128 Fries Rearrangement
The mechanism for that step is closely related to that of the Friedel–Crafts acylation. Upon subsequent hydrolysis the o-substituted Lewis acid-coordinated phenolate 7 is converted to the free o-acylphenol 2. By an analogous route, involving an electrophilic aromatic substitution para to the phenolate oxygen, the corresponding para-acylphenol is formed. Since the Fries rearrangement is a equilibrium reaction, the reverse reaction may be used preparatively under appropriate experimental conditions.2,6 An instructive example,2 which shows how the regioselectivity depends on the reaction temperature, is the rearrangement of m-cresyl acetate 8. At high temperatures the ortho-product 9 is formed, while below 100 Ž C the para-derivative 10 is formed: O O
OH
CH3
O
OH
165 °C H3C
25 °C AlCl3
AlCl3
CH3
CH3
CH3
CH3
O
8
10
9
A photochemical variant, the so-called photo-Fries rearrangement,7 proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ortho-derivative is formed in an analogous way:
O O
O R
O
R
O O C R
1
11
12