindoles.

Transition-metal-catalyzed hydroarylation of unsymmetrical internal alkynes remains challenging because of the difficulty in controlling regioselectivity and stereoselectivity. Moreover, the enantioselective hydroarylation of alkynes using organoboron reagents has not been reported. Herein, we report for the first time that palladium compounds can catalyze the hydroarylation of 1-alkynylindoles with organoborons for the synthesis of chiral C-N atropisomers. A series of rarely reported vinylindole atropisomers was synthesized with excellent regio-, stereo- (Z-selectivity), and enantioselectivity under mild reaction conditions. The ready availability of organoborons and alkynes and the simplicity, high stereoselectivity, and good functional group tolerance of this catalytic system make it highly attractive.

Catalytic Asymmetric Synthesis of Atropisomers Featuring an Aza Axis.

ConspectusAtropisomers bearing a rotation-restricted axis are common structural units in natural products, chiral ligands, and drugs; thus, the prevalence of asymmetric synthesis has increased in recent decades. Research into atropisomers featuring an N-containing axis (N-X atropisomers) remains in its infancy compared with the well-developed C-C atropisomer analogue. Notably, N-X atropisomers could offer divergent scaffolds, which are extremely important in bioactive molecules. The asymmetric synthesis of N-X atropisomers is recognized as both appealing and challenging. Recently, we devoted our efforts to the catalytic asymmetric synthesis of N-X atropisomers, benzimidazole-aryl N-C atropisomers, indole-aryl N-C atropisomers, hydrogen-bond-assisted N-C atropisomers, pyrrole-pyrrole N-N atropisomers, pyrrole-indole N-N atropisomers, and indole-indole N-N atropisomers. To obtain the N-C atropisomers, an asymmetric Buchwald-Hartwig reaction of amidines or enamines was employed. Using a Pd(OAc)2/(S)-BINAP or Pd(OAc)2/(S)-Xyl-BINAP catalyst system, benzimidazole-aryl N-C atropisomers and indole-aryl N-C atropisomers were readily obtained. To address the issue of the reduced stability of the diarylamine axis, a six-membered intramolecular N-H-O hydrogen bond was introduced into the N-C atropisomer scaffold. A tandem N-arylation/oxidation process was used for the chiral phosphoric acid (CPA)-catalyzed asymmetric synthesis of N-aryl quinone atropisomers. For N-N atropisomers, a copper-mediated asymmetric Friedel-Crafts alkylation/arylation reaction was developed. The desymmetrization process was completed successfully via a Cu(OTf)2/chiral bisoxazoline or (CuOTf)·Tol/bis(phosphine) dioxide system, thereby achieving the first catalytic asymmetric synthesis of N/N bipyrrole atropisomers. Asymmetric Buchwald-Hartwig amination of enamines was utilized to provide N-N bisindole atropisomers with excellent stereogenic control. This was the first asymmetric synthesis of N-N atropisomers featuring a bisindole structural scaffold using the de novo indole construction strategy. The asymmetric N-N heterobiaryl atropisomer synthesis was substantially facilitated using palladium-catalyzed transient directing group (TDG)-mediated C-H functionalization. Atropisomeric alkenylation, allylation, or alkynylation was accomplished using the Pd(OAc)2/l-tert-leucine system. Herein, we summarize our work on the palladium-, copper-, and CPA-catalyzed asymmetric syntheses of N-C and N-N atropisomers. Furthermore, the application of our work in the synthesis of bioactive molecule analogues and axially chiral ligands is demonstrated. Subsequently, the stability of the chiral N-containing axis is briefly discussed in terms of single crystals and obtained rotational barriers. Finally, an outlook on the asymmetric N-X atropisomer synthesis is provided.

Palladium-Catalyzed Double N-Arylation to Access Unsymmetric N,N'-Bicarbazole Scaffolds.

Herein, the palladium-catalyzed double C-N coupling of 9H-carbazol-9-amines and 2,2'-dibromo-1,1'-biphenyl is reported. This protocol offers access to N,N'-bicarbazole scaffolds, which have frequently been used as linkers in the construction of functional covalent organic frameworks (COFs). A variety of substituted N,N'-bicarbazoles were synthesized in moderate to high yields based on this chemistry, and the potential application of this method was showcased by the synthesis of COF monomers like tetrabromide 4 and tetraalkynylate 5.

Enantioselective Synthesis of N-N Atropisomers by Palladium-Catalyzed C-H Functionalization of Pyrroles.

The catalytic asymmetric construction of N-N atropisomeric biaryls remains a formidable challenge. Studies of them lag far behind studies of the more classical carbon-carbon biaryl atropisomers, hampering meaningful development. Herein, the first palladium-catalyzed enantioselective C-H activation of pyrroles for the synthesis of N-N atropisomers is presented. Structurally diverse indole-pyrrole atropisomers possessing a chiral N-N axis were produced with good yields and high enantioselectivities by alkenylation, alkynylation, allylation, or arylation reactions. Furthermore, the kinetic resolution of trisubstituted N-N heterobiaryls with more sterically demanding substituents was also achieved. Importantly, this versatile C-H functionalization strategy enables iterative functionalization of pyrroles with exquisite selectivity, expediting the formation of valuable, complex, N-N atropisomers.

Palladium-catalyzed enantioselective rearrangement of dienyl cyclopropanes.

Vinyl cyclopropanes (VCPs) are among the most useful three-carbon building blocks in organic synthesis. They are commonly used as dienophiles in a range of cycloaddition reactions. However, VCP rearrangement has not received much attention since its discovery in 1959. In particular, the enantioselective rearrangement of VCP is synthetically challenging. Herein, we report the first palladium-catalyzed regio- and enantioselective rearrangement of VCPs (dienyl or trienyl cyclopropanes) for the construction of functionalized cyclopentene units in high yields and with excellent enantioselectivities and 100% atom economy. The utility of the current protocol was highlighted by a gram-scale experiment. Moreover, the methodology provides a platform for accessing synthetically useful molecules containing cyclopentanes or cyclopentenes.

The Asymmetric Buchwald-Hartwig Amination Reaction.

Over the past few decades, the Buchwald-Hartwig reaction has emerged as a powerful tool for forging C-N bonds, and has been vital to the pharmaceuticals, materials, and catalysis fields. However, asymmetric Buchwald-Hartwig amination reactions for constructing centered chirality, planar chirality, and axial chirality remain in their infancy owing to limited substrate scope and laggard ligand design. The recent surge in interest in the synthesis of C-N/N-N atropisomers, has witnessed a renaissance in asymmetric Buchwald-Hartwig amination chemistry as the first practical protocol for the preparation of C-N atropisomers. This review highlights reported asymmetric Buchwald-Hartwig amination protocols and provides a brief overview of their chemical practicality.

Atroposelective Synthesis of N-Arylated Quinoids by Organocatalytic Tandem N-Arylation/Oxidation.

Diarylamines and related scaffolds are ubiquitous atropisomeric chemotypes in biologically active natural products. However, the catalytic asymmetric synthesis of these axially chiral compounds remains largely unexplored. Herein, we report that a BINOL-derived chiral phosphoric acid (CPA) successfully catalyzed the atroposelective coupling of quinone esters and anilines through direct C-N bond formation to afford N-aryl quinone atropisomers with an unprecedented intramolecular N-H-O hydrogen bond within a six-membered ring in good yields and enantioselectivities with the quinone ester as both the electrophile and the oxidant. A gram-scale experiment demonstrated the utility of this synthetic protocol. Moreover, this methodology provides a platform for the synthesis of structurally diverse secondary amine atropisomers by nucleophilic addition.

Enantioselective Synthesis of N-N Bisindole Atropisomers.

N-N Atropisomers are a common motif in natural products and represent a significant dimension for exploration in modern pharmaceutical and medicinal chemistry. However, the catalytic atroposelective synthesis of such molecules remains challenging, hampering meaningful development. In particular, an enantioselective synthesis of N-N bisindole atropisomers is unprecedented. Herein, the first enantioselective synthesis of N-N bisindole atropisomers via the palladium-catalyzed de novo construction of one indole skeleton is presented. A wide variety of N-N axially chiral bisindoles were generated in good yields with excellent enantioselectivities via a cascade condensation/N-arylation reaction. Structurally diverse indole-pyrrole, indole-carbazole, and non-biaryl-indole atropisomers possessing a chiral N-N axis were accessed using this protocol. Moreover, investigations using density functional theory (DFT) calculations provided insight into the reaction mechanism and enantiocontrol.

Palladium/Amino Acid Co-catalyzed Atroposelective C-H Olefination to Access Tetra-Ortho-Substituted Atropisomers Featuring 2,2'-Difluoro-1-biaryl Scaffolds.

Despite the great advancement in atroposelective synthesis in the past decades, the enantioselective synthesis of 2,2'-difluoro-1-biaryls is unprecedented. Herein, a palladium and chiral amino acid catalyzed atroposelective C-H olefination to construct the axially chiral 2,2'-difluoro-1-biaryls is reported. A variety of polyfluoro-substituted biaryls were forged under mild conditions in good yields with excellent enantioselectivity (up to 99% ee). The potential application was demonstrated by a gram-scale synthesis and synthetic transformations.

Synthesis and application of starch-stablized Fe-Mn/biochar composites for the removal of lead from water and soil.

Starch-stablized and Fe/Mn bimetals modified biochar derived from corn straw (SFM@CBC and SFM@CBC-350) were firstly prepared, characterized (FTIR, XRD, SEM, EDS, BET and XPS), and applied in Pb removal from water and soil. SFM@CBC and SFM@CBC-350 displayed highly effective adsorption performance of Pb2+ from wastewater with the maximum adsorption capacity of 170.91 mg g-1 and 190.17 mg g-1, respectively, which were much greater than that of FM@CBC (149.25 mg g-1) and CBC (101.10 mg g-1). Studies of adsorption kinetics, isotherms and thermodynamics indicated that the absorption of Pb2+ by SFM@CBC and SFM@CBC-350 was spontaneous and endothermic reaction, and it was controlled by monolayer chemisorption. The mechanism studies indicated that Pb2+ removal involved with multiple mechanism, including complexation (dominant process confirmed by XPS analysis), physical adsorption, electrostatic attraction, and cation exchange. The reusability test demonstrated that SFM@CBC and SFM@CBC-350 had very good stability and reusability. In addition, in order to further explore Pb removal performance of the modified biochar, SFM@CBC-350 was used in soil-ryegrass pot systems. Compared with the controls, the addition of SFM@CBC-350 reduced Pb content in soil and ryegrass, increased the biomass and total chlorophyll content, reduced the activity of antioxidant enzymes (CAT, SOD, MDA and POD) and ROS fluorescence intensity of ryegrass, thus alleviating Pb stress of ryegrass. Besides, the addition of SFM@CBC-350 could increase the richness and diversity of soil microorganisms, which was beneficial to the growth of ryegrass. Hence, SFM@CBC-350 has the potential of being used as a green, efficient and promising adsorbent in Pb removal from wastewater and soil.

Cu(I)-Catalyzed Asymmetric Arylation of Pyrroles with Diaryliodonium Salts toward the Synthesis of N-N Atropisomers.

We report herein that copper(I) catalysis using a bis(phosphine) dioxide ligand can catalyze the desymmetric C-H arylation of prochiral bipyrroles. More than 50 nitrogen-nitrogen atropisomers were achieved in good to excellent yields with excellent enantioselectivities (≤97% yield, ≤98% ee). The reaction proceeds under mild conditions with good functional group compatibility on arenes and diaryliodonium salts. Moreover, this principle enables iterative arylation of the bipyrroles to enantioselectively arylate different positions during the catalysis of copper.

Removal of U(vi) from aqueous solutions by an effective bio-adsorbent from walnut shell and cellulose composite-stabilized iron sulfide nanoparticles.

FeS nanoparticles were easily aggregated and oxidized in the natural environment. It was important to stabilize the iron sulfide nanoparticle composite with a stabilizer. Biochar could be used as an effective carrier to inhibit the agglomeration and oxidization of FeS nanoparticles. An efficient and novel bio-adsorbent (CFeS-WS) from walnut shell (WS) and cellulose composites-stabilized iron sulfide nanoparticle was synthesized by the modified method. The removal of U(vi) ions from an aqueous solution by CFeS-WS was carried out. The experimental results indicated that numerous functional groups were observed on the surface of CFeS-WS. In addition, the biochar was loaded successfully with cellulose and FeS nanoparticle composites. The cellulose and biochar effectively prevented the agglomeration of FeS nanoparticles. The adsorption process of U(vi) ions by CFeS-WS was more consistent with the pseudo second-order kinetic model and Langmuir isotherm model. The adsorption process of U(vi) ions was an endothermic and chemical reaction process. The proposed reaction mechanism of the U(vi) ion removal by CFeS-WS mainly consisted of the ion exchange reaction, reduction reaction, hydrogen bonding and functional group, and pore of the adsorbent filling. According to the results of the recycle experiment, it indicated that the chemical stability of CFeS-WS was good.

Functionalized porous nanoscale Fe3O4 particles supported biochar from peanut shell for Pb(II) ions removal from landscape wastewater.

The large amounts ofheavy metal from landscape wastewater have become serious problems of environmental pollution and risks for human health. The development of efficient novel adsorbent is a very important for treatment of heavy metal. The functionalized porous nanoscale Fe3O4 particles supported biochar from peanut shell (PS-Fe3O4) for removal of Pb(II) ions from aqueous solution was investigated. The characterization of PS-Fe3O4 composites showed that biochar was successfully coated with porous nanoscale Fe3O4 particles. The pseudo second-order kinetic model and Langmuir model were more fitted for describing the adsorption process of Pb(II) ions in solution. The adsorption process of Pb(II) ions removal by PS-Fe3O4 composites was a spontaneous and endothermic process. The adsorption mechanisms of Pb(II) ions by PS-Fe3O4 composites were mainly controlled by the chemical adsorption process. The maximum adsorption capacity of Pb(II) ions removal in solution by PS-Fe3O4 composites reached 188.68 mg/g. The removal mechanism included Fe-O coordination reaction, co-precipitation, complexation reaction, and ion exchange. PS-Fe3O4 composites were thought as a low-cost, good regeneration performance, and high efficiency adsorption material for removal of Pb(II) ions in solution.

Biochar-supported starch/chitosan-stabilized nano-iron sulfide composites for the removal of lead ions and nitrogen from aqueous solutions.

Novel materials that nano-FeS and starch (or chitosan) loaded on peanut shells biochar(Starch-FeS@PSB and Chitosan-FeS@PSB) were prepared and applied for removal of Pb(II) and nitrogen(NO3-N and NH4-N) in wastewater. It showed that Starch-FeS@PSB and Chitosan-FeS@PSB had excellent absorptive effects compared with PSB. The maximum adsorption capacity of Pb(II) by Starch-FeS@PSB and Chitosan-FeS@PSB reached 91.74 mg/g, 98.04 mg/g, respectively. Absorption of Pb(II) by Starch-FeS@PSB and Chitosan-FeS@PSB were controlled by monolayer chemisorption. Mechanism studies showed that complexation, electrostatic attraction, REDOX and physical absorption happened on the adsorbent surface. In addition, the maximum adsorption capacity of NO3-N and NH4-N by Starch-FeS@PSB and Chitosan-FeS@PSB reached 16.89 mg/g, 15.65 mg/g, and 18.45 mg/g, 18.28 mg/g, respectively. Absorption of N by Starch-FeS@PSB and Chitosan-FeS@PSB were controlled by multilayer chemisorption. Mechanism studies showed that complexation, electrostatic attraction and physical absorption happened on the adsorbent surface. Starch-FeS@PSB and Chitosan-FeS@PSB can be utilized in Pb(II) and N wastewater treatment.