Phenyl triplet states

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

The irradiation of 3-phenyl-4-acetyl-5-methylisoxazole (49) gave the isomeric oxazole (50) (Scheme 22) (75JA6484 76HCA2074). The reaction can involve the formation of the biradical intermediate starting from the triplet state, in agreement... 

The product mixture contained 81% of the c/j-3-p-methoxyphenyl-f/my-2-phenyl derivative. Again a reactive triplet state was involved, as evidenced by sensitization and quenching studies (kr = 1.5 x 1010 sec-1). 

Again two products were obtained from a reactive triplet state with a rate constant for photoisomerization of 2 x 1011. This rate, however, is over six times greater than that obtained for the unsubstituted phenyl derivative and indicates that the p-cyano-phenyl group facilitates reaction by stabilization of the intermediate biradical. 

In addition, it was observed that the sensitized photolysis produced the same distribution of products with the same efficiency (fingerprint characteristic of the triplet state). From quenching studies the specific rate constant for the rearrangement could be obtained. Phenyl migration rearrangement is of intermediate efficiency, interposed between the more efficient and less efficient type A processes (Table 7.4). The type of mechanism proposed for this transformation is as follows ... 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

Al-Soufi W, Grellmann KH, Nickel B (1991) Keto-enol tautomerization of 2-(2 -hydroxy-phenyl)benzoxazole and 2-(2 -hydroxy-4 -methylphenyl) benzoxazole in the triplet state hydrogen tunneling and isotope effects. 1. Transient absorption kinetics. J Phys Chem 95 10503-10509... 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

The triplet state of 4-nitrobiphenyl has been observed in laser flashed benzene solution (. max 540 nm, t 10 ns at room temperature) 32). 4-Nitrobiphenyl and 4,4 -dinitrobiphenyl have been photoreduced by sodium formate in buffered aqueous methanolic solution 43i>) 15% 4-aminobiphenyl and 11% 4,4 -azobi-phenyl as well as 49% 4-amino-4 -nitrobiphenyl and 20% 4,4 -(p-nitrophenyl)-azoxybenzene, respectively, could be isolated and identified by comparison with authentic samples. 

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

The homolysis of iodonaphthalene can be photosensitized in the presence of ketones.216 Homolysis of phenyl-iodine and phenyl-bromine bonds is a general photochemical reaction217,218 but even in the case of aromatic ketones,217 it is not as yet known whether the triplet state is involved in the direct photolysis. 

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