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31.5.1.3.5 Method 5: Rearrangement of Phenolic Esters

DOI: 10.1055/sos-SD-031-00288

González-Bello, C.; Castedo, L.Science of Synthesis, (200731326.

The Fries rearrangement of phenolic esters provides a useful route to acylphenols.[‌30‌] The reaction requires the use of Brønsted acids (e.g., H3PO4, HF, TsOH, Nafion-H, or MsOH)[‌31‌‌35‌] or Lewis acids (e.g., AlCl3, BF3, TiCl4, SnCl4, FeCl2, ZnCl2, or mixtures of AlCl3 and ZnCl2 supported on silica gel).[‌36‌,‌37‌] However, aluminum trichloride is the most frequently used catalyst. The acids, especially the Lewis acids, are used in excess because they form complexes with both the starting material and the products. This rearrangement affords a mixture of ortho- and para-substituted acylphenols, the ratio of which depends on the temperature and the reaction medium. Usually, high temperatures favor the ortho-rearrangement products, and the para-derivatives are obtained at lower temperatures. An exception is the zirconium(IV) chloride mediated rearrangement, which affords mainly the ortho-product at room temperature.[‌38‌] For example, treatment of 3-tolyl acetate (37) with 2 equivalents of aluminum trichloride affords mainly the 4-hydroxyacetophenone 39 at 25°C and mainly the 2-hydroxyacetophenone 38 at 160°C (Scheme 12).[‌39‌,‌40‌] The latter is also the main product obtained using zirconium(IV) chloride at 20°C.[‌38‌]

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References