Vanadium-catalyzed oxidative bromination promoted by Brønsted acid or Lewis acid

The oxidative bromination of arenes was induced by a vanadium catalyst in the presence of a bromide salt and a Brønsted acid or a Lewis acid under molecular oxygen, which provides an eco-friendly bromination method as compared with a conventional bromination one with bromine. This catalytic reaction could be applied to the bromination of alkenes and alkynes to give the corresponding vic-bromides. Use of aluminum halide as a Lewis acid in place of a Brønsted acid was demonstrated to provide a more practical protocol for the oxidative bromination. From ketones, α-bromination products were obtained. AlBr₃ was found to serve as both a bromide source and a Lewis acid to induce the bromination smoothly. ⁵¹V NMR experiment showed that this catalytic bromination is likely to depend on the redox cycle of a vanadium catalyst under molecular oxygen.     

Lewis acid-promoted Kharasch-Curran additions: a competition kinetics study of bromine atom transfer addition of N-alpha-bromoacetyl-oxazolidinone to 1-hexene

Lewis acids can efficiently promote free radical atom transfer reactions of an oxazolidinone imide substrate, 1, derived from alpha-bromo acetic acid. Thus, 1 undergoes a radical chain addition to 1-hexene giving the atom transfer addition compound, 6, in the presence of scandium or ytterbium triflate in 1,2-dichloroethane or a cosolvent mixture of 1/9 THF/dichloromethane. In 1,2-dichloroethane the solution is heterogeneous, while the cosolvent mixture gives a homogeneous solution, even at temperatures of -78 degrees C. Competition experiments were carried out in both solvent systems with added carbon tetrachloride to study how Lewis acid affected the product distribution. In the presence of carbon tetrachloride, chloride 7 is formed in addition to 6 and the ratio of these two products depends on the amount of Lewis acid present. In the presence of ytterbium triflate, in the cosolvent system, the reaction rate of bromine atom transfer was enhanced up to 400-fold compared to the reaction without added Lewis acid. Significant rate enhancements were also obtained in the solvent 1,2-dichloroethane, although the analysis of the system is complicated by the heterogeneous nature of the medium. Computation of C-Br bond dissociation energies (BDE) of the complexed and uncomplexed oxazolidinone bromide suggest that complexation lowers the BDE due to the effect of the strong electron-withdrawing group on the C-Br bond dipole.     

Multicomponent and one-pot synthesis of trisubstituted pyridines through a Pd-catalyzed cross-coupling/cross-coupling/cycloaddition sequence

Trisubstituted pyridines are regioselectively synthesized through multicomponent and one-pot processes promoted by a bifunctional Pd-catalyst. The process involves formation of an enamine by Pd-catalyzed amination of an alkenyl bromide, formation of a 2-aza-1,3-butadiene by Pd-catalyzed cross-coupling of a trimethylsilylimine with an alkenyl bromide, and Lewis acid catalyzed cycloaddition between the enamine and the azadiene.     

Regio‐selective peroxybromination of poly(vinyl methyl ketone) as versatile tool for generation active ATRP initiation sites on solid surfaces

Peroxybromination or so‐called radical bromination is an environmentally friendly process which involves the use of in situ generated bromine by action of hydrogen peroxide on sodium or ammonium bromide in acid medium. The reaction takes place at room temperature without eliminating hydrobromic acid and no needs the use of elemental bromine. The reaction with poly(vinyl methyl ketone) in biphasic system was demonstrated to result in quantitative bromination exclusively at the methyne carbon of the polymer. The brominated polymer was successfully used as multifunctional macroinitiator for atom transfer radical polymerization (ATRP) of styrene and MMA to give bottlebrush polymers, as evidenced by ¹H NMR and GPC. This strategy was demonstrated to provide a means of easy bromination of solid polystyrene microspheres (210–420 μm) constituting with vinyl methyl ketone copolymer segments. Bromoalkyl groups generated (1.3 mmol g^?1) in aqueous mixture were used for surface initiated ATRP of glycidyl methacrylate and styrene monomers to give dense graft chains tethered to the surfaces with hydrolysis‐proof linkages. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3892–3900     

Stereoselective allylation for preparation of L-hexose derivatives

The reaction of allyltrimethylsilane with α,β-dialkoxyaldehydes is catalyzed by magnesium bromide, and provides for a highly stereoselective allylation which is predicted by α-chelation of the Lewis acid. The observed stereocontrol is opposite to that generally obtained from a variety of common allylation reagents.     

Evolution of Anolyte Composition in the Oxidative Electrolysis of Sodium Bromide in a Sulfuric Acid Medium

The oxidation of a 10 mM aqueous solution of sodium bromide in a sulfuric acid medium on the surface of a platinum electrode in a cell with separated spaces was studied. The process is important in view of the use of the bromine–bromide redox couple in redox flow batteries. The study was performed by cyclic voltammetry, potentiostatic chronoamperometry with optical absorption spectrum recording, and measurements of the potential of the redox reference electrode. A numerical procedure for processing the experimental spectra of the solution was developed to separate them into the spectrum of molecular bromine and the residual signal. The latter was attributed to the absorption of the tribromide anion based on the literature data. The experimental dependences of the Br₂ and Br₃^- concentrations for the oxidative electrolysis of the NaBr solution in the sulfuric acid medium agreed well with the theoretical predictions. The current efficiency of bromine formation was evaluated.

     

Atom transfer cyclization catalyzed by InCl3 via halogen activation

[reaction: see text] Indium trichloride was found to be an efficient catalyst for the cyclization of allylic halides and alkynes with atom transfer in methylene chloride. Mechanistic evidence supports a cationic reaction pathway with Lewis acid activation of the allylic halogen. Concomitant nucleophilic attack by the alkyne and trapping with halide led to atom transfer cyclization products. Depending on alkyne substitution, a bromine atom was transferred from the substrate or a chlorine atom was transferred from the solvent.     

Nucleophilic and electrophilic substitutions of difluoropropargyl bromides

Fundamental organic reactions like nucleophilic and electrophilic substitutions have seldom been studied on fluorinated propargyl or allenyl modules, when the carbon atom undergoing substitution is bonded to two fluorine atoms. Herein we report a practical synthesis of difluoropropargyl bromides from substituted acetylenes and dibromodifluorometane using a wide variety of alkyl, aryl or silyl substrates. The synthesis of O-, S- and carboxylic acid derivatives of difluoropropargyl bromide is also described. These compounds are suitable starting materials for the synthesis of electrophilically substituted difluoropropargyl derivatives via magnesium and fluoride promoted reactions. An indium-mediated reaction of silyldifluoropropargyl bromide, followed by electrophilic trapping with bromine led to a very useful bromoallene, which was then used in reactions with nucleophiles (C, O, N, P, S, Hal) to yield a de facto bimolecular nucleophilic substitution of a difluoropropargyl bromide.     

Reaction of ethyl o-benzoylenephosphite with bromine

Conclusions The reaction of ethyl o-benzoylenephosphite with bromine proceeds by the scheme of the Arbuzov reaction to give the acid bromide of ethyl-o-bromoformylphenylphosphoric acid, which then decomposes to ethyl bromide and the acid bromide of o-benzoylenephosphoric acid.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 437–439, February, 1971.     

Kinetics and mechanism of the oxidation of nitrous acid by bromine in aqueous sulfuric acid

The kinetics and mechanism of the reaction between nitrous acid and bromine are studied in dilute sulfuric acid medium, using both the stopped‐flow method and conventional spectrophotometry. The partial reaction order with respect to Br₂ moderately differs from 1, showing a saturation at a higher concentration of bromine. The second order of the reaction towards nitrous acid has been observed. Hydrogen and bromide ions significantly suppress the rate of reaction. Despite the apparent simplicity, the mechanism is rather complex, with two reaction pathways proposed. The first one is represented by the reaction of bromine with the intermediate dinitrogen trioxide. A direct nucleophilic attack of NO₂⁻ ion towards the bromine molecule is suggested as the second pathway. The proposed mechanism accounts for the observed behavior; in almost all cases a satisfactory quantitative agreement with the experiments is obtained. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 279–285, 2000     

Designing Homogeneous Bromine Redox Catalysis for Selective Aliphatic C−H Bond Functionalization

The potential of homogeneous oxidation catalysis employing bromine has remained largely unexplored. We herein show that the combination of a tetraalkylammonium bromide and meta‐chloroperbenzoic acid offers a unique catalyst system for the convenient and selective oxidation of saturated C(sp³)−H bonds upon photochemical initiation with day light. This approach enables remote, intramolecular, position‐selective C−H amination as demonstrated for 20 different examples. For the first time, an N‐halogenated intermediate was isolated as the active catalyst state in a catalytic Hofmann–Löffler reaction. In addition, an expeditious one‐pot synthesis of N‐sulfonyl oxaziridines from N‐sulfonamides was developed and exemplified for 15 transformations. These pioneering examples provide a change in paradigm for molecular catalysis with bromine.     

The reaction of triethylamine with bromine. An evaluation of the formation constant of the transient 1:1 complex and a study of its conversion into products.

Triethylamine and bromine in 1,2-dichloroethane give a non isolable 1:1 charge transfer complex with K_f>10⁷ M^?1, which undergoes a fast intramolecular oxidation to N,N-diethylethyldeneiminium bromide and hydrogen bromide.     

1-Phenylthio-3-vinyl-3-cyclohexenol, a new reagent for bis-annelation of silyl enol ethers

1-Phenylthio-3-vinyl-cyclohex-1-en-3-ol (2) has been synthesized and investigated as a new bis-annelation reagent for silyl enol ethers. Reagent 2 can be synthesized by a Grignard reaction of vinyl magnesium bromide with 3-phenylthiocyclohexenone. The reaction with silyl enol ethers takes place under Lewis acid catalysis and generally proceeds in good yields. The resulting phenylthiodienes can be hydrolyzed to enones, which have been cyclized in a homologous aldol reaction to polycyclic compounds.     

Ligand and solvent effects on cobalt(I)-catalysed reactions: Alkyne dimerisation versus [2+2+2]-cyclotrimerisation versus Diels-Alder reaction versus [4+2+2]-cycloaddition

The cobalt catalysed conversion of phenyl acetylene led to linear enyne dimerisation products when CoBr₂(dppe) was activated with magnesium in the absence of a Lewis acid. In contrast, in the presence of a Lewis acid the cyclotrimerisation process is favoured. Among several ligand systems and solvents tested the best results were obtained using a catalyst system consisting of a diimine cobalt bromide complex, zinc and zinc iodide in acetonitrile. With 2-5 mol% of the cobalt catalyst at ambient temperatures 1,2,4-triphenylbenzene could be obtained in 99% yield and in excellent regioselectivity (95:5) in 10 min reaction time. Competition experiments of phenylacetylene and isoprene were performed. A preference for the cyclotrimerisation reaction was found for the diimine cobalt complex in acetonitrile, while the Diels-Alder reaction is favoured with the cobalt(dppe) complex in dichloromethane. Also a regioselectively substituted cyclooctatriene product was formed in a [4+2+2]-cycloaddition process and isolated which allows assumptions on the reaction mechanism.     

Determination of bromide ions in seawater using flow system with chemiluminescence detection

A simple flow-based procedure with chemiluminescence (CL) detection is proposed for bromide ion determination in seawater. The procedure was based on the oxidation of bromide to bromine by chloramine-T followed by the reaction of bromine with luminol resulting in CL emission. Since no significant reaction within chloramine-T and luminol was observed, the detection was carried out without bromine extraction from the oxidant medium. The proposed flow system had a sampling rate of 40 determinations per hour, reagents consumption of 100 μg luminol and 60 μg chloramine-T per determination, a limit of detection of 0.5 mg l⁻¹ bromide ions, a linear concentration range (r = 0.999 and n = 7) between 0 and 100 mg l⁻¹, and a coefficient of variance better than 2.5% (for 10 measurements of a 10 mg l⁻¹ Br⁻ solution) were achieved. The analytical system was applied for the determination of bromide in seawater and estuarine-water samples, obtaining an analyte recovery ranging from 94 to 102% and comparing the results with a reference spectrophotometric method no significant difference was observed in 95% confidence level.     

催化裂化汽油的相转移催化氧化脱硫反应研究

研究了相转移催化应用于催化裂化汽油氧化脱硫反应的循环模型, 并从影响循环效应的主要因素入手, 筛选出了催化氧化效果较好的相转移催化剂四丁基溴化铵和氧化剂过氧甲酸; 动力学研究表明, 以四丁基溴化铵作为相转移催化剂, 过氧甲酸氧化脱除催化裂化汽油中含硫化合物的反应为表观一级反应, 反应的活化能为E_a＝59.39 kJ/mol, 脱硫率可达98.36%.     

Kinetics and mechanism of reaction of toluidine blue with acidic bromate

The detailed kinetics of the reaction of toluidine blue {phenothiazine-5-ium, 3-amino-7(dimethylamino)-2-methyl chloride, tolonium chloride, TB⁺Cl⁻} with potassium bromate and with aqueous bromine reaction were studied. In most of the experiments, the kinetics were monitored by following the rate of consumption of TB⁺ at 590 nm with excess acid and bromate. The reaction exhibited complex kinetic behavior. Initial reaction was slow and after an induction time, the TB⁺ concentration decreased fast. It had first-order dependence on both TB⁺ and bromate, and second-order dependence on H⁺. Under excess bromate conditions, the stoichiometric ratio of TB⁺ to bromate was 1:1. Demethylated sulfoxides were found at the reaction products. Sharp increase in the overall potential synchronized with the increase in bromine levels and the fast depletion of [TB⁺]. The role of bromide ion and bromine in the reaction was established. A multi-step reaction mechanism is proposed consistent with the experimental results. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 111–120, 1998.     

Preparation of carbon-11 labelled acetate and palmitic acid for the study of myocardial metabolism by emission-computerised axial tomography

Methods have been developed for the labelling of acetate and palmitic acid with the positron-emitting radionuclide,¹¹C (T=20.4 min). Labelling was achieved via carbonation of the appropriate alkyl magnesium bromide (methyl magnesium bromide or n-pentadecyl magnesium bromide) with¹¹C-labelled carbon dioxide produced by the¹⁴N(p, α)¹¹C nuclear reaction. The radiochemical yield and speed of each method of labelling are such that a radiochemically pure product is obtained in injectable form and in activity (>10 mCi) suitable for the study of myocardial metabolism by emission-computerised axial tomography. High pressure liquid chromatography and thin layer chromatography were used to assess the radiochemical purity of each radiopharmaceutical. The specific activity of¹¹C-labelled acetate was estimated by an enzymic procedure to be greater than 0.5 Ci/μmole.     

Palladium‐catalysed Suzuki cross‐coupling of primary alkylboronic acids with alkenyl halides

The Suzuki reaction of primary alkylboronic acids with alkenyl halides proceeds nicely using the air‐stable catalyst PdCl(C₃H₅)(dppb), Cs₂CO₃ as base and toluene or xylene as solvent. A minor effect of the substituent position of the alkenyl bromide was observed. Quite similar yields were observed in the presence of α‐ or β‐substituted alkenyl bromides such as 2‐bromobut‐1‐ene or 1‐bromo‐2‐methylprop‐1‐ene with this catalyst. This reaction proceeded with a variety of alkylboronic acids such as 2‐phenylethylboronic acid or n‐octylboronic acid. Lower yields of coupling products were obtained in the presence of an alkenyl chloride. Copyright © 2008 John Wiley & Sons, Ltd.     

A short synthesis of (+/-)-epiasarinin

[reaction: see text] Epiasarinin, an endo-endo furofuran, has been synthesized from piperonal via a five-step route with good stereocontrol. The sequence involves Darzens condensation, alkenyl epoxide-dihydrofuran rearrangement, and a Lewis acid mediated cyclization.