Photochemistry photo Fries rearrangement

The photo-Fries rearrangement of polymers such as 1 leads to yellowing, an undesirable behavior for many potential applications. The photochemistry of analogous MCLC polymers has been reported [30] in which the alkoxyphenyl ester groups of 1 have been replaced by 4,4 -franj-disubstituted cyclohexyl... 

Miranda, M. A., Galindo, F. The photo-Fries rearrangement. Molecular and Supramolecular Photochemistry 200Z, 9,43-131. 

Quantum yield values measured in solution may not necessarily apply to polymer films, the usual environment for practical application of this photochemistry. McKean et al. have adapted the indicator dye method to the measurement of quantum yields for Bronsted acid photogeneration in poly-(4-tert-butoxycarbonyloxystyrene) [20], As with the solution photochemistry of diphenyliodonium salts [71], an inverse dependence of quantum yield on exposure intensity was observed absolute quantum yields from 0.26 to 0.40 were measured at 254 nm, which extrapolate to approximately 0.45 at zero intensity, comparable to the value estimated by Dektar and Hacker [82b] in solution. McKean et al. [20b] note that similar quantum yields in solution and polymer films below Tg have also been reported for photo-Fries rearrangements [84] and photodissociation of diacyl peroxides [85]. 

Direct Photochemistry of PC Excitation of PC at short wavelength (e.g., 254 nm) involves mainly two consecutive photo-Fries rearrangements of the aromatic carbonate units leading successively to the formation of phenylsalicylate (Li) and dihydroxybenzophenone (L2) units as shown in Scheme 15.9. Photo-Fries products have been well defined by definite maxima in the UV and in the carbonyl range of the IR domain Lj at 320 nm and 1689 cm-1 andL2 at 355 nm and 1629 cm-1. 

Reinvestigation of the photochemistries of N-acetyl and N-benzoyl carbazoles has confirmed that only the photo-Fries rearrangement products (carbazole and... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

A plano-convex lens was used to collimate and focus, an IR mirror was employed to eliminate non-productive IR irradiation and minimize associated heat, and a variety of UV filters could be employed to control wavelength. After evaluation of 728 reactions, they identified five which were further explored. Examples included a wavelength dependent [2-I-2]-cycloaddition of indole 27 to afford a mixture of indole 28 cyclobutane 29 (Scheme 5A). They observed that the photo-Fries rearrangement was predominant at shorter wavelengths while the [2 -I- 2]-cycloaddition was favoured at longer wavelengths. A second reaction they identified was an oxa-di-7i-methane reaction with bicycle 39 to afford the caged cyclopropane 35 in 70% yield after 2 min (Scheme 5B). In the screen they found both direct excitation and triplet sensitization conditions for the reaction. They also identified a 1,3-acyl shift with ketol 32 to afford cyclohutanone 33 in modest yield (Scheme 5C). This platform demonstrated that flow photochemistry can enable automated reaction discovery. 

This reaction is important in polymer photochemistry, because it can occur with significant efficiency even in glassy, solid polymers. It is believed to contribute significantly to photo-induced yellowing in aromatic polymers, particularly polycarbonates (Kobsa, 1964 Li and Guillet, 1977). The photo-Fries rearrangement has also been identified as a source of photo-yellowing in aromatic polyamides (Wiles and Carlsson, 1980). 

Avnir, D., de Mayo, R, and Ono, I., Biphasic photochemistry the photo-Fries rearrangement on silica gel,/. Chem. Soc., Chem. Commun., 1109, 1978. 

Iwasaki, S. (1976) Photochemistry of imidazolides. I. The photo-Fries-type rearrangement of N-substituted imidazoles. Helvetica Chimica Acta, 59 (8), 2738-2752. 

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