Bromination of aromatic compounds

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

The major organic reactions of BrCl consist of electrophilic brominations of aromatic compounds. Many aromatic compounds do not react in aqueous solution unless the reaction involves activated aromatic compounds (an example being phenol). Bromine chloride undergoes free-radical reactions more readily than bromine. 

BROMINATION OF AROMATIC COMPOUNDS WITH ALUMINA-SUPPORTED COPPER(II) BROMIDES... 

Our recent studies on effective bromination and oxidation using benzyltrimethylammonium tribromide (BTMA Br3), stable solid, are described. Those involve electrophilic bromination of aromatic compounds such as phenols, aromatic amines, aromatic ethers, acetanilides, arenes, and thiophene, a-bromination of arenes and acetophenones, and also bromo-addition to alkenes by the use of BTMA Br3. Furthermore, oxidation of alcohols, ethers, 1,4-benzenediols, hindered phenols, primary amines, hydrazo compounds, sulfides, and thiols, haloform reaction of methylketones, N-bromination of amides, Hofmann degradation of amides, and preparation of acylureas and carbamates by the use of BTMA Br3 are also presented. 

SIDE-CHAIN BROMINATION OF AROMATIC COMPOUNDS Benzylic bromination of arenes... 

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

Iron(III) bromide is a catalyst in bromination of aromatic compounds. 

Similarly, bromination of aromatic compounds as well as of electron-deficient olefins was carried out in good to excellent yields using (n-Bu4N)2S20g/Br2 or LiBr at 25 °C (equation 36)" . 

Interaction within encounter pairs. The above arguments have assumed that there is no interaction between the components in the encounter pairs A. B and B.X (Scheme 3) but this is probably unrealistic. Stabilization of the encounter pair A. B by charge-transfer interaction should favour the pre-association path by reducing the value of k 3 and possibly also increasing that of k4 [cf. equation (46)].22 Thus, in the nitration of neutral amine molecules (X) by nitronium ions (B) in concentrated sulphuric acid, interaction between the components in the encounter pair ArNHJ.NOJ should increase the acidity of the N—H hydrogens and facilitate the formation of the free amine. In the bromination of aromatic compounds (B) by HOBr(A), interaction between the components should increase the concentration of the encounter pair ArH. HOBr and facilitate the protonation of the hypobromous acid. 

Poly(4-vinylpyridine)-supported bromate in nonaqueous solution has been used in the bromination of aromatic compounds with potassium bromide <2005SC1947>. This bromination method is simple, efficient, mild, and selective for methoxyarenes, anilines, and phenols. 

As shown in [11,12] the NOz/NO catalytic system may be used in the iodination or bromination of aromatic compounds under the following conditions Aromatics - HCIO4 -NaNOz - NaI(NaBr) - O2. At the same time attempts to chlorinate aromatics were unsuccessful. 

Koros and Orban have studied (1978-1, 2, 1980) the uncatalyzed bromination of aromatic compounds and observed oscillations. 

The following mild procedure is recommended for nuclear bromination of aromatic compounds containing hydroxy, alkoxy, or amino groups a stream of air, C02, or N2 laden with bromine vapor is passed through an aqueous solution or suspension of the aromatic compound this bromination can also be carried out in absence of water if CaCl2 tubes are placed in front of and behind the reaction vessel. 

Choudary, B.M., Someshwar, T., Venkat Reddy, Ch., Lakshmi Kantam. M Jeeva Ratnam. K. and Sivaji, L.V. (2003). The first example of bromination of aromatic compounds with unprecedented atom economy using molecular bromine. Applied Catalysis A General. 251,397-409. 

The bromination of aromatic compounds using N-bromosuednimide (NBS) occurs in high yield in ionic liquids such as dibutylimidazohum tetrafluoroborate [110]. The yields of the monobrominated product were all in the 80% to 98% range. NBS was also used in the Wohl-Ziegler Q -bromination of aryl methyl groups [111]. The reaction in [BMIM][PF6] typically gave a 5 to 10% improvement in the yield, compared with a solvent-free reaction. 

Accordingly the reversibility of sulphonation and bromination of aromatic compounds (cf. also the Perrin s order) the quenching 22b) and (22d)-ion salts with methanol or methanol/diethylamine leads to hexamethylbenzene... 

The addition of Br j and the release of Br are probably not concerted, but the mechanism goes through an unstable intermediate ArHjBrj. This is well known in brominations of aromatic compounds by molecular bromine in acetic acid or aqueous acetic acid, where the whole bromine molecule is still present in the rate-determining transition state (equation 16). 

Bromination of Aromatic Compounds. Phenols, anilines, and other electron-rich aromatic corrqiounds can be monobromi-nated using NBS in DMF with higher yields and higher levels of para selectivity than with Br2 iV-TrimethyIsilylanilines and aromatic ethers are also selectively brominated by NBS in CHCI3 or ecu. A-Substituted pyrroles are brominated with NBS in THF to afford 2-bromopyrroles (1 equiv) or 2,5-dibromopyrroles (2 equiv) with high selectivity, whereas bromination with Br2 affords the thermodynamically more stable 3-bromopyrroles. The use of NBS in DMF also achieves the controlled bromination of imidazole and nitroimidazole. Thiophenes are also selectively brominated in the 2-position using NBS in acetic acid-chloroform. ... 

Benzylic Bromination of Aromatic Compounds. An efficient and fast microwave-assisted method for the preparation of benzylic bromides has appeared The 2-trimethylsilylethyl substituent on the benzenoid ring of 3 undergoes benzylic bromination followed by elimination of MesSiBr and addition of bromine to produce the dibromo compound (4) (eq 31). The ketone (5) is also observed from the hydrolysis of (4). ... 

Bromination of Aromatic Compounds. Studies on the bromination of monocyclic and polycyclic aromatic compounds with NBS have continued and in particular the bromination of phenols and naphthols has received attention, e.g., the conversion of 6 into 7 (eq 35). 

Aromatic bromination is also achieved using NBS, in some cases using strong acids as catalysts. Deactivated aromatic compounds are brominated by NBS in trifluoroacetic acid and sulphuric acid. NBS and aqueous sodium hydroxide is used to brominate activated benzoic acid derivatives. An intriguing effect of lithium perchlorate dispersed on silica gel on the bromination of aromatic compounds with NBS has been reported. Finally a method for the i/JS o-substitution of phenyl boronic acids (8) with NBS leading to the aromatic bromides (9) has appeared (eq 36). ... 

Solid State and Related Reactions. The area of solid/solid organic reactions has been explored. " Results on the solid state nuclear bromination of aromatic compounds with NBS as well as some theoretical insights into the mechanism of the reaction have been reported. NBS on a solid support has been used to sythesize benzylic bromides under neutral conditions and for the functionalization of a-oxoaldehyde-supported silicas. ... 

In macroscopic chemistry, the experimental procedures for the bromination of aromatic compounds depend greatly on the nature and reactivity of the starting material. Activated aromatics such as phenol and aniline can be brominated to the tri-and tetrabrominated derivatives by using dilute aqueous solutions of bromine, whereas a controlled monobromination is very challenging and often requires cryogenic conditions. On the other hand, thermally controlled brominations of less activated aromatics such as toluene are rather slu ish reactions. Th often require photoinitiation and the use of Lewis adds as catalysts. 

Bromination of Aromatic Compounds. Phenols, anilines, and other electron-rich aromatic compounds can be monobromi-nated using NBS in DMF with higher yields and higher levels of para selectivity than with Br2 M-Trimethylsilylanilines and aro-... 

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