Halogenation of <Emphasis Type="Italic">N</Emphasis>-substituted <Emphasis Type="Italic">p</Emphasis>-quinone monoimines and <Emphasis Type="Italic">p</Emphasis>-quinone monooxime ethers: XV. Synthesis and bromination of 4-(cinnamoyloxyimino)-cyclohexa-2,5-dienones

New 4-(cinnamoyloxyimino)cyclohexa-2,5-dien-1-ones were synthesized, and their bromination afforded bromine addition products to the syn- and anti-C=C bonds of the quinoid ring. In all cases, bromine addition to the C=C double bond of the cinnamoyl fragment was observed.     

Mechanism of isomerization of ortho or para bromo phenols in superacids

In superacids ortho or para bromo phenols are isomerized into the meta bromo isomers. In SbF₅-HF, the process is intramolecular and proceeds by a 1,2-Br shift. In CF₃SO₃H, the mechanism involves loss of bromine, followed by meta bromination.     

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.     

Unexpected regioselectivity observed in the bromination and epoxidation reactions of p-benzoquinone-fused norbornadiene: An experimental and computational study

The bromination reaction of p-benzoquinone-fused norbornadiene was studied at various temperatures (?78, ?50, 0, 25, and 77 °C). At room temperature, the double bonds of the p-benzoquinone units were mainly brominated. The double bond of the norbornene unit also underwent a bromination reaction in a yield of only 2%. However, the reaction at ?78 °C resulted in the formation of products derived from the attack of bromine on the norbornene double bond with higher charge density. In contrast to the bromination reaction, the epoxidation reaction of the same compound with m-chloroperbenzoic acid and dimethyldioxirane exclusively resulted in the formation of products derived from the addition to the double bond of norbornadiene. The regioselectivity observed was investigated and the results were supported by theoretical calculations.     

Organoboranes for synthesis. 11.1 preparation of alkyl bromides in the dark reaction of bromine with organoboranes2,3 exceptional reactivity toward radical bromination of the alpha hydrogen in trialkylboranes

The reaction of the three isomeric tributylboranes (tri-n-butyl, triisobutyl and tri-sec-butyl) with bromine in the dark gives rise to both butyl bromide and hydrogen bromide when carbon tetrachloride is used as a solvent. The rate of disappearance of the borane and bromine are essentially equal and decreases in the ordersec-butyl >n-butyl > isobutyl. However, the corresponding butyl bromide appears at a much slower rate and the formation of hydrogen bromide is quite rapid during the initial stages of the reaction. The amount of hydrogen bromide produced in the reaction reaches a peak in 1 h and then decreases with time. Similar results are obtained in cyclohexane. In methylene chloride, the rate of initial disappearance of bromine and tributylborane compares closely to the results obtained in carbon tetrachloride and cyclohexane. However, butyl bromide is formed with essentially the same rate as the rate of disappearance of the borane. Moreover, hydrogen bromide is formed in only minor amounts and the yields of alkyl bromides are high. In tetrahydrofuran, tri-n-butylborane and tri-sec-butylborane react at a rate similar to the rate similar to the rate of formation of the corresponding bromobutanes. This reaction is proposed to involve a slow, direct electrophilic attack of bromine, or its complex with THF, on tributylborane. Whereas in carbon tetrachloride, cyclohexane and methylene chloride, a fast, initially free-radical bromination, followed by a slow cleavage of the resulting a-bromoorganoborane with hydrogen bromide, takes place. Evidence supporting this mechanism is given. Competitive bromination studies reveal that the α-hydrogen in trialkylboranes is highly reactive toward free-radical bromination in the dark reaction. As an important synthetic application of this new reaction, the preparation of alkyl bromides is presented.     

New insights into bromination process: effective preparation of Ambroxol

Ambroxol used as an expectorant in treating respiratory diseases was effectively prepared with a total yield of 62%, with o-toluidine as the feedstock via successive procedures of electrophilic bromination, acetylation, radical benzylic bromination, N-alkylation and hydrolysis processes. The addition of aqueous hydrogen peroxide could enhance the utilisation of liquid bromine in the electrophilic bromination of o-toluidine, avoiding the hazardous HBr generated as a by-product. In addition, liquid bromine promoted by MnO₂ was used efficiently for the radical benzylic bromination of N-acetyl-N-(2,4-dibromo-6-methylphenyl)acetamide under mild conditions.     

Bromination of phenols in bromoperoxidase-catalyzed oxidations

Phenol and ortho-substituted derivatives furnish products of selective para-bromination, if treated with sodium bromide, hydrogen peroxide, and the vanadate(V)-dependent bromoperoxidase I from the brown alga Ascophyllum nodosum. Relative rates of bromination in morpholine-4-ethane sulfonic acid (MES)-buffered aqueous tert-butanol (pH 6.2) increase by a factor 32, as the ortho-substituent in a phenol changes from F via Cl, OCH₃, C(CH₃)₃, and H to CH₃. The polar effect in phenol bromination by the enzymatic method, according to a Hammett-correlation (ρ=−3), compares to reactivity of molecular bromine under identical conditions (ρ=−2). Hypobromous acid is not able to electrophilically substitute bromine for hydrogen at pH 6.2 in aqueous tert-butanol. The tribromide anion behaves in MES-buffered aqueous tert-butanol as electrophile (ρ∼−3), showing a similar polar effect in phenol bromination as molecular bromine.     

The bromination,debromination and debromosilylation of silylstyrenes and other vinylsilanes

The bromination of some vinylsilanes, and the debrominations and debromosilylations of their dibromides are stereospecific. Based on the crystal structure of the dibromide derived form trans-triphenylsilylstyrene, it is shown that bromination and debromination of silylstyrenes occur with syn stereochemistry, and that debromosilylation occurs with anti stereochemistry in polar solvents. Different stereochemistries may prevail with other vinylsilanes. Other elimination and substitution reactions are also described.     

Kinetik der Bromierung von Phenolen und phenolischen Mekrkernverbindungen

The bromination of 15 dinuclear phenolic compounds (dihydroxydiphenylmethanes, methylene bisphenols) by molecular bromine in acetic acid was studied kinetically at 22°C. In all compounds the electrophilic substitution occurred inortho-position to the phenolic hydroxy group of the methyl phenol unit while the non reacting neighboring unit was differently substituted by H, CH₃,t-Bu and NO₂. A decrease in the reaction rate was observed in 2,2′-dihydroxydiphenylmethanes, where the +M-effect of the hydroxy group is diminished by an intramolecular hydrogen bond. The strength of this hydrogen bond may be influenced mainly by steric factors. Strong electron withdrawing substituents like NO₂ show a rate decreasing influence on the reactivity of the neighboring unit also in 2,4′- and 4,4′-dihydroxydiphenylmethanes.     

Unsaturated derivatives of trifluoromethanesulfonimide

N-Allyl- and N-propargyltrifluoromethanesulfonimides were synthesized by reaction of trifluoromethanesulfonic anhydride with allylamine and propargylamine. Some reactions of the resulting unsaturated derivatives were studied, in particular bromination, dehydrobromination of the bromination product, and nucleophilic substitution of bromine.     

XPS STUDIES ON SURFACE MODIFIED POLY [1-(TRIMETHYLSILYL)-1-PROPYNE] MEMBRANES Ⅱ SURFACE MODIFICATION BY BROMINE VAPOR*

Surface modification of poly [1-(trimethylsilyl)-1-propyne] (PTMSP) membranes bybromine vapor has been studied. It is shown that Br/C atomic ratio at the surfaces increased withthe time of bromination until about 60 min, then it reached a plateau. The results of XPS and IRstudies indicated that the addition of bromine to double bonds and the replacement of H on CH_3 bybromine had taken place so that a new peak at 286.0 eV (C--Br)in C_(1s) spectra and some newbands, e. g. at 1220 and 580cm~(-1) in IR spectra were formed. The fact,t Po_2, permeability ofoxygen, decreased and α_(O_2/N_2), separation factor of oxygen relative to nitrogen, increased withbromination time, shows that surface modification of PTMSP by bromine may be an efficient approach to prepare PTMSP membranes used for practical gas separations.     

Oxidative bromination reaction using vanadium catalyst and aluminum halide under molecular oxygen

The vanadium-catalyzed oxidative bromination reaction of arenes, alkenes, and alkynes was performed in the presence of AlBr₃ to provide an alternative method for conventional bromination using hazardous bromine. The catalytic cycle is formed under molecular oxygen, which is more advantageous to vanadium bromoperoxidase (VBrPO) requiring hydrogen peroxide as a terminal oxidant.     

Iodine(III)/AlX3-mediated electrophilic chlorination and bromination of arenes. Dual role of AlX3 (X = Cl,Br) for (PhIO)n depolymerization and as the halogen source

An efficient chlorination and bromination of arenes mediated by in situ-formed PhI(X)OAlX₂ (X = -Cl, -Br), which is proposed as a plausible halogenating species, is described. The proposed dual role displayed by AlX₃, enables the Iodosylbenzene [(PhIO)_n] depolymerization while also acting as the halogen source by transferring the chlorine or bromine atoms to the iodine(III) center. This process allowed the chlorination and bromination of different arenes and heteroarenes under mild and open flask conditions. To the best of our knowledge, this is the first report describing a dual role of aluminum salts applied to the direct C-H chlorination and bromination of arenes.     

Asymmetric synthesis of 3-methylthio alcohols by intramolecular Michael addition reactions

Chiral 3-methylthio alcohols have been synthesized through a known intramolecular sulfur transfer reaction that has been carried out in di- and trisubstituted α,β-unsaturated N-enoyl oxazolidinethiones as substrates, giving rise to syn/anti-diastereomers. The anti-diastereomer is favored and obtained via a highly diastereoselective protonation step.     

Metabromation du dimethyl-2,6 phenol et de son ether methylique en milieu superacide

In SbF₅-HF, reaction of bromine with 2,6-dimethylphenol gives the meta bromo compound 1b which is further halogenated to 3,4-dibromo-2,6-dimethylphenol 1c. In the same conditions ether 2a yields successively 2b and 2d. Meta bromination of compounds 1a,2a,2b implies electrophilic attack on the O-protonated substrate, whereas reaction of the neutral phenol 1b leads to the p-bromoderivative 1c. All products are obtained in excellent yields (> 85%).     

Evidence for a nucleophilic anti-attack on the cleaved C(2)-oxygen bond in Cl2AlH-catalyzed ring-opening of 2-substituted 1,3-dioxolanes

The Cl₂AlH-mediated ring-opening of 2-substituted cis-4-methyl-5-trifluoromethyl-1,3-dioxolanes was found to occur with regioselective cleavage of the O1-C2 bond by attack of the aluminum hydride from the direction anti to the departing oxygen. This stereochemical outcome, which appears to be unprecedented in the reductive cleavage of chiral acetals by aluminum reagents, is interpreted on the basis of theoretical calculations.     

Surveying approaches to the formation of carbon-carbon bonds between a pyran and an adjacent ring

We have examined several methods for the stereoselective formation of carbon-carbon bonds between contiguous rings where a stereogenic center is already present. The approaches investigated were a [1,3] oxygen to carbon rearrangement of cyclic vinyl acetals, an intermolecular enolsilane addition into an in situ generated oxocarbenium ion, an intramolecular conjugate addition of tethered alkoxy enones, and epimerization of several α-pyranyl cycloalkanones. These routes have been found to be complementary in several cases and have enabled formation of both the trans:anti and cis:anti stereoisomers in good to excellent yields and varying diastereoselectivities. We have proven C2-C2′ relative stereochemistry of 1-2 via a chemical correlation.     

The determination of phenol and kinetic studies on the monobromination of phenol by a pulse coulometric technique

A method for the coulometric determination of phenol is described in which constant current pulses are used to electrogenerate bromine and a current recorder is used to follow the bromination reaction of phenol. Solutions containing 10^-4 to 7.10^-6M phenol are determined with a precision of 1 %. A general method for determining the kinetics of bromination reactions is postulated and used for the determination of the rate constant of the monobromination of phenol. The rate constant for the reaction in which the reacting species are the neutral phenol molecule and the bromine molecule, was found to be 1.6.10⁵1/mole sec.     

Plasma Bromination of Graphene for Covalent Bonding of Organic Molecules

Plasma-chemical bromination applied to graphitic materials, in particular to highly ordered pyrolytic graphite is reviewed. The resulting bromination efficiency of the plasma-chemical treatment was subject of systematic process optimization. The plasma of elemental bromine vapour produced bromine concentrations on graphene surfaces of more than 30 % Br/C using either inductively or capacitively coupled low-pressure radio-frequency plasmas. Plasma brominated graphite surfaces have been studied by Near Edge X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The introduction of bromine into graphene-like graphite layers and its binding situation were investigated. To study the physical effect of the plasma bromination process, Kr plasma was also used because of its chemical inertness but similar atomic mass. Covering the samples with a Faraday cage or with a LiF window should help to differentiate between physical, chemical and radiation effects of the plasma. Bromination was assigned to radical or electrophilic addition of bromine onto fully substituted aromatic double bonds (sp² C) with exothermal reaction enthalpy. Low bromination shows a strong decay of aromatic double bonds, higher bromination percentage let disappear all aromatic rings. The formed C–Br bonds were well suited for efficient grafting of organic molecules by post-plasma wet-chemical nucleophilic substitution. This grafting onto the graphene surface was demonstrated using aminosilane and different diamines. The bromination of double bonds changes the hybridization of carbon atoms from plane sp² to tetrahedral sp³ hybridization. Thus, the plane topography of graphene is destroyed and the conductivity is lost.     

室温下二氧化氮催化带有给电子取代基的苯和萘的氧化溴化

在室温下,以8.2%的二氧化氮为催化剂,反应管中残留的空气为氧化剂,分子溴为溴化试剂,研究了带有给电子取代基的苯和萘的氧化溴化.使用的低沸点非金属催化剂易于从产物中去除,很少玷污最终产品;但反应后有少量的苯环硝化副产物生成,这会造成催化剂二氧化氮的损耗.反应具有较高的原子经济性,大部分溴原子被转化到产物中.反应产物具有可控性:可通过改变溴源的用量,分别得到单溴化和二溴化产物.初步的机理分析结论为:反应可能经历分子溴和芳环的反应,生成溴代芳烃和溴化氢;然后在氧化氮类物种催化下,生成的溴化氢被氧气氧化为具有反应活性的溴.