- Mechanisms of hydride abstractions by quinones
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The kinetics of the hydride abstractions by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) from 13 C-H hydride donors (acyclic 1,4-dienes, cyclohexa-1,4-dienes, dihydropyridines), tributylstannane, triphenylstannane, and five borane complexes (amine-boranes, carbene-boranes) have been studied photometrically in dichloromethane solution at 20 °C. Analysis of the resulting second-order rate constants by the correlation log k2(20°C) = sN(E + N) (J. Am. Chem. Soc. 2001, 123, 9500) showed that the hydride abstractions from the C-H donors on one side and the Sn-H and B-H hydride donors on the other follow separate correlations, indicating different mechanisms for the two reaction series. The interpretation that the C-H donors transfer hydrogen to the carbonyl oxygen of DDQ while Sn-H and B-H hydride donors transfer hydride to a cyano-substituted carbon of DDQ is supported by quantum-chemical intrinsic reaction coordinate calculations and isotope labeling experiments of the reactions of D8-cyclohexa-1,4-diene, Bu3SnD, and pyridine·BD3 with 2,5-dichloro-p-benzoquinone. The second-order rate constants of the reactions of tributylstannane with different quinones correlate linearly with the electrophilicity parameters E of the quinones, which have previously been derived from the reactions of quinones with -nucleophiles. The fact that the reactions of Bu3SnH with quinones and benzhydrylium ions are on the same log k2 vs E (electrophilicity) correlation shows that both reaction series proceed by the same mechanism and illustrates the general significance of the reactivity parameters E, N, and sN for predicting rates of polar organic reactions.
- Guo, Xingwei,Zipse, Hendrik,Mayr, Herbert
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- 1-Methyl-1,4-cyclohexadiene as a Traceless Reducing Agent for the Synthesis of Catechols and Hydroquinones
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Pro-aromatic and volatile 1-methyl-1,4-cyclohexadiene (MeCHD) was used for the first time as a valid H-atom source in an innovative method to reduce ortho or para quinones to obtain the corresponding catechols and hydroquinones in good to excellent yields. Notably, the excess of MeCHD and the toluene formed as the oxidation product can be easily removed by evaporation. In some cases, trifluoroacetic acid as a catalyst was added to obtain the desired products. The reaction proceeds in air and under mild conditions, without metal catalysts and sulfur derivatives, resulting in an excellent and competitive method to reduce quinones. The mechanism is attributed to a radical reaction triggered by a hydrogen atom transfer from MeCHD to quinones, or, in the presence of trifluoroacetic acid, to a hydride transfer process.
- Baschieri, Andrea,Amorati, Riccardo,Valgimigli, Luca,Sambri, Letizia
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p. 13655 - 13664
(2019/10/28)
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- Reactivity of iPrPCPIrH4 with para-benzoquinones
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In the interest of investigating new hydrogen acceptors for pincer–iridium catalyzed dehydrogenations with the ability to be catalytically recycled, a series of para-benzoquinones have been reacted with iPrPCPIrH4 in various solvents and conditions. Preliminary results indicate that a wide range of quinones are capable of dehydrogenating iPrPCPIrH4, and that several turn-overs in alcohol dehydrogenation by iPrPCPIr are possible at room temperature using benzoquinone acceptors. However, strong acceptor–catalyst interactions are inhibitory toward catalysis when the acceptor is used in excess. A new class of (bis)-η2 pi-adducts, formed between iPrPCPIr and benzoquinones, nicknamed “barber-chairs”, has been identified and 3 examples have been characterized.
- Wilklow-Marnell, Miles,Brennessel, William W.,Jones, William D.
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p. 209 - 214
(2017/11/24)
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- Kinetic mechanism of the dechlorinating flavin-dependent monooxygenase HadA
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The accumulation of chlorophenols (CPs) in the environment, due to their wide use as agrochemicals, has become a serious environmental problem. These organic halides can be degraded by aerobic microorganisms, where the initial steps of various biodegradation pathways include an oxidative dechlorinating process in which chloride is replaced by a hydroxyl substituent. Harnessing these dechlorinating processes could provide an opportunity for environmental remediation, but detailed catalytic mechanisms for these enzymes are not yet known. To close this gap, we now report transient kinetics and product analysis of the dechlorinating flavin-dependent monooxygenase, HadA, from the aerobic organism Ralstonia pickettii DTP0602, identifying several mechanistic properties that differ from other enzymes in the same class. We first overexpressed and purified HadA to homogeneity. Analyses of the products from single and multiple turnover reactions demonstrated thatHadAprefers 4-CP and 2-CP over CPs with multiple substituents. Stopped-flow and rapid-quench flow experiments of HadA with 4-CP show the involvement of specific intermediates (C4a-hydroperoxy-FAD and C4a-hydroxy-FAD) in the reaction, define rate constants and the order of substrate binding, and demonstrate that the hydroxylation step occurs prior to chloride elimination. The data also identify the non-productive and productive paths of the HadA reactions and demonstrate that product formation is the rate-limiting step. This is the first elucidation of the kinetic mechanism of a two-component flavin-dependent monooxygenase that can catalyze oxidative dechlorination of various CPs, and as such it will serve as the basis for future investigation of enzyme variants that will be useful for applications in detoxifying chemicals hazardous to human health.
- Pimviriyakul, Panu,Thotsaporn, Kittisak,Sucharitakul, Jeerus,Chaiyen, Pimchai
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p. 4818 - 4832
(2017/04/03)
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- Diels-Alder trapping of in situ generated dienes from 3,4-dihydro-2H-pyran with p-quinone catalysed by p-toluenesulfonic acid
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This comprehensive study portrays that p-toluenesulfonic acid is a more efficient catalyst for the reaction between p-quinones and 3,4-dihydro-2H-pyran, than the Lewis acids. The products were accomplished by the Diels-Alder cycloaddition reaction and their mechanistic pathways have been formulated. The impact of C2 and C2,5 substituents of the p-quinones on the cycloaddition reaction has been explored. Remarkably, it is the first report to explore this kind of in situ generated diene for the Diels-Alder cycloaddition reaction.
- Mohan Raj, Radhakrishnan,Balasubramanian, Kalpattu K.,Easwaramoorthy, Deivanayagam
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supporting information
p. 1115 - 1121
(2017/02/10)
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- Cycloacylation of chloro-substituted hydroquinone dimethyl ethers with dichloromaleic anhydride
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Under the drastic conditions of Zahn—Ochwat cycloacylation of 2-chloroand 2,3-dichlorohydroquinones with dichloromaleic anhydride (a melt of anhydrous AlCl3 and NaCl, 185—195 °C), the substrates undergo various degrees of disproportionation, which reduces the yields of the target triand tetrachloronaphthazarins. Quantum chemical calculations showed that the cycloacylation in question proceeds as a double aromatic electrophilic substitution of the vicinal protons with the corresponding oxocarbenium ions (acylium cations).
- Novikov,Balaneva,Shestak,Anufriev, V. Ph.,Glazunov
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p. 993 - 1003
(2017/01/11)
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- Characterization of chlorophenol 4-monooxygenase (TftD) and NADH:FAD oxidoreductase (TftC) of burkholderia cepacia AC1100
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Burkholderia cepacia AC1100 completely degrades 2,4,5-trichlorophenol, in which an FADH2-dependent monooxygenase (TftD) and an NADH:FAD oxidoreductase (TftC) catalyze the initial steps. TftD oxidizes 2,4,5-trichlorophenol (2,4,5-TCP) to 2,5-dichloro-p-benzoquinone, which is chemically reduced to 2,5-dichloro-p-hydroquinone (2,5-DiCHQ). Then, TftD oxidizes the latter to 5-chloro-2-hydroxy-p-benzoquinone. In those processes, TftC provides all the required FADH2. We have determined the crystal structures of dimeric TftC and tetrameric TftD at 2.0 and 2.5 A resolution, respectively. The structure of TftC was similar to those of related flavin reductases. The stacked nicotinamide:isoalloxazine rings in TftC and sequential reaction kinetics suggest that the reduced FAD leaves TftC after NADH oxidation. The structure of TftD was also similar to the known structures of FADH2-dependent monooxygenases. Its His-289 residue in the re-side of the isoalloxazine ring is within hydrogen bonding distance with a hydroxyl group of 2,5-Di-CHQ.AnH289Amutation resulted in the complete loss of activity toward 2,5-DiCHQ and a significant decrease in catalytic efficiency toward 2,4,5-TCP. Thus, His-289 plays different roles in the catalysis of 2,4,5-TCP and 2,5-DiCHQ. The results support that free FADH2 is generated by TftC, and TftD uses FADH2 to separately transform 2,4,5-TCP and 2,5-DiCHQ. Additional experimental data also support the diffusion of FADH2 between TftC and TftD without direct physical interaction between the two enzymes.
- Webb, Brian N.,Ballinger, Jordan W.,Kim, Eunjung,Belchik, Sara M.,Lam, Ka-Sum,Youn, Buhyun,Nissen, Mark S.,Xun, Luying,Kang, Chulhee
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scheme or table
p. 2014 - 2027
(2011/02/22)
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- Rate constants of elementary steps of the reversible chain reaction of N-phenyl-1,4-benzoquinonemonoimine with 2,5-dichlorohydroquinone
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The kinetics of reversible chain reactions in quinoneimine-hydroquinone systems has first been studied for the reaction of N-phenyl-1,4- benzoquinonemonoimine with 2,5-dichloro-hydroquinone used as an example. The dependences of the reaction rate on the concentration of the initial reactants, initiator, and each product were studied. The reliable estimates of the rate constants of 11 (of 12) elementary steps of this reaction were obtained from the experimental data using the earlier derived formulas and the method of equal concentrations developed in the present work.
- Antonov,Varlamov
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p. 883 - 889
(2008/09/18)
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- The kinetics of the reversible chain reaction between 2,5-dichloroquinone and 4-hydroxydiphenylamine
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The paper presents the results obtained in a study of the kinetics of the reversible chain reaction between 2,5-dichloroquinone and 4-hydroxydiphenylamine (K eq = 3.2). We studied the dependence of the reaction rate on the concentrations of the initiator, initial reagents, and all products. The equations obtained earlier for the rate of reversible chain reactions and the method of equal concentrations suggested in this work were used to estimate the rate constants of most of the reaction mechanism elementary steps from the experimental data. The results obtained were shown to closely agree with and agument the data obtained earlier for the kinetics of the chain reaction between N-phenyl-1,4-benzoquinonemonoimine and 2,5-dichlorohydroquinone. On the whole, all the elementary steps of these two (forward and back) reversible chain reactions were characterized by rate constant values.
- Antonov,Varlamov
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p. 1968 - 1975
(2008/03/13)
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- Determination of dissociation energies of N-H bond in the 4-anilinodiphenylaminyl radical and O-H bond in the 2,5-dichloro-4- hydroxyphenoxyl radical from the equilibrium constants of chain reactions in quinoneimine-hydroquinone systems
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The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)-2,5- dichlorohydroquinone systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine was estimated from these data (ΔH = - 14.4±1.6 kJ mol-1) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (DNH = 278.6±3.0 kJ mol-1). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation energy in 2,5-dichlorohydroquinone: DOH = 362.4±0.9 kJ mol-1. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: DOH = 253.6±1.9 kJ mol-1.
- Antonov,Gadomsky,Varlamov
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p. 1723 - 1728
(2008/02/09)
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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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- 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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- Properties of chlorinated dihydroxybenzenes - components of pulp bleaching effluents
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Gas chromatography and mass spectrometry data are given for the chlorodihydroxybenzenes which are components of wood pulp bleaching effluents and biologically-treated effluents, and are proposed intermediates in the chlorination of humic acids. The chlorohydroxybenzenes include the nine chlorocatechols, the six chlorohydroquinones and the seven known chlororesorcinols. The 22 chlorinated compounds were generally well separated on a phenyl methyl silicone column with the exception of three dichloro compounds. The chloro compounds with the same level of chlorine substitution were not able to be distinguished on the basis of their electron impact mass spectra.
- Smith, Terrence J.,Wearne, Ross H.,Wallis, Adrian F. A.
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p. 1555 - 1560
(2007/10/03)
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- ELECTROCHEMICAL BEHAVIOUR OF SUBSTITUTED HYDROQUINONES
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The oxidation potentials of thirteen hydroquinones substituted by chlorine, pyrazol-1-yl and 3,5-dimethylpyrazol-1-yl groups were experimentally determined by cyclic voltammetry.The estimated half-wave potentials E1/2 have been discussed taking into account the inductive and conjugative substituent effects.Key words: Cyclic voltammetry, Hydroquinones, Redox, Pyrazoles
- Claramunt, R. M.,Escolastico, C.,Maria, M. D. Santa,Lopez, V.
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p. 368 - 371
(2007/10/02)
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- Electron-Transfer Photochemistry of Thianthrene. Nucleophile-Assisted Photooxidation to Sulfoxide
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The photochemistry of thianthrene (1) in the presence of a variety of electron acceptors in acetonitrile/water has been investigated.Diffusion-limited rates of fluorescence quenching were observed on excitation of 1 (with fumaronitrile and with methyl phthalate).Alternatively, addition of 1 led to quenching of the fluorescence emission of ketones, biacetyl, and fluorenone and to the quenching of the excited triplet states of quinones such as 2,5-dichloro-p-benzoquinone (5) and 9,10-anthraquinone (3).Steady irradiation of 1 with selected acceptors results in photooxidation yielding the sulfoxide (2) and the expected photoreduction products.The electron-transfer mechanisms were further studied by laser flash photolysis.The quenching of ketone triplets by 1 resulted in formation of the radical cation of 1 (λmax 540 nm).The decay of this species and its dependence on the concentration of water present were determined; the bimolecular rate constant for trapping of 1.+ by water in acetonitrile-water solutions is 4.9x104 M-1s-1 under conditions in which 5 acts as a sensitizer.The nucleophilic trapping of the 1 radical cation by imidazole (k=3.6x107 M-1s-1) was also investigated.The mechanism of photoinduced two-electron oxidation of 1 and the role played by nucleophiles in facilitating the reaction are discussed in detail.
- Jones, Guilford,Huang, Bin,Griffin, Susan F.
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p. 2035 - 2042
(2007/10/02)
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- REACTION OF PHOSPHORUS PENTACHLORIDE WITH QUINONES
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Phosphorus pentachloride reacts with 1,4-benzoquinone and 2-chloro-1,4-benzoquinone at room temperature.The basic products are dichlorophosphoranes (1a, b).Intermediates have been ascertained.The scheme of the process has been confirmed by model reactions.Key words: Quinones; Phosphorus pentachloride; Mechanism; Infrared; Dimethylaniline; Mass spectra.
- Kutyrew, A. A.,Fomin, S. G.,Moskva, V. V.
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- PHOTOSENSITIZED DIMERIZATION OF PHENOL IN PORPHYRIN-QUINONE-PHENOL SYSTEM
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Photosensitized dimerization of 4-methoxyphenol was observed upon irradiation of porphyrin in the presence of quinone.Quantum yield for dimer was depended upon the concentration and reduction potential of quinones used.Free radical coupling mechanism was confirmed by means of ESR and CIDNP techniques.
- Maruyama, Kazuhiro,Furuta, Hiroyuki
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p. 243 - 246
(2007/10/02)
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- Mechanism of the Oxidation of NADH by Quinones. Energetics of One-Electron and Hydride Routes
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The kinetics of NADH oxidation by 7 o-benzoquinones and 14 p-benzoquinones were studied by using buffered aqueous solutions and UV/vis spectroscopy.For each quinone the rate law was first order in NADH and first order in quinone.The rate constants varied from 0.0745 to 9220 M-1s-1.Variation of the pH from 6 to 8 gave no change in rate.The use of 4-D and 4,4-D2NADH revealed kinetic isotope effects.The dideutero data gave kH/kD in the range 1.6-3.1 for p-quinones and 4.2 for 3,5-di-tert-butyl-o-quinone.When p-quinones were used, the log k was a linear function of Eo for the quinone/hydroquinone monoanion (Q/QH(1-)) couple with a slope of 16.9 V-1. o-Quinones reacted about 100 times more rapidly, but the same linear relationship with a slope of 16.4 V-1 was observed.Comparisons to data for one-electron-transfer reactions indicate that such mechanisms are not involved.A hydride-transfer mechanism accommodates all the data, and rate-limiting hydrogen atom transfer followed by electron transfer cannot be ruled out.
- Carlson, Brian W.,Miller, Larry L.
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p. 479 - 485
(2007/10/02)
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- ON THE 1,6-ADDITION OF ALKYLALUMINIUM COMPOUNDS TO para-QUINONES
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Ethyl, n-butyl and i-butylaluminium dichlorides undergo 1,6-addition to a conjugated bond system O=C-C=C-C=O of para-quinones.Methylaluminium dichloride is inactive in this addition, and triethylaluminium gives low yields.The reactivities of the quinones vary with their electron affinities, and the highest yields of 1,6-addition are obtained in the reactions of chlorine derivatives of 1,4-benzoquinone.The results are discussed in terms of a radical mechanism involving a homolytic cleavage of the Al-C bond in the donor-acceptor complex formed between the reactants followed by combination of alkyl radicals and aluminium derivatives of semiquinone within a cage.The stable donor-acceptor complexes and aluminium derivative of semiquinone were isolated and characterized from the reactions of aluminium trichloride with 2,3,5,6-tetramethyl-1,4-benzoquinone and 2,3,5,6-tetrachloro-1,4-benzoquinone, respectively.
- Florjanczyk, Zbigniew,Szymanska-Zachara, Ewa
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p. 127 - 138
(2007/10/02)
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- Products and Kinetic Substituent Effects in the Reactions of Diaryldiazomethanes with 2,5-Dichloro-p-benzoquinone
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Substituted diphenyldiazomethanes were found to react with 2,5-dichloro-p-benzoquinone at its conjugated C=C and C=O bonds to give bicyclic diones and poly(2,5-dichlorohydroquinone benzhydryl ether)s recpectively, obeying the second-order kinetic low.The product ratios of the polyethers to the bicyclic diones increased from 1.1 to 8.1 with the electron-donating ability of the substituents.The individual rate constants were well correlated with the Hammett equation: log k3 = -2.15-1.55? (r = 0.996) and log k4 = -2.03-1.85 (?0+0.515ΔR+) (r = 1.00) for the bicyclic diones and polyethers processes respectively.The presence of methanol as a trapping agent stopped the polymerization completely and afforded the methanolysis product, though no essential change was found in the formation of the bicyclic dione.
- Oshima, Takumi,Nagai, Toshikazu
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p. 551 - 554
(2007/10/02)
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- Benzoquinones and Related Compounds. Part 4. Thermolysis of the Diels-Alder Adduct of 2-Acetyl-5,6-dichloro-1,4-benzoquinone and Cyclopentadiene: Evidence for a Partial Retro-diene Reaction
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Addition of chlorine to (2-methyl-1,3-dioxolan-2-yl)-1,4-benzoquinone occurs at the unsubstituted double bond.Subsequent enolisation and cleavage of the acetal affords 2-acetyl-5,6-dichlorohydroquinone in 50percent overall yield.Oxidation of this gives the corresponding 1,4-benzoquinone which with cyclopentadiene yields, predominantly, the 1 : 1 Diels-Alder adduct (6) by endo-addition to the 2,3-double bond.Thermolysis of this adduct in benzene results in disproportionation to cyclopentadiene and the spiro-acetal (13); thermolysis in acetic acid also yields (13), but the major product is the dihydrobenzofuran (14), an isomer of the Diels-Alder adduct.Mechanisms for the formation of these products are discussed.
- Beddoes, Roy L.,Bruce, J. Malcolm,Finch, Harry,Heelam, Leslie M. J.,Hunt, Ian D.,Mills, Owen S.
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p. 2670 - 2676
(2007/10/02)
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