- Linear Paired Electrolysis—Realising 200 % Current Efficiency for Stoichiometric Transformations—The Electrochemical Bromination of Alkenes
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The generation of bromine by oxidation of bromide anions at the anode and reduction of molecular oxygen at the cathode to hydrogen peroxide resulted in the overall formation of two molecules of Br2 (=four electron oxidation) by passing just two electrons through the solution. The bromine was used for the bromination of alkenes and thereby a linear paired electrolysis was attained which resulted in current efficencies of up to 200 %. Also, the diiodination of cyclohexene as well as the electrophilic aromatic bromination of an electron-rich arene were realised both in 168 % current efficiencies.
- Strehl, Julia,Abraham, Marvin L.,Hilt, Gerhard
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supporting information
p. 9996 - 10000
(2021/03/31)
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- A Highly Efficient Method for the Bromination of Alkenes, Alkynes and Ketones Using Dimethyl Sulfoxide and Oxalyl Bromide
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The pairing of DMSO and oxalyl bromide is reported as a highly efficient brominating reagent for various alkenes, alkynes and ketones. This bromination approach demonstrates remarkable advantages, such as mild conditions, low cost, short reaction times, provides excellent yields in most cases and represents a very attractive alternative for the preparation of dibromides and α-bromoketones.
- Ding, Rui,Li, Jiaqi,Jiao, Wenyi,Han, Mengru,Liu, Yongguo,Tian, Hongyu,Sun, Baoguo
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p. 4325 - 4335
(2018/11/21)
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- Dibromination of alkenes with LiBr and H2O2 under mild conditions
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Electron-rich and electron-poor alkenes, and alkenes bearing protecting groups can be efficiently and stereoselectively converted to trans-dibromides using LiBr/H2O2 and AcOH as a proton source in 1,4-dioxane. For most substrates addition of 0.1 mol% of PhTeTePh enhances the reaction rate and the yield of the products. Experimental data suggest that the brominating agent prepared in situ is molecular bromine and that LiBr assists the activation of H2O2 allowing bromination to occur using AcOH as a mild proton source in uncatalyzed experiments. Scale-up is feasible: 10.0 mmol of 1-octene was quantitatively converted to 1,2-dibromooctene in one hour of reaction at room temperature.
- Martins, Nayara Silva,Alberto, Eduardo E.
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supporting information
p. 161 - 167
(2017/12/28)
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- A Mechanistic Study of Halogen Addition and Photoelimination from π-Conjugated Tellurophenes
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The ability to drive reactivity using visible light is of importance for many disciplines of chemistry and has significant implications for sustainable chemistry. Identifying photochemically active compounds and understanding photochemical mechanisms is important for the development of useful materials for synthesis and catalysis. Here we report a series of photoactive diphenyltellurophene compounds bearing electron-withdrawing and electron-donating substituents synthesized by alkyne coupling/ring closing or palladium-catalyzed ipso-arylation chemistry. The redox chemistry of these compounds was studied with respect to oxidative addition and photoelimination of bromine, which is of importance for energy storage reactions involving X2. The oxidative addition reaction mechanism was studied using density functional theory, the results of which support a three-step mechanism involving the formation of an initial η1 association complex, a monobrominated intermediate, and finally the dibrominated product. All of the tellurophene derivatives undergo photoreduction using 430, 447, or 617 nm light depending on the absorption properties of the compound. Compounds bearing electron-withdrawing substituents have the highest photochemical quantum efficiencies in the presence of an alkene trap, with efficiencies of up to 42.4% for a pentafluorophenyl-functionalized tellurophene. The photoelimination reaction was studied in detail through bromine trapping experiments and laser flash photolysis, and a mechanism is proposed. The photoreaction, which occurs by release of bromine radicals, is competitive with intersystem crossing to the triplet state of the brominated species, as evidenced by the formation of singlet oxygen. These findings should be useful for the design of new photochemically active compounds supported by main-group elements.
- Carrera, Elisa I.,Lanterna, Anabel E.,Lough, Alan J.,Scaiano, Juan C.,Seferos, Dwight S.
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supporting information
p. 2678 - 2689
(2016/03/12)
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- An optimised procedure for PTFE phase vanishing reactions: An improved reaction design and the use of reagents adsorbed on silica
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While the phase-vanishing (PV)-PTFE reaction design works well with a broad range of substrates and reaction conditions, there are occasional problems. A description of the problems and their importance, including their effects on the reaction outcome and ways to address them, are discussed. Details of an improved design, a hybrid of previously reported PV-PTFE and solvent-free PV-PTFE designs, is presented, as well as the use of silica-supported reagents.
- Parsons, Brendon A.,Smith, Olivia Lin,Dragojlovic, Veljko
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p. 574 - 581
(2015/11/27)
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- Oxidative bromination of alkenes mediated with nitrite in ionic liquids
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The oxidative bromination of C2-C8 alkenes with HBr-NaNO2-O2 in solutions of BMImBr, HMImBr or BMImBF 4 containing 16-28 wt% H2O was studied using volumetric method, GC-MS analysis, 14N NMR and UV-VIS spectroscopy. The optimal conditions to conduct the reaction at high selectivity for 1,2-dibromoalkanes in BMImBr were determined. The composition of ionic liquid affects the catalytic performance. Although in BMImBF4 the reaction runs with equal rate as in bromide ionic liquid, the fraction of bromohydrin in the reaction products increases to 20 %. Generated from NaNO2, NOx operated as a catalyst in the oxidation of Br- and was oxidized to catalytically inert NO3 - anions when complete conversion of HBr was attained. Graphical Abstract: Oxidative bromination of alkenes [Figure not available: see fulltext.]
- Kuznetsova, Lidia I.,Kuznetsova, Nina I.,Zudin, Vladimir N.,Utkin, Viktor A.,Trebushat, Dmitry V.,Fedotov, Martin A.,Larina, Tatyana V.
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p. 1499 - 1506,8
(2014/11/08)
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- High quantum yield molecular bromine photoelimination from mononuclear platinum(IV) complexes
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Pt(IV) complexes trans-Pt(PEt3)2(R)(Br)3 (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe3 analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br2. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination.
- Raphael Karikachery, Alice,Lee, Han Baek,Masjedi, Mehdi,Ross, Andreas,Moody, Morgan A.,Cai, Xiaochen,Chui, Megan,Hoff, Carl D.,Sharp, Paul R.
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p. 4113 - 4119
(2013/05/09)
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- Solvent-free phase-vanishing reactions with PTFE (Teflon) as a phase screen
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In a solvent-free phase-vanishing reaction with PTFE (polytetrafluoroethylene, Teflon) tape as the phase screen, a thermometer adapter is utilized to insert a PTFE-sealed tube into the vapor phase above the substrate. Besides avoiding use of solvents, the experimental design is not dependent upon the densities of the reactants and the procedure generates little or no waste while providing the reaction products in high yield and in high purity.
- Pels, Kevin,Dragojlovic, Veljko
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scheme or table
(2010/04/22)
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- The role of neat substrates in phase-vanishing and tandem phase-vanishing reactions
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Phase-vanishing reactions are triphasic reactions, which involve a reagent, a liquid perfluoroalkane as a phase screen and a substrate. Aromatization, isomerization and halogenation of neat substrates under phase-vanishing conditions gave the expected products in good to excellent yields. In tandem single-phase-phase-vanishing reaction, two reactants, placed in the top phase, afforded the intermediate, which in a subsequent phase-vanishing reaction reacted with the reagent from the bottom phase to give the final product. The reaction worked well under solvent-free conditions on liquid substrates and intermediates. With solids, results were better if an additional solvent was employed.
- Windmon, Nicole,Dragojlovic, Veljko
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scheme or table
p. 6543 - 6546
(2009/04/06)
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- Reaction of Aromatic and Unsaturated Compounds with the Potassium Permanganate/HCI (HBr) Acetonitrile Reagent
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Addition of hydrochloric or hydrobromic acid to a solution of potassium permanganate in acetonitrile produced a homogeneous mixture, which is suitable for laboratory chlorination or bromination, respectively. Aromatic compounds more reactive than alkylbenzenes can be chlorinated or brominated without additional catalyst. Alkenes and alkynes give the corresponding vicinal dihaloalkanes and vinyl halides. All reactions complete within two hours under mild condition (25-60 °C) with excellent to moderate yields.
- Liu, Lilian Kao,Lin, Ching-Shan
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- Diphenylpyridylmethyl radicals. Part 1. Synthesis, dimerization and ENDOR spectroscopy of diphenyl(2-, 3- or 4-pyridyl)methyl radicals; bond dissociation enthalpies of their dimers
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ortho-ortho hydrogen van der Waals repulsions are the origin of the propeller shape of the triphenylmethyl radical and the main reason for the low bond dissociation enthalpy (BDH) of its dimer 1 (44.8 J mol-1).In order to reduce these steric repulsions (eliminating some aromatic hydrogens), diphenyl(2-, 3- or 4-pyridyl)methyl radicals 2-4 were prepared through reductive dehalogenation of the corresponding triarylchloromethanes 2-4a with silver in benzene.They form α,p-dimers 2-4e exclusively through the pyridine ring.ENDOR spectroscopy shows that the structure of the radicals, does not deviate substantially from that of the parent radical, PH3C-radical.In contrast, the BDH values of the dimers (measured using EPR spectroscopy) show strengthening of the central C-C bond in 2e (88.7 kJ mol-1) and 3e (90.0 kJ mol-2) and a silimar value for 4e (46.4 kJ mol-1) with respect to the trityl dimer 1.This is a consequence of the ground state stabilization of the dimers 2-4e due to relief of strain (elimination of ring hydrogens), whereas in the case of 4e, this stabilization is probably compensated by the formation of a weaker C-N bond with respect to the C-C bond.The above dimers undergo easy 1,5-H-rearrangement, autocatalysed by the basic pyridyl groups themselves.
- Tzerpos, Nikolaos I.,Zarkadis, Antonios K.,Kreher, Richard P.,Repas, Liesel,Lehnig, Manfred
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p. 755 - 762
(2007/10/02)
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- Surface-Mediated Reactions. 4. Hydrohalogenation of Alkynes
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The use of appropriately prepared silica gel and alumina has been found to mediate the addition of hydrogen halides to alkynes.The technique has been rendered even more convenient by the use of various organic and inorganic acid halides that react in the presence of silica gel or alumina to generate hydrogen halides in situ.Treatment in this fashion of 1-propynylbenzene (1), which underwent no reaction in CH2Cl2 solution saturated with HCl, readily afforded the syn addition product, alkenyl chloride (E)-4a.On extended treatment (E)-4a underwent subsequent iaomerization to the thermodynamically more stable Z isomer.Thus either isomer of 4a could be obtained in good yield depending on the reaction conditions.In a similar way bromides (E)- and (Z)-4b were obtained without competing formation of the radical products (E)- and (Z)-5, which occurred in solution.In contrast with solution-phase hydroiodination of alkyne 1, which slowly afforded iodide (E)-4c, surface-mediated addition readily afforded (E)-4c, followed by isomerization to the Z isomer.E ->/- Z equilibration of the alkenyl halides 4 was shown to involve, at last in part, addition-elimination via the gem-dihalides 13.Analogous behavior was exhibited by the phenylalkynes 2 and 3 on surface-mediated hydrohalogenation.Surface-mediated addition of HBr and HI to the internal alkylalkyne 14 afforded principally the anti addition products (Z)-15b,c.Treatment of the terminal alkynes 17 and 22 with (COBr)2 over alumina gave the dibromides 20 and 24/25, respectively, whereas use of acetyl bromide as the HBr precursor afforded the alkenyl bromides 18b and 23.
- Kropp, Paul J.,Crawford, Scott D.
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p. 3102 - 3112
(2007/10/02)
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- Stereochemistry of Polymethylated β,β'-Dibromoethers Formed by Bromination of Alkene-Epoxide Mixtures
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Examination of β,β'-dibromoethers formed when bromine is added to alkene-epoxide mixtures in pentane at -78 deg C revealed that opening of the bromonium ion and the epoxide occurs under high stereochemical control, whilst approach of the epoxide and the bromomonium ion is subject to lower or no stereochemical control.
- Neill, David,Thomas, Susan E.
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p. 2620 - 2622
(2007/10/02)
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- Outer-sphere and inner-sphere processes in reductive elimination. Direct and indirect electrochemical reduction of vicinal dibromoalkanes
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The reduction of vicinal dibromoalkanes is investigated as an example of the dichotomy between outer-sphere and inner-sphere processes in reductive elimination. As a result from the analysis of the kinetic data, outer- sphere reagents such as carbon electrodes and aromatic anion radicals react with vicinal dibromoalkanes according to an "ET" mechanism in which the rate-determining step is a concerted electron- transfer bond-breaking reaction leading to the β-bromoalkyl radical. The latter is then reduced very rapidly, in a second step, most probably along another concerted electro- transfer bond-breaking pathway leading directly to the olefin in the heterogeneous case and through halogen atom expulsion in the homogeneous case. In the absence of steric constraints, the reduction goes entirely through the antiperiplanar conformer because the resulting β-bromoalkyl radical is then stabilized by delocalization of the unpaired electron over the C-C-X framework due to a favorable interaction between the pz orbital of the radical carbon and the σ* orbital of the C-Br bond. This interaction is enhanced by alkyl substitution at the reacting carbons, resulting in an approximately linear correlation between the reduction potential and the C-X bond energy of OlX2 on one hand and the vertical ionization potential of the olefin on the other. The stabilization energy is of the order of 0.2-0.3 eV for the anti conformers. It can also be taken as a measure of the rotation barrier around the C-C bond responsible for the loss of stereospecificity in the reduction. This competes with the reduction of the two stable conformers of the OlX* radicals and for the expulsion of the halogen atom. There is a remarkably good agreement between the ensuing prediction of the E:Z olefin ratio that should be found upon reduction of threo and erythro OlX2 isomers by outer-sphere reagents such as aromatic anion radicals and the experimental data. Although members of perfectly reversible redox couples, iron(I), iron("0"), and cobalt(I) porphyrins offer typical examples of inner-sphere reagents in their reaction with vicinal dibromoalkanes. They indeed react much more rapidly than outer-sphere electron donors (aromatic anion radicals) of the same standard potential. On the basis of steric hindrance experiments, it was shown that they do not react according to an SN2 rate-determining step involving the transient formation of an organometallic species. Complete stereospecificity is obtained, showing that they react along a halonium transfer E2 elimination mechanism rather than by an E1 elimination or a halogen atom transfer mechanism. As shown on a quantitative basis, this is related to the large driving force offered to halonium abstraction by the strong affinity of the iron(III) and cobalt (III) complexes toward halide ions. In regards to catalysis, the investigated systems provide typical examples showing the superiority of inner-sphere (chemical) catalysis over outer-sphere (redox) catalysis of electrochemical reactions. Not only is the catalytic efficiency much better since the rate constants of the key steps are larger, given the standard potential of the catalyst, but also selectivity is dramatically improved.
- Lexa, Doris,Savéant, Jean-Michel,Sch?fer, Hans J.,Su, Khac-Binh,Vering, Birgit,Wang, Dan Li
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p. 6162 - 6177
(2007/10/02)
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- Nitrodibromoacetonitrile: An Agent for Bromination and for the Formation of Adducts Formally Derived from Cyanonitrocarbene
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The reactions of nitrodibromoacetonitrile (NDBA) with alkenes, aromatic compounds, amines, and sulphides have been studied.Products were formally derived from bromine or nitrocyanocarbene except for bis(2-bromocyclohexyl)nitroacetonitrile (13), an adduct (2:1) obtained from cyclohexene and NDBA. 2,3-Dimethylbut-2-ene was converted into 3-cyano-4,4,5,5-tetramethyl-4,5-dihydroisoxazole 2-oxide (15) and 2,3-dibromo-2,3-dimethylbutane (16). meso-1,2-Dibromo-1,2-diphenylethane was obtained from trans-stilbene.In a reaction catalysed by copper, benzene and NDBA gave benzoyl cyanide.From anthracene, phenol, and cupric acetylacetonate, 9,10-dibromoanthracene, 4-bromophenol, and cupric bromoacetylacetonate were obtained respectively.Dimethyl sulphide, tetrahydrothiophene, and triethylamine afforded the corresponding sulphonium-(30) and (31) and ammonium cyanomethylide-(33) derivatives and, with the latter, co-formation of triethylammonium bromide.Rationales for the reactions are discussed.
- Boyer, Joseph H.,Manimaran, Thanikavelu
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p. 1381 - 1385
(2007/10/02)
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- A UNIQUE FORMATION OF AN ISOXAZOLINE-N-OXIDE FROM NITRODIBROMOACETONITRILE AND TETRAMETHYLETHYLENE
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Tetramethylethylene and nitrodibromoacetonitrile in dichloromethane at 25 deg C gave 3-cyano-4,4,5,5-tetramethyl-2-isoxazolin-2-oxide (45percent) and 2,3-dibromo-2,3-dimethylbutane (42percent).
- Boyer, Joseph H.,Manimaran, Thanikavelu
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p. 2059 - 2062
(2007/10/02)
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