Quinoid systems in chemistry and pharmacology

Asymmetric Formal Coupling of β-Ketoesters with Quinones Promoted by a Chiral Bifunctional N-Heterocyclic Olefin

A highly enantioselective formal coupling of β-ketoesters with quinones was accomplished by a chiral bifunctional N-heterocyclic olefin organocatalyst. With as low as 1 mol % catalyst loading, a number of enantioenriched quinone derivatives were afforded in good yields with high enantioselectivities and regioselectivities (up to 96% yield, 98% ee, and 19:1 rr). Gram-scale synthesis and the high inhibitory effect of several products on the viability of cancer cells demonstrate the potential utility of the current method.

Aqueous Redox Flow Batteries: Small Organic Molecules for the Positive Electrolyte Species

There are a number of critical requirements for electrolytes in aqueous redox flow batteries. This paper reviews organic molecules that have been used as the redox-active electrolyte for the positive cell reaction in aqueous redox flow batteries. These organic compounds are centred around different organic redox-active moieties such as the aminoxyl radical (TEMPO and N-hydroxyphthalimide), carbonyl (quinones and biphenols), amine (e. g., indigo carmine), ether and thioether (e. g., thianthrene) groups. We consider the key metrics that can be used to assess their performance: redox potential, operating pH, solubility, redox kinetics, diffusivity, stability, and cost. We develop a new figure of merit - the theoretical intrinsic power density - which combines the first four of the aforementioned metrics to allow ranking of different redox couples on just one side of the battery. The organic electrolytes show theoretical intrinsic power densities which are 2-100 times larger than that of the VO2+ /VO2 + couple, with TEMPO-derivatives showing the highest performance. Finally, we survey organic positive electrolytes in the literature on the basis of their redox-active moieties and the aforementioned figure of merit.

Sensitive photodetection below silicon bandgap using quinoid-capped organic semiconductors

High-sensitivity organic photodetectors (OPDs) with strong near-infrared (NIR) photoresponse have attracted enormous attention due to potential applications in emerging technologies. However, few organic semiconductors have been reported with photoelectric response beyond ~1.1 μm, the detection limit of silicon detectors. Here, we extend the absorption of organic small-molecule semiconductors to below silicon bandgap, and even to 0.77 eV, through introducing the newly designed quinoid-terminals with high Mulliken-electronegativity (5.62 eV). The fabricated photodiode-type NIR OPDs exhibit detectivity (D^*) over 10¹² Jones in 0.41 to 1.2 μm under zero bias with a maximum of 2.9 × 10¹² Jones at 1.02 μm, which is the highest D^* for reported OPDs in photovoltaic-mode with response spectra beyond 1.1 μm. The high D^* in 0.9 to 1.2 μm is comparable to those of commercial InGaAs photodetectors, despite the detection limit of our OPDs is shorter than InGaAs (~1.7 μm). A spectrometer prototype with a wide measurable region (0.4 to 1.25 μm) and NIR imaging under 1.2-μm illumination are demonstrated successfully in OPDs.

Sustainable Synthesis, Antiproliferative and Acetylcholinesterase Inhibition of 1,4- and 1,2-Naphthoquinone Derivatives

This work describes the design, sustainable synthesis, evaluation of electrochemical and biological properties against HepG2 cell lines, and AChE enzymes of different substituted derivatives of 1,4- and 1,2-naphthoquinones (NQ). A microwave-assisted protocol was optimized with success for the synthesis of the 2-substituted-1,4-NQ series and extended to the 4-substituted-1,2-NQ family, providing an alternative and more sustainable approach to the synthesis of naphthoquinones. The electrochemical properties were studied by cyclic voltammetry, and the redox potentials related to the molecular structural characteristics and the biological properties. Compounds were tested for their potential anti-cancer activity against a hepatocellular carcinoma cell line, HepG2, using MTT assay, and 1,2-NQ derivatives were found to be more active than their 1,4-NQ homologues (3a-f), with the highest cytotoxic potential found for compound 4a (EC₅₀ = 3 μM). The same trend was found for the inhibitory action against acetylcholinesterase, with 1,2-NQ derivatives showing higher inhibition_50µM than their 1,4-NQ homologues, with 4h being the most potent compound (Inhibition_50µM = 85%). Docking studies were performed for the 1,2-NQ derivatives with the highest inhibitions, showing dual binding interactions with both CAS and PAS sites, while the less active 1,4-NQ derivatives showed interactions with PAS and the mid-gorge region.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Exploration of brominated Plastoquinone analogs: Discovery and structure-activity relationships of small antimicrobial lead molecules

In the fight with the antimicrobial resistance, our continuous effort to find quinone analogs with higher inhibitory activity has previously led us to the promising Plastoquinone analogs. The 1,4-quinone moiety substituted with alkoxy substituent(s) plays an important role in the field of antimicrobial and anticancer drug discovery and development. Thus, an extensive series of 1,4-quinones, substituted in different positions with a variety of alkoxy substituents, has been designed, synthesized, and evaluated for their antimicrobial activity. Here, we describe the synthesis of brominated Plastoquinone analogs (BrPQ1-15) based on the dimethyl-1,4-quinone scaffold by employing two different paths. We also present here the in vitro antimicrobial activity of these analogs (BrPQ1-15) against a panel of pathogenic organisms. These studies resulted in several new selective antibacterial inhibitors and gave valuable insights into the structure-activity relationships. Among all the analogs studied, two analogs BrPQ1 with a methoxy substituent and BrPQ14 with a cyclic dioxy stand out as the most promising antibacterial molecules against Staphylococcus aureus and Staphylococcus epidermidis. Afterwards, two analogs were selected for a further investigation for biofilm evaluation. Finally, molecular docking studies for BrPQ1 and BrPQ14 with probable target S. aureus PNPase (5XEX) and predictive ADMET studies were also carried out.

The Different Facets of Metal-Catalyzed C-H Functionalization Involving Quinone Compounds

Metal-catalysed C-H functionalization has emerged as a powerful platform for the derivatization of quinones, a class of compounds with wide-ranging applications. This review organises and discusses the evolution of this chemistry from early Fujiwara-Moritani reactions, through to modern directing-group assisted C-H functionalization processes, including C-H functionalization reactions directed by the quinone ring itself. Mechanistic details of these reactions are provided to afford insight into how the unique reactivity of quinoidal compounds has been leveraged in each example.

Acceleration of microiron-based dechlorination in water by contact with fibrous activated carbon

Zero-valent iron (ZVI) is widely applied for reduction of chlorohydrocarbons in water. Since the dechlorination occurs at the iron surface, marked differences in rate constants are commonly found for nanoscale and microscale ZVI. It has already been shown for trichloroethene (TCE) adsorbed to activated carbon (AC) that the dechlorination reaction is shifted to the carbon surface simply by contacting the AC with highly reactive nanoscale ZVI particles. Transfer of reactive species to the adsorbed pollutant was discussed. The present study shows that even low price and very low reactive microscale ZVI can also be utilized for an effective dechlorination process. Compared to the reaction rate at the iron surface itself, an enormous acceleration of the dechlorination rate for chlorinated ethenes was observed, reaching activity levels such as known for nanoscale ZVI. When fibrous AC is brought into direct contact with microscale ZVI the iron-surface-normalised dechlorination rate constants increased by up to four orders of magnitude. This implies that the dechlorination reaction is fully transferred to the AC surface. At the same time, the anaerobic corrosion of the same material was not substantially affected. Thus, the utilization of iron's reduction equivalents towards dechlorination (dechlorination efficiency) can be considerably enhanced. A screening with various AC types showed that the extent of rate acceleration depends strongly on the surface chemistry of the AC. By means of temperature-programmed desorption, it could be shown that concentration and type of oxygen surface groups determine the redox-mediation properties. Quinone/hydroquinone groups were identified as being the main drivers for electron-transfer processes, but to some extent other redox-active groups such as chromene and pyrone can also act as redox mediators. AC overall plays the role of a catalyst rather than a reactant. The present study derives recommendations for practical application of the findings in water-treatment approaches.

Cyanobacteria-specific algicidal mechanism of bioinspired naphthoquinone derivative, NQ 2-0

To mitigate cyanobacterial blooms, the naphthoquinone derivative, NQ 2-0, which has selective algicidal activity against cyanobacteria, has been developed. However, due to a lack of information on its algicidal mechanisms, there are significant gaps in our understanding of how this substance is capable of selectively killing cyanobacteria. Here, we investigated the selective algicidal mechanisms of NQ 2-0 using target (Microcystis aeruginosa) and non-target (Cyclotella sp. and Selenastrum capricornutum) species. NQ 2-0 showed selective algicidal activity against only M. aeruginosa, and this activity was strongly light-dependent. This NQ compound has selectively reduced the oxygen evolution rate and photosystem II (PSII) efficiency of M. aeruginosa throughout blocking electron transfer from the photosynthetic electron transport system, and significantly (p ≤ 0.05) increased levels of reactive oxygen species (ROS), resulting in membrane damage through lipid peroxidation. In ultrastructural observations, thylakoid membranes were disintegrated within 12 h after NQ 2-0 treatment, and cytoplasmic vacuolation and disintegrated cellular membrane were observed at 24 h. These findings suggest that increased ROS levels following NQ 2-0 treatment may induce cell death. Interestingly, compared to non-target eukaryotic cells, M. aeruginosa showed relatively late antioxidant response to reduce the increased ROS level, this may enhance algicidal activity against this cyanobacterium.

Crystal structure of (E)-2-(tert-butyl-amino)-4-(tert-butyl-imino)-naphthalen-1(4H)-one

The title compound, C18H24N2O, is the first example of a naphtho-quinone imine derivative isolated in the 4-imine/2-amine tautomeric form having bulky alkyl substituents at the N atoms. The mol-ecular conformation is stabilized by an intra-molecular hydrogen bond between the amine and a carbonyl group and by London attraction between the two tert-butyl groups. Only van der Waals inter-actions were identified in the crystal packing.

Profiling quinones in ambient air samples collected from the Athabasca region (Canada)

This paper presents new findings on polycyclic aromatic hydrocarbon oxidation products-quinones that were collected in ambient air samples in the proximity of oil sands exploration. Quinones were characterized for their diurnal concentration variability, phase partitioning, and molecular size distribution. Gas-phase (GP) and particle-phase (PM) ambient air samples were collected separately in the summer; a lower quinone content was observed in the PM samples from continuous 24-h sampling than from combined 12-h sampling (day and night). The daytime/nocturnal samples demonstrated that nighttime conditions led to lower concentrations and some quinones not being detected. The highest quinone levels were associated with wind directions originating from oil sands exploration sites. The statistical correlation with primary pollutants directly emitted from oil sands industrial activities indicated that the bulk of the detected quinones did not originate directly from primary emission sources and that quinone formation paralleled a reduction in primary source NO_x levels. This suggests a secondary chemical transformation of primary pollutants as the origin of the determined quinones. Measurements of 19 quinones included five that have not previously been reported in ambient air or in Standard Reference Material 1649a/1649b and seven that have not been previously measured in ambient air in the underivatized form. This is the first paper to report on quinone characterization in secondary organic aerosols originating from oil sands activities, to distinguish chrysenequinone and anthraquinone positional isomers in ambient air, and to report the requirement of daylight conditions for benzo[a]pyrenequinone and naphthacenequinone to be present in ambient air.

Naphthoquinones of Sinningia reitzii and Anti-inflammatory/Antinociceptive Activities of 8-Hydroxydehydrodunnione

Chemical investigation of the tubers of Sinningia reitzii led to the isolation of five new naphthoquinones, 8-hydroxydehydrodunnione (1), 7-hydroxydehydrodunnione (2), 5-hydroxy-6,7-dimethoxy-α-dunnione (3), 5-hydroxy-6,7-dimethoxydunniol (4), and 8-hydroxy-7-methoxy-2-O-methylstreptocarpone (5). Three known naphthoquinones, 7-hydroxy-α-dunnione, 8-hydroxydunnione, and 6,8-dihydroxy-7-methoxy-2-O-methyldunniol, were also identified. When tested for anti-inflammatory activity in a mouse model, compound 1 (50-500 pg/paw) reduced the edema induced by carrageenan in a dose-dependent fashion. The highest dose showed a similar inhibition to that observed for the positive control dexamethasone. At lower doses (5-10 pg/paw), 1 also dose dependently reduced the mechanical hyperalgesia induced by carrageenan. Compound 1 (15 pg/paw) abolished the mechanical hyperalgesia induced by prostaglandin E₂ and dopamine, but not that induced by dibutyryl cyclic AMP. Dipyrone (320 μg/paw) completely abolished the hyperalgesia induced by these algogens. Additionally, compound 1 did not alter heat-induced nociception. These results suggest that this new naphthoquinone exhibits important anti-inflammatory and antinociceptive activities, which is dissimilar to that of most known analgesics.

Di- vs. tetra-substituted quinonediimines: a drastic effect on coordination chemistry

N,N′-Di-substituted benzoquinonediimine ligands behave differently in coordination chemistry compared to the well-known N,N′,N′′,N′′′-tetra-substituted analogues.

A quinone mediator drives oxidations catalysed by alcohol dehydrogenase-containing cell lysates

Spontaneous electron transport to molecular oxygen led to regeneration of oxidised nicotinamide cofactor in cell lysates that contain an alcohol dehydrogenase, a quinone reductase and a quinone mediator. This concept allows the efficient oxidation of alcohols in the presence of alcohol dehydrogenase-containing E. coli lysates and catalytic amounts of the quinone lawsone.