Fries reaction, photochemical

Aryl sulfonates RS03Ar (R = C6H5, CH3) were found to undergo photochemical rearrangement upon irradiation with uv-light.8,9 In the case of phenyl p-toluenesulfonate (20), products originating from the photo-Fries reaction were identified as 2-hydroxy-4 -methyldiphenyl sulfone (21), 4-hydroxy-4 -methyldiphenyl sulfone (22), and phenol (23). 

A photochemical substitution reaction that does involve radical intermediates, but in a different way, is the photo-Fries reaction... 

Secondary rearrangements apparent isomerizations through radical recombination reactions. In the rearrangement reactions considered so far, the isomerization step is the primary photochemical process, except when a biradical is formed as an intermediate for in that case the primary photochemical process is really a dissociation, even though the fragments cannot separate. There are however cases of overall isomerizations which result from the recombinations of separated free radicals formed through a process of photodissociation. The photo-Fries reaction is an important example of this mechanism, and is illustrated in Figure 4.43. 

Sinha SC, Sun J, Miller GP, Wartmann M, Lemer RA. Catalytic antibody route to tbe naturally occurring epotbilones total synthesis of epothilones A-F. Chem. Eur. J. 2001 7(8) 1691—1702. Haga N, Takayanagi H. Mechanisms of the photochemical rearrangement of diphenyl ethers. J. Oig. Chem. 1996 61(2) 735—745. Dickerson TJ, Tremblay MR, Hoffman TZ, Ruiz Dl, Janda KD. Catalysis of the photo-Fries reaction antibody-mediated stabi-hzation of high eneigy states. J. Am. Chem. Soc. 2003 125(50) 15395-15401. 

The suitability of the photo-Fries reaction for the conversion of phenyl succinates (278)-(283) to the phenols (284)-(289), respectively, has been examined and compared with the corresponding thermal reaction catalysed by aluminium trichloride. Photochemists will be pleased to read that the authors conclude that the photochemical route is more general than the ground state procedure... 

Two other photochemical reactions that appear to proceed through a bond homolysis are the photo-Claisen reaction of phenyl allyl ether (106, equation 12.75), which produces phenol (107) plus o- (108) and p-allylphenol (109) ° and the photo-Fries reaction of phenyl acetate (110,equation 12.76) to produce o- (111) and p-hydroxyacetophenone (112) plus phenol. In the case of the photo-Fries reaction, both concerted and dissociative mechanisms were proposed for the formation of the acylphenols. 

More conventional liquid-liquid biphasic conditions have been also conveniently employed to increase the efficiency of the photo-Fries reaction. For instance, p-methoxyphenyl esters of a, 5-unsaturated carboxyhc acids (30) undergo photochemical rearrangement, followed by basic cyclization to give... 

Stenberg, V. I., Photo-Fries reaction and related arrangements, Org. Photochem., 1, 127, 1967. 

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