Photo-Fries formates

A photochemical variant, the so-called photo-Fries rearrangement, 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 ort/zo-derivative is formed in an analogous way ... 

Chain cleavage with subsequent formation of phenolic products, rather than the photo-Fries rearrangement to form salicylates and dihydroxybenxophenones, has been identified as the major initial degradation pathway of PC exposed to natural weathering conditions. 

Irradiation of at longer wavelengths (>280 nm) provided phenyl formate (6) as a major volatile product, together with minor amounts of phenol and phenoxyacetone (4), as well as other products. A possible pathway for formation of phenyl formate by oxidation and subsequent cleavage is provided in Scheme III. Phenoxyacetic acid (7) was also identified as a minor product by mass-gc analysis. Photolysis of phenoxyacetone ( ) and phenoxyacetic acid (7)12 yields phenol together with photo-Fries products (also shown in Scheme III). 

Quantum Yields (p) for Product Formation. The quantum yields for formation of the corresponding photoproducts b (arylamine), c (ortho photo-Fries), and d (para photo-Fries) upon photolysis of la-4a are given in Table II. In each case, for photolysis at 254 nm the sum (S>total) of the quantum yield for photoproducts (fy,... 

The broad band absorbance between 330 and 360 nm may be due, at least in part, to formation of the ortho photo-Fries product. Contributions from other species may also be important in this region and final assignment depends on kinetic studies in progress. 

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... 

Ortho- and para-rearrangement and phenol formation on uv-irradiation of aryl esters are accompanied in several cases by decarboxylation,37,60,62,64,80,81 represented for 3,5-di-t-butylphenyl benzoate by the equation 118 -> 119-122. It was shown that this reaction cannot be sensitized,64 but the dramatic differences in product distribution could be observed by changing of the solvent.60,84 The results in Table VI indicate that in polar solvents the decarboxylation process is minimized while the formation of the photo-Fries rearrangement 119 is enhanced. The reverse appears to be true when nonpolar ethereal solvents are used. A considerable amount of biaryls are formed, and hence this reaction may prove useful for the preparation of biaryls and alkylary Is. 

From the point of view of quantum yields calculation, photo-Fries rearrangement, including phenol and products formation, represents a photoreaction (expressed by Eq. 3) in which all products absorb intensively in the absorption region of the starting phenyl ester A. 

A considerable amount of attention has also been paid to the photo-Fries rearrangement of polymer pendant groups. For example, the rearrangement of poly (phenyl acrylate) (10,11) in solution or in the solid-state, is usually incomplete and results in the formation of both the ortho and the para-hydroxyphenone rearranged products in amounts which vary with the conditions of the photolysis. A concurrent side-reaction, which we term the Fries degradation, also results in the liberation of small amounts of phenol (Scheme 2). Similar results have been obtained with poly (phenyl methacrylate) and other substituted aryl acrylates (4,9,12). 

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. 

Irradiation of lower molecular weight samples in the fluid N phase at 313 or 366 nm led to an unusual result [21]. In the first few seconds of irradiation the perturbed spectrum of the N phase exhibited hyperchromism (an increase in absorbance) and its shape became similar to that of the spectrum of the isotropic melt. This effect is also observed upon triplet sensitization which, like 366-nm irradiation, suppresses photo-Fries rearrangement [28]. It has not yet been proved that this effect is accompanied by a phase change from N to I induced by photoproducts essentially acting as impurities in the mesophase. The effect could be at the microscopic level where formation of a cyclobutane dimer or other photoproduct could interrupt H-type aggregated chromophore stacks, or confor-... 

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. 

The photolysis of phenyl benzoate 8.88 is an example of the photo-Fries rearrangement, in which the acyl radical is created close to a phenoxy radical 8.89. The C—O bond is weakened by the presence of an electron in the n orbital of the ester conjugated system. The radical coupling then shows the usual selectivity for C—C bond formation, with the para selectivity in line with the coefficients of the SOMO of the phenoxy radical (see page 291). 

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... 

Arenes of the type ArX-Y, where X may be O, S, or NR, frequently undergo facile photoinduced homolytic cleavage of the X-Y bond with the Y radical subsequently attacking the aromatic ring. The photo-Fries reaction is the most common process of this type and has been reported within the year for aromatic esters which form part of a ptolymer chain or are pendant groups on a polymer chain. In the former case, the rearrangement of fluorene-based polyacrylates [for example, (292)] was studied. Formation of the o-hydroxybenzophenone moiety in the product (293) was monitored by u.v. and... 

The photo-Fries reaction of N-acylindoies favours the formation of... 

Carboxylic acid formation [quantum yield 8xl0 m elnst ( )] is a much more efficient process than CO elimination or Photo Fries rearrangement observed during vacuum irradiation (Table l). Carboxylic acid formation in PmPiPA films and fibers has been observed by IR spectroscopy (at 1T20 cm ) and by potentiometrlc titrations on dissolved samples ( ). 

---