Photorearrangement Reactions of Aromatic Compounds

Irradiation of 3-metiioxyphenol 34 in presence of AlBrs gives lumiketone t)tpe product, 4-meflioxybicyclo[3.1.0]-hex-3-en-2-one 35. This reaction provides a useful method for synthesis of bicyclo[3.1.0]-hex-3-en-2-ones [40]. 

The reaction is believed to proceed through a para-protonated Lewis complex. The presence of excess AlBrs prevents the cyclo-reversion of the product. 

The light-induced rearrangement of phenyl esters 36 into hydroxy aryl ketones 37 and 38 is called the photo- Fries rearrangement [41]. Because of low yields, this procedure has little commercial importance. The reaction is usually carried out in an aprotic solvent and involves a radical mechanism by the homolytic cleavage of the (9-acyl bond from its singlet excited state [41]. 

The rearrangement is found to be of intramolecular nature. The radical pair remains in the solvent cage and their recombination in the cage affords the acyl migration products, while hydrogen abstraction by the phenoxy radical from the solvent leads to the formation of phenol as by-product. When the reaction of phenyl acetate was carried out with deuterated phenol in methanol, the major products were ortho- and para-hydroxy acetophenones and phenol. Only trace amounts of crossover products were obtained [42]. The presence of methoxy substiment at meta- and para-positions increases the yield of ortho-Vries product [43]. For example, 39 gives 40. 

This reaction occurs in nature when a plastic bottle made of polycarbonate (polyethyleneterephthalate) exposed to sunlight (wavelength 310 nm) leads to leaching of phthalate from the plastic [44]. 

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This chapter deals with the photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement reactions of aromatic compounds and related photoreactions. 

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