Halogen/metal interconversion reactivity

Another important feature of the halogen/metal interconversion is that it proceeds almost exclusively on reactants RX where X is iodine or bromine. In the case of haloarenes, a few examples of chlorine/metal exchange have been reported in very particular cases—strong electron depletion in the arene [31-33] or buttressing effect [34] (vide infra)—but fluorides are unreactive. Iodides are the most reactive toward exchange and are several orders of magnitude faster than bromides. When... 

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