Aromatic compounds, bromination

Ozone ALkenes, aromatic compounds, bromine, diethyl ether, ethylene, HBr, HI, nitric oxide, nitrogen dioxide, rubber, stibine... 

Recently, several selective bromination reagents for reactive aromatic amines have been developed, for example, 2,4,4,6-tetrabromocyclohexa-2,5-dienone (35), iV-bromosuccinimide-dimethylformamide (36), and hexabro-mocyclopentadiene (37). Although molecular bromine is too reactive to perform selective bromination (mono- versus polybromination), the combined used of bromine and zeolites X and Y has been reported to be applicable to the selective bromination of halobenzenes and alkylbenzenes (38). This zeolite method, however, was not successful in the selective bromination of highly active aromatic compounds. Bromine preadsorbed on zeolite 5A (Ca type) was found to monobrominate aniline in carbon tetrachloride with excellent regioselectivity (91-93% para selectivity) in the presence of organic base, pyridine or 2,6-lutidine (Table XII) (39). The preadsorption of bromine on zeolite 5A is necessary for selective bromination, because the inverse procedure of adding bromine to aniline that had been adsorbed on zeolite beforehand caused a nonselective reaction. 

Given the value of aryl chlorides, bromides, and iodides as substrates in organic transformations, a host of protocols have been devised for their synthesis. One of the classic methods for the synthesis of aryl halides were versions of the halodediazoniation reactions [109-111]. For a range of electron-rich arenes as well as many common aromatic compounds, bromination is commonly achieved simply through the addition of NBS to the arene in DMF [112]. Yields are good to excellent, and no transition metal catalyst was needed. In addition to NBS, elemental bromine has also been successfully used for the bromination... 

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. 

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

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. 

Fluorine has also been added successfully to perfluorinated aromatic compounds, as illustrated in equation 6 [57, 58, 59, 60, 61], Also bromine trifluoride adds fluorine to pentafluorophenol [56] and octatluoronaphthalene [62]... 

The isoxazole nucleus is also halogenated in the 4-position by N-bromosuccinimide provided there is no substituent in this position. This reaction does not proceed homolytically, as might have been expected, and appears to represent a simple electrophilic substitution by the bromine cation. Similar cases have been previously described for the bromination of certain aromatic compounds with A -bromo-succinimide. ... 

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

The usual aromatic bromination are performed by free bromine in the presence of a catalyst, most often iron. However, liquid bromine is not easy to handle because of its volatile and toxic character. On the other hand, alumina-supported copper(II) bromide can be treated easily and safely as a solid brominating reagent for aromatic compounds. The advantages of this procedure using the solid reagent are simple workups, mild conditions, and higher selectivities. Products can be isolated in good yield by simple filtration and solvent evaporation, and no extraction steps are required. 

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. 

Combined effect of BTMA Br3 and ZnCl2 in acetic acid provides a new excellent bromination procedure for arenes. That is, while such reactive aromatic compounds as phenols, aromatic amines, aromatic ethers, and acetanilides have been easily brominated by BTMA Br3 in dichloromethane in the presence of methanol, the reaction of arenes, less reactive compounds, with BTMA Br3 in dichloromethane-methanol did not proceed at all, even under reflux for many hours. However, arenes could be smoothly brominated by use of this agent in acetic acid with the aid of the Lewis acid ZnCl2 (Fig. 13) (ref. 16). 

Since thiophene derivatives, heterocyclic aromatic compounds, are sensitive toward electrophilic substitution reactions, the bromination of these compounds generally gives a mixture of mono-, di-, and other poly-substituted bromination products (ref. 19). However, we have recently found that BTMA Br3 is a useful... 

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. 

The chlorine content of the fly ash is 4.9 %, the bromine content 0.065 %. A series of brominated aromatic compounds has been adsorbed on this fly ash and treated for 1 h. with air at 300°C. The extraction and analysis yield mixed brominated/chlorinated as well as completely chlorinated products. This is shown for 5 aromatic bromine compounds in Table 2. 

The studies on the effect of brominated aromatic compounds on the activity of ALA-D and ALA-S provide an introduction to the examination of porphyrogenic effect of these compounds. Disturbance in these enzymes as well as in URO decarboxylase activity according to some authors, might function as an introduction in development of liver porphyrias. 

Chlorine and Bromine. Aromatic compounds can be brominated or chlorinated by treatment with bromine or chlorine in the presence of a catalyst, most often iron. However, the real catalyst is not the iron itself, but the ferric bromide or ferric chloride formed in small amounts from the reaction... 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

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