High temperature bromination. Part 18: Bromination of benzonorbornadiene derivatives: Polybrominated benzonorbornenes and benzonorbornadienes

The low and high temperature bromination reactions of bromobenzonorbornadiene derivatives were studied and the possible role of a neighboring group in rearrangements was investigated. New polybrominated benzonorbornadiene and benzonorbornene derivatives were synthesized. All compounds were characterized properly using NMR spectroscopy.     

High temperature bromination. Part 22: Bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene

The high-temperature bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene and its carboethoxy derivative was studied. Reaction of the title compound with 1 mol of bromine in refluxing carbon tetrachloride resulted in the formation of ring-opening products. In the case of the carboethoxy derivative, bromination took place both regio- and stereospecifically at the benzylic positions, the cyclopropane ring did not undergo bond cleavage. A mechanism for the formation of the products and their dehydrobromination reactions is discussed.     

Functionalization of saturated hydrocarbons

Alkanes and cycloalkanes (isobutane, butane, isopentane, isohexane, and methylcyclopentane) react with benzene or bromobenzene at 0–20 °C in the presence of RCO⁺Al₂X₇ ^– complexes (R=Me, Pr, or Ph; X=Cl or Br) to give products of the alkylacylation of arenes. The yields of alkylated aromatic ketones reach 60–87 % in 5–30 min, whereas the yields of unalkylated aromatic ketones (the competitive reaction) reach 0–40 %. The reactions of isobutane or isopentane with benzene result exclusively inpara isomers oft-BuC₆H₄COR or a mixture of Me₂(Et)CC₆H₄COR and Me(i-Pr)CHC₆H₄COR isomers (11), respectively. The reaction of isobutane with benzene also proceeds regioselectively and gives only one isomer, 2-Br-t-BuC₆H₄COR.For Part 2 seeIzv. Akad. Nauk, Ser. Khim., 1991, No 1, 105 [Bull. Acad. Sci. USSR. Div. Chem. Sci., 1991, No 1, 90 (Engl. Transl)].Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1253–1257, July, 1993.The authors express their gratitude to B. I. Bakhmutov for his assistance in interpreting the spectra.     

Functionalization of saturated hydrocarbons by means of aprotic superacids 1. Ionic bromination of alkanes and cycloalkanes

Aprotic organic superacids with the composition AcBr·2AlX₃ (X=Cl, Br) are efficient catalysts (Cat) for the bromination of n-alkanes and cycloalkanes by molecular bromine. Under the given conditions, the reactions afford (predominantly or exclusively) monobromides in high yields.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2028–2033, September, 1989.     

Functionalization of saturated hydrocarbons by aprotic superacids

The inefficient formylation of adamantane by CO initiated by superacid systems based on polyhalomethanes and aluminum halides becomes selective in the presence of methylcyclopentane and affords 1-adamantanecarbaldehyde in an almost quantitative yield under mild conditions.     

Low and high temperature bromination of exocyclic dienes: high temperature bromination. Part 16

The electrophilic addition of bromine to an exocyclic diene, 5,6-dimethylenebicyclo[2.2.1]hept-2-ene, in CCl₄ at 0°C led to the formation of non-rearranged (73%) and rearranged products (27%). However, high temperature bromination of the exocyclic diene at 77°C suppressed the formation of the rearranged products. Similarly, bromination of 9,10-dimethylenetricyclo[6.2.1.0^2,7]undeca-2,4,6-triene at −10°C gave only the exo-1,2-addition product. Bromination at +5°C resulted in the formation of a mixture consisting of exo-1,2- and 1,4-addition products in a ratio of (1:4). High temperature bromination at 77°C resulted in the formation of the endo-1,2-addition product. Furthermore, it has been shown that the 1,4-addition product converts smoothly to the 1,2-addition product. The formation mechanism of the products is discussed and supported by calculations.     

Orientation of bromination in bay-region of perylene diimides

We found that the entering position of the bromination in the bay-region (e.g., 1,6,7,12-positions) was precisely determined by the orientation director(s), either individually or cooperatively. In particular, a combination of two directors shows precisely positioning for the third entering bromo with an effect of ‘1+1>2’.     

Liquid phase bromination of aromatics over zeolite H-beta catalyst

The liquid phase bromination of chlorobenzene, toluene and xylenes (o-, m-, p-) is catalyzed using zeolite as catalyst and N-bromosuccinimide (NBS) as the brominating agent. In addition, the bromination of toluene has been investigated over various zeolites using both NBS and liquid Br₂ as brominating agents. A comparison under similar reaction conditions with H₂SO₄, in the absence of catalyst and FeCl₃ (in the case of toluene) is also investigated for each reaction. Zeolite H-beta is found to be selective compared to the conventional catalysts and other zeolites in the bromination of chlorobenzene and toluene whereas selectivity for 4-bromo-o-xylene (4-BOX/3-BOX) over H-beta and H₂SO₄ was found nearly comparable in the bromination of o-xylene. In the bromination of toluene, acidic H-beta favours the formation of nuclear products whereas in the absence of any catalyst, in the presence of weakly acidic H–Y and potassium exchanged zeolites K-beta and K–L, the concentration of side-chain product, œ-bromotoluene, is enhanced. The conversion of NBS, rate of NBS conversion (mmol g⁻¹ h⁻¹) and selectivity for products are strongly influenced by the reaction parameters. As the reaction time, catalyst amount, reaction temperature and molar ratios of NBS/toluene are increased, an increase in the conversion of NBS is noticed. Presumably, the catalytic bromination of aromatics proceeds by an electrophile (Br⁺) which is generated by the heterolytic cleavage of NBS/Br₂ by an acidic zeolite. Thus, the generated Br⁺ attacks the aromatic ring resulting in the formation of brominated nuclear products.     

Unforeseen formation of 2-bromo-3-hydroxybenzaldehyde by bromination of 3-hydroxybenzaldehyde

Contrary to literature reports, bromination of 3-hydroxybenzaldehyde can afford both 2-bromo-5-hydroxybenzaldehyde and 2-bromo-3-hydroxybenzaldehyde, but 4-bromo-3-hydroxybenzaldehyde was not detected. 2-Bromo-3-hydroxybenzaldehyde was converted into 2-(benzyloxy)-1-bromo-5-methoxy-7-methylnaphthalene. X-ray crystallographic analysis supports the identity of 2-bromo-3-hydroxybenzaldehyde.     

Regioselective and high-yielding bromination of aromatic compounds using hexamethylenetetramine-bromine

A regioselective and highly efficient method for bromination of aromatic compounds in the presence of a stoichiometric amount of hexamethylenetetramine-bromine (HMTAB) as an efficient reagent in dichloromethane is reported. The selectivity depends on the temperature and nature of the substituent on the substrate. The reactivity of this reagent was increased by supporting it to silica gel for bromination of less activated compounds.     

Selective O-demethylation during bromination of (3,4-dimethoxyphenyl)(2,3,4-trimethoxyphenyl)methanone

(3,4-Dimethoxyphenyl)(2,3,4-trimethoxyphenyl)methanone was synthesized and its bromination was investigated in different conditions. Nine new products (bromophenol derivatives) were isolated in the reactions. These bromophenol derivatives were obtained by selective O-demethylation in its bromination with bromine. The products and their formations were discussed.     

A fast and efficient bromination of isoxazoles and pyrazoles by microwave irradiation

A fast and efficient method has been developed for the bromination of isoxazoles and pyrazoles using microwave irradiation. In this method, N-bromosuccinimide was used in different acid solvents according to the reactivity of the substrates to give mono-brominated isoxazoles and pyrazoles in good yields. Trifluoroacetic acid was found to be the best solvent for highly unreactive isoxazoles and pyrazoles.     

A novel method for the bromination of thiophenes

A novel, fast and convenient method for the bromination of thiophenes and oligothiophenes with N-bromosuccinimide (NBS) using ultrasonic irradiation is elaborated. The yield of bromothiophenes strongly depends on the initial thiophene structure and nature of the solvent.     

Cu-mediated selective bromination of aniline derivatives and preliminary mechanism study

A simple and efficient bromination of aniline, aniline derivatives, and analogs have been developed. Forty three examples were given and the highest yield reached was 98%. Different substrates including substituted aniline, pyridin-amine, N-substituted aniline, N,N-disubstituted aniline, N-phenyl-amide, N-phenyl-sulfonamide, and nitrogen-containing heterocycles were all reactive and selectively generated desired bromo-products. The method can be applied to synthesize drug intermediate and quinoxaline derivatives.     

A green bromination method for the synthesis of benzylic dibromides

Reported herein is the identification of a new methodology for the dibromination of benzylic diols. This method proceeds in moderate to good yields for a wide variety of electron-deficient, electron-neutral, and electron-rich aromatic substrates. Moreover, the reagent, 1,3-dibromo-5,5-dimethylhydantoin, and the solvent, tetrahydrofuran, are substantially more environmentally benign than traditional solvents and reagents used for bromination. The utility of this methodology was demonstrated in the high-yielding synthesis of a key intermediate in the synthesis of omeprazole.     

Variation in the regioselectivity of levulinic acid bromination in ionic liquids

The reaction of levulinic acid and its esters with bromine in ionic liquids results in the formation of 3-bromo derivatives as the major products and not the 5-bromo substituted isomers, which are typically formed in organic solvents. The bromination of levulinic acid in ionic liquids in the presence of urea leads to the formation of 5-bromolevulinic acid.     

Polyvinylpolypyrrolidone-bromine complex: Mild and efficient polymeric reagent for bromination of activated aromatic compounds

Mild and efficient method for bromination of electron-rich aromatic compounds is described using polyvinylpolypyrrolidone-bromine complex（PVPP-Br₂）.The reaction proceeded smoothly with phenols and N,N-alkylated amines to afford the corresponding monobrominated product in good yields at ambient temperature.     

Reactivity of pyrrole pigments. Part 5: Electrophilic substitution—Nitration and bromination—Of some pyrromethenones and 5-arylmethylene-3,4-dimethyl-3-pyrrolin-2-ones

Some 5-arylmethylene-3,4-dimethyl-3-pyrrolin-2-ones react with both bromine and nitronium tetrafluoroborate (NO₂BF₄) to give 5-(aryl)nitromethylene-3-pyrrolin-2-ones and 5-(aryl)bromomethylene-3-pyrrolin-2-ones, respectively. The use of bromine in methanol affords 5-(aryl)bromomethyl-3,4-dimethyl-5-methoxy-3-pyrrolin-2-ones. Whereas pyrromethenones react mainly on the pyrrole ring, ethyl 3,4-demethyl-5-[(3,4-dimethyl-5-oxo-3-pyrrolin-2-yl)methylene]-1H-pyrrole-2-carboxylate reacts as the aryl derivatives, however, with bromine in methanol the addition of two methoxy groups at the exocyclic double bond takes place. 3,4-Dimethyl-5-(2-pyridylmethylene)-3-pyrrolin-2-one does not react with bromine or NO₂BF₄, but reacts as the aryl derivatives with bromine in methanol. The reactivity patterns are in agreement with the theoretical ones obtained from MINDO/3 calculations, using theFukui frontier orbital model. The obtained results are used to explain the reactivity of rubins (biladienes-a,c) and verdins (bilatrienes-a,b,c) in front of electrophiles.Einige 5-Arylmethylen-3,4-dimethyl-3-pyrrolin-2-one reagieren sowohl mit Brom als auch mit Nitroniumtetrafloroborat (NO₂BF₄). Man erhält 5-(aryl)bromomethylen-oder 5-(aryl)nitromethylen-3-pyrrolin-2-one. Bei Verwendung einer methanolischen Bromlösung werden 5-(aryl)bromomethyl-3,4-dimethyl-5-methoxy-3-pyrrolin-2-one gebildet. Pyrromethenone reagieren hauptsächlich am Pyrrolring, Ethyl 3,4-dimethyl-5-[(3,4-dimethyl)-5-oxo-3-pyrrolin-2-yl)methylen]-1H-pyrrol-2-carboxylat hingegen verhält sich wie ein Arylderivat, mit methanolischen Bromlösung jedoch erfolgt Eintritt zweier Methoxygruppen an der exocyclischen Doppelbindung.5-(2-Pyridyl)methylen-3,4-dimethyl-3-pyrrolin-2-on reagiert nicht mit Brom oder NO₂BF₄, wohl aber mit einer methanolischen Bromlösung und verhält sich unter diesen Bedingungen wie ein Arylderivat; 3- und 4-Pyridylderivate verhalten sich analog. Die Reaktivität ist in Übereinstimmung mit theoretischen Werten aus MINDO/3-Rechnungen unter Verwendung des Fukui frontier orbital model. Die Reaktivität von Rubinen (Biladiene-a, c) und Verdinen (Bilatriene-a,b,c) gegenüber Elektrophilen werden im Zusammenhang mit den erhaltenen Resultaten diskutiert.

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Reaktivität der Pyrrolpigmente, 5. Mitt.: Elektrophile Substituierung (Nitrierung und Bromierung) von einigen Pyrromethenonen und 5-Arylmethylen-3,4-dimethyl-3-pyrrolin-2-onen

     

Effect of surface modification by bromination and metalation on Kevlar fibre-epoxy adhesion

This research uses metalation, bromination, and grafting to modify the surface of Kevlar fibre. The resulting fibre was characterized with tensile measurement, infrared spectroscopy, intrinsic viscosity, and scanning electron microscopy (SEM). From the SEM micrographs, the surface morphology of the bromine etched Kevlar fibre has a rougher surface than the untreated fibre. Furthermore, the tensile strength of Kevlar fibre decreases with the increase of bromine treating time. The interlaminar shear strength (ILSS) of the fibre-reinforced epoxy composite was measured. The results show that the ILSS of the bromoacetic acid-grafted Kevlar sample can be increased by 12%, while that of epichlorohydrin-grafted Kevlar can be increased by 8%.     

A green bromination process for synthesis of a novel drug candidate

A bromination process has been developed using a salt-based brominating reagent and a non-chlorinated solvent system. The process affords enhanced selectivity and enables product isolation by simple filtration of the reaction mixture, resulting in a substantially reduced waste stream.