Halogenation of aromatic compounds

The halogenation of a wide variety of aromatic compounds proceeds readily in the presence of ferric chloride, aluminum chloride, and related Friedel-Crafts catalysts. Halogenating agents generally used are elemental chlorine, bromine, or iodine and interhalogen compounds (such as iodine monochloride, bromine monochloride, etc.). These reactions were reviewed554 and are outside the scope of the present discussion. 

Generally, electrophilic halogenation of phenols leads to the corresponding ortho-and para-substituted products. The synthesis of meta-substituted products is considered difficult.474,555 

Jacquesy and co-workers561 have reacted aniline with Br2 in the presence HF-SbF5 and isolated monobrominated anilines (ortholmetal para 3 24 50). Isomerization was not observed under the reaction conditions ( 40°C), but the products equilibrate at room temperature through 1,2-Br shift. Dibromination yields 2,5-dibromoaniIine (10%) and 3,4-dibromoaniline (55%), with the second bromination being controlled by the first bromine introduced. 

Barluenga et al.565 have reported the selective monoiodination of arenes with bis (pyridine)iodonium(I) tetrafluoroborate [I(py2)BF4] in excess superacids (2 equiv.) [Eq. (5.210)]. Comparable results were found for activated compounds with both HBF4 and triflic acid, whereas triflic acid was more effective in the iodination of deactivated aromatics. For example, nitrobenzene and methyl benzoate are unreactive in HBF4 but give the corresponding iodo derivatives in triflic acid (83% and 84% yields, respectively, in 14 h). Iodination of phenol required low temperature (-60°C). 

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Halogenation of aromatic compounds Rearrangement of N-haloamines (Orton)... 

Scheme 39 Halogenation of aromatic compounds catalyzed by FeCl3...

Dolfing J (1998) Halogenation of Aromatic Compounds Thermodynamic, Mechanistic and Ecological Aspects. FEMS Microbiol Lett 167 271... 

The halogenation of aromatic compounds has probably a mechanism similar to that of the Friedel-Crafts reaction, a Lewis acid acting as catalyst. For example ... 

Electrophilic aromatic substitution (Sections 16.1-16.4) (a) Halogenation of aromatic compounds (Section 16.1)... 

The reagents have been used to effect halogenation of aromatic compounds examples ... 

Halogenation of aromatic compounds differs from the halogenation of alkenes, which do not require a Lewis Acid catalyst. The formation of the arenium ion results in the temporary loss of aromaticity, which has a higher activation energy compared to carbocation formation in alkenes. In other words, alkenes are more reactive and do not need to have the Br-Br or Cl-Cl bond weakened. Also, if the ring contains a strongly activating substituent such as -OH, -OR or amines, a catalyst is not necessary, however if a catalyst is used with excess bromine then a tribromide will be formed. 

The exchange of halogens of aromatic compounds, promoted by copper (I) or (II) [3-... 

The sp -hybridized domains on the surface exist in parts as bent, condensed aromatic structures, and in parts as isolated double bonds. Therefore, a possible strategy might comprise the application of typical reachons of olefins or aromatic compounds like, for instance, the Diels-Alder reachon or other cycloadditions, the alkylation, or the halogenation of aromatic compounds. However, there is a certain drawback to this approach usually it is only van der Waals forces connecting the graphitic structures situated on the surface to the particle s actual core. Hence, the functionality is only attached to a kind of shell, and stability problems may arise, especially if the respective material is to be employed in mechanically demanding applications. 

In the nuclear halogenation of aromatic compounds the operation must be conducted in the absence of light to avoid side-chain halogenation. 

More progress has been reported on the halogenation of aromatic compounds over zeolite catalysts. Jang et al. [55] studied the vapor phase catalytic chlorination of chlorobenzene using solid-acid catalysts such as silica-alumina, alumina, zeolites and modified clay (bentonite) impregnated with FeClj Dichlorobenzene selectivity was higher over the zeolite catalysts. 

A recently reported procedure for a Lewis acid-catalyzed selective halogenation of aromatic compounds was applied to the synthesis of 2-fluoropyrrole (57). Thus, reaction of pyrrole with NFSi in the presence of a catalytic amount of ZrCLi in CH2CI2 for 12 h gave a 55% yield of 2-fluoropyrrole (Fig. 3.25). 

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