Activated aromatics, iodination

Fluorine has been used for the generation of extremely strong electrophilic halogenating agents in electrophilic iodination and bromination of deactivated aromatic substrates in highly acidic reacton media. Polyhalogenation of more activated aromatic substrates is also possible (Fig. 90) [231-233]. 

A catalytic amount of acid is sufficient, on the other hand, for the halogenation of activated aromatics such as polyalkylbenzenes by all the three NXSs. NIS iodinates mesitylene in presence of the very mild acid hydroxy(tosyloxy)iodobenzene (Koser s reagent, HTIB)7 2. NBS performs much better in the presence of / -toluenesulfonic acid and NCS shows the highest yields when trifluoromethanesulfonic acid is used as catalyst743. This difference in reactivity leads to a novel method for preparation of a mixed halogenated mesitylene (equation 100). 

A large variefy of cafalysfs are effective in reactions between activated aromatic or heteroaromatic substrates and acyl chlorides or anhydrides. They include metals such as iron and copper, nonmetals such as iodine, metal halides such as ferric chloride and zinc chloride, and acids such as hydroiodic acid. 

Aromatic iodination. Activation of iodine is necessary for this reaction. For this purpose, mercuric nitrate appears to be quite effective. 

Reaction sequence for the radioiodination of a protein with lodobeads. In the first step, the A/-chioro substituent of the benzenesuifonamide is substituted by radioiodide. The covaiently bound iodine reacts subsequentiy with an activated aromatic side chain of the protein. The degree of damage to the protein is restricted because the reaction oniy proceeds on the surface of the poiymer beads. 

Sulfonic acids are prone to reduction with iodine [7553-56-2] in the presence of triphenylphosphine [603-35-0] to produce the corresponding iodides. This type of reduction is also facile with alkyl sulfonates (16). Aromatic sulfonic acids may also be reduced electrochemicaHy to give the parent arene. However, sulfonic acids, when reduced with iodine and phosphoms [7723-14-0] produce thiols (qv). Amination of sulfonates has also been reported, in which the carbon—sulfur bond is cleaved (17). Ortho-Hthiation of sulfonic acid lithium salts has proven to be a useful technique for organic syntheses, but has Httie commercial importance. Optically active sulfonates have been used in asymmetric syntheses to selectively O-alkylate alcohols and phenols, typically on a laboratory scale. Aromatic sulfonates are cleaved, ie, desulfonated, by uv radiation to give the parent aromatic compound and a coupling product of the aromatic compound, as shown, where Ar represents an aryl group (18). 

An unactivated aryl fluorine may be activated by complexauon with chro-mium(VI). Replacement of the fluonne in the complexed system occurs readily, and the uncomplexetl aromatic product can be generated by treatment with iodine [84] (equation 46)... 

Catalysis by hydrogen chloride or iodine monochloride in chlorination in carbon tetrachloride has also been examined. For the chlorination of pentamethylbenzene, the reaction was first-order in both aromatic and chlorine and either three-halves, or mixed first- and second-order in hydrogen chloride, but iodine monochloride was more effective as a catalyst and the chlorination of mesitylene was first-order in iodine monochloride the activation energy for this latter reaction (determined from data at 1.2 and 25.0 °C) was only 0.4 273. 

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