- Unusual Photochemistry of 4-Chlorobiphenyl in Water
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The photochemistry of 4-chlorobiphenyl in deoxygenated, doubly distilled water has been studied.When the reaction is carried to completion, equimolar amounts of 3- and 4-hydroxybiphenyl are produced; no biphenyl is formed, however.Experiments proved that arenium ions, arynes, and addition-elimination mechanisms are not involved in the formation of 3-hydroxybiphenyl.A plausible alternative is that 4-chlorobiphenyl isomerizes to the 3-isomer which then undergoes photohydrolysis.This was shown to be the case by capillary gas chromatography.The unique properties of water appear to be responsible for the unusual behavior of 4-chlorobiphenyl in water.
- Moore, Theresa,Pagni, Richard M.
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- Ligand-Free C–C Coupling Reactions Promoted by Hexagonal Boron Nitride-Supported Palladium(II) Catalyst in Water
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A micron-scale palladium(II) material has been successfully prepared using Schiff base-modified hexagonal boron nitride as a support and used for the first time as an efficient and recyclable catalyst in organic synthesis. The morphology, composition, metal loading and thermal stability of the catalyst were studied using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), infrared spectroscopy (FT-IR), inductively coupled plasma (ICP) and thermogravimetric (TG) analyses. Then, the micron material was tested in various C–C cross-coupling reactions and exhibited excellent catalytic activities in the Suzuki and Heck reactions. Moreover, the catalyst could be easily recovered by simple filtration and reused at least ten times without significant loss of its catalytic activity. In general, this work demonstrates the possibility of using Schiff-base@hexagonal boron nitride as an efficient support for heterogeneous catalysts. (Figure presented.).
- Cheng, Xu,Li, Weijian,Nie, Ruifang,Ma, Xiaojun,Sang, Rui,Guo, Li,Wu, Yong
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supporting information
p. 454 - 466
(2017/02/10)
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- Novel cyclodextrin-modified h-BN@Pd(II) nanomaterial: An efficient and recoverable catalyst for ligand-free C-C cross-coupling reactions in water
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An environmentally friendly palladium(II) catalyst supported on cyclodextrin-modified h-BN was successfully prepared. The catalyst was characterized by FT-IR, SEM, TG, XRD and XPS, and the loading level of Pd in h-BN@β-CD@Pd(II) was measured to be 0.088?mmol g?1 by ICP. It exhibits excellent catalytic activity for the Suzuki and Heck reactions in water, and can be easily separated and consecutively reused for at least nine times. In addition, a series of pharmacologically interesting products were successfully synthesized using this catalyst to demonstrate its potential applications in pharmaceutical industries. Above all, this work opens up an interesting and attractive avenue for the use of cyclodextrin-functionalized h-BN as an efficient support for hydrophilic heterogeneous catalysts.
- Ma, Xiaojun,Lv, Guanghui,Cheng, Xu,Li, Weijian,Sang, Rui,Zhang, Yong,Wang, Qiantao,Hai, Li,Wu, Yong
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- A dinuclear iron(II) complex bearing multidentate pyridinyl ligand: Synthesis, characterization and its catalysis on the hydroxylation of aromatic compounds
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A dinuclear iron(II) complex Fe2L2(μ2-Cl)2Cl2 (L = N,N-bis(pyridin-2-ylmethyl)prop-2-yn-1-amine) was prepared and fully characterized by UV–Vis spectroscopy, elemental analysis, electrochemical analysis and X-ray single crystal diffraction analysis. The catalytic activity of the complex was assessed for the hydroxylation of aromatic compounds by using aqueous H2O2 as an oxidant in acetonitrile. The catalytic system was applicable in a wide range of substrates including aromatic compounds with both electron-donating and electron-withdrawing substituents and showed moderate to good catalytic activity and selectivity in the oxidation reactions. Particularly, in the case of benzene the selectivity of phenol achieve to 74% with the reaction conversion of 24.8%.
- Gu, Erxing,Zhong, Wei,Ma, Hongxia,Xu, Beibei,Wang, Hailong,Liu, Xiaoming
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p. 159 - 165
(2018/03/29)
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- Palladium(II)-Schiff base complex immobilized covalently on h-BN: An efficient and recyclable catalyst for aqueous organic transformations
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A moisture- and air-stable palladium(II)-Schiff base complex supported on h-BN was simply prepared by using commercially available reagents. This nanomaterial was applied as an excellent and recyclable heterogeneous catalyst for the Suzuki and Heck cross-coupling reactions. And it has been characterized by FT-IR, XRD, SEM, XPS, TG and ICP-AES techniques. High yields, ligand-free, low reaction time, water as solvent, non-toxicity and recyclability of the catalyst are the main merits of these protocols. In addition, a series of pharmacologically relevant products were successfully synthesized using this catalyst. Above all, this work opens up an interesting and attractive avenue for the use of h-BN as an efficient support for heterogeneous catalysts.
- Li, Weijian,Lv, Guanghui,Cheng, Xu,Sang, Rui,Ma, Xiaojun,Zhang, Yong,Nie, Ruifang,Li, Jie,Guan, Mei,Wu, Yong
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supporting information
p. 8557 - 8564
(2016/12/07)
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- Base-catalyzed dehydration of 3-substituted benzene cis -1,2-dihydrodiols: Stabilization of a cyclohexadienide anion intermediate by negative aromatic hyperconjugation
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Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by σ- values with = 3.2. Solvent isotope effects for the reactions are kH2O/kD 2O = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ～1011 s-1, corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 1011-1012 s -1. From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pKa of 30.8 ± 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion.
- Kudavalli, Jaya Satyanarayana,Rao, S. Nagaraja,More Oferrall, Rory A.,Bean, David E.,Fowler, Patrick W.,Sharma, Narain D.,Boyd, Derek R.,Kamerlin, Shina Caroline Lynn,Keeffe, James R.,Gronert, Scott
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supporting information
p. 14056 - 14069,14
(2020/08/31)
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- Birch reductive alkylation of biaryls: Scope and limitations
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(Chemical Equation Presented) Birch reductive alkylation of biaryls has been carried out by varying the nature of the substituents on the aromatic rings. Our investigations have focused on electron-rich substituents such as OMe, OH, and NR2 gro
- Lebeuf, Raphael,Dunet, Julie,Beniazza, Redouane,Ibrahim, Dawood,Bose, Gopal,Berlande, Muriel,Robert, Frederic,Landais, Yannick
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experimental part
p. 6469 - 6478
(2010/01/16)
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- Direct hydroxylation of substituted benzenes to phenols with air and CO using molybdovanadophosphates as a key catalyst
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A direct synthetic method of cresols from toluene by hydroxylation with air using CO as a reducing agent was developed. The reaction of toluene with air (15 atm) and CO (5 atm) in the presence of catalytic amounts of H 4PMo11VO40·31H2O and Pd/C in aqueous acetic acid at 120°C for 2 h afforded a mixture of o-, m-, and p-cresols in 9.9% yield at 83% selectivity. Cresols were obtained in 19% yield by recharging air and CO under these conditions. A variety of substituted benzenes were hydroxylated by this method to give the corresponding phenol derivatives in higher selectivity.
- Mita, Shuichi,Sakamoto, Takao,Yamada, Shingo,Sakaguchi, Satoshi,Ishii, Yasutaka
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p. 7729 - 7732
(2007/10/03)
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- Acid-catalysed Aromatisation of Benzene cis-1,2-dihydrodiols: a Carbocation Transition State poorly stabilised by Resonance
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Acid-catalysed dehydration of 3-substituted benzene cis-1,2-dihydrodiols exhibits a Hammett plot with ρ=-8.2, consistent with reaction via a benzenonium ion-like intermediate; however, correlation of +M resonance substituents such as Me and MeO by ?p rather than ?+ constants indicates a marked imbalance between resonance and inductive stabilisation of the transition state.
- Boyd, Derek R.,Blacker, John,Byrne, Briege,Dalton, Howard,Hand, Mark V.,et al.
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p. 313 - 314
(2007/10/02)
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- Directing Effects of Phenyl Substitution in the Reaction of OH Radical with Aromatics: The Radiolytic Hydroxylation of Biphenyl
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The initial yields for formation of 2-, 3-, and 4-hydroxybiphenyl following the radiolytic oxidation of biphenyl in the presence of ferricyanide are respectively, 1.19, 1.13, and 1.55 molecules/100 eV.These yields reflect relative rates of 0.71:0.37:1 for *OH radical attack on each of the ortho, meta, and para positions of biphenyl.Comparison of the corresponiding partial rate constants to that of benzene shows that the phenyl substitution has a significant effect in directing *OH addition to the ortho and para positions of aromatic systems.Comparison with phenol shows that a phenyl substituent has only a slightly smaller effect than does OH in increasing the partial rate constant at the ortho and para positions but is substantially less effective in decreasing the rate of *OH reaction at the meta position.
- Chen, Xiaofeng,Schuler, Robert H.
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p. 421 - 425
(2007/10/02)
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- Desulfurization of Benzo- and Dibenzothiophenes with Nickel Boride
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Nickel boride, prepared from the reduction of nickel chloride hexahydrate with sodium borohydride in methanol-tetrahydrofuran, reduces benzothiophenes to alkylbenzenes and dibenzothiophenes to biphenyls.The reaction is rapid at or below room temperature and does not require protection from the atmosphere.Best results are obtained when the nickel boride is generated in situ in the presence of the sulfur compound.Hydroxyl, carboxyl, ester, and amino groups are unaffected while chloro, bromo, and nitro substituents are also reduced under these conditions.A short-livedintermediate, possibly a nickel hydride species, appears to be required in the desulfurization.Complexation of the substrate to the nickel boride surface, followed by stepwise reduction of the two C-S bonds, occurs.The faster disappearance of dibenzothiophene containing the lighter 32S isotope compared to that with 34S (k(32S)/k(34S) = 1.005 to 1.006) suggests that C-S bond cleavage is the rate-determining step.
- Back, Thomas G.,Yang, Kexin,Krouse, H. Roy
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p. 1986 - 1990
(2007/10/02)
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- Further Studies on the Photoisomerization and Hydrolysis of Chlorobiphenyls in Water. Common Ion Effect in the Photohydrolysis of 4-Chlorobiphenyl
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The photochemistry of 14C-labeled 2-chlorobiphenyl (2-ClBP) and 3-chlorobiphenyl (3-ClBP) in water was studied by isotopic dilution. 2-ClBP yielded 2-hydroxybiphenyl (2-HOBP) exclusively. 3-ClBP, on the other hand, yielded 3-HOBP, 2-HOBP, and 4-HOBP and a small amount of biphenyl (2percent).Thus, 2-ClBP reacts exclusively by a heterolytic process, whereas 3-ClBP reacts not only by a heterolytic pathway but also by competing isomerization (to 2-ClBP and 4-ClBP) and homolytic pathways.The photochemistry of 4-ClBP in 80:20 (v:v) H2O-CH3CN with added HCl or NaCl was also studied.These reactions were suppressed in the presence of HCl or NaCl.Further experiments demonstrated that the suppression was not due to any great extent to the ionic strength of the solution, spin orbit effects of heavy atoms, and proton quenching.These results are compatible with a mechanism originally proposed by Kropp in which the excited state of 4-ClBP, likely the triplet excited state, undergoes homolysis of the aryl carbon-chlorine bond to form a radical pair.Once formed the radical pair undergoes an electron transfer from the aryl radical to the chlorine atom to form a highly reactive aryl cation and chloride.Reaction of the aryl cation with added chloride then regenerates 4-ClBP.The overall reaction is equivalent to a common ion effect.The role of other mechanisms in the observed chemistry is also discussed.
- Orvis, Jessica,Weiss, Joseph,Pagni, Richard M.
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p. 1851 - 1857
(2007/10/02)
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