Ortho/para ratio Fries rearrangement

Banks, M. R. Fries rearrangement of some 3-acetoxy- and 3-(propionyloxy)thiophenes. J. Chem. Soc., Perkin Trans. 11986, 507-513. Dawson, I. M., Gibson, J. L., Hart, L. S., Waddington, C. R. Aromatic rearrangements in the benzene series. Part 5. The Fries rearrangement of phenyl benzoate the rearranging species. The effect of tetrabromoaluminate ion on the ortho/para ratio the noninvolvement of the proton as a cocatalyst. J. Chem. Soc., Perkin Trans. 2 1989, 2133-2139. 

Dealumination of the ZSM-5 zeolite shows a great effect on ortho/ para selectivity in the acylation of phenol by AAC. Thus, for a phenol conversion of 20%, ortho/para selectivity is 7.0 when ZSM-5(41.8) is utilized, and becomes 13.0 in the presence of ZSM-5(42.4). This unexpected increase in the ortho/para ratio can only be explained by postulating that ortho-HAP and para-HAP result from different pathways. The ortho isomer is mainly produced in the pores, whereas para isomer production occurs only on the external acid sites. The ortho isomer can be formed by direct C-acylation of phenol with AAC this selective reaction can be related to the general mechanism reported in Scheme 5.1. ortho-HAP can also be obtained by the Fries rearrangement. On the contrary, the para-isomer is a secondary product and, therefore, it results from the acylation of phenol by PA according to Scheme 5.6. 

A further example of the crucial role played by solvent polarity in controlling the ortho/para ratio in photo-Fries rearrangement is represented by the behavior of variously substituted PAs. For example, irradiation of... 

Heating a phenol ester in the presence of Nafion-H gave good yields of the hydroxy phenyl ketone.This Fries rearrangement gave a mixture of the ortho and para hydroxy compounds in a 1 2-2.5 ratio (Eqn. 22.19). A study of the rearrangement of phenyl benzoate over a number of solid acid catalysts indicated that Nafion-H was more effective than montmorilIonite or amorphous aluminum silicate for this reaction. " ... 

Ortho-specific metal-promoted Fries rearrangement can be achieved, starting from orffeo-bromophenyl esters and sec-butyllithium. The reaction is performed at -95°C with 4 1 1 tetrahydrofuran diethyl ether hexane ratio followed by stirring for 30 min, followed by an additional 30 min at 78°C. The orfto-hydroxyaryl ketones are the sole isomers obtained, and the regioisomeric para-hydroxyaryl ketones are not obtained (Table 5.11). 

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