Catalytic activity of anion-exchange resins modified with metal-porphine in oxidative reactions of phenols

Anion-exchange resins modified with metal-porphine (M-Pr) have been investigated to develop a solid catalyst in the oxidative reaction of phenols by O2 in air. Co-Pr, which is easily prepared and separable from the reaction mixture, has been proved to accelerate the oxidative reaction of phenols such as 3,5-di-tertbutyl-4-hydroxyanisole. The resulting main oxidative products were identified to be quinones by using the GC-MS method.     

Direct Synthesis of Hydroquinones from Quinones through Sequential and Continuous‐Flow Hydrogenation‐Derivatization Using Heterogeneous Au–Pt Nanoparticles as Catalysts

Pt–Au bimetallic nanoparticle catalysts immobilized on dimethyl polysilane (Pt–Au/(DMPSi‐Al₂O₃)) have been developed for selective hydrogenation of quinones to hydroquinones. High reactivity, selectivity, and robustness of the catalysts were confirmed under continuous‐flow conditions. Various direct derivatizations of quinones, such as methylation, acetylation, trifluoromethanesulfonylation, methacrylation, and benzoylation were successfully performed under sequential and continuous‐flow conditions to afford the desired products in good to excellent yields. Especially, air‐sensitive hydroquinones, such as anthrahydroquinones and naphthohydroquinones, could be successfully generated and derivatized under closed sequential and continuous‐flow conditions without decomposition.     

Hypercoordinate germanium complexes with phenanthrene-9,10-diolate ligands: synthesis,structure, and electronic properties

The synthesis, molecular structure, and electronic properties of sodium tris(phenanthrene-9,10-diolato)germanate(iv) are described. The germanate complex is readily oxidized in air to produce 9,10-phenanthrenequinone, and the resulting quinones and the ligands reveal intermolecular π-stacking interaction in the polymeric association in the solid state. Addition of phenanthroline to the germanate complex leads to a monomeric structure.     

Highly efficient and chemoselective reductive bis-silylation of quinones by silyltellurides

Silyltellurides serve as new silicon-based chemoselective reducing agents and reduce quinones to the corresponding bis-silylated hydroquinones. The reaction proceeds under ambient thermal conditions without the need of any additional promoters or catalysts and gives the products in excellent yields. Several control experiments suggest that the reaction is initiated by a single electron transfer from silyltellurides to quinones.     

Theoretical prediction of the hydride affinities of various p- and o-quinones in DMSO

The hydride affinities of 80 various p- and o-quinones in DMSO solution were predicted by using B3LYP/6-311++G (2df,p)//B3LYP/6-31+G* and MP2/6-311++G**//B3LYP/6-31+G* methods, combined with the PCM cluster continuum model for the first time. The results show that the hydride affinity scale of the 80 quinones in DMSO ranges from -47.4 kcal/mol for 9,10-anthraquinone to -124.5 kcal/mol for 3,4,5,6-tetracyano-1,2-quinone. Such a long scale of the hydride affinities (-47.4 to -124.5 kcal/mol) indicates that the 80 quinones can form a large and useful library of organic oxidants, which can provide various organic hydride acceptors that the hydride affinities are known for chemists to choose in organic syntheses. By examining the effect of substituent on the hydride affinities of quinones, it is found that the hydride affinities of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma: DeltaGH-(Q) approximately -16.0Sigmasigmai - 70.5 (kcal/mol) for p-quinones and DeltaGH-(Q) approximately -16.2Sigmasigmai - 81.5 (kcal/mol) for o-quinones only if the substituents have no large electrostatic inductive effect and large ortho-effect. Study of the effect of the aromatic properties of quinone on the hydride affinities showed that the larger the aromatic system of quinone is, the smaller the hydride affinity of the quinone is, and the decrease of the hydride affinities is linearly to take place with the increase of the number of benzene rings in the molecule of quinones, from which the hydride affinities of aromatic quinones with multiple benzene rings can be predicted. By comparing the hydride affinities of p-quinones and the corresponding o-quinones, it is found that the hydride affinities of o-quinones are generally larger than those of the corresponding p-quinones by ca. 11 kcal/mol. Analyzing the effect of solvent on the hydride affinities of quinones showed that the effects of solvent (DMSO) on the hydride affinities of quinones are mainly dependent on the electrostatic interaction of the charged hydroquinone anions (QH-) with solvent (DMSO). All the information disclosed in this work should provide some valuable clues to chemists to choose suitable quinones or hydroquinones as efficient hydride acceptors or donors in organic syntheses and to predict the thermodynamics of hydride exchange between quinones and hydroquinones in DMSO solution.     

THE CHLOROPHYLL-SENSITIZED REACTION BETWEEN BENZOQUINONE AND ETHANOL*

The electron-transfer reaction between triplet excited chlorophyll and quinones has been extensively studied as a model of the primary reaction in photosystem II. There has also been reported a minor reaction in which the chlorophyll cation radical ostensibly oxidizes the alcohol solvent or even water, leading to a gradual net reduction of quinone, but the exact mechanism and even the existence of this reaction has been uncertain. We have examined the consequences of prolonged irradation of ethyl chlorophyllide and benzoquinone in acidulated ethanol, and find a chlorophyllide-sensitized reaction which is not analogous to the better-known autosensitized reduction of quinones in blue or UV light. In the chlorophyllide-sensitized reaction, benzoquinone is apparently converted to ethoxy-substituted quinones and quinols, and polymeric material. Ethyl chlorophyllide (or chlorophyll) is simultaneously oxidized to more polar products which themselves continue to photosensitize the reaction of quinones. The production of acetaldehyde could not be demonstrated in the sensitized reaction. Chlorophyllide-sensitized reaction of (l-hydroxyethyl)benzoquinone, ethoxybenzoquinone and 2.5-diethoxybenzo-quinone were examined for additional information. A reaction sequence, tentatively proposed to accommodate the known facts, starts with oxidative attack by quinone on an oxidized chlorophyllide radical formed by loss of a hydroxyl proton from alcohol bound as a ligand to Mg²⁺. It is not likely that this reaction is closely related to events at the oxidizing side of photosystem II.     

Quinine‐Catalyzed Asymmetric Synthesis of 2,2′‐Binaphthol‐Type Biaryls under Mild Reaction Conditions

Simple quinine as an organocatalyst mediates the addition of various naphthols to halogenated quinones to afford non‐C₂‐symmetrical, axially chiral biaryl products, which are promising compounds as chiral ligands and organocatalysts. The rotational barrier required to have two distinct atropisomers has been evaluated in the products generated from the addition of naphthols to various quinones by means of DFT calculations and HPLC. The use of halogenated quinones as reagents was necessary to have configurationally stable enantiomeric products which can be obtained in good yield and stereoselectivity. These compounds have also been prepared in gram quantities and recrystallized to near enantiopurity.     

Design of synthetic molecular units including quinones towards the construction of artificial photosynthesis

Quinones are essential components in many biological systems, notably in photosynthesis. This is largely due to the characteristic proton-coupled redox chemistry of quinones. This review article overviews the use of quinones in studies on artificial photosynthesis, as one-electron electron acceptors, reversible proton/electron carriers, and replacements for sacrificial oxidant and reductants in photosynthetic chemical conversion. Topics included are the early attempts on intramolecular photoinduced electron transfer involving quinones, subsequent reactions after photoinduced electron transfer between pigments and quinones, photochemistry in molecular assemblies containing quinones, and photochemical quinone/hydroquinone interconversion.     

Determination of the spectroscopic properties and chromatographic sensitivities of substituted quinones by hexachlorate(IV) oxidation of hydroquinones

HPLC studies of the oxidation of substituted hydroquinones show that the corresponding quinones can, in most cases, be produced quantitatively by two electron transfer to hexachloroiridate(IV). As a consequence the chromatographic and spectroscopic properties of substituted quinones that are not readily available can be determined without the necessity of preparation of an isolatable sample. Absorption spectra and extinction coefficients of bromoquinone, chloroquinone, hydroxyquinone and carboxyquinone anion at pH 7 are reported as illustrative examples. All have intense absorption bands at approximately 250nm that are characteristic of quinones. Oxidation of carboxyhydroquinone at low pH is, however, anomalous in that a hydroxylated carboxyquinone is produced as the result of four electron transfer to hexachloroiridate.     

Simultaneous measurement of tocopherols and tocopheryl quinones in tissue fractions using high-performance liquid chromatography with redox-cycling electrochemical detection

Tocopherols and tocopheryl quinones in lipid extracts of biological samples have primarily been measured using relatively insensitive ultraviolet detection methods. Oxidative electrochemical detection increases both the sensitivity and selectivity when measuring the tocopherols. We have developed an electrochemical detection system which sequentially reduces and oxidizes tocopheryl metabolites eluted from a reversed-phase high-performance liquid chromatography column, achieving sensitivities of about 0.05 pmol for both the tocopherols and their quinones. Using a rapid and mild extraction procedure, endogenous levels of alpha- and gamma-tocopherol as well as their respective quinones were measured in homogenates of chicken liver and muscle, and in dilute preparations of rat liver microsomes. The principle of the detection system could be applied to the determination of tocopheryl dihydroquinones, ubiquinols and ubiquinones with slight modifications to the mobile phase buffer and the electrode potentials of the detector.     

Activation of Electron‐Deficient Quinones through Hydrogen‐Bond‐Donor‐Coupled Electron Transfer

Quinones are important organic oxidants in a variety of synthetic and biological contexts, and they are susceptible to activation towards electron transfer through hydrogen bonding. Whereas this effect of hydrogen bond donors (HBDs) has been observed for Lewis basic, weakly oxidizing quinones, comparable activation is not readily achieved when more reactive and synthetically useful electron‐deficient quinones are used. We have successfully employed HBD‐coupled electron transfer as a strategy to activate electron‐deficient quinones. A systematic investigation of HBDs has led to the discovery that certain dicationic HBDs have an exceptionally large effect on the rate and thermodynamics of electron transfer. We further demonstrate that these HBDs can be used as catalysts in a quinone‐mediated model synthetic transformation.     

Synthesis and conductivity of poly acene quinone radical polymers

Polymers with conductivities from 8.6 × 10⁰ to 7.3 × 10^?6 (ω cm)^?1 at 2–12 kbar were prepared by zinc-chloride-catalyzed condensation of polycyclic aromatic hydrocarbons, heterocycles, and quinones with pyromellitic dianhydride, tetrabromophthalic anhydride, and tetrachlorophthalic anhydride at 450°C. These materials are stable in air and exhibit strong electroactive character without addition of oxidizing or reducing agents (dopants).     

Chemical Modifications of Biopolymers by Quinones and Quinone Methides

Many quinones and their precursors, which are transformed oxidatively into quinones and/or quinone methides, are important natural products. As secondary metabolites, they frequently possess antibiotic and cytotoxic activities, in addition to acting sometimes as pathogens. Several plants and animals, especially insects, use quinonoid substances for defense, often with spectacular results. On the macromolecular level, quinone methides have a key role in the plant kingdom in lignin biosynthesis; the biosynthesis of melanoproteins exemplifies the reactions of o-quinones in the animal kingdom. In insects, cross-linking of structural proteins through quinones and quinone methides results in the construction of the sclerotized exoskeleton. For mankind, the reactivity of quinones in biological systems has far-reaching consequences of pharmaceutical, toxicological, and technical relevance. The examples in this review show that a common principle underlies these reactions, namely, the chemical modification of biopolymers. As demonstrated particularly well in a more detailed discussion of the chemical principles of insect cuticle sclerotization, several major and important new results have emerged from the development and applications of modern methods of sample separation and from solid-state NMR spectroscopy.     

Synthesis, structures, and properties of peripheral o-dimethoxy-substituted pentiptycene quinones and their o-quinone derivatives

A series of peripheral o-dimethoxy-substituted pentiptycene quinones and their o-quinone derivatives have been synthesized. Especially, it was found that if two o-dimethoxybenzene moieties were situated at the same side of the pentiptycene quinones, one of them was only oxidized by excess CAN in aqueous acetonitrile. Moreover, the pentiptycene quinones with unique 3D rigid structure could all self-assemble into a 3D microporous structure in the solid state. For the pentiptycene quinones containing the dimethoxybenzene unit(s) and the quinone group(s) simultaneously, interesting intramolecular charge transfer interactions and electrochemical properties were also shown. These peripheral-substituted pentiptycene quinones and their o-quinone derivatives can be used as new useful building blocks and will find wide applications in material science and host-guest chemistry.     

Site-specific binding of quinones to proteins through thiol addition and addition-elimination reactions

Ubiquinone-0, menaquinone-0, and 2,3,5-trimethyl-1,4-benzoquinone were site-specifically bound to free cysteine of proteins (yeast iso-1 cytochrome c as a model protein) through thioether bond formation. Model thioether quinone conjugates showed unexpected reactivity to cysteine of proteins as their parent quinones by thiol addition-elimination reaction. Cyclic voltammetry studies of the model compounds showed only minor differences in their redox potentials as compared to their parent quinones. Thioether ligation provides a general, simple, and fast method to construct model quinone protein systems. In addition, these studies also contribute to the understanding of biological activities, toxicity, and anti-cancer mechanism of quinones and thioether quinone adducts.     

Mechanism of catalytic hydrogenation of nitrobenzene in an aprotic medium in the presence of quinones

Conclusions The hydrogenation of nitrobenzene in an aprotic medium in the presence of quinones is realized by a reduction-protonation mechanism through alternating stages of electron transfer and protonation. The function of proton donor is fulfilled by the corresponding hydroquinones formed from the quinones under the reaction conditions.Translated from IzvÉstiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1500–1505, July, 1981.     

Effects of nonionic surfactants on electrochemical behavior of ubiquinone and menaquinone incorporated in a carbon paste electrode

A carbon paste electrode, in which the carbon particles were coated with a thin layer of a nonionic surfactant (NIS), was constructed with a pasting liquid containing ubiquinone (UQ) or menaquinone (MQ). It has revealed that the layer acts not only as a diffusion barrier but also as a matrix for the redox reaction of quinones at electrode surface, and its effects on the electrochemical behavior of quinones depend on both the physico-chemical structure of a surfactant and the kind of quinones. Further, such a modification was applied to the preparation of an enzyme electrode in which the quinone molecule act as a redox mediator and the influences on the sensitivity of the glucose biosensor was demonstrated.     

Electrocoagulation of quinone pigments

Some representative quinones, viz. one naphthoquinone (plumbagin) and five anthraquinones (alizarin, purpurin, chrysazin, emodin, and anthrarufin), were subjected to electrocoagulation. It was found that the rate and extent of coagulation of these compounds appears to correlate with the number and relative position of their phenolic substituent groups, and that all of the coagulated quinones could be recovered. Attempts were then made to electrochemically isolate three quinones, namely plumbagin, morindone and erythrolaccin, from natural sources.