Halo arenes reduction

The enzymatic migration of chloro and bromo groups, but not of fluoro substituents, by a cationoid mechanism is supported by observations on aluminum chloride-catalyzed isomerizations of halo arenes (22), where all halogens except fluorine migrate by a mechanism involving cationoid intermediates. For the enzymes, however, fluorine appears to be removed reductively, so that it is lost as fluoride ion rather than as positive fluorine. 

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