Mechanism photocyclization

When stilboestrol (diethylstilbestrol, 62) was photolysed in aqueous methanol at 254 nm it gave the stable 4a,4b-dihydrophenanthrene dione (63). The mechanism required a photo trans-cis isomerization, photocyclization and spontaneous enol-keto tautomerism [53,54]. Previous workers had carried out a similar irradiation in dilute acetic acid and obtained the expected aromatic product (64) [55],... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

Considerable effort has been expended on elucidating the mechanism of the photocyclizations. Di-p-tolylamine can be cyclized to 3,6-dimethylcar-bazole either photolytically in petrol or thermally at 880°C. Each process was viewed as electrocyclic, proceeding via cis and trans versions of 312 (R = Me, = H) produced by dis- or conrotatory processes, respectively. ... 

The remaining mechanism involves the preferential photocyclization of a special conformer of the cis-1,2-diarylethylene. The parent compound cannot be planar and must be considered as a mixture of several interconverting conformers. In solution, there is a slight preference for the cis-syn-conformations over the cis-anti-conformations 68). 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

A number of reactions have been investigated which could form a thiophene ring by forming a bond y to the sulfur atom, but which have not been generalized. Photocyclization of phenylthioethylenes has led to benzothiophenes, usually in low yields, and sometimes accompanied by rearrangement (68JOC2218). Irradiation of (115 R = H or Me, and R1 = Ph) in the presence of iodine gave a mixture of (116) and (117) in about 11% maximum yield. If R1 = H or Me, no product (117) was found, and if R was Ph, only traces of cyclized products were detected. A complex mechanism was proposed to account for these results. 

The photocyclization of stilbene and its simple derivatives to phenanthrene and substituted phenanthrenes is now a well-documented and useful synthetic reaction the scope and mechanism of this reaction have been thoroughly reviewed by Stermitz.344 There... 

Nonoxidative photocyclization also occurs in 2-diphenyl isocyanate (367) to form carbazole (368) and phenanthridone (369).390 The mechanism of this transformation is uncertain, and, in fact, little attention has been given to the photochemistry of isocyanates. Carbazole is also the principal product of nonsensitized photolysis of 2-azidobiphenyl391 the reaction presumably involves the cyclization of an intermediate nitrene. 

A number of interesting photocyclizations have been reported in aromatic nitro compounds. The first authenticated example is probably that of Tanasescu392 who showed that photolysis of 2-nitrocinnamic acid (370) led to the formation of 3-hydroxy-3/7-indole-2-carboxylic acid 1-oxide (371). Although the mechanism of... 

The photocyclization of 1,3-dienes both cyclic and acyclic has proved to be a very general reaction. The yields are often quite good, particularly for cyclic 1,3-dienes. Very little is known about the mechanism of the valence tautomerization in which 1,3-dienes are cyclized to cyclobutenes. 

Thiones, in general, do not undergo similar photoreactions. An exception to this behavior has been found in the thiolactone (483), which on irradiation in methanol is converted to the ether (484).422 Photocyclization of a different type has been observed in the thione (485), leading to the formation of the thiophen (486).423 Details of the mechanism of this and related photo-cyclizations424 are not completely clear. 

Photocyclization arising by intramolecular elimination of HC1, HBr, or HI has been extensively used in the synthesis of nitrogen-containing heterocycles, but much less so in oxygen and sulfur systems. The mechanism of these transformations is not always clear some proceed by way of an initial carbon-halogen bond homolysis, whereas others involve cyclization and... 

Optically active hosts 12 [10] and 13 [10] are also useful for an enantioselective photocyclization of 9 [11], The mechanism of the enantioselective reaction of 9 in an inclusion complex with 12 has also been studied by x-ray analysis [12],... 

It is difficult to distinguish between these two mechanisms, in which the initial step may be very similar. Furthermore, the occurrence of both mechanisms may also be conceivable. On the other hand, finding model systems, e.g. lignin model compounds showing photocycling behavior both in solution and applied to pulps and other carbohydrate matrices, would support mechanism 1. Efforts will be made in that direction in future work. 

The acid-catalyzed cyclization to benzimidazole AT-oxides may also be conducted photolytically with good results.40 Denitration is not observed here and very clean reaction products are obtained. In a few cases, however, the corresponding benzimidazole and not its A-oxide is the sole product. Since these A-oxides are photostable under the reaction conditions, it appears that a different mechanism operates for the photocyclization. A plausible alternative (101)-(88) may be visualized by analogy to the photolytic rearrangement of o-nitro to o-nitroso compounds.43 The o-nitroso-A-oxide (110) may... 

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