Aromatic Building Blocks Research Articles

Tunable Isometric Donor-Acceptor Wurster-Type Covalent Organic Framework Photocathodes.

Covalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster-type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor-acceptor-donor (D-A-D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady-state photoluminescence revealed a tunable red-shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical (PEC) water splitting studies exhibited a doubled current density (8.1 μA cm-2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of Wurster D-A-D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of chemical functionality-reactivity-property relationships in this promising class of photoactive materials.

Tunable Isometric Donor‐Acceptor Wurster‐Type Covalent Organic Framework Photocathodes

AbstractCovalent organic frameworks (COFs) offer remarkable versatility, combining ordered structures, high porosity, and tailorable functionalities in nanoscale reaction spaces. Herein, we report the synthesis of a series of isostructural, photoactive Wurster‐type COFs achieved by manipulating the chemical and electronic nature of the Wurster aromatic amine building blocks. A series of donor‐acceptor‐donor (D‐A‐D) Wurster building block molecules was synthesized by incorporating heteroaromatic acceptors with varying strengths between triphenylamine donor groups. These tailored building blocks were integrated into a 2D COF scaffold, resulting in highly crystalline structures and similar morphologies across all COFs. Remarkably, this structural uniformity was also achieved in the synthesis of homogeneous and oriented thin films. Steady‐state photoluminescence revealed a tunable red‐shift in film emission exceeding 100 nm, demonstrating effective manipulation of their optical properties. Furthermore, photoelectrochemical (PEC) water splitting studies exhibited a doubled current density (8.1 μA cm−2 at 0.2 VRHE) for the COF with the strongest acceptor unit. These findings highlight the potential of Wurster D‐A‐D COFs in photoelectrochemical water splitting devices and pave the way for further exploration of chemical functionality‐reactivity‐property relationships in this promising class of photoactive materials.

Assessing the potential for upscaling the continuous production of lignin oils through reductive catalytic depolymerization

Lignin represents up to one third of lignocellulosic biomass and is a sustainable carbon source available at a sufficient scale to replace an impactful amount of fossil resources when converted into materials and chemicals. The molecular structure of lignin renders it ideal to obtain renewable aromatic building blocks for the polymer industry. Nevertheless, in polymeric form, lignin poses serious challenges for material applications and chemical conversion, namely poor solubility, and low reactivity. This problem can be circumvented by depolymerization of isolated lignins. To meet the demand for renewable aromatic building blocks, it is crucial to utilize technical lignin streams generated on large scale. This will eventually allow the development of continuous, high-throughput processes which are required for viable industrial-scale operations. Our work demonstrates for the first time that “technical grade” hydrolysis lignin from an operating biorefinery can be successfully subjected to continuous reductive catalytic depolymerization (RCD) in a packed bed reactor using a commercially available Pd/Al2O3 catalyst. The impact of catalyst amount, feed concentration (in methanol), and temperature with regards to process stability and product quality was investigated. The product was characterized by NMR, GPC, and GC-FID. Feed concentrations up to 7 wt% could be continuously processed for 77 h at 200 °C whereas at 225 °C, the operation time was restricted to 21 h. In all cases, a stable output was obtained over time in terms of molar mass, OH-group distributions, and monomer content. At 200 °C, carbon yields above 90 % of the soluble lignin fraction were feasible.

Harnessing in vivo synthesis of bioactive multiarylmethanes in Escherichia coli via oxygen-mediated free radical reaction induced by simple phenols

BackgroundXanthenes and multi-aryl carbon core containing compounds represent different types of complex and condensed architectures that have impressive wide range of pharmacological, industrial and synthetic applications. Moreover, indoles as building blocks were only found in naturally occurring metabolites with di-aryl carbon cores and in chemically synthesized tri-aryl carbon core containing compounds. Up to date, rare xanthenes with indole bearing multicaryl carbon core have been reported in natural or synthetic products. The underlying mechanism of fluorescein-like arthrocolins with tetra-arylmethyl core were synthesized in an engineered Escherichia coli fed with toluquinol remained unclear.ResultsIn this study, the Keio collection of single gene knockout strains of 3901 mutants of E. coli BW25113, together with 14 distinct E. coli strains, was applied to explore the origins of endogenous building blocks and the biogenesis for arthrocolin assemblage. Deficiency in bacterial respiratory and aromatic compound degradation genes ubiX, cydB, sucA and ssuE inhibited the mutant growth fed with toluquinol. Metabolomics of the cultures of 3897 mutants revealed that only disruption of tnaA involving in transforming tryptophan to indole, resulted in absence of arthrocolins. Further media optimization, thermal cell killing and cell free analysis indicated that a non-enzyme reaction was involved in the arthrocolin biosynthesis in E. coli. Evaluation of redox potentials and free radicals suggested that an oxygen-mediated free radical reaction was responsible for arthrocolins formation in E. coli. Regulation of oxygen combined with distinct phenol derivatives as inducer, 31 arylmethyl core containing metabolites including 13 new and 8 biological active, were isolated and characterized. Among them, novel arthrocolins with p-hydroxylbenzene ring from tyrosine were achieved through large scale of aerobic fermentation and elucidated x-ray diffraction analysis. Moreover, most of the known compounds in this study were for the first time synthesized in a microbe instead of chemical synthesis. Through feeding the rat with toluquinol after colonizing the intestines of rat with E. coli, arthrocolins also appeared in the rat blood.ConclusionOur findings provide a mechanistic insight into in vivo synthesis of complex and condensed arthrocolins induced by simple phenols and exploits a quinol based method to generate endogenous aromatic building blocks, as well as a methylidene unit, for the bacteria-facilitated synthesis of multiarylmethanes.

Benign and Selective Amination of Lignins towards Aromatic Biobased Building Blocks with Primary Amines.

Lignin is a widely available second-generation biopolymer and the main potential source of renewable aromatic building blocks. Lignin-based polyamines offer great potential in applications based on chemical and materials sciences. However, common aminations techniques for lignin usually involve toxic chemicals and generate hindered and low reactivity amines. In this study, we developed two new, simple, and benign 2-step methodologies for the elaboration of lignin-based polyamines from different technical lignins (kraft, soda and organosolv) with a selectivity towards reactive primary amines. These methods involve grafting amide groups onto lignin followed by a hydrolysis step. Non-toxic heterocyclic compounds N-acetyl-2-oxazolidinone and 2-methyl-2-oxazoline were used as amidation agents. Hydrolysis was performed in acetone-water mixtures. Reactions were studied on model compounds and optimized on lignins. Aminated lignins were fully characterized and primary amines were quantified using quantitative 19F NMR. Our methods generated aminated lignins with low apparent molar masses and high solubility in water and solvents. Nitrogen contents of the products ranged between 2.0 and 3.5 mmol/g with reactive primary amines counts up to 1.7 mmol/g. These soluble and reactive lignin-based polyamines offer great potential as a replacement for fossil-based polyamines in e.g., the synthesis of aromatic polymer materials or as potential chelating, antibacterial agents.

Benign and Selective Amination of Lignins towards Aromatic Biobased Building Blocks with Primary Amines

Lignin is a widely available second‐generation biopolymer and the main potential source of renewable aromatic building blocks. Lignin‐based polyamines offer great potential in applications based on chemical and materials sciences. However, common aminations techniques for lignin usually involve toxic chemicals and generate hindered and low reactivity amines. In this study, we developed two new, simple, and benign 2‐step methodologies for the elaboration of lignin‐based polyamines from different technical lignins (kraft, soda and organosolv) with a selectivity towards reactive primary amines. These methods involve grafting amide groups onto lignin followed by a hydrolysis step. Non‐toxic heterocyclic compounds N‐acetyl‐2‐oxazolidinone and 2‐methyl‐2‐oxazoline were used as amidation agents. Hydrolysis was performed in acetone‐water mixtures. Reactions were studied on model compounds and optimized on lignins. Aminated lignins were fully characterized and primary amines were quantified using quantitative 19F NMR. Our methods generated aminated lignins with low apparent molar masses and high solubility in water and solvents. Nitrogen contents of the products ranged between 2.0 and 3.5 mmol/g with reactive primary amines counts up to 1.7 mmol/g. These soluble and reactive lignin‐based polyamines offer great potential as a replacement for fossil‐based polyamines in e.g., the synthesis of aromatic polymer materials or as potential chelating, antibacterial agents.

One-Pot Synthesis of Diverse Anion-Binding Macrocycles

AbstractA one-pot synthetic protocol to access a diverse library of diamide–diester macrocycles from the same starting materials is reported. The molecular symmetry can be readily tuned based on the reaction sequence, while the core structure can be varied using amino acids and aromatic building blocks. The first class of macrocycles with C2 axes in their molecular plane were obtained in 24 h with 40–70% yield, while another class with C2 axes perpendicular to the plane were synthesized in 18 h in 10–30% yield. The phenyl- and serine-derived macrocycles of the first class could bind acetate, chloride, and phosphate ions. These macrocycles can be functionalized with hydrophobic groups and potentially be used as ion transporters.

Modular Synthesis of Conjugated Aromatic Boronate Esters by Radical Xanthate Addition

AbstractConjugated aromatic boronate esters serve as crucial intermediates in the synthesis of pharmaceuticals and polymer materials. Traditional methods for their synthesis typically involve organoboration of aromatic ring compounds, where pre-constructions of aromatic rings are required. Here, we present a general strategy for the synthesis of diverse conjugated aromatic boronate esters based on the radical addition of xanthates. Through this method, we synthesized various boronate ester xanthates that could be utilized as a platform to furnish conjugated aromatic building blocks, including thiophenes, pyrroles, tetralones, naphthols, and naphthylamines. This cost-effective strategy holds promise as a viable method for the industrial-scale production of (hetero)aromatic boronate esters.

Three-dimensional covalent organic frameworks with nia nets for efficient separation of benzene/cyclohexane mixtures

The synthesis of three-dimensional covalent organic frameworks with highly connected building blocks presents a significant challenge. In this study, we report two 3D COFs with the nia topology, named JUC-641 and JUC-642, by introducing planar hexagonal and triangular prism nodes. Notably, our adsorption studies and breakthrough experiments reveal that both COFs exhibit exceptional separation capabilities, surpassing previously reported 3D COFs and most porous organic polymers, with a separation factor of up to 2.02 for benzene and cyclohexane. Additionally, dispersion-corrected density functional theory analysis suggests that the good performance of these 3D COFs can be attributed to the incorporation of highly aromatic building blocks and the presence of extensive pore structures. Consequently, this research not only expands the diversity of COFs but also highlights the potential of functional COF materials as promising candidates for environmentally-friendly separation applications.

Covalent organic polyrotaxanes based on β-cyclodextrin for iodine capture.

Herein, covalent organic polyrotaxanes (COPRs) were integrated with supermolecule self-assembly and dynamic imine bond formation to act as absorbents that captured radioactive iodine from water. The aromatic building blocks were initially complexed with β-cyclodextrin (β-CD) to form pseudorotaxanes, which were then condensed with aromatic tri-aldehyde via mechanical grinding and solvothermal synthesis in sequence. The threading of β-CD throughout the polymer skeleton effectively reduced the usage of expensive building blocks and significantly lowered the cost, while also remarkably enhancing the skeleton polarity, which is closely related to many special applications. Impressively, the threading of CD improved the water dispersibility of COPRs, which displayed an abnormally high iodine adsorption capacity. This novel synthetic strategy allows the incorporation of mechanically interlocked CDs into porous polymeric materials, which provides access to low-cost preparations of COPRs with a brand new structure for specific applications.

Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of spherical aromatic building blocks. Persistence of aromaticity upon cluster gathering.

Formation of cluster-based materials requires a fundamental understanding of the resulting cluster aggregation processes. The Sn94- Zintl-ion structure can be viewed as a building block featuring a spherical aromatic species, leading to a cluster gathering upon oxidative coupling and/or mediated by transition metals. Here, we evaluate the spherical aromatic properties of [Sn9-Sn9]6-, [Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of two Sn9 building units held together via oxidative coupling and mediated by a Ag(I) transition metal center. Our results from magnetic criteria of aromaticity show that the inherent spherical aromatic characteristics of the parent Sn94- cluster are persistent in the overall aggregate where the enabled shielding cones ascribed to each Sn9 unit are able to interplay between them, leading to an overlap of the shielding regions. Hence, the two approaches for bringing cluster units together are able to retain the inherent spherical aromatic features for each Sn9 unit, leading to a cluster-based dimer where the parent properties remain. Thus, further cluster-based materials can be envisaged from aggregation upon oxidative coupling and/or mediated by transition metals, where the constituent building blocks retain their initial features, useful to guide the formation of more complex cluster-based aggregates.

Development of Porous Organic Polymers as Metal‐Free Photocatalysts for the Aromatization of N‐Heterocycles

AbstractPorous organic polymers (POPs), and especially covalent triazine frameworks (CTFs), are being developed as the next generation of metal‐free heterogeneous photocatalysts. However, many of the current synthetic routes to obtain these photoactive POPs require expensive monomers and rely on precious metal catalysts, thus hindering their widespread implementation. In this work, a range of POPs was synthesized from simple unfunctionalized aromatic building blocks, through Lewis acid‐catalyzed polymerization. The obtained materials were applied, for the first time, as heterogeneous photocatalysts for the aromatization of N‐heterocycles. With the use of the most active material, denoted as CTF‐Pyr, which consists of photoactive pyrene and triazine moieties, a wide range of pyridines, dihydroquinoline‐5‐ones, tetrahydroacridine‐1,8‐diones and pyrazoles were obtained in excellent yields (70–99 %). Moreover, these reactions were carried out under very mild conditions using air and at room temperature, highlighting the potential of these materials as catalysts for green transformations.

Synthesis of electron-rich benzbisimidazole-containing amorphous poly(Schiff-base) networks for efficient iodine capture

In this work, two benzbisimidazole-containing amorphous poly(Schiff-base) networks (namely ATA-BA and ATA-BZ) are synthesized by reacting benzbisimidazole-containing aromatic tetra-aldehyde building block (ATA) with benzene-1,4-diamine and benzidine, respectively, under solvothermal conditions. The resultant ATA-BA and ATA-BZ networks possess the specific surface areas of 113 and 87 m2/g, respectively. Owing to the presence of abundant imine bond, alkyl-substituted imidazole ring, and extended π-conjugated system, ATA-BA network can effectively capture iodine in both vapor and solution phases with the capture capacities up to 5.29 g g−1 and 2343 mg g−1, respectively, although its specific surface area is not high. The exceptional iodine capture performance of ATA-BA network can be attributed to the strong charge-transfer interactions between imine bond/imidazole ring/π-conjugated system and iodine molecules, as demonstrated by spectroscopic analysis and density functional theory (DFT) calculations. In addition, the iodine capture by ATA-BA network in cyclohexane solution is systematically investigated. The experimental results indicate that the iodine adsorption kinetics and capture behavior obey pseudo-second-order model and Freundlich isotherm model, respectively.

Gene amplifications cause high-level resistance against albicidin in gram-negative bacteria.

Antibiotic resistance is a continuously increasing concern for public healthcare. Understanding resistance mechanisms and their emergence is crucial for the development of new antibiotics and their effective use. The peptide antibiotic albicidin is such a promising candidate that, as a gyrase poison, shows bactericidal activity against a wide range of gram-positive and gram-negative bacteria. Here, we report the discovery of a gene amplification-based mechanism that imparts an up to 1000-fold increase in resistance levels against albicidin. RNA sequencing and proteomics data show that this novel mechanism protects Salmonella Typhimurium and Escherichia coli by increasing the copy number of STM3175 (YgiV), a transcription regulator with a GyrI-like small molecule binding domain that traps albicidin with high affinity. X-ray crystallography and molecular docking reveal a new conserved motif in the binding groove of the GyrI-like domain that can interact with aromatic building blocks of albicidin. Phylogenetic studies suggest that this resistance mechanism is ubiquitous in gram-negative bacteria, and our experiments confirm that STM3175 homologs can confer resistance in pathogens such as Vibrio vulnificus and Pseudomonas aeruginosa.

Skeletal Transformation of Unactivated Arenes Enabled by a Low-Temperature Dearomative (3 + 2) Cycloaddition.

Simple aromatic compounds like benzene are abundant feedstocks, for which the preparation of derivatives chiefly begins with electrophilic substitution reactions or, less frequently, reductions. Their high stability makes them particularly reluctant to participate in cycloadditions under ordinary reaction conditions. Here, we demonstrate the exceptional ability of 1,3-diaza-2-azoniaallene cations to undergo formal (3 + 2) cycloadditions with unactivated benzene derivatives below room temperature, providing thermally stable dearomatized adducts on a multi-gram scale. The cycloaddition, which tolerates polar functional groups, activates the ring toward further elaboration. On treatment with dienophiles, the cycloadducts undergo a (4 + 2) cycloaddition-cycloreversion cascade to yield substituted or fused arenes, including naphthalene derivatives. The overall sequence results in the transmutation of arenes through an exchange of the ring carbons: a two-carbon fragment from the original aromatic ring is replaced with another from the incoming dienophile, introducing an unconventional disconnection for the synthesis of ubiquitous aromatic building blocks. Applications of this two-step sequence to the preparation of substituted acenes, isotopically labeled molecules, and medicinally relevant compounds are demonstrated.

The preparation of 3,4‐ethylenedioxy thiophene (EDOT)‐based conjugated microporous polymers by direct arylation polymerization and fluorescence‐sensing nitro‐aromatic compounds and iodine

AbstractThe 3,4‐ethylenedioxy thiophene (EDOT)‐based conjugated microporous polymers (CMPs) were readily constructed via facile one‐step direct arylation polymerication of CH from simple EDOT as a linker and commercially available aryl bromides of different geometries as the knots, without preactivation of CH bonds of aromatic building blocks using organometallic reagents. In contrast to conventional coupling reactions, this synthesis strategy not only has atomic efficiency but also can use simple thiophene derivatives as building blocks for synthesizing thiophene‐based CMPs of different structures, which avoids the synthesis of complex thiophene derivatives and are important for applications, for example, fluorescent sensors. The resulting EDOT‐based CMPs were used to fluorescent sensing to 2,4‐dinitrophenol (DNP), I2, and p‐nitrophenol (p‐NP), with high optimal Stern‐Volmer quenching constants.

1,4-Dithianes: attractive C2-building blocks for the synthesis of complex molecular architectures.

This review covers the synthetic applications of 1,4-dithianes, as well as derivatives thereof at various oxidation states. The selected examples show how the specific heterocyclic reactivity can be harnessed for the controlled synthesis of carbon-carbon bonds. The reactivity is compared to and put into context with more common synthetic building blocks, such as 1,3-dithianes and (hetero)aromatic building blocks. 1,4-Dithianes have as yet not been investigated to the same extent as their well-known 1,3-dithiane counterparts, but they do offer attractive transformations that can find good use in the assembly of a wide array of complex molecular architectures, ranging from lipids and carbohydrates to various carbocyclic scaffolds. This versatility arises from the possibility to chemoselectively cleave or reduce the sulfur-heterocycle to reveal a versatile C2-synthon.

Solvent Fractionation and Depolymerization Provide Liquid Lignin Fractions Exploited as Bio-based Aromatic Building Blocks in Epoxies

In the context of a green transition, alternatives to fossil-based aromatic compounds have to be implemented. In this study, the combination of sequential solvent fractionation followed by depolymerization via catalytic hydrogenolysis of lignin led to liquid oligomeric lignin structures with chemical properties governed by the choice of lignin soluble fraction. Lignin species of increasing molecular weight (MW) were obtained with MW reduction up to 72% compared to the parent lignin, with a corresponding increase in total hydroxyl content up to 18% obtained through cleavage of lignin ether bonds. Hydrogenolysis led to lignin depolymerization oils comprising macromolecular lignin fragments and app. 15–18 wt % lignin monomers, resulting in a liquid product mixture. The lignin species were epoxidized to show their potential as a bio-based aromatic substitute to fossil-based bisphenol A. Liquid epoxy resins were obtained with viscosities between 2 × 103 and 4 × 105 Pa·s, which after curing resulted in thermoset networks with Young’s moduli between 0.9 and 1.4 GPa depending on the lignin fraction. Optimal viscosity to mechanical performance was obtained for ethyl acetate and ethanol solvent fractionated and depolymerized lignins as lower viscosity allows for reduced use of volatile organics.

Bioderived furanic compounds as replacements for BTX in chemical intermediate applications

Aromatic building blocks are essential chemicals to deliver high performances for a defined application. The implementation of biobased alternatives is a crucial factor for the reduction of carbon emissions and sustainability.

Uncanonical Semireduction of Quinolines and Isoquinolines via Regioselective HAT-Promoted Hydrosilylation.

Heterocycles are the backbone of modern medical chemistry and drug development. The derivatization of "an olefin" inside aromatic rings represents an ideal approach to access functionalized saturated heterocycles from abundant aromatic building blocks. Here, we report an operationally simple, efficient, and practical method to selectively access hydrosilylated and reduced N-heterocycles from bicyclic aromatics via a key diradical intermediate. This approach is expected to facilitate complex heterocycle functionalizations that enable access to novel medicinally relevant scaffolds.