Fries, photoproducts

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. 

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,... 

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... 

Compound 217 when irradiated with a high-pressure mercury lamp takes another course, as Constenla has found. In the first step the acetyl group migrates to position 2, but the stereochemistry of this photoproduct 218 has not yet been determined. This step is analogous to a photochemical Fries rearrangement. In the next step the phosphorus residue is split off and 2.4.6-triphenyltoluene 219 can be isolated. 

The enhanced stability of the photo-Fries rearrangement products was again confirmed in a thorough study by Guillet and co-workers (13) who attributed it to the high extinction coefficient of both the ortho and the para photoproducts and to their ability to dissipate the absorbed energy by non-photochemical pathways. 

Recently, Ramamurthy and Weiss and their coworkers reported the photo-Fries rearrangement of three 1-naphthyl phenylacelates (Fig. 36) in cation-ex-changed zeolite Y and high-density polyethylene films [193], When the substrates were irradiated in hexane to <30% conversion, the eight photoproducts in Fig. 36 were detected. The photoproduct distributions from polyethylene or a Y-zeolite are drastically different from those in solution. Cage-escape products (54 and 55) are absent in both constrained media, and in zeolite Y, only 49 was detected. The... 

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. 

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-... 

Figure 6 UV-Vis spectral changes upon 313- and 366-nm irradiation of glassy nematic films of HMW1. (a) Before irradiation, (b) After irradiation. Residual absorption at ca. 280 nm is attributed to cyclobutane photoproducts. Residual absorption beyond ca. 340 nm is mostly due to photo-Fries rearrangement products. (Reprinted with permission from Creed et al. [23]. Copyright 1990 American Chemical Society.)...

In each of the aryl esters discussed above, the acyl radical formed upon lysis of the excited singlet state of the ester loses CO very slowly at the temperatures of the irradiations. At 296K, the rates of loss of CO by acetyl and propanoyl radicals in the gas phase are 4.0 and 2.1 x 10 s respectively." As aresult, no products from decarbonylation and rearrangement are expected" (or have been found) when either of the NA or NM isomers is irradiated in liquid solvents or bulk polymers, and kinetic information from photoproducts alone is limited to relative rates of radical pair processes (Scheme 13.3). For example, if no Fries products from 1-NA or 1-NM emanate from reencounter of radicals that have escaped from their initial cages, [2-AN]/[4-AN]/[l-NOL] = /c2a/ 4a/ nol-... 

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... 

Aromatic polyurethanes exhibit a strong tendency to yellow in sunlight. This may be accounted for by two mechanisms based on formation of different photoproducts. The first one was found to be a photo-Fries reaction caused by a short wave length light [572]. The second one, caused by longer wave length light, is formation of quinonoid structures [573] ... 

The photoproducts, [2]catenanes 37 and 38, contain two disconnected rings since the photochemical reaction leads to decomplexation of the bpy chelate from the ruthenium(II) centre, fri a typical reaction, a degassed CH2CI2 solution of 38 and NEt4. cr was irradiated with visible Ught, at... 

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]. 

Inorganic mesoporous materials such as zeolites have also served as hosts for aromatic esters whose irradiation leads to changes in the selectivity of the photoproducts ortho preference) in comparison with the rearrangement in homogeneous media., - Finally, the photochemical Fries rearrangement of naphthyl acetate has shown the occurrence of solvent-solute clustering in supercritical carbon dioxide. " ... 

---