Electrosynthesis of a Biaurone by Controlled Dimerization of Flavone: Mechanistic Insight and Large-Scale Application.

The electrochemistry of flavone (1) has been carefully investigated at glassy carbon cathodes in dimethylformamide containing 0.10 M tetra-n-butylammonium tetrafluoroborate as supporting electrolyte. In this medium, a cyclic voltammogram for a reduction of 1 exhibits a reversible cathodic process (Epc = -1.58 V and Epa = -1.47 V vs SHE) that is followed by an irreversible cathodic peak (Epc = -2.17 V vs SHE). When water (5.0 M) is introduced into the medium, the first peak for 1 becomes irreversible (Epc = -1.56 V vs SHE), and the second (irreversible) peak shifts to -2.07 V vs SHE. Bulk electrolyses of 1 at -1.60 V vs SHE afford flavanone, 2'-hydroxychalcone, 2'-hydroxy-3-phenylpropionate, and two new compounds, namely (Z)-1,6-bis(2-hydroxyphenyl)-3,4-diphenylhex-3-ene-1,6-dione (D1) and (Z)-2,2'-(1,2-diphenylethene-1,2-bis(benzofuran-3(2H))-one) (D2), obtained in significant amounts, that were characterized by means of 1H and 13C NMR spectrometry as well as single-crystal X-ray diffraction. Along with the above findings, we have proposed a mechanism for the electroreduction of 1, which has been further corroborated by our quantum mechanical study.

Chemo-preventive and therapeutic effect of the dietary flavonoid kaempferol: A comprehensive review.

Kaempferol, a natural flavonoid present in several plants, possesses a wide range of therapeutic properties such as antioxidant, anticancer, and anti-inflammatory. It has a significant role in reducing cancer and can act as a therapeutic agent in the treatment of diseases and ailments such as diabetes, obesity, cardiovascular diseases, oxidative stress, asthma, and microbial contamination disorders. Kaempferol acts through different mechanisms: It induces apoptosis (HeLa cervical cancer cells), decreases cell viability (G2/M phase), downregulates phosphoinositide 3-kinase (PI3K)/AKT (protein kinase B) and human T-cell leukemia/lymphoma virus-I (HTLV-I) signaling pathways, suppresses protein expression of epithelial-mesenchymal transition (EMT)-related markers including N-cadherin, E-cadherin, Slug, and Snail, and metastasis-related markers such as matrix metallopeptidase 2 (MMP-2). Accordingly, the aim of the present review is to collect information pertaining to the effective role of kaempferol against various degenerative disorders, summarize the antioxidant, anti-inflammatory, anticancer, antidiabetic, and antiaging effects of kaempferol and to review the progress of recent research and available data on kaempferol as a protective and chemotherapeutic agent against several ailments.

Resveratrol as an anti-cancer agent: A review.

Owing to their antimicrobial, antioxidant, and anti-inflammatory activity, grapes (Vitis vinifera L.) are the archetypal paradigms of fruits used not only for nutritional purposes, but also for exclusive therapeutics. Grapes are a prominent and promising source of phytochemicals, especially resveratrol, a phytoalexin antioxidant found in red grapes which has both chemopreventive and therapeutic effects against various ailments. Resveratrol's role in reducing different human cancers, including breast, cervical, uterine, blood, kidney, liver, eye, bladder, thyroid, esophageal, prostate, brain, lung, skin, gastric, colon, head and neck, bone, ovarian, and cervical, has been reviewed. This review covers the literature that deals with the anti-cancer mechanism of resveratrol with special reference to antioxidant potential. Furthermore, this article summarizes the literature pertaining to resveratrol as an anti-cancer agent.

Electroreductive Remediation of Halogenated Environmental Pollutants.

Electrochemical reduction of halogenated organic compounds is gaining increasing attention as a strategy for the remediation of environmental pollutants. We begin this review by discussing key components (cells, electrodes, solvents, and electrolytes) in the design of a procedure for degrading a targeted pollutant, and we describe and contrast some experimental techniques used to explore and characterize the electrochemical behavior of that pollutant. Then, we describe how to probe various mechanistic features of the pertinent electrochemistry (including stepwise versus concerted carbon-halogen bond cleavage, identification of reaction intermediates, and elucidation of mechanisms). Knowing this information is vital to the successful development of a remediation procedure. Next, we outline techniques, instrumentation, and cell designs involved in scaling up a benchtop experiment to an industrial-scale system. Finally, the last and major part of this review is directed toward surveying electrochemical studies of various categories of halogenated pollutants (chlorofluorocarbons; disinfection byproducts; pesticides, fungicides, and bactericides; and flame retardants) and looking forward to future developments.

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO2-to-HONO Conversion on Soil Surfaces.

Humic acid (HA) is thought to promote NO2 conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO2 reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO2 to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO2 reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO2-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO2 reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO2-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.

Electroreductive dimerization of coumarin and coumarin analogues at carbon cathodes.

Electrochemical reduction of coumarin (1), 6-methylcoumarin (2), 7-methylcoumarin (3), 7-methoxycoumarin (4), and 5,7-dimethoxycoumarin (5) at carbon cathodes in dimethylformamide containing 0.10 M tetra-n-butylammonium tetrafluoroborate has been investigated by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1-5 exhibit two irreversible cathodic peaks: (a) the first peak arises from one-electron reduction of the coumarin to form a radical-anion intermediate, which is protonated by the medium to give a neutral radical; (b) although most of this radical undergoes self-coupling to yield a hydrodimer, reduction of the remaining radical (ultimately to produce a dihydrocoumarin) causes the second cathodic peak. At a potential corresponding to the first voltammetric peak, bulk electrolysis of 1-5 affords the corresponding hydrodimer as a mixture of meso and dl diastereomers. Although the meso form dominates, the dl-to-meso ratio varies, due to steric effects arising from substituents on the aromatic ring. Electroreduction of an equimolar mixture of 1 and 4 gives, along with the anticipated symmetrical hydrodimers, an unsymmetrical product derived from the two coumarins. A mechanistic scheme involving both radical-anion and radical intermediates is proposed to account for the formation of the various products.

Electrocatalytic oxygen activation by carbanion intermediates of nitrogen-doped graphitic carbon.

Nitrogen-doped graphitic carbon has been intensively studied for potential use as an electrocatalyst in fuel cells for the oxygen reduction reaction (ORR). However, the lack of a mechanistic understanding on the carbon catalysis has severely hindered the progress of the catalyst development. Herein we use a well-defined graphene nanostructure as a model system and, for the first time, reveal an oxygen activation mechanism that involves carbanion intermediates in these materials. Our work shows that the overpotential of the electrocatalytic ORR is determined by the generation of the carbanion intermediates, and the current by the rate the intermediates activate oxygen.

Electrochemical determination of trihalomethanes in water by means of stripping analysis.

A protocol has been developed and evaluated for the determination of trihalomethanes (THMs) at the submicromolar concentration level in water. This method is based on a three-step stripping analysis that utilizes a single electrochemical cell and that entails (a) direct electrochemical reduction of a trihalomethane at a silver cathode to form halide ions in an aqueous sample containing tetraethylammonium benzoate, (b) capture of the released halide ions as a silver halide film on the surface of a silver gauze anode, and (c) cathodic reduction and quantitation of the silver halide film by means of differential pulse voltammetry. Using this procedure, we have determined THMs individually; bromoform and chloroform have been successfully quantitated in 30 min and with a detection limit of 3.0 µg L(-1) (12 nM) and 6.0 µg L(-1) (50 nM), respectively. In addition, we have employed our methodology to determine the total trihalomethane (TTHM) content in a prepared water sample at a level commensurate with the maximum allowable annual average of 80 µg L(-1) mandated by the United States Environmental Protection Agency. We have compared our TTHM results to those obtained by an independent testing laboratory.

Octopods versus concave nanocrystals: control of morphology by manipulating the kinetics of seeded growth via co-reduction.

Au/Pd octopods and concave core@shell Au@Pd nanocrystals have been prepared by coupling for the first time a seed-mediated synthetic method with co-reduction. The integration of these two methods is central to the formation of these binary Au/Pd nanocrystals wherein the kinetics of seeded growth are manipulated via the co-reduction technique to control the final morphology of the nanocrystals. Significantly, the synthesis of these structures under similar reaction conditions illustrates that they are structurally related kinetic products. Detailed characterization by STEM-EDX analysis highlights the unique structural features of these nanocrystals and indicates that Pd localizes on the higher-energy features of the nanocrystals. Optical and electrocatalytic characterization also demonstrates their promise as a new class of multifunctional nanostructures.

A comprehensive review of the health perspectives of resveratrol.

Many natural products present in our diet, including flavonoids, can prevent the progression of cancer and other diseases. Resveratrol, a natural polyphenol present in various fruits and vegetables, plays an important role as a therapeutic and chemopreventive agent used in the treatment of various illnesses. It exhibits effects against different types of cancer through different pathways. It additionally exerts antidiabetic, anti-inflammatory, and anti-oxidant effects in a variety of cell types. Furthermore, the cardiovascular protective capacities of resveratrol are associated with multiple molecular targets and may lead to the development of novel therapeutic strategies for atherosclerosis, ischemia/reperfusion, metabolic syndrome, and heart failure. Accordingly, this article presents an overview of recent developments in the use of resveratrol for the prevention and treatment of different diseases along with various mechanisms. In addition, the present review summarizes the most recent literature pertaining to resveratrol as a chemotherapeutic agent against multiple diseases and provides an assessment of the potential of this natural compound as a complementary or alternative medicine.

Nickel Complexes of C-Substituted Cyclams and Their Activity for CO2 and H+ Reduction.

Several nickel(II) complexes of cyclams bearing aryl groups on the carbon backbone were prepared and evaluated for their propensity to catalyze the electrochemical reduction of CO2 to CO and/or H+ to H2, representing the first catalytic analysis to be performed on an aryl-cyclam metal complex. Cyclic voltammetry (CV) revealed the attenuation of catalytic activity when the aryl group bears the strong electron-withdrawing trifluoromethyl substituent, whereas the phenyl, p-tolyl, and aryl-free derivatives displayed a range of catalytic activities. The gaseous-product distribution for the active complexes was determined by means of controlled-potential electrolysis (CPE) and revealed that the phenyl derivative is the most active as well as the most selective for CO2 reduction over proton reduction. Stark differences in the activity of the complexes studied are rationalized through comparison of their X-ray structures, absorption spectra, and CPE profiles. Further CV studies on the phenyl derivative were undertaken to provide a kinetic insight.

Electrochemical Reduction of Mono- and Dihalothiophenes at Carbon Cathodes in Dimethylformamide. First Example of an Electrolytically Induced Halogen Dance.

Cyclic voltammetry and controlled-potential electrolysis have been employed to probe the electrochemical reduction of a number of mono- and dihalothiophenes at carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Reduction of 2-bromo-, 3-bromo-, 2-chloro-, 3-chloro-, and 2-iodothiophene gives rise to a single irreversible cyclic voltammetric wave for each compound that corresponds to the two-electron cleavage of the carbon-halogen bond, and thiophene is obtained as the only product. Cyclic voltammograms for the reduction of 2,3-dibromo-, 2,4-dibromo-, 2,5-dibromo-, 3,4-dibromo-, 2-bromo-5-chloro-, and 3-bromo-2-chlorothiophene each exhibit a pair of irreversible two-electron waves. Electrolyses of either 2,3-dibromo- or 2,4-dibromothiophene at potentials corresponding to the first voltammetric wave yield a two-to-one mixture of 3-bromo- and 3,4-dibromothiophene; under similar conditions, electrolyses of 2,5-dibromothiophene give a mixture of 2-bromo-, 3-bromo-, and 3,4-dibromothiophene, electrolyses of 2-bromo-5-chlorothiophene afford a mixture of 3-bromo-, 3,4-dibromo-, 3-bromo-2-chloro-, 4-bromo-2-chloro-, and 2-chlorothiophene, and electrolyses of 3-bromo-2-chlorothiophene yield 2-chlorothiophene. Aside from the last result, these product distributions appear to arise from an electrolytically induced halogen dance. When electrolyses of the dibromothiophenes and of 2-bromo-5-chloro- and 3-bromo-2-chlorothiophene are performed at potentials that correspond to the second voltammetric wave, thiophene is the only product obtained.

Electrosynthesis of substituted 1H-indoles from o-nitrostyrenes.

A novel procedure has been devised for the synthesis of derivatives of 1H-indole that is based on the direct, room-temperature electrochemical reduction of substituted o-nitrostyrenes at carbon cathodes in N,N-dimethylformamide containing tetramethylammonium tetrafluoroborate as supporting electrolyte and in the presence of a 10-fold molar excess of a proton donor (phenol or methyl 3-oxobutanoate).

Catalytic reduction and intramolecular cyclization of haloalkynes in the presence of nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide.

[reaction: see text] Pentylidenecyclopentane can be conveniently prepared in up to 86% yield via the catalytic reduction of 1-iodo- or 1-bromo-5-decyne by [[2,2'-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N',O,O']nickelate(I) electrogenerated at a carbon cathode in dimethylformamide containing tetramethylammonium tetrafluoroborate. This electrosynthesis can be accomplished at potentials for which the haloalkynes are electroinactive, and it can be completed within 30 min at room temperature. Attempts to synthesize pentylidenecyclobutane and pentylidenecyclohexane from 1-halo-4-nonynes and 11-halo-5-undecynes, respectively, under similar conditions afford the carbocycles in very low yields (2% and 6%, respectively). Other products derived from the various haloalkynes are dimers, alkynes, and 1-alkenynes. Dimers (alkadiynes) arising from 1-halo-4-nonynes and 11-halo-5-undecynes are formed in yields ranging from 80% to 89%, whereas icosa-5,15-diyne (the dimer obtained from a 1-halo-5-decyne) is found in significantly lower yield (

Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(i) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide.

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate and characterize the reductive intramolecular cyclization of ethyl 2-bromo-3-(3',4'-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) promoted by (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)nickel(I), [Ni(tmc)](+), electrogenerated at glassy carbon cathodes in dimethylformamide containing tetraalkylammonium salts. Cyclic voltammograms for reduction of [Ni(tmc)](2+) in the presence of 1 reveal that [Ni(tmc)](+) catalytically reduces 1 at potentials more positive than those required for direct reduction of 1. During controlled-potential electrolyses of solutions containing [Ni(tmc)](2+) and 1, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon-bromine bond to form a radical intermediate that undergoes cyclization to afford 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methylenetetrahydrofuran (2). In the presence of a base (either electrogenerated or deliberately added as potassium tert-butoxide), 2 rearranges to give 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3). A mechanistic scheme is proposed to explain the results obtained by means of cyclic voltammetry and controlled-potential electrolysis.

Metal-ligand charge-transfer-promoted photoelectronic Bergman cyclization of copper metalloenediynes: photochemical DNA cleavage via C-4' H-atom abstraction.

Metal-to-ligand charge-transfer (MLCT) photolyses (lambda > or = 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), [Cu(bpod)(2)]PF(6) (2), and [Cu(bpod)(2)](NO(3))(2) (3) yield Bergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)(2)(CH(3)COO)(2) compound (4) are photochemically inert under the same conditions. In the case of 4, sensitized photochemical generation of the lowest energy (3)pi-pi state, which is localized on the enediyne unit, leads to production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemical studies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between E(p) values of -1.75 and -1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E(1/2) = +0.15 and +0.38 V, respectively. Combined with the observed Cu(I)-to-pyridine(pi) MLCT and pyridine(pi)-to-Cu(II) ligand-to-metal charge transfer (LMCT) absorption centered near approximately 315 nm, the results suggest a mechanism for photo-Bergman cyclization that is derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediates produced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5'-phosphate and 3'-phosphoglycolate termini that are derived from H-atom abstraction from the 4'-position of the deoxyribose ring rather than redox-induced base oxidation.