Electron-rich Aromatic Rings Research Articles

Preparation and evaluation of surface-bonded phenylglycine zwitterionic stationary phase.

4-Hydroxy-D-phenylglycine was modified with methacrylic anhydride and then immobilized on silica through thiol-initiated surface polymerization; the prepared material was applied as stationary phase for HPLC. The obtained stationary phase was characterized by elemental analysis, infrared spectroscopy, and thermogravimetric analysis. The chromatographic performance of the packed column was evaluated in reversed-phase liquid chromatograph (RPLC) and hydrophilic interaction liquid chromatograph (HILIC) mode; this column has shown excellent selectivity to both the hydrophobic and hydrophilic solutes. The selectivity towards polycyclic aromatic hydrocarbons relative to that towards alkylbenzenes exhibited by the prepared column was higher than the corresponding selectivity exhibited by commercial C18 column, which could be explained by electronic π-π interaction between phenylglycine and electron-rich aromatic rings. On the other hand, the prepared column has also shown better selectivity for polar compounds, which was based on the multiple interaction and retention mechanisms. It was also used to separate sulfonamides and organic acid compared with a commercial C18 and HILIC column; the results show its great chromatographic performance with distinctive selectivity. All the results indicated the prepared column had potential application in a wide range.

Exploiting arene-perfluoroarene interactions for dispersion of carbon black in rubber compounds

Coupling agents are intended to promote filler dispersion by providing a bridge between the filler and the rubber phase. This study investigated the ability of a novel physical coupling agent, poly(butadiene-graft-pentafluorostyrene) in a mixture with polypentafluorostyrene, to improve rubber-filler interactions and suppress filler-filler networking in carbon-black-reinforced styrene-butadiene rubber (SBR), and thereby decrease hysteresis. The electron-rich aromatic rings of carbon black are involved in arene-perfluoroarene interactions with the electron-poor pentafluorostyrene aromatic rings of the coupling agent. The SBR chains in the rubber compound have an affinity for the polybutadiene backbone of the coupling agent. The interactions between carbon black and the coupling agent were analyzed using Raman spectroscopy, transmission electron microscopy, zeta potential measurements, surface area measurements, and scanning electron microscopy. Filler flocculation analysis showed that the coupling agent improves the dispersion and lowers the energy of dissipation. The hysteresis loss, quantified in terms of loss tangent values at 60 °C, was reduced by up to 12% due to the coupling agent's promotion of better filler-rubber interactions. The influence of the PPFS graft length was also studied.

Palladium-Catalyzed Csp3–H Bond mono-Aroyloxylation of O-Alkyl Substituted 2,4,6-Trimethoxybenzaldoxime Ethers

A palladium-catalyzed Csp3–H bond mono-aroyloxylation of O-alkyl substituted oxime ethers has been developed by using 2,4,6-trimethoxybenzaldoxime as an exo-type directing group with exclusive site-selectivity. With the wide range of masked aliphatic alcohol substrates and aromatic acid coupling partners, the protocol allows rapid access to various 2-alkyl substituted glycol derivatives in synthetically useful to good yields. The employed directing group is readily removed, accordingly affording valuable functionalized aliphatic alcohols. When the solvent from hybrid DCE/HFIP to CH3CN, non-directed oxidative cross-coupling is observed between the electron-rich aromatic ring of substrates and aromatic acid partners.

Selective reaction of benzyl alcohols with HI gas: Iodination, reduction, and indane ring formations

Reactions of benzyl alcohols with HI in solvent-free conditions were examined. Three types of reactions (iodination, reduction, and ring formation) occurred depending on the degree of crowding around the benzyl position and the benzylic stabilization of substrates. Results also showed that the ring formation to give indanes proceeded efficiently when HI was used, and that compounds with electron-rich aromatic rings gave indane derivatives in good yields.

Use of SF6 for the production of electrophilic 18F-fluorination reagents

Electrophilic 18F-fluorination is an important method for production of tracers for positron emission tomography. The most commonly employed 18F-fluorination reagents [18F]F2, [18F]Selectfluor bis(triflate) and [18F]NFSi allow for the synthetically simple and fast introduction of fluorine-18 into organic molecules, in particular those containing electron rich aromatic rings. For the time being the best method for the production of [18F]F2 gas, the most common electrophilic reagent, is a post-target production which applies a high voltage electrical discharge to promote the 19F/18F isotopic exchange between [18F]MeF and carrier F2 gas.The aim of this study was to develop a new method for the production of electrophilic 18F-fluorination reagents which would avoid use of toxic and very reactive carrier F2 gas. In this study the use of SF6 as an alternative to carrier F2 gas was tested. Experiments were carried out in two different matrix gases: Ne and Xe. The electrophilic 18F-fluorination reagent produced was used for the labelling of two model compounds (6-[18F]fluoro-L-DOPA and [18F]Selectfluor bis(triflate)) in order to prove its feasibility.This proof-of-concept study showed that SF6 in Xe can be used in place of F2 for the production of electrophilic [18F]F2 gas. When SF6 in Ne was used only trace amounts of [18F]F2 was observed.

Through-Space Activation Can Override Substituent Effects in Electrophilic Aromatic Substitution.

Electrophilic aromatic substitution (EAS) represents one of the most important classes of reactions in all of chemistry. One of the "iron laws" of EAS is that an electron-rich aromatic ring will react more rapidly than an electron-poor ring with suitable electrophiles. In this report, we present unique examples of electron-deficient arenes instead undergoing preferential substitution in intramolecular competition with more electron-rich rings. These results were made possible by exploiting the heretofore unknown propensity of a hydrogen-bonding OH-arene interaction to switch to the alternative HO-arene interaction in order to provide activation. In an extreme case, this through-space HO-arene activation is demonstrated to overcome the deactivating effect of a trifluoromethyl substituent, making an otherwise highly electron-deficient ring the site of exclusive reactivity in competition experiments. Additionally, the HO-arene activation promotes tetrabromination of an increasingly more electron-deficient arene before the unactivated "control" ring undergoes monobromination. It is our hope that these results will shed light on biological interactions as well as provide new strategies for the electrophilic substitution of aromatic rings.

Liquid and solid-state study of antioxidant quercetin donor and TCNE acceptor interaction: Focusing on solvent affect on the morphological properties

Quercetin (Que), a naturally occurring flavonoid, has attracted considerable interest due to its diverse biological properties. Structurally, Que possesses conjugated electron rich aromatic rings which make it a good electron donor. The present study investigated and compared the structural morphology and spectroscopic characteristics of the charge–transfer (CT) complex of the Que as a donor with TCNE as an acceptor in ethanol (EtOH), methanol (MeOH), and acetonitrile solvents at room temperature. The obtained CT product was characterized in both solid and liquid state using UV–visible, IR spectroscopy, and CHN elemental analyses. Infrared spectral bands were assigned and proved that the interaction between Que and TCNE take place via the electron rich aromatic rings of Que donor to the CN group of the TCNE acceptor. The morphological properties of these products have been visualized using SEM and TEM techniques. The outcome shows that the complexation mode and morphologic characteristics of the CT product are markedly affected by the solvent. The band gap of CT complex presented in this study has been estimated from the absorbance coefficient data as a function of wavelength using Tauc relation.

ChemInform Abstract: Selective Metal‐Free Deoxygenation of Unsymmetrical 1,2‐Dicarbonyl Compounds by Chlorotrimethylsilane and Sodium Iodide.

AbstractThe reaction generally proceeds with excellent α‐selectivity at the carbonyl group proximal to the more electron‐rich aromatic ring.

Direct Metal-Free Entry to Aminocyclobutenes or Aminocyclobutenols from Ynamides: Synthetic Applications.

The [2+2] cycloaddition of ynamides with the highly polarized reagent Tf2 C=CH2 has been developed to regioselectively afford bis(triflyl)aminocyclobutenes in the absence of catalyst under mild conditions. Incidentally, with the ynamides bearing electron-rich aromatic rings at the C-terminal, an interesting reactivity switch was observed; a cyclization/hydroxylation sequence yielded 2-amino-3-(triflyl)cyclobut-2-enols. Aminocyclobutene construction with addition of alcohols resulted in the formation of aminocyclobutenyl ethers through a cyclization/hydroalkoxylation process. Moreover, the utility of functionalized aminocyclobutenes as precursors for further elaboration was demonstrated with the preparation of α-amino-β,γ-unsaturated ketones and 3-(triflyl)buta-1,3-dien-2-amines through 4 π-electrocyclic ring opening.

Syntheses of methylated catechins and theaflavins using 2-nitrobenzenesulfonyl group to protect and deactivate phenol.

An efficient and versatile synthetic method for labile polyphenols was established using 2-nitrobenzenesulfonate (Ns) as a protecting group for phenol. This methodology provides regio- and stereoselective access to a range of methylated catechins, such as methylated epigallocatechin gallates, that are not readily available from natural sources. In addition, biomimetic synthesis of theaflavins from catechins was accomplished using Ns protection to minimize undesired side reactions of electron-rich aromatic rings during oxidation, enabling construction of the complex benzotropolone core in a single-step oxidative coupling reaction. Availability of these compounds will aid detailed structure-biological activity relationship studies of catechins.

Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron-Rich and -Deficient Pore Surface.

A porous covalent triazine framework (CTF) consisting of both an electron-deficient central triazine core and electron-rich aromatic building blocks is reported. Taking advantage of the dual nature of the pore surface, bimodal functionality has been achieved. The electron deficiency in the central core has been utilized to address one of the pertinent problems in chemical industries, namely separation of benzene from its cyclic saturated congener, that is, cyclohexane. Also, by virtue of the electron-rich aromatic rings with Lewis basic sites, aqueous-phase chemical sensing of a nitroaromatic compound of highly explosive nature (2,4,6-trinitrophenol; TNP) has been achieved. The present compound supersedes the performance of previously reported COFs in both the aspects. Notably, this reports the first example of pore-surface engineering leading to bimodal functionality in CTFs.

Secondary Structures in a Freeze-Dried Lignite Humic Acid Fraction Caused by Hydrogen-Bonding of Acidic Protons with Aromatic Rings.

A lignite humic acid (HA) was separated from inorganic and non-HA impurities (i.e., aluminosilicates, metals) and fractionated by a combination of dialysis and XAD-8 resin. Fractionation revealed a more homogeneous structure of lignite HA. New and more specific structural information on the main lignite HA fraction is obtained by solid-state nuclear magnetic resonance (NMR) spectroscopy. Quantitative (13)C multiple cross-polarization (multiCP) NMR indicated oxidized phenyl propane structures derived from lignin. MultiCP experiments, conducted on potassium HA salts titrated to pH 10 and pH 12, revealed shifts consistent with carboxylate and phenolate formation, but structural changes associated with enolate formation from aromatic beta keto acids were not detected. Two-dimensional (1)H-(13)C heteronuclear correlation (2D HETCOR) NMR indicated aryl-aliphatic ketones, aliphatic and aromatic carboxyl groups, phenol, and methoxy phenyl ethers. Acidic protons from carboxyl groups in both the lignite HA fraction and a synthetic HA-like polycondensate were found to be hydrogen-bonded with electron-rich aromatic rings. Our results coupled with published infrared spectra provide evidence for the preferential hydrogen bonding of acidic hydrogens with electron-rich aromatic rings rather than adjacent carbonyl groups. These hydrogen-bonding interactions likely result from stereochemical arrangements in primary structures and folding.

9-Anthracenecarboxamide fluorescent probes for selective discrimination of picric acid from mono- and di-nitrophenols in ethanol

A new class of ‘switch off’ anthracenecarboxamide-based sensing probes (1–3) for the detection of picric acid (PA) was designed and synthesized. Probe 1 exhibited the highest sensitivity and selectivity toward PA, enabling complete fluorescence quenching and exhibiting a high association constant (4.14×105M−1) with a 1:1 stoichiometric ratio of 1 to PA in EtOH. Probe 1 was able to detect PA through multiple synergic interactions such as hydrogen bonding, π–π interactions between the electron-rich aromatic rings of anthracene and the electron-deficient aromatic ring of PA that resulted in a favorable fluorescence resonance energy transfer from the latter to the former. The ability of probe 1 to sense PA was investigated by UV–visible, fluorescence, and 1H NMR spectroscopies, as well as DFT calculations.

Thiophene-based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes.

Molecule-metal junctions are inevitable for the realization of single-molecule electronics. In this study, we developed new tripodal anchors with electron-rich aromatic rings to achieve robust contact with gold electrodes, an effective hybridization of the π orbital with gold electrodes (π channel), and hole transport through π-channel hybridization. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements of the monolayers indicated that the thiophene-based tripodal molecule exhibits anchoring characteristics as expected. The electrical conductance of thiophene-anchored bistripodal molecules using the scanning tunneling microscope (STM)-based break junction technique confirmed the formation of molecular junctions. The Seebeck coefficient of this compound estimated from thermoelectric voltage measurements using a STM was determined to be a positive value, which indicates that the charge carriers are holes. On the contrary, the corresponding pyridine-anchored molecules showed electron transport. These results reveal the versatility of π-channel tripodal anchors for the control of charge-carrier type in single-molecule electronics.

Formylation of electron-rich aromatic rings mediated by dichloromethyl methyl ether and TiCl4: scope and limitations.

Here the aromatic formylation mediated by TiCl4 and dichloromethyl methyl ether previously described by our group has been explored for a wide range of aromatic rings, including phenols, methoxy- and methylbenzenes, as an excellent way to produce aromatic aldehydes. Here we determine that the regioselectivity of this process is highly promoted by the coordination between the atoms present in the aromatic moiety and those in the metal core.

Aromaticity and amyloid formation: Effect of π-electron distribution and aryl substituent geometry on the self-assembly of peptides derived from hIAPP22–29

A comprehensive investigation of peptides derived from the 22–29 region of human islet amyloid polypeptide (hIAPP) that contain phenylalanine analogs at position 23 with a variety of electron donating and withdrawing groups, along with heteroaromatic surrogates, has been employed to interrogate how π-electron distribution effects amyloid formation. Kinetic aggregation studies using turbidity measurements indicate that electron rich aromatic ring systems consistently abolish the amyloidogenic propensity of hIAPP22–29. Electron poor systems modulate the rate of aggregation. Raman and Fourier transform infrared spectroscopy confirm the parallel β-sheet secondary structure of aggregates derived from peptides containing electron poor phenylalanine analogs and provide direct evidence of ring stacking. Transmission electron microscopy confirms the presence of amyloid fibrils. The effect of aryl substituent geometry on peptide self-assembly reveals that the electronic nature of substituents and not their steric profile is responsible for failure of the electron donating group peptides to aggregate. Non-aggregating hIAPP22–29 peptides were found to inhibit the self-assembly of full-length hIAPP1–37. The most potent inhibitory peptides contain phenylalanine with the p-amino and p-formamido functionalities. These novel peptides may serve as leads for the development of future aggregation inhibitors. A potential mechanism for inhibition of amylin self-assembly by electron rich hIAPP22–29 peptides is proposed.

The effects of σ(C-Se)-π hyperconjugation in 1-methoxy- and 1-nitro-4-[(phenylselanyl)methyl]benzene.

The structures of 1-methoxy-4-[(phenylselanyl)methyl]benzene, C14H14OSe, (1), and 1-nitro-4-[(phenylselanyl)methyl]benzene, C13H11NO2Se, (2), were determined at 130 K. The two structures, which differ in that (1) contains an electron-rich aromatic ring and (2) contains an electron-deficient aromatic ring, both adopt conformations which allow for σ(C-Se)-π hyperconjugation. However, although there are significant differences in the (77)Se chemical shifts for these two compounds, they do not display significantly different H2C-Se or H2C-C(ar) bond lengths, suggesting that the effects of σ(C-Se)-π hyperconjugation in (1) and (2) are not strong enough to be manifested in measurable differences in the structural parameters.

The effects of N-pendants and electron-rich amidine motifs in 2-(p-alkoxyphenyl)-2-imidazolines on mild steel corrosion in CO2-saturated 0.5 M NaCl

A novel series of diethylenetetramine-(I), and tetraethylenepentamine-derived imidazolines (II) having p-octyloxy-, dodecyloxy and octadecyloxy-phenyl pendants as hydrophobes, have imparted very good inhibition of mild steel corrosion in CO2–0.5M NaCl (40°C, 1atm; 120°C, 10bar). Pendants to the ring-nitrogen of I and II showed no change in inhibition efficiencies, while the presence of electron-rich aromatic ring in conjugation with the amidine motifs and increasing hydrophobe chain lengths imparted increasing corrosion inhibition. The formation of an imidazoline film covering the metal surface is corroborated by an XPS study. The imidazolines cover the metal surface before reaching their critical molar concentrations.

Understanding the Reactivity of Enol Ether Radical Cation: Investigation of Electrochemical [2 + 2] Cycloaddition Reactions

Electrochemical process has proven to be powerful tool to form carbon-carbon bond and has widely used to construct various carbon skeletons. Especially, the electrochemical carbon-carbon bond forming reactions have demonstrated to afford complicated ring systems effectively, including naturally occurring compounds. For example, enol ether radical cations are readily produced in anodic oxidations of enol ethers and provide versatile reactive intermediates. This reaction is synthetically intriguing due to the umpolung process that polarity of nucleophilic enol ethers is reversed to give enol ether radical cations which function as electrophilic species. We have developed the electrochemical reactions in lithium perchlorate/nitromethane (LPC/NM) electrolyte solution, which exhibits a remarkable accelerating property as a Lewis acid catalyst and stabilize carbocation intermediates. The feature of LPC/NM allows facile synthesis of biologically relevant compounds, such as dihydrobenzofuran derivatives1 and azanucleoside derivatives.2 In LPC/NM, unactivated olefins can serve as efficient nucleophiles to trap anodically generated cationic species. In this context, we also have developed electrochemical [2 + 2] cycloaddition reactions between enol ethers and unactivated olefins, affording cyclobutane products(Scheme 1).3 The reaction was initiated by anodic oxidation of enol ether (1) and thus generated enol ether radical cation (2) was trapped by unactivated olefin nucleophile to give transient cyclobutyl radical cation (3). In order for successful cyclobutane formation, electron rich aromatic ring was required as an electron donor to form relatively long lived aromatic ring radical cation (4), which was finally reduced by cathode or starting enol ether, affording neutral cycloadduct (5) (Scheme 2).Although our previous work have focused on the use of enol ether (1) for electrochemical [2 + 2] cycloaddition reactions, the reactivity of enol ether radical cation (2) still remains unclear. To investigate the reactivity of such enol ether radical cations, we designed and synthesized several types of new substrates and then subjected them to the electrochemical [2 + 2] cycloaddition reactions(Scheme 3). Synthetic and mechanistic details would be discussed in the presentation. Reference (1) Kim, S.; Hirose, K.; Uematsu, J.; Mikami, Y.; Chiba,K. Chem. – Eur. J. 2012, 18, 6284–6288.(2) Kim, S.; Shoji, T.; Kitano, Y.; Chiba, K. Chem. Commun. 2013, 49, 6525–6527.(3) (a) Yamaguchi, Y.; Okada, Y.; Chiba, K. J. Org. Chem., 2013, 78, 2626–2638. (b) Yamaguchi, Y.; Okada, Y.; Chiba, K. Electrochemistry 2013, 81, 331–333 and references therein.

The smart precursors of energetic–energetic cocrystals from eutectic precursors

Abstract The selected 18 energetic compounds were theoretically investigated by using the density functional theory (DFT) quantum mechanical code, DMol 3 , and the Hansen solubility parameters (HSPs) analyses. The results showed that 4-nitrotoluene, 4-nitrophenol, N , N ′-dimethyl- N , N ′-diphenylurea and N , N ′-diethyl- N , N ′-diphenylurea contain relatively electron-rich aromatic rings. Four satisfactory energetic precursors with electron-rich rings were quickly and effectively found by electrostatic potential (ESP) surfaces and HSPs analyses. The results also indicated that the absolute value of the lowest unoccupied molecular orbital (LUMO) of the energetic precursors with electron-rich rings often was less than 3.00 eV, and the absolute value of LUMO of the energetic precursors with electron deficient rings was often more than 3.00 eV. Additionally, we found that with at least two eutectic points was a prerequisite for two precursors to form a cocrystal.