Benzene photoisomerization

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

When an aqueous solution containing 1,2,3,4-tetrachlorobenzene and a nonionic surfactant micelle (BriJ 58, a polyoxyethylene cetyl ether) was illuminated by a photoreactor equipped with 253.7-nm monochromatic UV lamps, no photoisomerization was observed. However, based on photodechlorination of other polychlorobenzenes under similar conditions, it was suggested that tri- and dichlorobenzenes, chlorobenzene, benzene, phenol, hydrogen, and chloride ions are likely to form. The half-life for this reaction, based on the first-order photodecomposition rate of 5.24 x lO Vsec, is 2.2 min (Chu and Jafvert, 1994). 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

Irradiation of a benzene solution of anhydro-4-benzoylamino-2-phenyl-l,3-dithiolium hydroxide (141, R = Ph, R = H, R = COPh) yields the 1,2-dithiole (145, 80%). This photoisomerization has been interpreted as involving the bicyclic intermediate 144. ... 

Compound 122 represents the product of a transposition of the substituents on the benzene ring. Such products are quite characteristic for this photodecarboxylation reaction. Their origin could be explained a priori by the well-known photoisomerization of substituted benzenes320 of type 121 and 126, which are formed primarily. Alternatively, it has been suggested60 that the phototransposition reaction occurs during the course of the decarboxylation. This supposition is based on two facts. First, no photo-Fries... 

Isomerization of unsaturated nitrogen-containing organic compounds, containing C=C double bonds, has been investigated less intensely than the isomerization of other related unsaturated compounds. Photoisomerization has been reported upon irradiation of a benzene solution of aqueous solution of its hydrochloride, and of the methiodide salt under nitrogen atmosphere. The corresponding cis-isomers are formed.308,309... 

E. Photoisomerization of benzene. Photolysis of liquid benzene by excitation to its (7t, jr ) states brings about interesting photochemical transform ations from vibronically excited singlet as well as triplet states (Figure 7.8). 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Substituted benzenes have been shown to undergo various valence photoisomerizations to yield bicyclo[2.2 Ojhexadienes, benzvalenes, and prismanes.223 The photochemistry of benzene itself is, at present, rather ill defined. Recently, it has been shown that benzene excited by 2537-A irradiation adds to olefins224-226 and to dienes227 to form 1,2-, 1-3-, and 1,4-bridged products. 

A remarkable series of papers has appeared over the last few years reporting the photoisomerization of substituted benzenes, largely in the condensed phase. One of the earliest observations of photoisomerization in benzenes was that of Van Tamelen and Pappas38 who irradiated an ether solution of 1,2,4-tri-I-butyl benzene (1) with a Hanovia Type L lamp and identified a product as the Dewar analog of tributyl benzene (2). This compound reverted thermally to the starting material... 

Irradiation of 2,6-dimethylpyran-4-one (357) in ethanol or benzene or in the solid state yields the cage-like molecule (358) from which the pyran-4-one is regenerated by hydrochloric acid (63JA1208). When the photodimerization is conducted in water but in the absence of air, a small amount of 4,5-dimethylfuran-2-aldehyde is also formed (63JA2956). Pyran-4-one is photoisomerized to pyran-2-one (359) the intermediates postulated resemble those suggested in the irradiation of pyrylium salts (Section 2.23.2.1) (79JA7521). 

Arnett, E. M., and J. M. Bollinger Evidence for a ladenburg benzene intermediate in the photoisomerization of 1.2.4.5-tetra-f-butylbenzene to 1.2.3.5-tetra-f-butylbenzene. Tetrahedron Letters 1964, 3803. 

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