Photo-Fries process

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-... 

There are a number of reactions related to the photo-Fries process, in which cleavage of a bond adjacent to a heteroatom ring substituent leads by way of radical intermediates to a ring-substituted product. As an illustration, N-phenylcaprolactam, which can be regarded as... 

The intramolecular cyclization of o-allylaniline derivatives (42) and (43) have been studied. The irradiations were carried out in hexane or acetonitrile and irradiation has supplied evidence for the involvement of intramolecular CT complexes. Irradiation of (42) affords the products (44) in the yields shown. The reactivity of (43) was also studied and its cyclization gave the analogous products (45). The influence of substituents on the photochemical reactions of some aniline derivatives (46) has been studied. The unsubstituted derivatives (46) predominantly undergo cyclization. The yield of product is dependent upon the mode of irradiation. Two conditions are described as (a) benzene/air and (b) ben-zene/argon. Under the aerated conditions oxidation of the cyclized product (47) occurs to yield (48). Cyclization is suppressed when the nitrogen is acylated these compounds (46, R = COMe) predominantly undergo a photo-Fries process to yield a mixture of the ortho and para products (49). Detailed studies showed that cyclization occurs from a CT state. 

The direct irradiation of the bicycloenones (62) results in a 1,3-acyl shift followed by decarbonylation. The photochemical and thermal reactivity of so-called o- and -acylcyclohexadienones has been studied in a further attempt to examine the mechanism of the photo-Fries process. 

The regioselectivity of photoaddition of the enol acetate of 40 depends on reaction temperature, and the ratios of 41 to 42 are 11 89, 2 3, and 51 49 at -70 °C, 25 °C, and 65 °C, respectivelyOf note is that the acetylation of 1,3-diketone 40 is not regiospecific, but the two enol acetates interconvert via a photo-Fries process. However, only the enol acetate leading to 41 and 42 participates in the cycloaddition. Fragmentation of adducts 41 and 42 gives diketone 44 and the aldol product 45 via diketone 43. 

When acylated arylamines are photolyzed, migration of an acyl group takes place" in a process that resembles the photo-Fries reaction (11-30). 

Acetylcarbazole has been prepared in yields up to 90% by the aluminium chloride catalyzed Fries rearrangement of 9-acetylcarbazole. Photo-Fries rearrangement of this substrate gave an equimolar mixture of 1- and 3-acetylcarbazoles in cyclohexane it has been used to prepare the 1-isomer by reaction in isopropanol. The Lewis acid-catalyzed Fries process was used to prepare 3-phenacetylcarbazole. ... 

While the possibility of a very rapid electron reorganization from a low energy triplet state is ruled out by the observed insensitivity of the photo-Fries rearrangement to paramagnetic quenchers (vide supra) and the usual rate of intersystem crossing (kST = 10s—1011 sec-1 43a), the remaining alternative to singlet processes for both Path A and Path B, i.e., reactions from... 

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. 

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. 

As in five-membered heterocycles, the introduction of a carbonyl group produces a dramatic effect on the photoreactivity. Pyran-2-ones undergo other electrocyclic processes in addition to those discussed in preceding sections.213 Photorearrangement, however, does not occur in the coumarin nucleus, although a photo-Fries rearrangement of 3-benzoyloxy-6,7-... 

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-... 

The reactions analyzed here (and used to generate the initial radical pairs in the polymer media) can be separated into two distinct categories those that involve lysis of one molecule into two radicals (such as the Norrish Type 1, photo-Fries, and photo-Claisen reactions) and those that require bimolecular processes in which a part of one molecule is abstracted by another (e.g., H-atom abstractions from a phenol or an amine by the lowest energy triplet state of benzophenone). Each reaction produces either singlet or triplet radical pairs and, thus, allows the influence of spin multiplicity on radical pair reaction rates to be separated somewhat from other influences, such as the natures of the polymer matrices and the radical structures. Different methods for extracting rates of processes for the radicals from both static and dynamic data will be discussed. ... 

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