Photo-Fries products

Photochemical treatment of an N-acylimidazole results in a 1,2-shift (N-C migration) of the acyl group to give a mixture of 2- and 5-acylimidazoles (photo-Fries products) [1],[2]... 

Irradiation of 1-acyl-1,2,4-triazoles, contrary to A-acylimidazoles affords no photo-Fries products, but instead forms aldehydes and other products via the corresponding acyl radicals.t9]... 

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). 

At present, the relevance of these results to photode-gradationl 0f condensates 1 is a matter of speculation. Of particular interest is identification of the photoproduct quencher(s) (PP, Scheme I). Possible candidates are salicylic acid derivatives, which exhibit the requisite absorptivity at about 300 nm, and which may be formed by oxidation of ortho-photo-Fries products (Scheme III), as illustrated in eq. 1. 

Arylamine Photodecomposition. A number of researchers have alluded to the fact that the products produced from photolysis of aromatic carbamates (i.e., la) also degrade upon irradiation (10), 17). Indeed, we found that the aryl amine 2b and the photo-Fries products 2c and 2d (resulting from photolysis of 2a) decomposed with respective disappearance quantum yields of 0.035, 0.004, and 0.003 when irradiated at 280 nm. These latter results agree with those of Schwetlick et al. (17), who found the rates of disappearance of lc and Id to be quite small. 

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. 

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). 

Photo-Fries products are also formed upon irradiation of some pyridine [112,113] and indole derivatives [114,115], This is shown in Schemes 41 and 42. 

Although the photochemistry of aryl selenoesters and aryl telluroesters is not fully developed, there are some photoreactions of these compounds that resemble a photo-Fries process. Se-/ ara-tolyl selenobenzoate (212) gives, upon irradiation, selenocresol (214), benzaldehyde, and the benzophenone 213 (Scheme 56), which is clearly a photo-Fries product [157,158], Starting from Se-phenyl 2-chlorosele-... 

Anhydrolycorine (73) was prepared by photolytic dehydrobromina-tion of the amide 70, which gave the lactam 72 as first cyclization product. Reduction (LiAlH4) of 72 led to 73 in a good overall yield. Attempts to induce photocyclization of the amide 71 to 73, failed, the photo-Fries product 74 being the only relevant reaction result (9). 

The corresponding esters are much less informative because the centers of chirality in their acyl radicals are structurally protected from racemization like that experienced by translational or rotational motions of prochiral alkyl radicals. In addition, the decarbonylated radicals derived from them are formed long after their acyl precursors have moved to orientations with respect to their aryloxy partners that result in a loss of the memory of their host stereochemistry within a cage see above. Thus, of the Claisen-like photoproducts from irradiation of (7 )-lb, only the BzON (i.e., 3b) retains a measurable amount of optical activity even in the solid phases of long -aIkane. However, in polyethylene hlms, all of the Claisen products from irradiation of (7 )-lb—2-BN, 4-BN, and 3b—exhibit signihcant ee values. In the same media, the photo-Fries products from lb retain virtually all of the enantiomeric purity of the... 

The allenic esters (169) yield the the cyclobuteneones (170) on irradiation. In these examples this is the major product and is accompanied by 1,1,4,4-tetraphenylbuta-l,3-diene. The cyclobutenones are photochemically unstable and C2in be completely destroyed by irradiation for 4-6 h. When the phenyl ester (171) was irradiated no cyclization occurred and the only compound obtained was the photo-Fries product (172). One interpretation of the cyclization process is that the ester undergoes Fission of the carbonyl-ester O bond yielding a radical pjur. Cyclization and recombination would afford the product. However, attempts to trap intermediates such as (173) were unsuccessful. ... 

The photochemistry of the para-substituted phenyl sulphamates (308)-(312) has been found to be substituent dependent. Photolysis of (308) and (309) in methanol gave photo-Fries products (i.e. the expected aniline sulphonic acids) as well as aniline, while the nitro derivative (311) was photochemically inert, and the halogen substituted sulphamates (310) all gave the photosolvolysis product (312)... 

A number of variables are involved in polycarbonate photo-aging (5). For instance, the degree of cross-linking has been found to be dependent on wavelength (2,5), as have the amounts of the photo-Fries products (6). Changes in the near UV (300-360 nm) have been reported to be dependent on the relative humidity (7,8) and on the presence or absence of oxygen (8). 

A. Changes in the Near-UV and Visible Region — Photoyellowing and Photo-Fries Products... 

Comparison of PC film exposed in nitrogen with films containing 0.2% each of model compounds for photo-Fries products. 

The photo-Fries products are not present in significant amounts in solid polycarbonate degraded under normal weathering conditions. 

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. 

In order to address this issue, a process which involves a much shorter-lived radical pair needed to be examined. Toward this end, Weedon, et al. examined the photo-Fries rearrangement of naphthyl acetate (Scheme 4.4-6) in SCCO2 at 35 and 46 C [56]. Photolysis of 1 leads to caged pair [2/3] reaction in-cage yields the photo-Fries products, 2- or 4-acetylnaphthol (4 and 5). However, cage escape, followed by hydrogen abstraction (isopropanol was present as a hydrogen atom donor) leads to a-naphthol (6). 

Photo-Fries products are also formed upon irradiation of pyridine [68, 69], indole [70, 71], and carbazole [72] derivatives. In the case of imides, only one of the acyl groups tends to migrate [73-76] however, macrocyclic imides give rise to photoproducts diacylated at the aromatic ring [77]. 

The photo-Fries products are themselves easily photo-oxidized, making it difficult to find evidence of the photo-Fries pathway [1848, 1849]. 

No photo-Fries product was detected after UV irradiation in air. The cross-linking reaction occurs only in the absence of oxygen. The photo-oxidation reaction of poly(l,3-phenylene isophthalamide) is accompanied by a rapid chain scission [369] ... 

Much attention has been paid to the photo-Fries rearrangement of aromatic polyesters due to the remarkable photostabilization produced by the resulting polymer-bound o-hydroxycarbonyl chro-mophores. This property has been attributed to the high extinction coefficient of the photo-Fries products (internal filter) and to their ability to dissipate the absorbed energy by non-photochemical pathways. 

Another field of application is the design of polymeric imaging systems. The lithographic potential of a photochemical reaction is based on the possibility of dissolving selectively either the exposed or the unexposed areas of a polymer film. Because aU the photo-Fries products are phenols, the irradiated polyester should be easily dissolved in aqueous base, while the unchanged starting material should remain undissolved. The key photochemical step of this sequence is shown above for the conversion of poly(p-acetoxystyrene) (34) into (35) (Scheme 9). ... 

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