Zinc powder Fries rearrangement

Thus, in the case of PA, ortho-HAF is obtained in 75% isolated yield when irradiated in a microwave oven for 3 min, whereas, when the reaction is carried out with the same quantities in an oil-bath at 60°C for 6 h, para-HAF is obtained in 70% isolated yield. Under microwave irradiation, para-migration takes place, whereas, under oil-bath conditions, ortho-migration is observed. Moreover, 1-naphthyl- and 2-naphthyl acetates undergo the Fries rearrangement at the ortho-position in 85% and 90% isolated yields, respectively, when subjected to microwave irradiation in the presence of zinc powder and dimethylformamide for 10 and 8 min. 

Paul, S. and Gupta, M. 2004. Selective Fries rearrangement catalyzed by zinc powder. Synthesis 1789-1792. 

The original Fries procedure has been modified extensively to occur under either UV irradiation (Photo-Fries Rearrangement) or anionic lithiation or lithium-bromo exchange (anionic ortho-Fries rearrangement). Other modifications include the application of microwaves,electron beam, BF3-H20, Y zeolite,zinc powder, Bi(OTf)3, and solid acid catalyst as the promoters. 

Paul and Gupta (2004) observed selective Fries rearrangement of acetylated phenols catalyzed by zinc powder in presence of N,N-dimethylformamide under the microwave heating. This reaction was completed in about 0.5-7 h under conventional conditions while it is over in 2.25-23 min in presence of microwaves. 

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