Hydroquinone Ethers Research Articles

A dodecamethoxy[6]cycloparaphenylene consisting entirely of hydroquinone ethers: unveiling in-plane aromaticity through a rotaxane structure

[n]Cycloparaphenylenes ([n]CPPs, where n is the number of phenylene groups), consisting of 1,4-linked phenylene unit, have attracted much attention due to their cyclic π-conjugated structures and physical properties. However, functionalizing of the benzene rings of smaller [n]CPPs (n < 7) has been a challenge due to ring strain and steric hindrance of the substituents that hampers their synthesis. Here we show successful synthesis of a new [6]CPP derivative with twelve methoxy groups at the 2,5-positions of all benzene rings by utilizing our developed CPP synthesis method via a macrocyclic gold complex. This molecule exhibited a significantly higher oxidation potential caused by the electron-donating ability of the methoxy groups and the tubular molecular conformation, allowing facile oxidation to give dicationic species with in-plane aromaticity. Furthermore, this molecule successfully included with the guest molecules with a flexible alkyl chain in the cavity, enabling the creation of a CPP-based rotaxane, which exploited its mechanically interlocked molecular structure to the first experimental observation that the in-plane aromaticity in the center of the macrocycle.

Synthesis, characterization and anti-HIV activity of polycarboxylic [60]fullerene derivatives obtained in the reaction of C60Cl6 with a hydroquinone ether

We report the preparation and characterization of five new [60]fullerene derivatives formed in the novel reaction of C60Cl6 with dimethyl 2,2′-(1,4-phenylenebis-(oxy))diacetate, which were isolated by preparative HPLC and characterized by APCI-MS, 1H, 13C, and 2D NMR spectroscopy. Acidic hydrolysis of the ester groups in the synthesized compounds produced a series of water-soluble fullerene derivatives, which demonstrated pronounced activities against Human Immunodeficiency Virus (HIV) replication. Correlations between the structures of the fullerene derivatives and their antiviral activity were established.

Effect of Chemical Structure on the Performance of Sulfonated Poly(aryl ether sulfone) Composite Nanofiltration Membranes.

This paper discusses the effect of the chemical structure of sulfonated poly(aryl ether sulfone) on the performance of composite nanofiltration membranes. The composite nanofiltration membranes were fabricated by coating sulfonated poly(aryl ether sulfone) solution onto the top surface of poly(phthalazinone ether sulfone ketone) support membranes. Three kinds of sulfonated poly(aryl ether sulfone)s with different amounts of phthalazinone moieties, namely, sulfonated poly(phthalazinone ether sulfone) (SPPES), sulfonated poly(phthalazinone biphenyl ether sulfone) (SPPBES), and sulfonated poly(phthalazinone hydroquinone ether sulfone)s (SPPHES), were used as coating materials. The solvents used in preparing the coating solution were investigated and optimized. The separation properties, thermal stability, and chlorine resistance of composite membranes were determined. The structures and morphologies of membranes were characterized with FTIR and SEM, respectively. The membrane prepared from SPPES with more phthalazinone moiety groups showed high water flux and salt rejection. The salt rejection of composite membranes followed the order SPPES > SPPHES > SPPBES. The rejection of the three composite membranes decreased slightly with the solution temperature rising from 20 to 90 °C, while the composite membrane with SPPES as the active layer showed a higher increase in flux than others. The results indicate that SPPES composite membranes show better thermal stability than others.

Influence of the hydroquinone ether moieties and Bisphenol A glycerolate diacrylate on the UV stability behavior of new polyurethane materials

A series of the polyurethane materials with different ratios of hydroquinone ether derivatives and bisphenol A glycerolate diacrylate were synthesized using a two-step polymerization procedure. The structures of the obtained polyurethanes were characterized by Fourier transform infrared spectroscopy (FTIR). The influence of the hard segments structure on the thermo-mechanical properties were studied by TGA and stress-strain testing. It was found that with the increase in bisphenol A glycerolate diacrylate content, the new polyurethane materials had improved thermo-mechanical properties, due to the fact that the material develops a crosslinked structure. The durability of the polyurethane films when exposed to ultraviolet radiation was tracked by mechanical behavior and changes in chemical structure. Bisphenol A glycerolate diacrylate-based polyurethane elastomers exhibit improved behavior under UV radiation exposure compared with conventional polyurethanes that are chain-extended with butanediol.

From Intramolecular (Circular) in an Isolated Molecule to Intermolecular Hole Delocalization in a Two‐Dimensional Solid‐State Assembly: The Case of Pillarene

AbstractTo achieve long‐range charge transport/separation and, in turn, bolster the efficiency of modern photovoltaic devices, new molecular scaffolds are needed that can self‐assemble in two‐dimensional (2D) arrays while maintaining both intra‐ and intermolecular electronic coupling. In an isolated molecule of pillarene, a single hole delocalizes intramolecularly via hopping amongst the circularly arrayed hydroquinone ether rings. The crystallization of pillarene cation radical produces a 2D self‐assembly with three intermolecular dimeric (sandwich‐like) contacts. Surprisingly, each pillarene in the crystal lattice bears a fractional formal charge of +1.5. This unusual stoichiometry of oxidized pillarene in crystals arises from effective charge distribution within the 2D array via an interplay of intra‐ and intermolecular electronic couplings. This important finding is expected to help advance the rational design of efficient solid‐state materials for long‐range charge transfer.

From Intramolecular (Circular) in an Isolated Molecule to Intermolecular Hole Delocalization in a Two-Dimensional Solid-State Assembly: The Case of Pillarene.

To achieve long-range charge transport/separation and, in turn, bolster the efficiency of modern photovoltaic devices, new molecular scaffolds are needed that can self-assemble in two-dimensional (2D) arrays while maintaining both intra- and intermolecular electronic coupling. In an isolated molecule of pillarene, a single hole delocalizes intramolecularly via hopping amongst the circularly arrayed hydroquinone ether rings. The crystallization of pillarene cation radical produces a 2D self-assembly with three intermolecular dimeric (sandwich-like) contacts. Surprisingly, each pillarene in the crystal lattice bears a fractional formal charge of +1.5. This unusual stoichiometry of oxidized pillarene in crystals arises from effective charge distribution within the 2D array via an interplay of intra- and intermolecular electronic couplings. This important finding is expected to help advance the rational design of efficient solid-state materials for long-range charge transfer.

A Tetrasubstituted Hydroquinone Ether Catholyte for Non-Aqueous Redox Flow Batteries: Bicyclic Substitution Enabling High Stability

The practical application of non-aqueous redox flow batteries (NRFBs) sets stringent requirements on the electrochemical stability of the redox active materials. In the redox pair for NRFBs, the high-potential species is referred to as the catholyte, which is oxidized into the radical cation state in the charging process. Hydroquinone ethers are a key class of catholyte materials, but the intermolecular reaction between the radical cations is a dominating decomposition pathway of the catholyte during functioning. In order to increase the electrochemical stability of the catholyte materials, the introduction of bulky alkyl spacers at 2,5- positions of the arene ring is commonly used as a strategy in the molecular design to suppress this type of undesirable side reaction. However, the unsubstituted 3,6- positions that are favored for the electrochemical reversibility are responsible for the radical cation reaction in the long-time cycling. Tetrasubstitution of the arene core with either primary or secondary alkyl groups often leads to a compromised redox reversibility. This talk will describe a tetrasubstituted hydroquinone ether-based catholyte molecule that retains excellent redox reversibility and illustrates exceptional stability. Unique bicyclic alkyl groups are incorporated to the molecular scaffold, eliminating the bimolecular reaction between the radical cations. A hybrid NRFB using such catholyte has been operated for over 150 charge-discharge cycles with minimal loss of capacity.

When Substituents Do Not Matter: Frontier Orbitals Explain the Unusually High and Invariant Oxidation Potential in Alkoxy-, Alkyl-, and H-Substituted Iptycenes.

Frontier molecular orbitals (FMOs) have played a critical role in predicting reactivity/selectivity of pericyclic reactions. Here we show that the structurally similar iptycene-based hydroquinone ether (HE), that is, MeOIpt and BOHE/BHHE, molecules have drastically different ordering of bisallylic and quinoidal FMOs. They are almost degenerate in BOHE/BHHE, while in MeOIpt, the bisallylic orbital lies far below the quinoidal HOMO. Oxidation of BOHE/BHHE induces coplanarization of the methoxy group and destabilizes the bisallylic HOMO, leading to a relatively low oxidation potential. In MeOIpt, considerable energy must be invested in coplanarization of the methoxy group to bring about orbital swapping, resulting in an oxidation potential higher than that in structurally similar BOHE/BHHE. As the quinoidal HOMO density does not extend to the substituent-bearing carbon in H-, alkyl-, and alkoxy-substituted iptycenes, their redox potentials remain invariant. This case study involving a simple visual inspection of the nodal arrangement as well as energetics of the FMOs and Walsh analysis could serve as a tool for the design of organic molecules with a desired redox potential.

Highly Stable Hydroquinone Ether-Based Catholyte for Non-Aqueous Redox Flow Batteries

Hydroquinone ethers are a key class of catholyte materials for non-aqueous redox flow batteries, showing high open-circuit potential, electrochemical reversibility and chemical tunability. However, the instability of many hydroquinone ethers, particular to electrochemical oxidation, limits their applicability. 2,5-dialkylation of the benzene core has been by far the only effective chemical approach to suppress the side reactions of the corresponding radical cations, restricting the chemical space of structures from which to choose. This talk will describe a bicyclo-alkyl substitution strategy that yields a hydroquinone ether with unprecedentedly high electrochemical stability. The influences of different substituents on the electrochemical properties are investigated. The electrochemical stability is determined via a full flow cell using a benzothiadiazole-based anolyte. In addition, the low impedance shown in the single electrolyte flow cell indicates its great promises as catholyte for high-performance non-aqueous redox flow batteries.

Quantitative generation of cation radicals and dications using aromatic oxidants: effect of added electrolyte on the redox potentials of aromatic electron donors

Redox titrations using robust aromatic oxidants allow for a quantitative analysis of redox and optical properties of organic electron donors in their oxidized states. Unlike spectroelectrochemistry, redox titrations can be performed without added electrolyte in relatively nonpolar solvents, affording quick access to the redox and optical properties of a given electron donor without the need of a complex electrochemical setup. However, the redox potentials obtained by the two methods are not the same. To establish the direction and magnitude of this discrepancy, we have performed a systematic case study using a set of tetraarylethylene donors and a tetrasubstituted hydroquinone ether cation radical (THEO+•) as a stable aromatic oxidant. We show that redox potentials (especially second and higher oxidation potentials) measured by electrochemical methods are systematically lower compared with the redox potentials obtained by redox titrations in the absence of electrolyte, because of the enhanced stabilization of dications and polycations by electrolyte. We have also uncovered that the smaller cation radicals (e.g., a para‐hydroquinone or ortho‐hydroquinone ether cation radicals) are much more effectively stabilized when compared with the cation radicals in which charge is delocalized over larger area (e.g., tetraarylethylene cation radicals) in the presence of electrolyte because of increased ionic strength of the solution. Copyright © 2015 John Wiley &amp; Sons, Ltd.

HPLC-UV Method for the Identification and Screening of Hydroquinone, Ethers of Hydroquinone and Corticosteroids Possibly Used as Skin-Whitening Agents in Illicit Cosmetic Products.

Corticosteroids, hydroquinone and its ethers are regulated in cosmetics by the Regulation 1223/2009. As corticosteroids are forbidden to be used in cosmetics and cannot be present as contaminants or impurities, an identification of one of these illicit compounds deliberately introduced in these types of cosmetics is enough for market survey control. In order to quickly identify skin-whitening agents present in illegal cosmetics, this article proposes an HPLC-UV method for the identification and screening of hydroquinone, 3 ethers of hydroquinone and 39 corticosteroids that may be found in skin-whitening products. Two elution gradients were developed to separate all compounds. The main solvent gradient (A) allows the separation of 39 compounds among the 43 compounds considered in 50 min. Limits of detection on skin-whitening cosmetics are given. For compounds not separated, a complementary gradient elution (B) using the same solvents is proposed. Between 2004 and 2009, a market survey on "skin-whitening cosmetic" was performed on 150 samples and highlights that more than half of the products tested do not comply with the Cosmetic Regulation 1223/2009 (amending the Council Directive 76/768/EEC).

Synthesis and properties of segmented polyurethanes with hydroquinone ether derivatives as chain extender

Two series of 4, 4′-diphenylmethane diisocyanate (MDI) type polyurethanes based on poly(tetramethyleneoxide) glycol (PTMO) and polybutylene adipate glycol (PBA) as soft segments were synthesized by one-step bulk polymerization process. Various hard segments were obtained from hydroquinone ether derivatives with or without ether oxygen structure bonded with the aromatic units in main chain, including hydroquinone (HQE), hydroquinone bis(b-hydroxyethy) ether (HQEE), and 4-hydroxyethyl ethyl oxygen-1-hydroxy ethyl benzene ether (HQEE-L) as chain extenders. The effect of chain extender structure on aggregation structure, thermal and mechanical properties was studied. ATR-FTIR suggested that hydrogen bonding interactions were significantly changed in the presence of flexible ether oxygen structure in hard segment. HQEE-based polyurethanes and HQEE-L-based polyurethanes displayed microphase separation and microphase mixing behavior respectively based on DMA and DSC results. XRD curves confirmed that flexible ether oxygen chain bonded with the rigid aromatic units played a balancing role in the steric effect caused by aromatic chain extender. Thermal properties of polyurethanes were affected mainly by the hard segment packing while mechanical properties were influenced by both phase separation and phase mixing which possess strong hydrogen bonding interactions. The excellent elongation in PTMO/HQEE-L and the high ultimate tensile strength in PBA/HQEE-L can be attributed to the phase mixing with strong hydrogen bonding interactions presented in HQEE-L-based polyurethanes.

Research Progress on the Synthesis of Pillar[n]arenes and Their Host-guest Chemistry

柱芳烃是由对苯二酚或对苯二酚醚通过亚甲基桥在苯环的对位连接而成的一类环状低聚物,是一类新型的大环主体分子.本文介绍了柱芳烃和功能化柱芳烃分子的合成,以及在分子识别、自组装等主客体化学方面的最新研究进展,并对其研究前景进行展望.

T‐Butyl Hydroperoxide Oxidative Dealkylation of Hydroquinone Ethers to 1,4‐Quinones.

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T‐Butyl Hydroperoxide Oxidative Dealkylation of Hydroquinone Ethers to 1,4‐Quinones

Various organoselenium compounds (diselenides, benzisoselenazol‐3(2H)‐ones, and 1,3,2‐thiaselenazolones) are effective catalysts for oxidative dealkylation of hydroquinone ethers with t‐butyl hydroperoxide to 1,4‐quinones. The reactions is most efficient when t‐butyl hydroperoxide in the presence of poly(bis‐1,2‐phenyl) diselenide is used as an oxidizing agent. In this way 1,4‐benzo and 1,4‐naphthoquinones are produced in good to high yields.

Chiroptical Properties of Organic Radical Cations. The Electronic and Vibrational Circular Dichroism Spectra of α-Tocopherol Derivatives and Sterically Hindered Chiral Hydroquinone Ethers

Qualitatively and quantitatively reliable electronic and vibrational circular dichroism (ECD and VCD) spectra of chiral organic radical cations were obtained for the first time with α-tocopherol de...

Synthesis of Hydroquinone‐, Biphenol‐, and Binaphthol‐Containing Aza Macroheterocycles via Regioselective Hydroformylation and Reductive Amination.

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Synthesis of hydroquinone-, biphenol-, and binaphthol-containing aza macroheterocycles via regioselective hydroformylation and reductive amination

Rhodium(I) catalyzed regioselective hydroformylation of diolefins and subsequent reductive amination of the dialdehydes in the presence of α,ω-diamines is applied to azamacroheterocyclic ring synthesis. Starting from aromatic diallyl ethers of hydroquinone, biphenol and binaphthol 20–28 membered macroheterocycles were obtained in up to 78% yield.

Total synthesis of yahazunol, zonarone and isozonarone

The synthesis of the marine natural products zonarone and isozonarone was achieved via (+)-albicanic acid, a sesquiterpene of the drimane type. Coupling of the appropiate drimane-synthon with lithiated hydroquinone ethers led to sesquiterpene arenes, which were further modified to the target molecules. Stereoselective epoxidation followed by regioselective opening of the oxirane ring yielded yahazunol.

One-electron transfer product of quinone addition to carotenoids: EPR and optical absorption studies

It was shown by EPR and UV–VIS studies that one-electron transfer reactions between carotenoids (β-carotene, 8′-apo-β-caroten-8′-al, canthaxanthin) and quinones [2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachlorobenzoquinone (CA)] include the formation of a charge-transfer complex (CTC), which exists in equilibrium with an ion–radical pair (Car +⋯Q −). UV spectra display a new absorption band in the near IR region (at 1030 nm) for the β-carotene–DDQ mixture which is assigned to a CTC. The absorption maximum of this band gradually shifts from 1030 nm to shorter wavelengths and finally corresponds to that of the β-carotene radical cation (1000 nm). A new absorption band at 340 nm, detected for all systems under study, is attributed to the product of quinone addition to carotenoid. The EPR spectra of the Car–DDQ mixture measured at 77 K when [Car]≤[DDQ] exhibit a structureless singlet line with g=2.0066±0.0002 which is attributed to a CTC. Increasing the temperature gives rise to a new five-line signal with g=2.0052±0.0002 and hyperfine coupling constant of 0.6 G due to the DDQ radical anion. At room temperature stable radicals (more than 24 h) with g=2.0049±0.0002 and Δ H p–p=4.3 G were detected. Electron nuclear double resonance (ENDOR) results indicate that these species contain both nitrogen and chlorine atoms. We attribute these signals to a carotenoid–quinone radical adduct. For chloranil paramagnetic species are generated only after irradiation. Both the carotenoid radical cation and the quinone radical anion were observed in this case. Analysis of the isolated reaction product made using the Beilstein test, 1 H -NMR and IR spectroscopies suggests that the major product is a carotenoid–hydroquinone ether. A mechanism of the Car–Q adduct formation is proposed.