Miscellaneous Photochemistry of Aromatic Systems

Horspool and F. Lend (ed.), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn., CRC Press, Boca Raton, Florida, 2004. 

Nalwa (ed.). Handbook of Photochemistry and Photobiology, American Scientific Publishers, Stevenson Ranch, California, 2003. 

Dtirr and H. Bouas-Laurent (ed.), Photochromism Molecules and Systems Revised Edition, Elsevier Science, Amsterdam, 2003. 

de Waele, M. Beutter, U. Schmidhammer, E. Riedle and J. Daub, Chem. Phys. Lett., 2004, 390, 328. 

Sakamoto, T. Yagi, S. Kobaru, T. Mino and T. Fujita, Tetrahedron Lett., 2004, 45, 4437. 

It has been previously reported that l,T-binaphthalene undergoes racemization through the triplet state,whereas this process for the 2,2 -diol derivative is proposed to involve an intermediate arising from intramolecular addition of a hydroxy group to the C2-C3 bond. Nevertheless, neither of these routes is considered to operate for the recently described efficient photoracemization of the naturally occurring l,r-biphenanthrene, blestriarene (200) in ambient light. Instead it is proposed that quinone-type species are the likely intermediates in this case. 

Quinone methides are frequently reported as intermediates from irradiation of arenes having a methyl or substituted methyl group in the 2-position to a carbonyl or nitro group (H-abstraction) or a hydroxy function (formal loss of water). The latter process has been studied with pyridoxine (201) and its derivatives (202) and (203), and the mechanism by which the loss occurs is found to depend upon the pH of the solution. In neutral solution, the formation of the quinone methide (204) from (201) arises either by excited state proton transfer to the aqueous methanol solvent and loss of OH from the phenoxide ion, or by intramolecular proton transfer and loss of water, while the reaction in alkaline solution involves dehydroxylation from the excited state of the phenoxide ion. 

It has previously been shown that in the triplet state a nitro group on a benzene ring is highly electron withdrawing at the 3- and 4- positions and induces decarboxylations, retro-Aldol type processes and a novel intramolecular redox-type reaction by way of nitrobenzyl carbanion-type intermediates in all cases. In this context, the irradiation of the three 4-nitrobiphenyl derivatives (217), (218) and (219) has been examined in order to assess the potential of photoexcited nitro groups to induce such processes across the biphenyl system.Indeed analogous reactions are observed in these biphenyls but with enhanced quantum efficiencies to those for the reactions in benzenoid compounds. Thus (217) yields the aldehydes (220) and (221) at pHs of 2 and 7, respectively, and (218) gives (222) and (223) under similar conditions, whereas (223) is also formed from (219) at pH 13 but no reaction is observed in neutral solution. The authors note that these results provide more evidence that the 

6- and 1,8-pyrene quinones (224) and (225), respectively, and that 1-naphthol, coumarin and 2-hydroxyquinones are formed from exposure of naphthalene aqueous solutions to natural sunlight. An earlier report had described the formation of benz[/]oxepins from 254 nm irradiation of 1,4-epoxy-1,4-dihyd-ronaphthalenes. Under similar conditions, however, the 2-cyano- and 2-car-boxymethyl derivative (226) and (227) are now reported to yield solely the indenes (228) and (229) by a route, it is proposed, which probably involves a decarbonylation step.  

Mizuno, H. Maeda, A. Sugimoto and K. Chiyonobu, Mol. Supramol. Photo-chem., 2001,8,127. 

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