Fries Rearrangement
The Fries Rearrangement enables the preparation of acyl phenols.
Mechanism of the Fries Rearrangement
The reaction is catalyzed by Brønsted or Lewis acids such as HF, AlCl3, BF3, TiCl4 or SnCl4. The acids are used in excess of the stoichiometric amount, especially the Lewis acids, since 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 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 sterically unhindered arenes are suitable substrates, since substituents will interfere with this reaction.
The requirement 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 newer protocols. Zeolites have proven to be unsuitable, since they are deactivated, but strong acids, such as sulfonic acids, provide a reasonable alternative.
An additional option for inducing a Fries Rearrangement is photochemical excitation, but this method is only feasible in the laboratory:
Recent Literature
An Improved Synthesis of Hydroxy Aryl Ketones by Fries Rearrangement with Methanesulfonic Acid/Methanesulfonic Anhydride
Selective Fries Rearrangement Catalyzed by Zinc Powder
Anionic ortho-Fries Rearrangement, a Facile Route to Arenol-Based Mannich Bases
Trifluoromethanesulfonic acid catalyzed Friedel-Crafts acylation of aromatics with β-lactams
Convenient Synthetic Approach to 2,4-Disubstituted Quinazolines
Fries rearrangement in methane sulfonic acid, an environmental friendly acid
A Photochemical Two-Step Formal [5+2] Cycloaddition: A Condensation-Ring-Expansion Approach to Substituted Azepanes
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