Exchange Reaction of Aryl Halides

Iodine-magnesium exchange of aryl iodides bearing a functional group proceeds at low temperatures to provide the corresponding arylmagneslum compounds, which react with electrophiles [121]. Some examples are shown in Tab. 3.9. 

In these examples, functional groups such as ester, amide, nitrile, or halogen act as electron-withdrawing groups. The exchange reaction thus goes to completion under mild conditions. 

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L-Proline (LI) was also identified as an effective additive to promote the reaction of aryl halides and CuCN at relatively low temperatures (80-120 °C), affording the corresponding arylnitriles in good yields (Scheme 9.10) [73], In this case, aryl bromides could be converted directly into arylnitriles, without the requirement of the extra halogen-exchanging process. 

Fig. 22 Seiected examples of the use of lithium alkyl magnesiates in metal-halogen exchange reactions with aryl halides...

Reaction of aryl organometallic compounds with halogens Aryl halide exchange halo-denitration halo-dehydroxylation Reaction between diazonium salts and iodide ion Heating of diazonium fluoroborates (Schiemann)... 

In the direct synthesis of aryl terminal alkynes via Pd-catalyzed cross-coupling of aryl halides with ethynylmetals, formation of diarylethynes is one of the potential side reactions. Indeed, the Kumada coupling of 2-iodo-5-methylthiophene (29) with ethynylmagnesium chloride gave the desired 2-ethynyl-5-methylthiophene (30) in only 35% yield, along with 24% of bis(5-methyl-2-thienyl)ethyne (31) [29], The high propensity for H-Mg exchange reaction to occur was blamed for the diarylethyne formation. 

In the copper catalyzed aromatic nucleophilic substitution of aryl halides bromoindole derivatives were converted to the appropriate cyanoindoles. Both 5-bromoindole and its 7V-tosyl derivative gave excellent yields, when a substoichiometric amount potassium iodide was added to the reaction mixture (6.80.), Pyrazole and benzothiophene showed a similar reactivity. The role of the added iodide is to activate the aromatic system through a bromine-iodine exchange.111... 

Halide exchange has been extended to vinylic halides with excellent stereospecificity. Thus, under the above conditions, cw- -bromostyrene is converted to d.s-/ -chlorostyrene and trans-fi-bvomostyrene is transformed into trans-fi-chlorostyrene in high yields. Recently, Cheng and colleagues211 reported the catalytic activity of nickel powder in the exchange reaction of potassium iodide with aryl bromides at 150 °C in DMF. Zinc powder, combined with nickel bromide, is also known to promote this exchange212. 

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