Photo-Fries mechanism

The results in this paper support an N-C bond cleavage mechanism (Schemes I and II) for the photolysis of both TDI and MDI based polyurethanes. The substituted anilinyl radicals observed no doubt are formed by diffusion from a solvent cage after the primary N-C bond cleavage. Although not specifically shown in this paper, the reported photo-Fries products (6) are probably formed by attack of the carboxyl radical on the phenyl ring before radical diffusion occurs. The solvent separated anilinyl radicals rapidly abstract hydrogens from the solvent to give the reported aromatic amine product (6). 

In addition, recent studies of amides include the 1,3-acyl migration of enamides to obtain the enamines through a photo-Fries like mechanism (equations 103160 and 104161). 

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... 

Other ring contractions that give azapentalene derivatives include the rearrangement of a fused oxazine to a pyrazolo[l,5-a]indol-4-one with acetic anhydride,165 and an interesting photochemical contraction of a cyclic lactone, which produces an isomeric fused indolone by a mechanism thought to involve the first meta photo-Fries rearrangement [Eq. (19)].166 The product was used as an intermediate for the synthesis of mitomycin antibiotics (Section VI,A). 

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. 

Blends of iso- and terephthalates give amorphous, transparent resins, mosdy yellow in color. Heat-deflection temperatures are higher than those of 100% PC resins and depend on the iso- to terephthalate ratio. For example, a resin with a 1 1 ratio has a value of 160°C. These resins are flame retardant mechanical and electrical properties are similar to those of PC resins. The notched Izod impacts are lower at 150—300 J/m (4.7—5.6 fflbf/in.), even in thick sections. Long-term uv radiation stabilities are excellent, but yellowness increases during initial exposure owing to photo-Fries rearrangements (80), wherein 0-hydroxy-benzophenone units are produced along the polymer chains. 

The API fenoprofen calcium is a diaryl ether. Degradation of fenopro-fen under intense ultraviolet light in solution yields a mixture of isomeric biphenyls via a photo-Fries rearrangement mechanism (Fig. 76) (117). 

When the crystal of 59c was irradiated at 0°C, a single regioisomer of 60c was obtained quantitatively. The photoproduct 60c showed an ee of 42%. Furthermore, a higher ee value of 60c (87% ee) was obtained in the reaction at -50°C. The radical mechanism has been confirmed in the photo-Fries rearrangement of aromatic amides. 

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. 

PC also shows photolytic (in this case photo-Fries) and photo-oxidation reactions, but when PC is irradiated with wavelengths above 300 nm, photolysis only plays a minor role. Photo-oxidation is the major degradation mechanism it was shown that the initiation of this photo-oxidation is not due to the photo-Fries rearrangement reaction. 

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. 

Photo-Fries migration has been reported for the sulphonate ester (316) to give (317) A homolytic cleavage-recombination mechanism akin to the photo-Fries rearrangement of sulphonamldes may... 

---