Photo-Fries, solvents

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). 

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. 

A thorough study of the photo-Fries rearrangement of poly(p-acetamido styrene) was made difficult by the lack of solubility of the polymer in solvents other than DMSO or DMF. Nevertheless, photoexposure of thin films of V cast from DMF solution confirmed the lack of reactivity of the polymer as only minimal conversions could be achieved even after long periods of exposure. 

Andrew, D. Des Islet, B. T. Margaritis, A. Weedon, A. C. Photo-Fries Rearrangement of Naphthyl Acetate in Supercritical Carbon Dioxide Chemical Evidence for Solvent-Solute Clustering. J. Am. Chem. Soc. 1995, 117, 6132-6133. 

Ramamurthy and coworkers studied the photo-Fries rearrangement of phenyl acetate and phenyl benzoate and photo-Claisen rearrangement of allyl phenyl ether (Fig. 34) included in two types of zeolite (faujasites X and Y and pen-tasils ZSM-5 andZSM-11) [192], The photolysis was performed with the zeolite slurry in either hexane or iso-octane. One of the most remarkable observations is that the product distribution is altered within zeolites from that in isotropic solvent. Furthermore, while in solution, nearly a 1 1 mixture of ortho and para isomers 40 and 41 (Fig. 34) was obtained, within zeolites one is able to direct the photoreaction selectively toward either the ortho or the para products by conducting the reaction either within faujasites or pentasils, respectively (Fig. 34). 

Photochemical processes of CD complexes,1 Differences in photochemical reactions conducted in solution and in CD complexes have been reviewed. For example, photo-Fries rearrangement of phenyl esters in solvents results in a mixture of o- and p-phenolic ketones via a radical reaction. Rearrangement of the same encapsulated ester results in exclusive rearrangement to the ortho-position (equa-... 

In methanol or benzene solution the major photolysis product is aniline while para-aminodiphenylsulphone is obtained as a minor product. However, when the reaction is performed in aqueous solution in the presence of /3-cyclodextrin relatively little aniline is produced and instead the major product is the para-aminodi-phenylsulphone along with substantial quantities of the ortho-isomer. Dramatically different results were obtained when the solid complex of the sulphanilide with p-cyclodextrin was photolysed in the absence of solvent under these circumstances the ortho-amino-diphenylsulphone is the exclusive product. In previous years this chapter has reported several examples of the photo-Fries reaction being conducted in cyclodextrin cavities but this degree of selectivity is much higher than that seen before. Unfortunately, the authors do not provide details of how the solid complexes were prepared. 

A full paper has appeared expanding on the previous report concerning the photo-Fries rearrangements of N-aroylcarbazoles. In polar solvents mixtures of carbazole and both (311) and (312) are obtained, while in non-polar solvents carbazole and low yields of (311) only are isolated instead. An intramolecular cyclisation reaction competed successfully with rearrangement in the case of N-(ortho-chlorobenzoyl)carbazole to yield (313) as the major product. In a report from a different laboratory the photo-Fries rearrangement of N-sulphonylcarbazole is described and the products identified as (314) and (315) in contrast to the results described above, these authors report that N-benzoylcarbazole is photochemically inert. 

A further example of the crucial role played by solvent polarity in controlling the ortho/para ratio in photo-Fries rearrangement is represented by the behavior of variously substituted PAs. For example, irradiation of... 

Chromanones.- Three new methods of preparing these compounds are noteworthy. Chromanone-2-acetaldehydes (94) were synthesized by the condensation of 2-hydroxyacetophenones with the vinologous formamidinium salt (93).103 The discovery that 4-chromanols (96) can be obtained by the reaction of salicylaldehyde with a dialkyla-ted enamine (95) at room temperature greatly increases the availability of the little known 3,3-dialkylchromanones (97) which are oxidation products of the chromanols. Bis(chromanone)piperazine (99) is similarly obtained from NN -bis(2-methylprop-1-enyl)piperazine (98).10 A photo-Fries rearrangement of aryl acrylates (100 R=H or Me) in a two-phase solvent system of benzene and aqueous alkali has given a convenient route to the chromanones (101) which have been reduced to the chromanol and then dehydrated.105... 

The photo-Fries enolization products are formed within the solvent cage, and diffusion out of the cage is necessary to give the other product, 1-naphthol. Outside the critical region the ratio of photo-Fries enolization products to naphthol is around four, but this increases to more than 12 near the critical density. 

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