- Oxidation using [bis(trifluoroacetoxy)]iodobenzene: A new and potentially practical approach to detection of polychlorinated phenols
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A novel oxidation for pentachlorophenol, 2,4,6-trichlorophenol and 2,3,5,6-tetrachlorophenol using [bis-(trifluoroacetoxy)]iodobenzene has been developed, and the oxidation products from pentachlorophenol and 2,3,5,6-tetrachlorophenol have been identified as tetrachloro-1,4-benzoquinone; this novel reaction can be applied in electrochemistry using glucose oxidase for sensitive determination and identification of PCP, one of the most toxic polychlorinated phenols.
- Saby, Coralie,Luong, John H. T.
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- Effect of oxalate and pH on photodegradation of pentachlorophenol in heterogeneous irradiated maghemite System
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Photochemical degradation in the system of iron oxides and oxalic acid (OX) is the important reaction for detoxification of organic pollutants in natural environments, including surface soils, surface water, and even aerosols, and it was more effective at low pH according to previous studies. However, in this study, the photodegradation of pentachlorophenol (PCP) proceeded rapidly at different pH conditions in the system with maghemite and OX under UV light illumination. It was observed that the removal of PCP was 77.7% ± 0.90%, 79.9% ± 0.80% and 74.3% ± 1.50% at initial pH of 3.5, 5.0 and 7.0, respectively. To explore the degradation mechanism, the interaction of OX and maghemite were systematically studied as a function of pH. The presence of OX of 1.2 mM effectively decreased the iso-electric point (iep) of the maghemite from 5.6 to 1.8. The maximum adsorption amount of maghemite adsorbing OX increased with increasing pH value from 208 mmol kg-1 at pH = 3.5 to 293 mmol kg-1 at pH = 9.0. However, PCP (0.0375 mM) inhibited the adsorption of oxalic acid at pH = 3.5 and pH = 5.0 but promoted it at pH = 7.0 and pH = 9.0. When the initial content of OX was 1.2 mM, the highly active compounds of Fe(C2O4)33- as Fe(III) and Fe(C2O4)22- as Fe(II) were the dominant species at different pH. The formation of H2O2 also relied on the value of pH and the concentration range of H2O2 during PCP degradation was 0-1.67 mg L-1, 0-1.16 mg L-1 and 0-0.16 mg L-1at initial pH of 3.5, 5.0 and 7.0, respectively. The low pH conditions favored the iron cycling, the H2O2 generation and the broken of aromatic ring of PCP, so as to enhance the degradation rates of PCP. At the high pH conditions, due to the slowdown of the iron cycling and the decreased amount of H2O2 formation, the direct photolysis was responsible for the enhanced degradation of PCP. The foundation of high photochemical efficiencies of OX and maghemite for PCP degradation at large-scale pH conditions improves the photochemical mechanisms of OX-iron oxide system and is of important for understanding the transformation of organic pollutants in light environments.
- Lan, Qing,Cao, Meiyuan,Ye, Zhijun,Zhu, Jishu,Chen, Manjia,Chen, Xuequan,Liu, Chengshuai
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- Azido bridge mediated catecholase activity, electrochemistry and magnetic behavior of a dinuclear copper(II) complex of a phenol based "end-off" compartmental ligand
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Abstract A dinuclear Cu(II) species [Cu2L2(H2O)2(N3)](NO3)2 (L = 2,6-bis(N-ethylpyrrolidine-iminomethyl)-4-methyl-phenolato) where two Cu centers are bridged by phenoxido and μ1,1-azido bridges with Cu-Cu separation of ～3 ? have been synthesized with the view to explore the role of azido bridge on catecholase activity and electrochemical property and the roles of both the bridging groups on magnetic coupling of two copper centers. The complex exhibits excellent catecholase activity in acetonitrile as well as in DMSO medium not only by oxidizing 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very thorny to oxidize, under aerobic conditions and becomes the first example of its own kind. CV study reveals three quasi-reversible reductive couples which are tentatively assigned as Cu2II to CuIICuI and CuICuI reduction followed by reduction of CuICuI complex to Cu0Cu0 species. Variable temperature magnetic study suggests the presence of an antiferromagnetic spin-exchange interaction between Cu(II) ions in the dimer via double bridge where the antiferromagnetic contribution of phenoxido bridge predominates over the ferromagnetic interaction of azido bridge.
- Chakraborty, Prateeti,Majumder, Ishani,Kara, Hulya,Chattopadhyay, Shyamal Kumar,Zangrando, Ennio,Das, Debasis
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- Activation of electron transfer reduction of p-benzoquinone derivatives by intermolecular regioselective hydrogen bond formation
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Electron transfer reduction of p-benzoquinones by cobalt tetraphenylporphyrin is enhanced significantly by the presence of o-bis(phenylcarbamoylmethyl)benzene (o-L) due to the regioselective hydrogen bond formation between the corresponding semiquinone ra
- Fukuzumi, Shunichi,Kitaguchi, Hironori,Suenobu, Tomoyoshi,Ogo, Seiji
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- Coexistence of Hydrogen Atom Transfer Reactions through and not through Triplet Ion Pair between p-Chloranil and Durene
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Mechanism of hydrogen atom abstraction reactions by triplet state p-chloranil (3CA) from durene (DH) were studied by picosecond and nanosecond laser photolysis and transient photoconductivity measurements. 3CA was quenched by DH through diffusional encounter to form a triplet ion pair (IP) between CA and DH, p-chloranil semiquinone radical (CAH.), and 2,4,5-trimethylbenzyl radical (D.).Ionic dissociation of IP was observed in 1,2-dichloroethane (DCE) as well as in acetonitrile.However, no transient species was observed by direct excitation of a charge-transfer (CT) band of the electron donor-acceptor (EDA) complex between CA and DH.The H-atom transfer leading to production of CAH. was found to proceed through two distinct mechanisms; H-atom transfer via IP (Mechanism I) and a more rapid transfer competing with IP formation (Mechanism II).The quantum yields of CAH. produced by Mechanism I and II and the first-order rate constants for proton transfer, ionic dissociation, and intersystem crossing competing with one another in the IP state were estimated to be (0.1 and 0.2) and (2,5, and 13)X106 s-1, respectively, in DCE at room temperature.
- Kobashi, Harumichi,Funabashi, Masa-aki,Kondo, Tomoyuki,Morita, Toshifumi,Okada, Tadashi,Mataga, Noboru
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- A flow injection (FI) biosensor system for pentachlorophenol (PCP) using a substance recycling scheme
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A flow injection (FI) biosensor system has been designed for the analysis of pentachlorophenol (PCP) using a substrate recycling scheme comprising immobilized bilirubin oxidase (BOX) in the presence of excess of NADH. PCP was efficiently converted to tetrachloro-p-benzoquinone (1,4-TCBQ) and then tetrachloro-p-hydroquinone (1,4-TCHQ) by bis(trifluoroacetoxy)iodobenzene (BTFAIB) and zinc powder, respectively. BOX immobilized on aminopropyl glass beads rapidly oxidized 1,4-TCHQ to 1,4-TCBQ, which in turn was readily reduced back to 1,4-TCHQ in the presence of excess NADH. This recycling scheme enabled one molecule of PCP to consume several NADH molecules leading to enhanced sensitivity. Under optimized conditions the rate of NADH uptake measured as the absorbance decrease at 340 nm yielded a detection limit for 1,4-TCHQ or oxidized PCP of 250 nM. The detection limit was improved to 25 nM for both analytes using a fluorescence detector with excitation and emission wavelengths of 345 and 450 nm, respectively. The PCP level in contaminated soil samples was measured using the FI biosensor system, and the results obtained compared well with capillary zone electrophoresis (CZE) analysis. A flow injection (FI) biosensor system has been designed for the analysis of pentachlorophenol (PCP) using a substrate recycling scheme comprising immobilized bilirubin oxidase (BOX) in the presence of excess of NADH. PCP was efficiently converted to tetrachloro-p-benzoquinone (1,4-TCBQ) and then tetrachloro-p-hydroquinone (1,4-TCHQ) by bis(trifluoroacetoxy)iodobenzene (BTFAIB) and zinc powder, respectively. BOX immobilized on aminopropyl glass beads rapidly oxidized 1,4-TCHQ to 1,4-TCBQ, which in turn was readily reduced back to 1,4-TCHQ in the presence of excess NADH. This recycling scheme enabled one molecule of PCP to consume several NADH molecules leading to enhanced sensitivity. Under optimized conditions the rate of NADH uptake measured as the absorbance decrease at 340 nm yielded a detection limit for 1,4-TCHQ or oxidized PCP of 250 nM. The detection limit was improved to 25 nM for both analytes using a fluorescence detector with excitation and emission wavelengths of 345 and 450 nm, respectively. The PCP level in contaminated soil samples was measured using the FI biosensor system, and the results obtained compared well with capillary zone electrophoresis (CZE) analysis.
- Labra-Espina, Marietta,Male, Keith B.,Luong, John H. T.
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- Peroxidase-Catalyzed Oxidation of Pentachlorophenol
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Pentachlorophenol (PCP) was shown to function as a reducing substrate for horseradish peroxidase (HRP) and to stimulate the HRP-catalyzed reduction of 5-phenyl-4-penten-1-yl hydroperoxide (PPHP) to 5-phenyl-4-penten-1-ol. HRP catalyzed the hydroperoxide-dependent oxidation of PCP, using H2O2, PPHP, or ethyl hydroperoxide as substrates, as evidenced by UV spectroscopic and reverse phase HPLC analysis of reaction mixtures. The major oxidation product was tetrachloro-1,4-benzoquinone which was identified on the basis of electronic absorption spectroscopy, mass spectrometry, and cochromatography with authentic standard. HRP-catalyzed oxidation of PCP yielded relatively stable, ESR-detectable pentachlorophenoxyl radical intermediates whose ESR spectra consisted of a symmetrical single line without hyperfine structure. Substitution of natural abundance isotopically-labeled PCP with 13C-labeled PCP resulted in broadening of the ESR signal line width from 6.1 G to 13.5 G. ESR spin trapping studies, with α-(1-oxy-4-pyridyl)-N tert-butylnitrone (4-POBN) as the spin trap demonstrated identical spectra using natural abundance isotopically-labeled PCP versus 13C-labeled PCP, suggesting oxyl addition, rather than carbon-centered radical addition to 4-POBN. The computer simulation of the observed spectra is consistent with two distinct 4-POBN adducts, with relative abundances of ca. 3:1, and hyperfine coupling constants of aN = (14.61 G)/aH = 1.83 G and aN = (14.76 G)/aH = 5.21 G, respectively. Mechanisms for the hydroperoxide-dependent, HRP-catalyzed oxidation of PCP are presented that are consistent with these results.
- Samokyszyn, Victor M.,Freeman, James P.,Maddipati, Krishna Rao,Lloyd, Roger V.
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- Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I
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The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH2) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H2) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH2) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr+-Mes) as a photoredox catalyst, and a cobalt(III) complex, CoIII(dmgH)2pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H2 and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H2 evolution from tetrachlorohydroquinone (Cl4QH2) with Acr+-Mes and CoIII(dmgH)2pyCl and H2O in deaerated MeCN was determined to be 10%. Photocatalytic H2 evolution is started by electron transfer (ET) from Cl4QH2 to the triplet ET state of Acr+-Mes to produce Cl4QH2?+ and Acr?-Mes with a rate constant of 7.2 × 107 M-1 s-1, followed by ET from Acr?-Mes to CoIII(dmgH)2pyCl to produce [CoII(dmgH)2pyCl]-, accompanied by the regeneration of Acr+-Mes. On the other hand, Cl4QH2?+ is deprotonated to produce Cl4QH?, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [CoII(dmgH)2pyCl]- to produce a cobalt(III) hydride complex, [CoIII(H)(dmgH)2pyCl]-, which reacts with H+ to evolve H2, accompanied by the regeneration of CoIII(dmgH)2pyCl. The formation of [CoII(dmgH)2pyCl]- was detected by electron paramagnetic resonance measurements.
- Fukuzumi, Shunichi,Hong, Young Hyun,Lee, Yong-Min,Nam, Wonwoo
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- Substrate recycling scheme for tetrachloro-p-benzoquinone using bilirubin oxidase and NADH: Application for pentachlorophenol assay
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A novel assay for tetrachloro-p-benzoquinone (TCBQ), the main oxidation product of pentachlorophenol (PCP), was developed using bilirubin oxidase (BOX) in the presence of excess NADH. TCBQ was easily and rapidly reduced by NADH to 1,4-tetrachlorohydroquinone (TCHQ), which was then recycled back to TCBQ by the enzyme. BOX exhibited no reactivity toward NADH while its catalytic activity for the oxidation of TCHQ was very high. Under an optimized condition (250 μM NADH, 0.3 U/mL BOX, and 25 mM sodium phosphate at pH 5.5), the rate of NADH consumption determined by measuring the absorbance decrease at 340 nm yielded a detection limit for TCBQ of 110 nM. Fluorescence detection of the NADH using a lower enzyme concentration (0.1 U/mL) with excitation and emission wavelengths of 345 and 450 nm, respectively, allowed for a TCBQ detection limit of 30 nM. PCP was oxidized to TCBQ with high yield using bis(trifluoroacetoxy)iodobenzene in 0.05 M trichloroacetic acid. Coupling this oxidation reaction to the BOX/NADH assay attained PCP detection limits of 170 and 50 nM using absorbance and fluorescence measurements, respectively. When tested on PCP-contaminated soil samples, the BOX assay compared very well with HPLC measurements. Chlorophenols constitute a major group of pollutants having been widely used as wood preservatives, pesticides, and herbicides. They are also formed as byproducts of many industrial activities including chlorination of potable water and paper bleaching. A novel assay for tetrachloro-p-benzoquinone (TCBQ), the main oxidation product of pentachlorophenol (PCP), was developed using bilirubin oxidase (BOX) in the presence of excess NADH. TCBQ was easily and rapidly reduced by NADH to 1,4-tetrachlorohydroquinone (TCHQ), which was then recycled back to TCBQ by the enzyme. BOX exhibited no reactivity toward NADH while its catalytic activity for the oxidation of TCHQ was very high. Under an optimized condition (250 μM NADH, 0.3 U/mL BOX, and 25 mM sodium phosphate at pH 5.5), the rate of NADH consumption determined by measuring the absorbance decrease at 340 nm yielded a detection limit for TCBQ of 110 nM. Fluorescence detection of the NADH using a lower enzyme concentration (0.1 U/mL) with excitation and emission wavelengths of 345 and 450 nm, respectively, allowed for a TCBQ detection limit of 30 nM. PCP was oxidized to TCBQ with high yield using bis(trifluoroacetoxy)iodobenzene in 0.05 M trichloroacetic acid. Coupling this oxidation reaction to the BOX/NADH assay attained PCP detection limits of 170 and 50 nM using absorbance and fluorescence measurements, respectively. When tested on PCP-contaminated soil samples, the BOX assay compared very well with HPLC measurements.
- Cybulski, David,Male, Keith B.,Scharer, Jeno M.,Moo-Young, Murray,Luong, John H. T.
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- Primary product of the horseradish peroxidase-catalyzed oxidation of pentachlorophenol
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Peroxidases are a class of enzymes that catalyze the oxidation of various phenolic substrates by hydrogen peroxide. They are common enzymes in soil and are also available commercially, so that they have been proposed as agents of phenolic pollutant transformation both in the environment and in engineered systems. Previous research on the peroxidase-catalyzed oxidation of pentachlorophenol (PCP) has suggested that tetrachloro-p-benzoquinone (chloranil) is the principal product and that a considerable fraction of the PCP added to reaction mixtures appears to be resistant to oxidation. In experiments employing alternative methods of product separation and analysis, we found that both of these observation s are artifacts of extraction and analytical methods used in previous studies. The major product of the horseradish peroxidase-catalyzed oxidation of pentachlorophenol from pH 4-7 was 2,3,4,5,6-pentachloro-4-pentachlorophenoxy-2,5-cyclohexadienone (PPCHD), which is formed by the coupling of two pentachlorophenoxyl radicals. The yield of chloranil and other soluble products was negligible. PPCHD is insoluble and unreactive in aqueous media but is reactive when dissolved in various organic solvents. Substantia amounts of chloranil were formed when PPCHD was dissolved in benzene, ethyl acetate, or methanol; less was formed in hexane and acetonitrile; and negligible amounts were formed in dimethylformamide. High-pressure liquid chromatography (HPLC) analysis of PPCHD indicated that it is capable of undergoing dissociation and reduction to pentachloro phenol under typical reversed-phase HPLC conditions. Chlorinated oligomeric products are formed when PPCHD is stored in acetonitrile, either alone or with added pentachlorophenol. Our results demonstrate that the removal of PCP in peroxidase-catalyzed reactions can be much higher than indicated in previous work, as long as the initial product is separated by filtration or other physical means. The major product of the horseradish peroxidase-catalyzed oxidation of pentachlorophenol from pH 4-7 was 2,3,4,5,6-pentachloro-4-pentachlorophenoxy-2,5-cyclohexadienone (PPCHD), which is formed by the coupling of two pentachlorophenoxyl radicals. PPCHD is insoluble and unreactive in aqueous media but is reactive when dissolved in various organic solvents. High pressure liquid chromatography (HPLC) analysis of PPCHD indicated that it is capable of undergoing dissociation and reduction to pentachlorophenol under typical reversed-phase HPLC conditions. The removal of pentachlorophenol in peroxidase-catalyzed reactions can be much higher as long as the initial product is separated by filtration or other physical means.
- Kazunga, Chikoma,Aitken, Michael D.,Gold, Avram
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- Kinetics of photodegradation and ozonation of pentachlorophenol
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The oxidation of 2,3,4,5,6-pentachlorophenol (PCP) has been carried out by a photodecomposition process using a polychromatic UV irradiation, and by an ozonation process. In the photodegradation process, the pH accelerated the decomposition rate and the approximate first-order rate constants were evaluated, with values between 0.16 ± 0.005 min-1 at pH = 3 and 0.26 ± 0.007 min-1 at pH = 9. A more rigorous kinetic study led to the determination of the quantum yields of the reaction, with values of 200 ± 7 x 10-3 mol/Eins for pH = 3 and 22 ± 1.1 x 10-3 mol/Eins for pH = 9. In the ozonation process, the rate constants for the reaction between ozone and PCP were determined by means of a competition kinetics, with values in the range from 0.67 x 105 to 314 x 105 l/mol s. The specific rate constants for the un-dissociated and dissociated forms of PCP were also calculated. Finally, in both processes, the intermediate reaction products were identified, the most important being tetrachlorocatechol, tetrachlorohydroquinone and tetra-p-chlorobenzoquinone. Free chloride ion released, which was favored at high pHs, was also followed in both processes.
- Benitez, F. Javier,Acero, Juan L.,Real, Francisco J.,Garcia, Juan
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- Toxicity changes during the UV treatment of pentachlorophenol in dilute aqueous solution
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Pentachlorophenol (PCP) was photolysed using a 1 kW photochemical reactor. The degradation of PCP (0.15 mM) was carried out using both direct UV photolysis and photolysis in the presence of H2O2 (3.0, 6.7, 18.0 and 37.2 mM). The decay of PCP and the formation of photoproducts were followed by high performance liquid chromatography, ion chromatography and UV absorbance detection. The first-order rate constants k1 for the decay of PCP were estimated under various conditions: for direct UV photolysis, k1 = 0.115 min-1; for photolysis in the presence of various concentrations of H2O2, the rate constant increased to a plateau (k1 ? 0.7 min-1) at H2O2 concentrations greater than 6 mM. The correlation between photodegradation and toxicity was studied using a bacterial toxicity test and a 96 h Fathead Minnow toxicity test. In both cases, the toxicity decreased as the concentration of PCP or the total organic chlorine concentration fell. These results indicate that the UV treatment of PCP either does not generate significant levels of acutely toxic intermediates, or any toxic intermediates are rapidly degraded under the specific conditions used in these studies. Pentachlorophenol (PCP) was photolysed using a 1 kW photochemical reactor. The degradation of PCP (0.15 mM) was carried out using both direct UV photolysis and photolysis in the presence of H2O2 (3.0, 6.7, 18.0 and 37.2 mM). The decay of PCP and the formation of photoproducts were followed by high performance liquid chromatography, ion chromatography and UV absorbance detection. The first-order rate constants k1 for the decay of PCP were estimated under various conditions: for direct UV photolysis, k1 = 0.115 min-1; for photolysis in the presence of various concentrations of H2O2, the rate constant increased to a plateau (k1≈0.7 min-1) at H2O2 concentrations greater than 6 mM. The correlation between photodegradation and toxicity was studied using a bacterial toxicity test and a 96 h Fathead Minnow toxicity test. In both cases, the toxicity decreased as the concentration of PCP or the total organic chlorine concentration fell. These results indicate that the UV treatment of PCP either does not generate significant levels of acutely toxic intermediates, or any toxic intermediates are rapidly degraded under the specific conditions used in these studies.
- Ho, Te-Fu L.,Bolton, James R.
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- DETECTION OF AN INTERMEDIATE TERNARY COMPLEX IN THE REDUCTION OF p-BENZOQUINONE DERIVATIVES BY AN NADH MODEL COMPOUND IN THE PRESENCE OF Mg2+ ION
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A transient charge-transfer (CT) band of a ternary complex due to the CT transition from an NADH model compound - Mg2+ ion complex to a p-benzoquinone derivative was observed in the reduction of a p-benzoquinone derivative by an NADH model compound in the presence of Mg2+ ion in acetonitrile.The role of the ternary complex in the reduction by an NADH model compound is reported.
- Fukuzumi, Shunichi,Nishizawa, Nobuaki,Tanaka, Toshio
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- Optimization and characterization of a flow injection electrochemical system for pentachlorophenol assay
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A flow injection (FI) electrochemical detection system has been developed and optimized for the determination of pentachlorophenol (PCP) in contaminated soil. PCP was oxidized to tetrachloro-1,4-benzoquinone (1,4- TCBQ) with a high yield using bis(trifluoroacetoxy)iodobenzene in 0.1 M tartaric acid, pH 2.0, at ambient temperature. Upon rapid reaction with immobilized glucose oxidase, the detection and amplification scheme was completed as the reduced form of 1,4-TCBQ or tetrachloro-1,4-hydroquinone was reoxidized to 1,4-TCBQ at the surface of the glassy carbon electrode (+ 0.40 V vs Ag/AgCl). Rapid electron exchange between the enzyme and its glucose substrate provided a non-rate-limiting current toward the electrode. The FI electrochemical system was linear up to 1 μM oxidized PCP with a detection limit of 10 nM and exhibited a reproducibility of ±0.6% over 165 repeated analyses during 14 h of continuous operation. When applied to PCP- contaminated soil samples, the results obtained from the FI electrochemical system compared well with those of the HPLC standard method.
- Male, Keith B.,Saby, Coralie,Luong, John H. T.
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- In Situ Electrochemical EPR Studies of Charge Transfer across the Liquid/Liquid Interface
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The in situ measurement of EPR spectra of radical ions generated at the polarized liquid/liquid interface is described in relation to the 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrachloro-p-benzoquinone (TCBQ), and 2,3,5,6-tetrafluoro-p-benzoquinone (TFBQ) radical anions and the tetrathiafulvalene (TTF) radical cation. TCNQ and TTF were chosen as model compounds with which to quantify the performance of a novel liquid/ liquid electrochemical EPR cell. The anion radical of TCNQ and the cation radical of TTF in 1,2-dichloroethane (DCE) were produced at the water interface by electron transfer from/to the aqueous-phase ferricyanide/ferrocyanide redox couple by applying a potential difference between the two phases with a four-electrode potentiostat. The EPR signal intensity (at constant magnetic field) of the resultant organic radicals was monitored during potential step experiments which indicated that the EPR data could be modeled in terms of diffusional transport. TCBQ and TFBQ were chosen as compounds to model the electron transfer behavior of biologically important quinones at the oil/water interface. The EPR and voltammetric data obtained from TCBQ/TCBQ-· and TFBQ/ TFBQ-· indicated unambiguously that the two semiquinones are stable at the DCE/water interface and do not undergo immediate protonation.
- Webster, Richard D.,Dryfe, Robert A. W.,Coles, Barry A.,Compton, Richard G.
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- Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes
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A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF 3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for parasubstituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.
- De Lijser, H.J. Peter,Kim, Jason S.,McGrorty, Suzanne M.,Ulloa, Erin M.
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- Photochemical mechanisms responsible for the versatile application of naphthalimides and naphthaldiimides in biological systems
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Despite the number and variety of their biological applications, the mechanisms of action of the photoactive naphthalenic imides have not yet been fully elucidated. In order to provide mechanistic insight, the photochemistry of several N-substituted 1,8-naphthalimides (NT) and 1,4,5,8-naphthaldiimides (NDI) has been studied using absorption and fluorescence spectroscopy and by laser flash photolysis (λ(exc) = 355 nm). The lowest singlet state (S1) is mainly ππ* in nature for NI whereas nπ* character predominates for the NDI. This difference exerts a profound effect on subsequent reaction mechanisms: upon irradiation, only the NDI molecules can undergo intramolecular γ hydrogen abstraction. In the case of NP-III, a bishydroperoxy NDI derivative, this photoprocess (Φ = 0.03) leads to concomitant formation of an oxygen-centered radical (ε = 21,600 M-1 cm-1 at 465 nm in acetonitrile) and release of the hydroxyl radical (.OH). All the compounds studied produce the triplet state (in acetonitrile, ε(T) ~ 10,500-11,500 M-1 cm-1 at 470 nm for NI and 485 nm for NDI). The quantum yield of intersystem crossing was determined to be close to unity except where intramolecular γ hydrogen abstraction was possible (Φ(isc) 0.8 for NI and > 0.5 for NDI). In the absence of quenchers, the triplet states react with the ground-state of starting material via electron-transfer with a high rate constant [k = (4-6) x 109 and 5 x 108 M-1 s-1 for NDI and NI, respectively] to give the radical anion and radical cation of the corresponding naphthalenic derivative. The high reactivity of the triplet states toward electron donors such as DABCO and their low ability for hydrogen abstraction are typical of a ππ* configuration. These mechanistic photochemistry results are discussed with regard to the photobiological effects observed for these compounds and show that the actual reaction leading to biological damage will depend on the microenvironment of the naphthalenic molecule.
- Aveline, Béatrice M.,Matsugo, Seiichi,Redmond, Robert W.
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- A radical intermediate in the conversion of pentachlorophenol to tetrachlorohydroquinone by sphingobium chlorophenolicum
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Pentachlorophenol (PCP) hydroxylase, the first enzyme in the pathway for degradation of PCP in Sphingobium chlorophenolicum, is an unusually slow flavin-dependent monooxygenase (kcat = 0.02 s-1) that converts PCP to a highly reactive product, tetrachlorobenzoquinone (TCBQ). Using stopped-flow spectroscopy, we have shown that the steps up to and including formation of TCBQ are rapid (5-30 s-1). Before products can be released from the active site, the strongly oxidizing TCBQ abstracts an electron from a donor at the active site, possibly a cysteine residue, resulting in an off-pathway diradical state that only slowly reverts to an intermediate capable of completing the catalytic cycle. TCBQ reductase, the second enzyme in the PCP degradation pathway, rescues this nonproductive complex via two fast sequential one-electron transfers. These studies demonstrate how adoption of an ancestral catalytic strategy for conversion of a substrate with different steric and electronic properties can lead to subtle yet (literally) radical changes in enzymatic reaction mechanisms.
- Rudolph, Johannes,Erbse, Annette H.,Behlen, Linda S.,Copley, Shelley D.
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- Generation and reactivity of the radical cations of coniferyl alcohol and isoeugenol in solution
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Nanosecond laser flash photolysis of coniferyl alcohol and isoeugenol in acetonitrile leads to the formation of transient species that are identified as the corresponding radical cations. These radical cations decay with rate constants of ca. 1 × 106 s-1 in dry acetonitrile. Both radical cations react rapidly with hydroxylic solvents like water and alcohols to give 4-vinylphenoxyl radicals, indicating that these reagents behave as bases rather than nucleophiles. In addition, anionic reagents (acetate, cyanide, and chloride) react rapidly with the radical cations with second-order rate constants that are close to diffusion controlled. The main products generated in the presence of the anionic reagents are again the 4-vinylphenoxyl radicals, suggesting that these reagents also behave as bases. The lifetime of the radical cations in acidic acetonitrile was found to increase dramatically due to a shift in the radical cation - vinyl phenoxyl radical acid-base equilibrium to the side of the radical cation. An estimate of the pKa of the radical cation in acetonitrile of 4.0 was obtained from the data.
- Schepp,Rodriguez-Evora
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Read Online
- Preparation method of regenerated chloranil
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The invention provides a preparation method of regenerated chloranil, and belongs to the technical field of chemical production. According to the method, tetrachlorohydroquinone is taken as a raw material, hydrogen peroxide is taken as an oxidizing agent, under the action of ferric trichloride and a phase transfer catalyst, and an oxidation reaction is carried out in an organic solvent which is insoluble in water so as to obtain chloranil. Specifically, the organic solvent which is insoluble in water is adopted, so that the solvent is convenient to recycle and reuse, and the environmental pollution is small; ferric trichloride and the phase transfer catalyst are matched with hydrogen peroxide for oxidation reaction, the production period is short, and the production efficiency is high. The method provided by the invention is simple and convenient, has high product yield and purity, is suitable for industrial large-scale production, and has good economic benefits.
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Paragraph 0035-0048
(2021/04/26)
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- Synthesis of 2,4-Diarylquinoline Derivatives via Chloranil-Promoted Oxidative Annulation and One-Pot Reaction
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An oxidative annulation for the synthesis of 2,4-diarylquinolines from o -allylanilines is disclosed that uses recyclable reagent Chloranil as the oxidant. The corresponding products are obtained in moderate to excellent yields. Furthermore, a one-pot access to 2,4-di aryl quinolines from easily available anilines and 1,3-diarylpropenes is described as a highly atom-efficient protocol that involves oxidative coupling, rearrangement, and oxidative annulation.
- Cheng, Dongping,Pu, Yueqi,Shen, Jing,Xu, Xiaoliang,Yan, Jizhong,Yan, Xianhang
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supporting information
p. 1833 - 1840
(2020/06/08)
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- Method for regenerating chloranil by oxidation of hydrogen peroxide
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The invention discloses a method for regenerating chloranil by oxidation of hydrogen peroxide and belongs to the technical field of preparation of compounds. The method mainly comprises the followingsteps of: 1) adopting chloranil as a raw material and adopting hydrogen peroxide as an oxidant, stirring in a methanol solvent to carry out reaction, wherein chloranil is almost not dissolved in methanol, so that with conduction of reaction, products are gradually precipitated out to obtain yellow solid; 2) separating reaction liquid obtained in the step 1) by simple filtering operation to obtaina product (chloranil) so as to realize regeneration of chloranil. The method disclosed by the invention has the beneficial effects that the cost is lower, the aftertreatment is simple and convenient,the yield of chloranil is higher, the solvent can be reused for multiple times, the operation is simple, the produced sewage is less, so that the method is environmentally friendly and is suitable forindustrial large-scale production.
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Paragraph 0021-0030; 0031-0040; 0041-0054; 0055; 0056
(2018/11/04)
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- Synthetic method for ethyleniminoquinone drug intermediate chloranil
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The invention discloses a synthetic method for the ethyleniminoquinone drug intermediate chloranil. The synthetic method comprises the following steps: adding 2,3,5,6-tetrachloro-1,4-dimethoxybenzeneand a potassium chloride solution into a reaction vessel, controlling a solution temperature, adding aluminum acetylacetonate powder in batches, controlling a stirring speed and carrying out a reaction; and adding a tributyl borate solution, raising the temperature, then adding a sodium nitrate solution, carrying out a reaction, subjecting the obtained solution to layering, lowering the temperature, carrying out washing with a hexyl ether solution a plurality of times, then carrying out washing with a 3-methylpyridine solution a plurality of times, then carrying out recrystallization in a chloroacetyl chloride solution, and carrying out dehydration with a dehydrating agent so as to obtain the finished chloranil.
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Paragraph 0013; 0018; 0019; 0022; 0023; 0024; 0026; 0027
(2018/07/30)
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- Metal-Free Oxidative C-C Coupling of Arylamines Using a Quinone-Based Organic Oxidant
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A variety of arylamines are shown to undergo oxidative C-C bond formation using quinone-based chloranil/H+ reagent as the recyclable organic (metal-free) oxidant system to afford benzidines/naphthidines. Arylamines (3°/2°) designed with various substituents were employed to understand the steric as well as electronic preferences of oxidative dimerization, and a mechanism involving amine radical cation has been proposed. The tetraphenylbenzidine derivative obtained via oxidative C-C coupling has been further converted to blue-emissive hole-transporting material via a simple chemical transformation. This study highlights the preparation of novel HTMs in a simple, economic, and efficient manner.
- Maddala, Sudhakar,Mallick, Sudesh,Venkatakrishnan, Parthasarathy
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supporting information
p. 8958 - 8972
(2017/09/11)
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- A Facile and Selective Procedure for Oxidation of Hydroquinones using Silica Gel Supported Catalytic Cerium(IV) Ammonium Nitrate
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A new procedure for oxidation of hydroquinones to quinones using a silica gel supported cerium(IV) ammonium nitrate-NaBrO3 reagent has been developed. This simple, easy to prepare and use, heterogeneous reagent is highly selective towards oxidation of 1,4-dihydroxybenzenes and produces high yields of quinones. Waste generated by using this procedure is minimal.
- Ali, Mohammed Hashmat,Welker, Andrea,York, Crystal
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p. 3207 - 3211
(2015/10/19)
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- Electron-transfer studies of a peroxide dianion
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A peroxide dianion (O22-) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)∪mBDCA-5t-H 6]2-, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)∪mBDCA-5t-H6] 2- with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22- occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O 22- within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O 2)∪mBDCA-5t-H6]2- as a chemical reagent.
- Ullman, Andrew M.,Sun, Xianru,Graham, Daniel J.,Lopez, Nazario,Nava, Matthew,De Las Cuevas, Rebecca,Mueller, Peter,Rybak-Akimova, Elena V.,Cummins, Christopher C.,Nocera, Daniel G.
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supporting information
p. 5384 - 5391
(2014/06/09)
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- Spectroscopic and thermal investigations on the charge transfer interaction between risperidone as a schizophrenia drug with some traditional π-acceptors: Part 2
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The focus of present investigation was to assess the utility of non-expensive techniques in the evaluation of risperidone (Ris) in solid and solution states with different traditional π-acceptors and subsequent incorporation of the analytical determination into pharmaceutical formulation for a faster release of risperidone. Charge-transfer complexes (CTC) of risperidone with picric acid (PA), 2,3-dichloro-5,6-dicyano-p-benzoquinon (DDQ), tetracyanoquinodimethane (TCNQ), tetracyano ethylene (TCNE), tetrabromo-p-quinon (BL) and tetrachloro-p-quinon (CL) have been studied spectrophotometrically in absolute methanol at room temperature. The stoichiometries of the complexes were found to be 1:1 ratio by the photometric molar ratio between risperidone and the π-acceptors. The equilibrium constants, molar extinction coefficient (εCT) and spectroscopic-physical parameters (standard free energy (ΔGo), oscillator strength (f), transition dipole moment (μ), resonance energy (RN) and ionization potential (ID)) of the complexes were determined upon the modified Benesi-Hildebrand equation. Risperidone in pure form was applied in this study. The results indicate that the formation constants for the complexes depend on the nature of electron acceptors and donor, and also the spectral studies of the complexes were determined by (infrared, Raman, and 1H NMR) spectra and X-ray powder diffraction (XRD). The most stable mono-protonated form of Ris is characterized by the formation of +NH (pyrimidine ring) intramolecular hydrogen bonded. In the high-wavenumber spectral region ～3400 cm-1, the bands of the +NH stretching vibrations and of the pyrimidine nitrogen atom could be potentially useful to discriminate the investigated forms of Ris. The infrared spectra of both Ris complexes are confirming the participation of +NH pyrimidine ring in the donor-acceptor interaction.
- El-Habeeb, Abeer A.,Al-Saif, Foziah A.,Refat, Moamen S.
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p. 464 - 477
(2013/04/23)
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- Structure and function of quinones in biological solar energy transduction: A high-frequency d-band EPR spectroscopy study of model benzoquinones
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Quinones are utilized as charge-transfer cofactors in a wide variety of reactions that are crucial for photosynthesis and respiration. In photosynthetic protein complexes, both Type I and Type II, including oxygenic and anoxygenic reaction centers contain quinone cofactors that are known to participate in electron- and proton-transfer processes. Type II reaction centers, purple bacterial reaction centers, and photosystem II utilize benzoquinone molecules, ubiquinone, and plastoquinone, respectively, to facilitate proton-coupled electron transfer reactions. Here, we report a systematic study of the principal components of the g-tensor of an extensive library of model benzosemiquinone anion radicals in both protic (2-isopropanol) and aprotic (dimethyl sulfoxide) solvents using high-frequency EPR spectroscopy. A detailed comparison of the experimental g-values of the benzosemiquinone models at D-band EPR frequency allows for the discrimination of substituent effects and solvent hydrogen bonds on the principal components of the g-tensor. Further, we compare the primary plastosemiquinone, QA-, of photosystem II with the substituent and solvent hydrogen bond effects of benzosemiquinone models in vitro. This study significantly extends the experimental basis for elucidating the role of both molecular structure and interactions with environment on the functional tuning of quinone cofactors in biological solar energy transduction.
- Chatterjee, Ruchira,Coates, Christopher S.,Milikisiyants, Sergey,Poluektov, Oleg G.,Lakshmi
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experimental part
p. 676 - 682
(2012/04/10)
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- Photocatalytic transformation of chlorophenols under homogeneous and heterogeneous conditions using palladium octadodecylthio phthalocyanine
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Homogeneous and heterogeneous photosensitized transformations of 4-chlorophenol (4-CP) and pentachlorophenol (PCP) using palladium octadodecylthiophthalocyanine (PdODPc) were investigated. Under heterogeneous conditions, the photosensitizer was supported on functionalized single walled carbon nanotubes (SWCNTs). Homogeneous photosensitization proved to be more effective than the heterogeneous reaction in terms of percentage of transformation achieved. The kinetics of heterogeneous catalysis proved that ads-PdODPc-SWCNT-COOH (where SWCNT has been functionalized with COOH groups) was reusable for 4-CP while its activity degenerated when reused for PCP. Singlet oxygen was confirmed as playing an active role in the reactions.
- Ogunbayo, Taofeek B.,Nyokong, Tebello
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experimental part
p. 49 - 55
(2012/01/13)
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- Photocatalytic removal of pentachlorophenol by means of an enzyme-like molecular imprinted photocatalyst and inhibition of the generation of highly toxic intermediates
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Pentachlorophenol (PCP) is a typical highly-toxic pollutant, and its direct photolysis and conventional photocatalysis may produce more toxic by-products such as dibenzodioxins. It is urgently needed to develop a photocatalytic process able to remove PCP without the generation of highly toxic by-products. To achieve this, enzyme-like molecular-imprinted photocatalysts were prepared by using structural analogues of PCP as pseudo templates. It was found that 2,4-dinitrophenol (DNP) was the best template among the tested analogues. The molecular imprinted polymer (MIP) coated P25 TiO2 photocatalyst DNP-P25 prepared with DNP as the template greatly accelerated the photocatalytic degradation of PCP and depressed the generation of toxic intermediates. It was confirmed that the amino groups at the footprint cavities provided a well-defined micro reaction environment, which made the benzene ring of the adsorbed PCP be better exposed to photo-generated reactive OH radicals, leading to easier cleavage of the benzene ring. Both the intermediate analysis and toxicity evaluation confirmed that the MIP-coated TiO2 can make the photocatalytic degradation a safe and green approach of removing PCP.
- Shen, Xiantao,Zhu, Lihua,Liu, Guoxia,Tang, Heqing,Liu, Shushen,Li, Weiying
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experimental part
p. 2278 - 2285
(2011/04/23)
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- Aerobic oxidation of hydroquinone derivatives catalyzed by polymer-incarcerated platinum catalyst
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(Chemical Equation Presented) It's a lock-in! A remarkably wide substrate scope of hydroquinones are oxidized to quinones in high yields in a platinum-catalyzed process with as low as 0.05 mol% catalyst. The aerobic oxidation is catalyzed by platinum nanoclusters trapped in a styrene-based polymer network (see scheme, PI Pt=polymer-incarcerated nanoclusters). The catalyst could be reused at least 13 times without any loss of catalytic activity.
- Miyamura, Hiroyuki,Shiramizu, Mika,Matsubara, Ryosuke,Kobayashi, Shu
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supporting information; experimental part
p. 8093 - 8095
(2009/04/13)
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- Charge transfer complex formation between p-chloranil and 1,n-di(9-anthryl)alkanes
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Dimer model compounds of polyvinylanthracenes (1,n-di(9-anthryl)alkanes, when n = 1-5) were synthesized to model the effects of distance and orientation between anthracene groups in polymeric systems. Charge transfer (CT) complexes of anthracene, 9-methylanthracene and 1,n-di(9-anthryl)alkanes with p-chloranil (p-CHL) have been investigated spectrophotometrically in dichloromethane. The colored products are measured spectrophotometrically at different wavelength depending on the electronic transition between donors and acceptor. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated by Van't Hoff equation. Stochiometries of the complexes formed between donors and acceptor were defined by the Job's method of the continuous variation and found in 1:1 complexation with donor and acceptor at the maximum absorption bands.
- Arslan, Mustafa,Masnovi, John
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p. 711 - 716
(2007/10/03)
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- ARYL AND BIARYL COMPOUNDS HAVING MCH MODULATORY ACTIVITY
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In one embodiment, this invention provides a novel class of compounds as antagonists of the MCH receptor, methods of preparing such compounds, pharmaceutical compositions containing one or more of the compounds, methods of preparing pharmaceutical formulations comprising one or more such compounds, and methods of treatment, prevention or amelioration or one or more of diseases associated with the MCH receptor. An illustrative inventive compound is shown below:
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- Photoreduction of p-Benzoquinones: Effects of Alcohols and Amines on the Intermediates and Reactivities in Solution
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The photochemistry of 1,4-benzoquinone (BQ) and alkyl-, Cl- and related derivatives, e.g. methyl-, 2,6-dimethyl-, chloro-, 2,5-dichloro-1,4-benzoquinone, duroquinone and chloranil, was studied in nonaqueous solvents by UV-vis spectroscopy using nanosecond laser pulses at 308 nm. The reactivity of the triplet state (3Q*) of the quinones with 2-propanol in the absence of water is largest for BQ and depends mainly on the quinone structure, whereas the rate constant of electron transfer from amines, such as triethylamine (TEA) or 1,4-diazabicyclo[2.2.2]octane, is close to the diffusion-controlled limit for BQ and most derivatives. Photoinduced charge separation after electron transfer from amines to 3Q* and the subsequent charge recombination or neutralization are supported by time-resolved conductivity measurements. The half-life of the decay kinetics of the semiquinone radical (.QH/Q.-) depends significantly on the donor and the medium. The photoconversion into the hydroquinones was measured under various conditions, the quantum yield, λirr = 254 nm, increases with increasing 2-propanol and TEA concentrations. The effects of quenching of 3Q*, the .QH/Q.- radicals and the photoconversion are outlined. The mechanisms of photoreduction of quinones in acetonitrile by 2-propanol are compared with those by TEA in benzene and acetonitrile, and the specific properties of substitution are discussed.
- Goerner, Helmut
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p. 440 - 448
(2007/10/03)
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- Photoinduced electron transfer between acenaphthylene and 1,4-benzoquinones. Formation of dimers of acenaphthylene and 1 : 1-adducts and effect of excitation mode on reactivity of the charge-transfer complexes
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Photochemical reactions of acenaphthylene (ACN) with 1,4-benzoquinones (BQs) of varying reduction potentials in solution have been investigated in order to determine final products and quantum yields of the reactions and to get an insight into the factors
- Haga, Naoki,Takayanagi, Hiroaki,Tokumaru, Katsumi
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p. 734 - 745
(2007/10/03)
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- Dielectric and spectral studies of molecular crystal - Chloranil systems forming charge transfer complexes
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Molecular crystal - chloranil systems forming charge transfer complexes were studied in solutions and as melts at precrystallization temperatures. Dielectric permittivity of chloranil mixtures with stilbene, naphthalene and diphenyl was measured as function of concentration and temperature, and absorption spectra of the same systems in CCl4 solution were analyzed. The results obtained suggest the presence of additional molecular ordering in the melts at chloranil concentrations 0,5-5 mol.%. A model of such ordering accounting for both specific and non-specific intermolecular interactions is proposed.
- Sidletskiy, O. Ts.,Lisetski,Malikov, V. Ya.,Stadnik,Panikarskaya,Budakovsky
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p. 101 - 109
(2007/10/03)
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- Reaction of semiquinoid compounds derived from N-arylsulfonyl-p-quinonemono- and diimines with tosylhydrazine
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Reaction of 4-arylsulfonylimino-2,3,5,6,6-pentachloro-2-cyclohexen-1-ones with tosylhydrazine gives rise to N-arylsulfonyl-N-p-tolylsulfonyl-2,3,5,6-tetrachloroaminophenols. With 4-arylsulfonylimino-2,2,3-trichloro-1,2,3,4-tetrahydronaphthalen-1-ones and 4-arylsulfonylimino-2-dihalo-1,2,3,4-tetrahydronaphthalen-1-ones reaction products are 4-arylsulfonylamido-2,3-dichloro-4-p-tolylsulfonylhydrazido-1,4-dihydronaphthalen-1-ones and 2-p-tolylsulfonyl-1,4-naphthoquinone-4-diazide respectively. First stage of the processes consists in dehydrohalogenation yielding the corresponding N-arylsulfonyl-p-quinonimines.
- Avdeenko,Zhukova
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p. 816 - 819
(2007/10/03)
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- Photoinduced hydrogen- and electron-transfer processes between chloranil and aryl alkyl sulfides in organic solvents. Steady-state and time-resolved studies
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The photochemical behavior of three aryl alkyl sulfides, thioanisole (TA), benzyl phenyl sulfide (BPS) and 4-methoxybenzyl phenyl sulfide (MBPS), sensitized by triplet chloranil (CA), was investigated by nanosecond laser flash photolysis and steady-state irradiation in CH2Cl2 and MeCN. The nature of the transients detected upon 355-nm laser excitation was independent of the molecular structure of the aryl alkyl sulfides but strongly affected by the solvent polarity. In particular, in CH2Cl2 the quenching process of triplet CA by aryl alkyl sulfides was accompanied by H- transfer, with formation of the CAH· and TA(-H)·/BPS(-H)·/MBPS(-H)· radicals. In contrast, a charge transfer process between triplet CA and aryl alkyl sulfides, with formation of the radical anion CA·- and radical cations of aryl alkyl sulfides, occurred in MeCN. In this solvent, a transient detected at long delay time was tentatively assigned to the anion CAH- formed by H-transfer between radical ions. In all experiments, transient species were characterized in terms of second-order decay rate constants and quantum yields of formation. Steady-state irradiation of the CA/TA system led to the stable photoadduct C6H5SCH2OC6Cl4OH in both CH2Cl2 and MeCN with quantum yields of 0.033 and 0.27, respectively. In contrast, aldehydes, thioacetals, and disulfides were the main products obtained upon irradiation of the CA/BPS and CA/MBPS systems. The photoaddition products were not observed, probably owing to their low stability. The nature of the photoproducts formed by irradiation of CA/aryl alkyl sulfides was independent of solvent properties, even though the reactivity was higher in MeCN than in CH2Cl2.
- Del Giacco, Tiziana,Elisei, Fausto,Lanzalunga, Osvaldo
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p. 1701 - 1708
(2007/10/03)
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- The reaction of the OH radical with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodophenol in water: Electron transfer vs. addition to the ring
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The OH-radical-induced dehalogenation of pentafluorophenol (F5C6OH), pentachlorophenol (Cl5C6OH), pentabromophenol (Br5C6OH) and 2,4,6-triiodophenol (I3H2C6OH) in water has been studied by pulse radiolysis in basic solution where these compounds are deprotonated and hence slightly water soluble. Hydroxyl radicals react with these phenolates both by electron transfer and by addition. Electron transfer yields hydroxide ions and the corresponding phenoxyl radicals (X5C6O and I3H2C6O); these were also generated independently, to the exclusion of OH-adduct radicals, by reacting the phenolates with N3 radicals [k(N3 + F5C6O-) = 4.9 × 109 dm3 mol-1 s-1, λmax(F5C6O) = 395 nm; k(N3 + Cl5C6O-) = 5.7 × 109 dm3 mol-1 s-1, λmax(Cl5C6O) = 452 nm; k(N3 + Br5C6O-) = 6.5 × 109 dm3 mol-1 s-1, λmax(Br5C6O) = 476 nm; k(N3 + I3H2C6O-) = 5.6 × 109 dm3 mol-1 s-1, λmax(I3H2C6O) = 540 nm]. Hydroxyl radical addition to the pentahalophenolates is followed by rapid halide elimination, giving rise to hydroxytetrahalophenoxyl radical anions (X4O-C6O). The latter exhibit absorption maxima near those of the pentahalophenoxyl radicals. This prevents a proper determination of the relative importance of the two processes by optical detection. However, these two processes distinguish themselves by their behaviour with respect to the stoichiometry and kinetics of the production of ionic conducting species. In basic solution, electron transfer causes a conductivity increase due to the formation of OH- whereas addition followed by HX elimination and deprotonation of the X4OHC6O radical results in a conductivity drop. The evaluation of the conductivity change at 8 μs after the radiolytic pulse has ended, reveals that about 27%, 53%, 73%, and 97% of the OH radicals react by electron transfer with F5C6O-, Cl5C6O-, Br5C6O- and I3H2C6O-, respectively. Further conductivity change occurs during the bimolecular termination of the halophenol-derived radicals (t1/2 9 and 4 × 109 dm3 mol-1 s-1) and continues into progressively longer times, owing to the hydrolysis of unstable HX-releasing products, on account of the replacement of OH- by halide/halophenolate ions. Additionally, further halide is released on a time scale of minutes and hours. The rates of the conductivity change in the time range from a few ms to several tens of seconds are proportional to the OH- concentration.
- Fang, Xingwang,Schuchmann, Heinz-Peter,Von Sonntag, Clemens
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p. 1391 - 1398
(2007/10/03)
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- Activated sterically strained C=N bond in N-arylsulfonyl-p-quinonimines and diimines: IV. Forecasts and some reactions
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We have forcasted with the use of precision three-dimensional atomic and molecular models (Tartu Models) a presence of activated sterically strained C=N bond in N-substituted p-quinonimines and diimines. The decisive role indicating the presence of such bond plays the bond angle C=N-X. If the angle is greater than 130° the C=N bond starts to behave as activated which is confirmed by a number of reactions. At the bond angle C=N-X over 145-150° the quinonimines get thermodinamically unstable, and their synthesis is impossible. The validity of the forecast is corroborated by X-ray diffraction study of 3,5-dimethyl-N-p-chlorophenylsufonyl-1,4-benzoquinonimine. Applying the atomic and molecular Tartu Models we determined the sterical features of the tetragonal carbon atom of the quinolide structure which forms by 1,2-addition characteristic of para-quinonimines with the activated sterically strained C=N bond. The forecast of the sterical situation at tetragonal carbon atom of qiunolide structure is demonstrated on reactions of some N-arylsulfonyl-p-quinonimines with alcohols.
- Avdeenko,Menafova,Yusina,Dementii
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p. 877 - 887
(2007/10/03)
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- Autooxidation of tetrachlorohydroquinone in aqueous media
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The oxidation of tetrachlorohydroquinone in an aqueous solution at pH 7.40 is an autocatalytic reaction (sigmoid kinetic curves). The interaction of the tetrachloro-1,4-semiquinone radical anion with dioxygen occurs with the rate constant k2 equal to 9±3 L mol-1 s-1 (22-37 °C). Superoxide dismutase does not affect the maximum rate of tetrachlorohydroquinone oxidation.
- Pisarenko
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p. 881 - 886
(2007/10/03)
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- Reactions of polychloro derivatives of semiquinoid compounds based on N-arylsulfonyl-p-quinonimines with N-containing heterocycles
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Semiquinoid derivatives based on N-arylsulfonyl-p-benzoquinonimines treated with N-containing five-membered heterocycles are hydrolyzed and then eliminate Cl2 molecule. Among the N-containing five-membered heterocycles with a conjugated benzene ring only benzotriazole reacts with semiquinoid derivatives of N-arylsulfonyl-p-benzoquinonimines affording 2,3-di(benzotriazol-1-yl)-5,5,6,6-tetrachlorocyclohexene-1,4-dione which occurs as crystalline cis-and trans-isomers. The structure of the stable transisomer was established by X-ray diffraction study. N-Arylsulfonyl-p-naphthoquinonimines do not react with N-containing heterocycles.
- Avdeenko,Yusina,Zhukova
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p. 585 - 590
(2007/10/03)
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- Photodynamics of the Paterno-Buechi cycloaddition of stilbene to quinone. Unusual modulation of electron-transfer kinetics by solvent and added salt
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Oxetanes are produced in the Paterno-Buechi cycloaddition of stilbene (S) to quinone (Q) via an efficient photoinduced electron transfer. Kinetics analysis of the time-resolved absorption spectra over three distinctive (ps, ns, μs) timescales establishes the coupling (kC) of the initially formed ion-radical pair 3[S+?, Q-?] to the 1,4-biradical ?SQ? as the critical step toward oxetane formation. The (rather slow) rate constant of kC ≤ 107 s-1 in acetonitrile must compete with other faster decay pathways of the ion pair involving ionic separation, ion exchange (with added salt) and back electron transfer. As such, solvent polarity and donicity as well as added salts play an unusually prominent role in modulating the ion-pair microdynamics. Donor-acceptor complexation of the photoexcited quinone with the solvent and cis→trans isomerization of (Z)-stilbene must also be considered in the overall photodynamics of electron transfer.
- Hubig, Stephan M.,Sun, Duoli,Kochi, Jay K.
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p. 781 - 788
(2007/10/03)
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- Steric control of electron transfer. Changeover from outer-sphere to inner-sphere mechanisms in arene/quinone redox pairs
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The various aromatic hydrocarbons (Chart 2) constitute a sharply graded series of sterically encumbered (unhindered, partially hindered, and heavily hindered) donors in electron transfer (ET) to quinones (Chart 1). As such, steric effects provide the quantitative basis to modulate (and differentiate) outer-sphere and inner-sphere pathways provided by matched pairs of hindered and unhindered donors with otherwise identical electron-transfer properties. Thus the hindered donors are characterized by (a) bimolecular rate constants (k2) that are temperature dependent and well correlated by Marcus theory, (b) no evidence for the formation of (discrete) encounter complexes, (c) high dependency on solvent polarity, and (d) enhanced sensitivity to kinetic salt effects - all diagnostic of outer-sphere electron-transfer mechanisms. Contrastingly, the analogous unhindered donors are characterized by (a) temperature-independent rate constants (k2) that are 102 times faster and rather poorly correlated by Marcus theory, (b) weak dependency on solvent polarity, and (c) low sensitivity to kinetic salt effects - all symptomatic of inner-sphere ET mechanisms arising from the preequilibrium formation of encounter complexes with charge-transfer (inner-sphere) character. Steric encumbrances which inhibit strong electronic (charge-transfer) coupling between the benzenoid and quinonoid π systems are critical for the mechanistic changeover. Thus, the classical outer-sphere/inner-sphere distinction (historically based on coordination complexes) is retained in a modified form to provide a common terminology for inorganic as well as organic (and biochemical) redox systems.
- Hubig, Stephan M.,Rathore, Rajendra,Kochi, Jay K.
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p. 617 - 626
(2007/10/03)
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- The kinetics and thermodynamics of quinone-semiquinone-hydroquinone systems under physiological conditions
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The steady-state concentration of semiquinones (Q·-) determined by EPR in the mixtures of eleven alkyl-, methoxy-and chloro-substituted 1,4-benzoquinones as well as 1,4-naphthoquinone (Q) with corresponding hydroquinones (QH2) in aqueous buffer. pH 7.40, was used to calculate a constant for equilibrium (1) Q + QH2?Q·- + Q·- + 2H+ (k1; 2k-1; K1 = k1/2k-1). The rale constants for comproportionation between Q and QH2, k1, were calculated from the combination of K1 determined in this work and 2k-1 reported previously. The Nernst equation was applied to calculate the change in one-electron reduction potential ΔE1 = E(Q/Q·-) - E(Q·-/QH2) in equilibrium (1). The E(Q·-/QH2) values were calculated from ΔE1 and the values of E(Q/Q·-) known from the literature. The correlations between E(Q·-/QH2) and E(Q/Q·-) as well as between ΔE1 (k1) and E(Q/Q·-) are discussed. The values of ΔE1 and k1 are suggested to be the key factors governing the autoxidation of QH2.
- Roginsky, Vitaly A.,Pisarenko, Leonid M.,Bors, Wolf,Michel, Christa
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p. 871 - 876
(2007/10/03)
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- Process for the preparation of optically active ortho-substituted 4-aryl-dihydropyridines
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The invention relates to a process for the preparation of optically active ortho-substituted 4-aryl- or heteroaryl-1,4-dihydropyridines by oxidation and subsequent reduction from their opposite enantiomers.
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- Kinetic isotope effects for electron-transfer pathways in the oxidative C-H activation of hydrocarbons
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Fast hydrogen atom transfers from various methylbenzenes (ArH) to photoactivated quinones Q* show primary kinetic isotope effects k(H)/k(D) of 2.4-5.6. The quantitative effects of added inert salt on the kinetics and on the yields of the intermediate cation radical ArH(+*) demonstrate that hydrogen transfer proceeds via a two-step sequence involving an initial electron transfer to form the ion-radical pair [ArH(+*), Q(-*)] which subsequently undergoes proton transfer according to the electron-transfer mechanism in Scheme 1.
- Bockman,Hubig,Kochi
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p. 2826 - 2830
(2007/10/03)
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- Conversion of pentahalogenated phenols by microperoxidase-8/H2O2 to benzoquinone-type products
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This study reports the microperoxidase-8 (MP8)/H2O2-catalyzed dehalogenation of pentafluorophenol and pentachlorophenol, compounds whose toxic effects and persistence in the environment are well documented. The primary products of this dehalogenation reaction appear to be the corresponding tetrahalo-p-benzoquinones. Under the conditions used, the fluorinated phenol and its intermediate products are more susceptible to degradation than the corresponding chlorinated analogue and its products. The main degradation products of tetrachloro-p-benzoquinone and tetrafluoro-p- benzoquinone were identified as trichlorohydroxyp-benzoquinone and trifluorohydroxy-p-benzoquinone, respectively. This secondary conversion of tetrafluoro-p-benzoquinone and tetrachloro-p-benzoquinone was not mediated by MP8, but was driven by H2O2. Evidence is presented for a mechanism where H2O2 molecules and not hydroxide anions are the reactive nucleophilic species attacking the tetrahalo-p-benzoquinones. In addition to the formation of the trihalohydroxy-p-benzoquinones, the formation of adducts of the tetrahalo-p-benzoquinone products with ethanol, present in the incubation medium, was observed. The adduct from the reaction of tetrachloro-p- benzoquinone with ethanol was isolated and identified as trichloroethoxyquinone. Thus, the present paper describes a system in which the formation of tetrahalo-p-benzoquinone-type products by an oxidative heme- based catalyst could be unequivocally demonstrated.
- Osman, Ahmed M.,Posthumus, Maarten A.,Veeger, Cees,Van Bladeren, Peter J.,Laane, Colja,Rietjens, Ivonne M.C.M.
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p. 1319 - 1325
(2007/10/03)
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- Charge-Transfer Complexes of Benzo[b]thiophene with σ- And π-Electron Acceptors
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The spectrophotometric properties of the charge-transfer (CT) complexes of benzo[6]thiophene with iodine at 24°C in different solvents such as cychlohexane, carbon tetrachloride, chloroform, dichloromethane, and 1,2-dichloroethane are studied. In addition the spectrophotometric and thermodynamic properties of the CT complexes of benzo[b]thiophene with tetracyanoethylene, chloranil, 2,3-dichloro-5,6-dicyano-p-benzoquinone and 7,7,8,8,-tetracyanoquinodimethane are studied. The results show that the equilibrium and the thermodynamic parameters and the wavelengths of the maximum absorption bands (λmax) of the complexes vary markedly with the solvent. The ionization potential and the donor sites of benzo[b]thiophene are also determined and discussed.
- Ayad, Mohamad M.
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p. 2369 - 2373
(2007/10/03)
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- Photoreduction of chloranil by benzhydrol and related compounds. Hydrogen atom abstraction vs sequential electron-proton transfer via quinone triplet radical ion-pairs
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The photoreduction of chloranil (Q) to the hydraquinone (QH2) in benzene by benzhydrols and by related arylmethanols has been investigated. The products of photooxidation of the benzhydrols are benzophenones, in lieu of formation of benzpinacols. Three distinct mechanisms of oxidation-reduction have been identified from quantum yield determinations and laser flash photolysis experiments, including quinone triplet quenching via H-atom and electron transfer paths. Direct excitation of ground state quinone complexes has also been investigated. The quenching of the triplet state of the quinone by benzhydrol proceeds normally (k(q) = 1.3 x 106 M-1 s-1) and gives semiquinone radical (QH., λ(max) = 435 nm) and the benzhydryl radical (λ(max) = 535 nm). The latter intermediate decays by pseudo-first-order kinetics through hydrogen atom transfer with ground state quinone (Q). Triplet quenching by bis(4-methoxyphenyl)methanol proceeds at a more rapid rate (k(q) = 5.5 x 109 M-1 s-1) leading to an intermediate that is identified as the chloranil radical anion (λ(max) = 450 nm). A similar intermediate is observed on Q quenching by 1-naphthylmethanol and acenapthenol with the appearance of an accompanying naphthalene radical cation absorption (ca. 670 nm). The radical ion transients, which are assigned to contact ion-pairs (triplet excited complexes) of the quinone and the various electron donors, decay to semiquinone radicals (QH.) by first-order processes occurring in the 100 ns time regime. The transient behavior is interpreted in terms of a hydrogen atom transfer mechanism for photoreduction with benzhydrol and, for the more robust electron donors, a mechanism involving electron transfer followed by proton transfer between geminate radical ions. For the electron transfer donors, ground state charge-transfer (CT) complexes can be observed (λ(max) ca. 500 nm). Selective CT excitation leads to quinone photoreduction with reduced quantum yield. The results are discussed in terms of the time resolution of sequential electron/proton transfer steps for photogenerated ion-pairs, the occurrence of one photon-two electron transfer photoredox mechanisms and the kinetically distinct pathways for decay of singlet and triplet intimate radical ion-pairs.
- Jones II, Guilford,Mouli, Nandini,Haney, William A.,Bergmark, William R.
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p. 8788 - 8794
(2007/10/03)
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- Oxidation mechanism of NAD dimer model compounds
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The oxidation of a dimeric N-benzyldihydronicotinamide with various oxidants such as quinones, triphenyl carbenium ions and a triplet exited tris(bipyridine) ruthenium(II) complex occurs via initial outer-sphere electron transfer followed by fast C-C bond cleavage and second electron transfer. The kinetic studies allow the determination of the oxidation potential of this compound.
- Patz, Matthias,Kuwahara, Yoshihiro,Suenobu, Tomoyoshi,Fukuzumi, Shunichi
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p. 567 - 568
(2007/10/03)
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- Direct observation and structural characterization of the encounter complex in bimolecular electron transfers with photoactivated acceptors
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The encounter complex between photoexcited quirtones Q* and various aromatic donors (ArH) is observed directly by time-resolved ps spectroscopy immediately before it undergoes electron transfer to the ion-radical pair [Q(°-) ArH(°+)]. The encounter complex (EC) is spectrally characterized by distinctive (near IR) absorption bands, and its temporal evolution is established by quantitative kinetics analysis. The structural characterization of the 1:1 encounter complex [Q*, ArH] identifies the cofacial juxtaposition of the donor and acceptor moieties for optimal overlap of their π-orbitals. Further comparisons of the (excited-state) encounter complex with the corresponding (ground-state) EDA complex of aromatic donors and quinones establish its charge-transfer character, which directly relates to electron transfer within the encounter complex. The mechanistic significance of the encounter complex to bimolecular electron transfer is discussed (Scheme 1).
- Rathore,Hubig,Kochi
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p. 11468 - 11480
(2007/10/03)
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