Pyridine Precursor Research Articles

Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) are known as a perspective material for a variety of applications in nanoelectronic devices, sensors, catalysts for carbon dioxide reduction, and flexible thermoelectrics. However, up to date most of the reports on the properties of N-MWCNTs are focused on a narrow niche of research, for example, a study of low-temperature magnetoresistance or room-temperature thermoelectrical properties. In this work, N-MWCNTs were synthesized using benzene:pyridine precursor in different ratios, and both magnetoresistance and thermoelectrical properties of the synthesized N-MWCNTs were systematically investigated in the temperature range 2-300 K and compared with the properties of undoped MWCNTs. Unexpected switching of the magnetoresistance of the N-MWCNTs at low temperatures from negative to positive values was observed, and the processes underlying this effect are discussed. The study of the thermoelectrical properties revealed n-type conductance in the N-MWCNTs, which was attributed to the impact of nitrogen defects incorporated in the MWCNT structure. Performed for the first-time investigations of the thermal stability of the Seebeck coefficient of N-MWCNTs in air revealed that the Seebeck coefficient retains its negative values and even increases after annealing of the N-MWCNTs in air at 500 °C. These findings illustrate the high potential of the presented in this work N-MWCNTs for applications in different devices in a wide range of temperatures.

One-Carbon Insertion and Polarity Inversion Enabled a Pyrrole Strategy to the Total Syntheses of Pyridine-Containing Lycopodium Alkaloids: Complanadine A and Lycodine.

Complanadine A and lycodine are representative members of the Lycopodium alkaloids with a characteristic pyridine-containing tetracyclic skeleton. Complanadine A has demonstrated promising neurotrophic activity and potential for persistent pain management. Herein we report a pyrrole strategy enabled by one-carbon insertion and polarity inversion for concise total syntheses of complanadine A and lycodine. The use of a pyrrole as the pyridine precursor allowed the rapid construction of their tetracyclic skeleton via a one-pot Staudinger reduction, amine-ketone condensation, and Mannich-type cyclization. The pyrrole group was then converted to the desired pyridine by the Ciamician-Dennstedt rearrangement via a one-carbon insertion process, which also simultaneously introduced a chloride at C3 for the next C-H arylation. Other key steps include a direct anti-Markovnikov hydroazidation, a Mukaiyama-Michael addition, and a Paal-Knorr pyrrole synthesis. Lycodine and complanadine A were prepared in 8 and 11 steps, respectively, from a readily available known compound.

Facile Synthesis and Characterization of a Bromine-Substituted (Chloromethyl)Pyridine Precursor towards the Immobilization of Biomimetic Metal Ion Chelates on Functionalized Carbons

Multidentate ligands involving tethered pyridyl groups coordinated to transition metal ions have been frequently used to mimic the 3-histidine (3H), 2-histidine-1-carboxylate (2H1C) brace motifs or other combinations of histidine and carboxylate endogenous ligating residues found in bioinorganic metalloenzymes. It is of interest to immobilize these ligand chelates onto heterogeneous supports. This, however, requires the use of bromine-substituted (chloromethyl)pyridines, whose current synthetic routes involve the use of extremely pyrophoric chemicals, such as n-butyllithium that require cryogenic reaction conditions, and toxic chemicals, such as thionyl chloride, that are challenging to handle and require extensive hazard controls. Herein, we report alternative methodologies towards the syntheses of 2-bromo-6-hydroxymethylpyridine and 2-bromo-6-chloromethylpyridine from inexpensive commercially available 2,6-dibromopyridine using isopropylmagnesium chloride lithium chloride complex (Turbo Grignard) and cyanuric chloride which are easier to handle and require milder reaction conditions than the conventional reagents. Gas chromatography-mass spectrometry (GC-MS) methods were developed and simple 1H- and 13C- nuclear magnetic resonance (NMR) and Fourier-transform infrared (FT-IR) spectroscopies were also used to monitor the conversion of both reaction steps and showed that products could be obtained and isolated through simple workups without the presence of unreacted starting material or undesired overchlorinated 2-chloro-6-chloromethylpyridine side product.

Green Fabrication of Silkworm Cocoon-like Silicon-Based Composite for High-Performance Li-Ion Batteries.

Designing yolk-shell nanostructures is an effective way of addressing the huge volume expansion issue for large-capacity anode and cathode materials in Li-ion batteries (LIBs). Previous studies mainly focused on adopting a SiO2 template through HF etching to create yolk-shell nanostructures. However, HF etching is highly corrosive and may result in a significant reduction of Si content in the composite. Herein, a silkworm cocoon-like silicon-based composite is prepared through a green approach in which Al2O3 was selected as a sacrificial template. The void space between the outer nitrogen-doped carbon (NC) shell formed by chemical vapor deposition using a pyridine precursor and the inside porous silicon nanorods (p-Si NRs) synthesized by magnesiothermic reduction of ordered mesoporous silica nanorods can be generated by etching Al2O3 with diluted HCl. The obtained p-Si NRs@void@NC composite is utilized as an anode material for LIBs, which exhibits a large initial discharge capacity of 3161 mAh g-1 at 0.5 A g-1, excellent cycling behavior up to 300 cycles, and super rate performance. Furthermore, a deep understanding of the mechanism for the yolk-shell nanostructure during the Li-alloying process is revealed by in situ transmission electron microscopy and finite element simulation.

Expanded Rosarin: A Versatile Fullerene (C60) Receptor.

An expanded rosarian (P3P6) with a bowl-like conformation has been prepared and characterized in a one-pot procedure that involves condensing a bispyrrole pyridine precursor (P1P2) with benzaldehyde, followed by oxidation. Single crystal X-ray diffraction analysis reveals a bowl-like conformation in the solid state with an upper rim diameter defined by the meso-phenyl substituents of ca. 13.5 Å and a depth of roughly 6.3 Å. P3P6 forms both 1:1 and 2:1 complexes with C60 in the solid state. DFT reveals similar energies for the two binding modes. A 1:1 binding stoichiometry dominates in 1,2-dichlorobenzene-d4 at the millimolar concentrations dictated by solubility consideration. The solution phase interactions between rosarian and C60 were studied using 1H NMR, UV-vis, and femtosecond transient absorption spectroscopies in 1,2-dichlorobenzene-d4 or toluene. To our knowledge, this is the first report of an unfunctionalized porphyrinoid that forms a well-defined complex with C60 in solution as well as in solid state.

Metal-Free Synthesis of C-4 Substituted Pyridine Derivatives Using Pyridine-boryl Radicals via a Radical Addition/Coupling Mechanism: A Combined Computational and Experimental Study.

Density functional theory investigations revealed that the pyridine-boryl radical generated in situ using 4-cyanopyridine and bis(pinacolato)diboron could be used as a bifunctional "reagent", which serves as not only a pyridine precursor but also a boryl radical. With the unique reactivity of such radicals, 4-substituted pyridine derivatives could be synthesized using α,β-unsaturated ketones and 4-cyanopyridine via a novel radical addition/C-C coupling mechanism. Several controlled experiments were conducted to provide supportive evidence for the proposed mechanism. In addition to enones, the scope could be extended to a wide range of boryl radical acceptors, including various aldehydes and ketones, aryl imines and alkynones. Lastly, this transformation was applied in the late-stage modification of a complicated pharmaceutical molecule.

From Carbon-Based Nanotubes to Nanocages for Advanced Energy Conversion and Storage.

Carbon-based nanomaterials have been the focus of research interests in the past 30 years due to their abundant microstructures and morphologies, excellent properties, and wide potential applications, as landmarked by 0D fullerene, 1D nanotubes, and 2D graphene. With the availability of high specific surface area (SSA), well-balanced pore distribution, high conductivity, and tunable wettability, carbon-based nanomaterials are highly expected as advanced materials for energy conversion and storage to meet the increasing demands for clean and renewable energies. In this context, attention is usually attracted by the star material of graphene in recent years. In this Account, we overview our studies on carbon-based nanotubes to nanocages for energy conversion and storage, including their synthesis, performances, and related mechanisms. The two carbon nanostructures have the common features of interior cavity, high conductivity, and easy doping but much different SSAs and pore distributions, leading to different performances. We demonstrated a six-membered-ring-based growth mechanism of carbon nanotubes (CNTs) with benzene precursor based on the structural similarity of the benzene ring to the building unit of CNTs. By this mechanism, nitrogen-doped CNTs (NCNTs) with homogeneous N distribution and predominant pyridinic N were obtained with pyridine precursor, providing a new kind of support for convenient surface functionalization via N-participation. Accordingly, various transition-metal nanoparticles were directly immobilized onto NCNTs without premodification. The so-constructed catalysts featured high dispersion, narrow size distribution and tunable composition, which presented superior catalytic performances for energy conversions, for example, the oxygen reduction reaction (ORR) and methanol oxidation in fuel cells. With the advent of the new field of carbon-based metal-free electrocatalysts, we first extended ORR catalysts from the electron-rich N-doped to the electron-deficient B-doped sp2 carbon. The combined experimental and theoretical study indicated the ORR activity originated from the activation of carbon π electrons by breaking the integrity of π conjugation, despite the electron-rich or electron-deficient nature of the dopants. With this understanding, metal-free electrocatalysts were further extended to the dopant-free defective carbon nanomaterials. Moreover, we developed novel 3D hierarchical carbon-based nanocages by the in situ MgO template method, which featured coexisting micro-meso-macropores and much larger SSA than the nanotubes. The unique 3D architecture avoids the restacking generally faced by 2D graphene due to the intrinsic π-π interaction. Consequently, the hierarchical nanocages presented superior performances not only as new catalyst supports and metal-free electrocatalysts but also as electrode materials for energy storage. State-of-the-art supercapacitive performances were achieved with high energy density and power density, as well as excellent rate capability and cycling stability. The large interior space of the nanocages enabled the encapsulation of high-loading sulfur to alleviate polysulfide dissolution while greatly enhancing the electron conduction and Li-ion diffusion, leading to top level performance of lithium-sulfur battery. These results not only provide unique carbon-based nanomaterials but also lead to in-depth understanding of growth mechanisms, material design, and structure-performance relationships, which is significant to promote their energy applications and also to enrich the exciting field of carbon-based nanomaterials.

Synthesis of Continuous Graphene Film Using Liquid Pyridine Precursor

Synthesis of Continuous Graphene Film Using Liquid Pyridine Precursor

Targeting Metal-Aβ Aggregates with Bifunctional Radioligand [11C]L2-b and a Fluorine-18 Analogue [18F]FL2-b

Interest in quantifying metal-Aβ species in vivo led to the synthesis and evaluation of [11C]L2-b and [18F]FL2-b as radiopharmaceuticals for studying the metallobiology of Alzheimer’s disease (AD) using positron emission tomography (PET) imaging. [11C]L2-b was synthesized in 3.6% radiochemical yield (nondecay corrected, n = 3), >95% radiochemical purity, from the corresponding desmethyl precursor. [18F]FL2-b was synthesized in 1.0% radiochemical yield (nondecay corrected, n = 3), >99% radiochemical purity, from a 6-chloro pyridine precursor. Autoradiography experiments with AD positive and healthy control brain samples were used to determine the specificity of binding for the radioligands compared to [11C]PiB, a known imaging agent for β-amyloid (Aβ) aggregates. The Kd for [11C]L2-b and [18F]FL2-b were found to be 3.5 and 9.4 nM, respectively, from those tissue studies. Displacement studies of [11C]L2-b and [18F]FL2-b with PiB and AV-45 determined that L2-b binds to Aβ aggregates differently from known radiopharmaceuticals. Finally, brain uptake of [11C]L2-b was examined through microPET imaging in healthy rhesus macaque, which revealed a maximum uptake at 2.5 min (peak SUV = 2.0) followed by rapid egress (n = 2).

Quinolinate Salvage and Insights for Targeting NAD Biosynthesis in Group A Streptococci

The essential coenzyme NAD plays important roles in metabolic reactions and cell regulation in all organisms. As such, NAD synthesis has been investigated as a source for novel antibacterial targets. Cross-species genomics-based reconstructions of NAD metabolism in group A streptococci (GAS), combined with focused experimental testing in Streptococcus pyogenes, led to a better understanding of NAD metabolism in the pathogen. The predicted niacin auxotrophy was experimentally verified, as well as the essential role of the nicotinamidase PncA in the utilization of nicotinamide (Nm). PncA is dispensable in the presence of nicotinate (Na), ruling it out as a viable antibacterial target. The function of the "orphan" NadC enzyme, which is uniquely present in all GAS species despite the absence of other genes of NAD de novo synthesis, was elucidated. Indeed, the quinolinate (Qa) phosphoribosyltransferase activity of NadC from S. pyogenes allows the organism to sustain growth when Qa is present as a sole pyridine precursor. Finally, the redundancy of functional upstream salvage pathways in GAS species narrows the choice of potential drug targets to the two indispensable downstream enzymes of NAD synthesis, nicotinate adenylyltransferase (NadD family) and NAD synthetase (NadE family). Biochemical characterization of NadD confirmed its functional role in S. pyogenes, and its potential as an antibacterial target was supported by inhibition studies with previously identified class I inhibitors of the NadD enzyme family. One of these inhibitors efficiently inhibited S. pyogenes NadD (sp.NadD) in vitro (50% inhibitory concentration [IC(50)], 15 μM), exhibiting a noncompetitive mechanism with a K(i) of 8 μM.

Theoretical investigation of formation mechanism of bipyridyl molecule on Ni(111) surface: implication for synthesis of N-doped graphene from pyridine

The formation mechanism of bipyridyl molecule catalyzed by nickel catalyst with pyridine precursor has been studied using density functional theory calculations. The formation of bipyridyl on Ni(111) surface from two pyridine molecules is considered as the initial process of N-doped graphene growth, and the minimum energy pathway for the formation has been investigated in detail. The whole formation processes mainly includes three steps, i.e., the dehydrogenation of the first pyridine, adsorption and dehydrogenation of the second pyridine, and formation of the bipyridyl molecule. It is found that the C-H bond of pyridine could be selectively dissociated while the C-C and C-N bond connections are retained during the catalytic processes. The N-doped graphene formed by pyridine only contains pyridine-like nitrogen atoms, suggesting a possible way to produce N-doped graphene with pure pyridine-like nitrogen atoms. The comparison of formation mechanisms between bipyridyl and biphenyl molecules was carried out, and the results imply a lower temperature process for synthesis of N-doped graphene from pyridine than that for graphene from benzene.

Uniformly Dispersed Pt−Ni Nanoparticles on Nitrogen-Doped Carbon Nanotubes for Hydrogen Sensing

Nitrogen-doped multiwall carbon nanotubes (NCNTs), having an average diameter of approximately 20 nm, were synthesized at 650 °C by chemical vapor deposition using a pyridine precursor. Pt−Ni alloyed nanoparticles with approximate diameter 3 nm and with different Pt to Ni molar ratios were deposited on the NCNTs by a microwave-polyol method. Electron microscopy revealed that the nanoparticles were deposited homogeneously on the outer surface of the NCNTs and were immobilized at active nitrogen sites. A dielectrophoresis technique was used to selectively align the Pt−Ni-coated NCNTs between metallic electrodes to form conductometric hydrogen gas sensors. Gas sensing measurements performed with different concentrations of hydrogen revealed that the sensor based upon Pt/NCNTs exhibited the fastest response and recovery and best sensitivity. The sensing mechanism in the Pt/NCNT sensors can be explained by a combination of responses from the nitrogen-induced defects and the supported Pt nanoparticles, with the latter providing significantly faster response and recovery.

(Dimethoxy‐ and Dihalopyridyl)boronic Acids and Highly Functionalized Heteroarylpyridines by Suzuki Cross‐Coupling Reactions

AbstractWe report the synthesis of (2,6‐dimethoxy‐3‐pyridyl)boronic acid (2), (2,3‐dimethoxy‐4‐pyridyl)boronic acid (4), (2,6‐difluoro‐3‐pyridyl)boronic acid (6), (2,6‐dichloro‐3‐pyridyl)boronic acid (8) and (2,3‐dichloro‐4‐pyridyl)boronic acid (10) by directed ortho‐metalation reactions on the corresponding disubstituted pyridine precursor, followed by the reaction with triisopropyl borate (TPB) or trimethyl borate. Thereactivity of the pyridylboronic acids with heteroaryl halides in Suzuki–Miyaura cross‐coupling reactions has been evaluated. New highly functionalized heteroarylpyridine derivatives have thereby been obtained in moderate to high yields. The reaction of 8 and 3‐amino‐2‐chloropyridine yielded the rare 5H‐pyrrolo[2,3‐b:4,5‐b′]dipyridine (i.e. 1,5‐diazacarbazole) ring system by sequential cross‐coupling and intramolecular cyclisation reactions. The X‐ray crystal structures are reported for the pyridylboronic acids 2, 4, 8 and 10. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Synergism of C5N Six-Membered Ring and Vapor−Liquid−Solid Growth of CNx Nanotubes with Pyridine Precursor

A series of bamboo-like CN(x)() nanotubes have been synthesized from pyridine precursor by chemical vapor deposition with bimetallic Fe-Co/gamma-Al(2)O(3) catalyst in the range of 550 approximately 950 degrees C. An unusual predomination of pyridinic nitrogen over graphitic nitrogen has been observed for the CN(x)() nanotubes with reaction temperature below 750 degrees C. The pyridinic nitrogen decreases and the graphitic nitrogen increases with rising reaction temperature. A synergism mechanism of C(5)N-six-membered-ring-based growth through surface diffusion and vapor-liquid-solid growth through bulk diffusion was accordingly deduced and schematically presented. This mechanism could not only explain our own experimental results, but also understand the CN(x)()-nanotube-related experimental phenomena in the literature, as well as be in accordance with the basic principle of diffusion kinetics. A promising route to the challenging topic for synthesizing regularly arranged C(5)N or high-N-content CN(x)() nanotubes has also been suggested.

Sol–gel synthesis of Pt/Al 2O 3 catalysts: Effect of Pt precursor and calcination procedure on Pt dispersion

Pt/Al 2O 3 catalysts are used in a wide variety of reactions. Tailoring the catalyst structure is important in order to efficiently and effectively utilize the noble metal. In this work, the effects of Pt precursor (Pt(NH 3) 4Cl 2, Pt(C 5H 5N) 4Cl 2, Pt(CH 3NH 2) 4Cl 2, or Pt(C 4H 9NH 2) 4Cl 2) and calcination procedure (heating rates of 2 °C/min or 10 °C/min in different atmospheres) have been investigated for 1.5 wt% Pt/Al 2O 3 catalysts prepared by sol–gel synthesis. After drying, calcination, and reduction, the Pt dispersion was measured by H 2 chemisorption. The catalyst structures were characterized using X-ray diffraction, N 2 adsorption, and transmission electron microscopy. The Pt precursors as well as the calcination procedures influenced the Pt dispersion. Higher dispersions were obtained using a lower heating rate (2 °C/min), an ammonia precursor, and a flowing gas stream (as opposed to static). Monitoring the emissions during calcination with time resolved mass spectrometry indicated that decomposition of the precursors could be achieved in helium and that subsequent treatment in oxygen was not required. Differential thermal analysis indicated that larger heat flows resulted at the higher heating rate (10 °C/min) and with the pyridine precursor, compared to the ammonia precursor. The larger heat flows may have caused more sintering and, thus, a lower dispersion. Toluene hydrogenation was used as a model reaction to demonstrate that the catalysts with higher dispersions had higher activities and better catalyst stability.

Total Synthesis of (+)‐Camptothecin.

A total synthesis of (+)-camptothecin is described. The approach is based on a room temperature LUMOdiene-controlled Diels–Alder cycloaddition of the electron deficient N-sulfonyl-1-aza-1,3-butadiene 6 with the electron rich dienophile 1,1,3,3-tetraethoxypropene (7) for assembly of a pyridine precursor to the D-ring pyridone. The 20(S) tertiary alcohol was installed through a Sharpless asymmetric dihydroxylation reaction on the methyl vinyl ether 12, using the 3,4,5-trimethoxyphenyl-derived pyrimidine DHQ dimer ligand 16. In a single reaction vessel, the C and E rings were closed using an acid-catalyzed deprotection of the benzylic ethers to afford the corresponding benzylic bromides (18) which underwent intramolecular nucleophilic displacement by the carboxylate and pyridone nitrogen to furnish (+)-camptothecin.

Total synthesis of (+)-camptothecin

A total synthesis of (+)-camptothecin is described. The approach is based on a room temperature LUMO diene-controlled Diels–Alder cycloaddition of the electron deficient N-sulfonyl-1-aza-1,3-butadiene 6 with the electron rich dienophile 1,1,3,3-tetraethoxypropene ( 7 ) for assembly of a pyridine precursor to the D-ring pyridone. The 20( S) tertiary alcohol was installed through a Sharpless asymmetric dihydroxylation reaction on the methyl vinyl ether 12 , using the 3,4,5-trimethoxyphenyl-derived pyrimidine DHQ dimer ligand 16 . In a single reaction vessel, the C and E rings were closed using an acid-catalyzed deprotection of the benzylic ethers to afford the corresponding benzylic bromides ( 18 ) which underwent intramolecular nucleophilic displacement by the carboxylate and pyridone nitrogen to furnish (+)-camptothecin.

The synthesis of 4‐benzylamino‐6‐methyl‐1H‐pyrrolo[3, 2‐c]pyridine and 4‐benzylamino‐6‐methyl‐1H‐pyrrolo[2, 3‐b]pyridine

Abstract4‐Benzylamino‐6‐methyl‐1H‐pyrrolo[3,2‐c]pyridine (2) and 4‐benzylamino‐6‐methyl‐1H‐pyrrolo[2,3‐b]pyridine (3) were synthesized as deaza analogues of the anxiolytic agent 4‐benzylamino‐2‐methyl‐7H‐pyrrolo[2,3‐d]pyrimidine (1). The 1‐deaza analogue (2) was prepared via a multi‐step procedure from a pyrrole precursor, 1‐benzyl‐2‐formylpyrrole (4) while the 3‐deaza analogue 3 was synthesized from a pyridine precursor, 2‐amino‐3,6‐dimethylpyridine (12).