Facile synthesis of arylboronic esters by palladacycle-catalyzed bromination of 2-arylbenzoxazoles and subsequent borylation of the brominated products

An efficient and facile synthesis of arylboronic esters bearing the benzoxazole moiety has been described using a new family of palladacycle: cyclopalladated ferrocenylimines as the catalysts. This reaction includes two concessive steps: bromination of 2-arylbenzoxazoles and subsequent borylation. The bromination of para-C-H bond was an electrophilic substitution process by using NBS as the brominating reagent, and the brominated products were determined by HMBC (¹H-detected heteronuclear multiple bond correlation) spectra. Particularly, the borylation step could be carried out successively only after removal of the solvent to afford the arylboronic esters in moderate to good yields.     

Highly Efficient Oxidative Bromination of Alkanes with the HBr-H2O2 System in the Presence of Catalyst

Various cycloalkanes and straight‐chain alkanes were efficiently brominated with an aqueous HBr‐H₂O₂ system. This oxidative brominating process was promoted by catalysis and irradiation with light. The cycloalkanes were converted to the corresponding bromo‐cycloalkanes in moderate yields and the straight‐chain alkanes produced dominantly secondary bromides. This simple but effective bromination method of alkanes is characterized by high atom efficiency, inexpensive reagents and the absence of organic waste, which make it a good alternative to the existing method for C? H activation through bromination.     

Regioselective bromination of aromatic compounds with Br2/SO2Cl2 over microporous catalysts

A new selective brominating system Br₂/SO₂Cl₂/zeolite, has been discovered. Partially cation-exchanged Ca²⁺-Y zeolite efficiently catalyzes the selective para-bromination of neat chlorobenzene (CB) by Br₂/SO₂Cl₂ affording a CB conversion of ∼89% and a para-selectivity of ∼97%. During the bromination reaction, SO₂Cl₂ oxidizes HBr, prevents its accumulation within the zeolite pores and yields a more active brominating species. The Ca²⁺-Y catalyst was found to be stable under the bromination conditions, and can easily be regenerated by calcination. The Br₂/SO₂Cl₂/Ca²⁺-Y brominating system could be applicable to other activated aromatic compounds such as o-xylene, toluene and fluorobenzene.     

Free radical bromination by the H2O2-HBr system on water

An aqueous solution of hydrogen peroxide and hydrogen bromide illuminated by a 40 W incandescent light bulb serves as a source of bromine radicals. Various substituted toluenes (H, Me, tBu, Br, COOEt, COPh, NO₂) were brominated at the benzyl position. This haloperoxidase-like system for benzylic bromination does not require the presence of metal ions or an organic solvent for efficient conversion of methyl-arenes to benzyl bromides.     

Electrophilic Bromination of Nitrobenzene Using Barium Tetrafluorobromate (III)

It has been shown that Ba (BrF₄)₂ acted as a highly-active brominating agent. In case of interaction with nitrobenzene, the pure 3-bromo-nitrotoluene is formed. It has been shown that typical electrophilic bromination of the aromatic compound with electron-donating and electron-accepting substituents occurred without any catalysts or hard conditions.     

Bromine formation in solid NaBr/KNO<Subscript>3</Subscript> mixture and assay of this reaction via bromination of activated aromatics

Bromine formation in the mixture of solid NaBr and KNO₃ was observed and the process was studied in different acidified organic solvent–water mixtures by monitoring the bromination of acetanilide and other compounds, containing activated aromatic substituents. This assay is based on fast bromination reaction of these aromatic compounds, as differently from the assay of Br₂, the brominated aromatics can be easily determined by conventional gas chromatography–mass spectrometry (GC–MS) methods. It was found that bromine was generated autocatalytically on the surface of salt crystals and the reaction was characterized by a lag period, the duration of which depended on reaction conditions, and importantly on the type of the organic solvent in the reaction mixture. As the bromine formation could be easily controlled by reaction conditions, it was suggested that the studied reaction might have practical applications as an environmentally friendly and economically feasible bromination method. It was also shown that the bromination of aromatics followed the mechanism of classical electrophilic aromatic substitution reaction.     

Trifluoromethylation of various aromatic compounds by CF3I in the presence of Fe(II) compound, H2O2 and dimethylsulfoxide

Trifluoromethylation of aromatic and hetero-aromatic compounds by CF₃I in the presence of Fe(II) compound, H₂O₂ and dimethylsulfoxide was investigated. Various trifluoromethylated benzene derivatives, six-membered nitrogen-containing aromatic compounds and five-membered hetero-aromatic compounds were obtained under mild conditions. General orientation of electrophilic substitution of aromatic compounds was observed similarly as reported in other radical trifluoromethylation previously.     

Efficient thiocyanation of phenols and anilines in the CeBr3 / H2O2 system

An efficient electrophilic thiocyanation of phenols and anilines in the CeBr₃ / H₂O₂ system has been developed. The system CeBr₃ / H₂O₂ generates reactive brominating species which would react with NH₄SCN to deliver the active SCN cations in situ. The Br^- anion can be further oxidized to Br⁺, thereby furnishing the catalytic cycle.     

Kinetics of N‐bromination of 2‐oxazolidinone

The kinetics of N‐bromination of 2‐oxazolidinone by transfer of Br from sodium hypobromite, N‐bromosuccinimide (NBS), or N‐bromoacetamide (NBA) were determined spectrophotometrically, at pH between 4.6 and 12.45 (depending on the brominating agent). The reaction with hypobromite was of first order with respect to both the hypobromite and the substrate. The bromination of oxazolidinone with NBS (or NBA) has been found to be a reversible process of order one with respect to both NBA (or NBA) and oxazolidinone in the forward direction, and order one with respect to SI (or ACAM) and the resulting N‐bromo‐oxazolidinone in the other. The pH dependence of the reaction rate was in keeping with a mechanism in which all the brominating agents (HOBr, BrO^?, NBS and NBA) react predominantly with the anion of the substrate. Bimolecular bromination rate constants increased in the order BrO^? < NBA < NBS < HOBr. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 642–649, 2004     

Regioselective, photochemical bromination of aromatic compounds using N-bromosuccinimide

Regioselective nuclear bromination of aromatic compounds is investigated with N-bromosuccinimide as the brominating agent under UV irradiation to afford the corresponding brominated compounds. The reaction proceeds at ambient temperature (30 ± 2 °C) without any catalyst. In most of the reactions, regioselectively mono-brominated products are obtained in good to high yields. The conversion and selectivity for bromination depend on the nature of the substituent on the aromatic ring.     

Bromination of Arenes Using I2O5-KBr in Water

An efficient and environmentally benign bromination of various aromatic compounds using aN aqueous I₂O₅-KBr system at room temperature has been developed in this work. A series of aromatic compounds such as acetophenones, benzaldehydes, benzoic acids, anilines, amides, and aminopyridine have been successfully brominated in excellent regioselectivities and good yields under the typical reaction conditions. The features of KBr as brominating reagent, water as solvent, and mild conditions make this system an attractive synthetic procedure.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]     

High selectively oxidative bromination of toluene derivatives by the H_2O_2-HBr system

An aqueous solution of hydrogen peroxide and hydrogen bromide illuminated by a 60 W incandescent light bulb serves as a source of bromine radicals.Various substituted toluenes（NO₂,Cl,Br,H,CH₃） were high selectively brominated at the benzyl position for monobromination in CH₂C1₂ at ice water with catalyst free.This simple but effective bromination of toluene derivatives with an aqueous H₂O₂-HBr system is characterized with the use of inexpensive reagents and a lower impact on the environment, which make it a good alternative to the existing bromination methods.     

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.     

Aromatization and Halogenation of 3,3a,4,5‐Tetrahydro‐3‐aryl‐2‐phenyl‐2H‐benzo[g]indazole Using I2/DMSO,CuCl2/DMSO,and N‐Bromosuccinimide

The treatment of 3,3a,4,5‐tetrahydro‐3‐aryl‐2‐phenyl‐2H‐benzo[g]indazoles 4 with I₂/DMSO led to the oxidation of the five‐member rings ( 5 ) as well as the iodination of N‐phenyl moieties along with oxidation of the five‐member rings ( 6 ). However, the reactions of 4 with CuCl₂/DMSO gave only compounds 5 . The reaction of N‐bromosuccinimide (NBS) with compounds 4 resulted in fully aromatization along with bromination at C‐5 of the indazole rings ( 7 ). The indazole six‐member rings in compounds 5 and 6 also underwent aromatization along with bromination by using NBS ( 7 and 8 ).     

Visible light induced ‘on water’ benzylic bromination with N-bromosuccinimide

Benzylic bromination of various 4-substituted toluenes (Me, tert-Bu, COOEt and COMe) was effectively conducted with NBS in pure water and with a 40 W incandescent light-bulb as an initiator of the radical chain process, while electron donating groups (OMe and NHAc) directed the reaction to electrophilic aromatic substitution.     

Bromination of ketones with the systems H2O2-LiBr-CeIII and H2O2-LiBr-CeIV

A new method for the synthesis of α-bromoketones was suggested. The C₅-C₁₁ linear and branched ketones in the reaction with the systems H₂O₂-LiBr-Ce^III and H₂O₂-LiBr-Ce^IV in acetonitrile were brominated at α-position. The reaction is highly selective.     

Na4H3[SiW9Al3(H2O)3O37]·12H2O/H2O: a new system for selective oxidation of alcohols with H2O2 as oxidant

This work describes a catalytic system consisting of both Na₄H₃[SiW₉Al₃(H₂O)₃O₃₇]·12H₂O(SiW₉Al₃) and water as solvents (a small quantity of organic solvents were used as co-solvent for a few substrates) that can be good for selective oxidation of alcohols to ketones (aldehydes) using 30% H₂O₂ without any phase-transfer catalyst under mild reaction conditions. The catalyst system allows easy product/catalyst separation. Under the given conditions, the secondary hydroxyl group was highly chemoselectively oxidized to the corresponding ketones in good yields in the presence of primary hydroxyl group within the same molecule, and hydroxides are selectively oxidized even in the presence of alkene. Benzylic alcohols were selectively oxidized to the corresponding benzaldehydes in good yields without over oxidation products in solvent-free conditions. Nitrogen, oxygen, sulfur-based moieties, at least for the cases where these atoms are not susceptible to oxidation, do not interfere with the catalytic alcohol oxidation.     

Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of P2O5/SiO2 under heterogeneous conditions

A convenient and efficient procedure for the Friedel-Crafts acylation of aromatic compounds with carboxylic acids in the presence of P₂O₅/SiO₂ is described. Both aromatic and aliphatic carboxylic acids reacted easily to afford the corresponding aromatic ketones. The use of non-toxic and inexpensive materials, simple and clean work-up, short reaction times and good yields of the products are the advantages of this method.     

On the apex angle in C2v-‘V’ structure of B2O3

Results of valence electron SCF-MO calculations in the INDO approximation predict a value of 102° for the apex angle of C_2v-‘V’ structure of B₂O₃, assuming bond lengths obtained from electron diffraction work. Calculated results for the dipole moment are reported and electron population analysis is used to discuss bonding.     

Synthesis of dibromo ketones by the reaction of the environmentally benign H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HBr system with oximes

It was found that oximes undergo deoximation in the presence of the H₂O_2aq-HBr_aq system to form ketones and bromo ketones. This reaction provided the basis for the synthesis of dibromo ketones in yields varying from 40% to 94%. This method is environmentally friendly, sustainable, and easy to perform. The results of this investigation extend the potential of the use of oximes for the protection of carbonyl group, thus offering the ability to perform not only conventional deoximation but also the subsequent bromination of ketones. The reaction is easily scaled up and dibromo ketones can be prepared in gram amounts.