Mechanisms of the fries rearrangement
The reaction is catalyzed by Bronsted or Lewis acids such as HF, AlCl3, BF4, TiCl4 or SnCl4.The acids are used in excess of the stoichiometric amount especially the Lewis acids, scince they form complexes with both the starting materials and products.
The complex can dissociate to form an acylium ion. Depending on the solvent, an ion pair for can form, and the ionic species can react with each other within the solvent cage. However, reaction with a more distant molecule is also possible
After hydrolysis, the product is liberated
The reaction is ortho, para-selective so that, for example , the site of acylation can be regulated by the choice of temperature. Only statically unhindered arenes are suitable substrates, since substituents will interfere with the reaction.
The requirements for equimolar quantities of the catalyst, the corrosive and toxic conditions (HF), and the violent reaction of the catalyst with water have prompted the development of never protocols. Zeolites have proven to be unsuitable since they are deactivated, but strong acids, such as sulfonic acids ,provide a reasonable alternative.
An addiction option for inducing a Fries Rearrangement is photochemical excitation, but this method is only feasible in the laboratory.
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