Photo Fries reaction

When acylated arylamines are photolyzed, migration of an acyl group takes place" in a process that resembles the photo-Fries reaction (11-30). 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

Photolysis of 2-naphthyl thioesters in the presence of a hydrogen donor such as 1,4-cyclohexadiene yields aldehydes (80-100%) no photo-Fries reaction products have been reported [60],... 

Chapter 2, by Miranda and Galinco, provides a critical survey of the photo-Fries reaction undergone by numerous aromatic esters, amides, and so forth. This chapter is a valuable companion to Chapter 5 by Fleming and Pincock, in Volume 3. Miranda and Galinco s chapter is the sixth chapter devoted to the photochemistry of aromatic compounds in this series. 

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). 

In contrast to If/-azepines the photolysis of N-substituted 5f/-dibenz[Z>,/]azepine under sensitized (PI12CO) conditions yields [2 + 2] cyclobutane dimers (77 R = Ac, CC Et, CONH2, COPh). The N-p- tosyl derivative, however, undergoes photo-Fries reaction to the 2-p-tosylamino derivative (74CRV101). 

Fries rearrangement of aromatic formate esters suggests that phenols are the major products (.24) obtained in the reaction. As poly(p-hydroxystyrene) is remarkably clear in the deep UV, it is likely that poly(p-formyloxystyrene) will not suffer from the same problem of photostabilization upon exposure as was the case with poly (p-acetoxystyrene). This expectation was confirmed by our study of the photo-Fries reaction of p-cresyl formate no ortho rearranged product was isolated after reaction while p-cresol and a small amount of starting material were obtained. 

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. 

A photo-Fries reaction has been used to synthesize the highly substituted benzopyrrolizinone 85.60... 

An impressive illustration of the influence of the medium on the photo-Fries reaction product distribution can be found in Sch. 5 where the products of photolysis of 1-naphthyl 2-methyl 2-phenyl propanoate in hexane solution and within NaY are compared. Remarkably, whereas in solution eight products are formed, within NaY zeolite a single product dominates the product mixture. Cations present within zeolites help to anchor the reactants, intermediates and products to the surfaces of the zeolite and... 

Similar photo-Fries reactions have been reported by Schell and co workers. Irradiation of vinylogous imide 50 led not only to the formation of the crossed photoadduct 51 (as outlined in Scheme 14) but also to the formation of 137, the product of acyl group migration (equation 18)32. 

Reaction of all of the radical pairs within their initial cages does not preclude one or both of the radicals from undergoing a structural change (N. B., step [6] in Scheme 13.1). In the case of photo-Fries reactions, the most commonly encountered structural change is loss of carbon monoxide (CO) from the acyl radical, leading to formation of an alkyl radical. The rates of decarbonylation of acyl radicals have been measured for a wide variety of acyl structures as a function of the medium viscosity... 

SCHEME 13.3 A simplified mechanism for photo-Fries reactions of 1-naphthyl esters, showing the central role of the initially formed singlet radical pairs. 

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