Halogenation aromatic rings

An imine moiety can, interestingly, be interposed in the ether linkage used to connect the two halogenated aromatic rings. The requisite oxime (55-1) is obtained in a straightforward fashion by reaction of imidazo acetophenone (54-2) with hydroxyl-amine alkylation with a,2,3-trichlorotoluene leads to oxiconazole (55-2) [60]. 

Arylation Reactions with Halogenated Aromatic Rings Coupling of Substituted Glycals with Aryl Groups... 

Arylation Reactions with Halogenated Aromatic Rings... 

We know from the chemistry of halogenated aromatic ring systems that fluorine can backbond to the aromatic. -electron system using one of its lone pairs. This ability is far less pronounced in chlorine, and entirely absent in the higher homo-logues bromine and iodine. The data in Table 2.5 corroborates this assumption for the interaction between phosphorus and halides. 

Figure 2 shows the strucmre of polybrominated biphenyl [11], Figure 3 shows the structure of polybrominated diphenyl ether [12], Both structures consist of halogenated aromatic rings. 

Scheme 3 shows the flexibility of this approach in more detail. Possible electrophiles for addition to metalated intermediate 4 include iodine to give the halogenated aromatic ring 6, Weinreb amides (e.g., 9) to access the aromatic ketone 10, DMF to give aldehyde 11, or various aldehydes, e.g., 8, 12, 15, or 16, to give differently substituted benzylic alcohols, e.g., 7, 13, 14, and 17, with diverse additional functional handles for further homologation. All these products may be readily further elaborated. Possible transformations include reductions, iodolactonizations, aldol reactions, allylation, epoxidation, or direct lactonization reactions. 

In the presence of a strong Lewis acid, bromine and chlorine halogenate aromatic rings. Bromina-tion requires both Br and a Lewis acid catalyst, FeBr. The catalyst generates a Lewis acid-Lewis base complex with a weakened Br—Br bond. The bromine atom bonded to iron carries a formal positive charge. It is the electrophile. 

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