Photodehalogenation Reactions

The mechanism is by no means completely clear, but the photodehalogenation reaction is postulated to occur through the formation of a pair of radical ions from an exciplex resulting in the excited state (Grimshaw and de Silva, 1981). The precursor of the reduction product (Aryl-H) is suggested to be a radical anion (Aryl-Ck-), while a radical cation (Aryl-Cl+ ) is postulated as the precursor of the substitution product (Aryl-OR). In a less polar solvent, e.g., iso-propanol, direct homolysis of the C-Cl bond occurring from the triplet state has been suggested, 

Not all chloroaromatic drugs appear to follow this type of reaction. For example, free chloride ion is not formed on irradiation of chlordiazepoxide for which an oxaziridine is the major photoproduct (Comelissen et al., 1979). Reports on other drugs that contain chlorine substituents vary. This can arise due to differences in the irradiation conditions. If an unfiltered mercury arc source is used, the sample will receive 254-nm irradiation and the C-Cl bond will certainly break, while under longer wavelength irradiation ( 300 nm), the bond may be stable. 

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A homolytic cleavage mechanism with aryl radicals as intermediates is generally accepted for the photodehalogenation reactions of haloaromatics, and this type of reaction is included in a review of modern methods of aryl-aryl bond formation. An example of such bond formation is seen in the photolysis of o- or p-(but not m-) chlorobenzonitrile, or of tetrachloro- or tetrafluoro-phthalonitrile (109), in the presence of methoxybenzenes this leads to the formation of biaryls, in... 

The reductive photodehalogenation of aryl halides has been actively investigated in recent years. Special attention has been given to (poly)halobenzenes and (poly)halo-biphenyls. The reactions are of interest in view of their mechanisms, and because of the importance of chlorinated aromatic hydrocarbons as environmental pollutants and the possibility of their photoinduced degradation. The photochemistry of aryl halides and related compounds in general14 and the photochemistry of polyhaloarenes in particular18 have been reviewed. 

In the dehalogenation of 4-chlorobiphenyl, 1-chloro- and 1-bromonaphthalene, 9-chloro- and 9-bromoanthracene and 4-chlorobenzonitrile, diethyl sulphide has been used as electron donor397. The involvement of radical anions in these reactions is evidenced by the incorporation of deuterium into the products when the reactions were performed in acetonitrile-deuterium oxide. Lithium diisopropylamide in hexane or tetrahydrofuran398 and sodium methyl siliconate (MeSi03Na3)399 have also been used as electron transfer reagents in the photodehalogenation of 4-chlorobiphenyl. 

The photodehalogenation of haloarenes has attracted the attention of scientists interested in reaction mechanisms, synthesis, and environmental processes [1]. Considerable effort has been devoted to developing our understanding of the mechanism of photodehalogenation of monohaloarenes. Since in the environment polyhaloarenes are of more importance than their monohalo parents, interest has been stimulated in applying current theory to achieving an understanding of the mechanistic features of the phototransformation of polyhaloarenes. 

The reactivity of a heteroexcimer has been exploited as a means of degrading low concentrations of chlorinated pollutants such as polychlorinated biphenyls and polychlorinated benzenes. Copolymers of vinylnaphthalene and styrenesulfonic acid form water-soluble, miceUar-hke copolymers with a hydro-phobic core. When the nonpolar chloro-compounds are dissolved in aqueous solutions containing the water-soluble copolymer, they are scavenged into the core, where they undergo photodehalogenation. In these reactions, essentially all the incident radiation is absorbed by the naphthalene chromophores, which form heteroexcimers with the polychloro-compound. The pattern of dechlorination is characteristic of electron transfer rather than direct homolysis. 

After many years of investigation of the photochemistry of alkyl hahdes RX (X = Cl, Br, it was found that the photochemical reaction of alkyl iodides in nonpolar solvent leads to radical products in polar solvent, to carbocationic products (Scheme 1). Photoinduced alcoholysis and photodehalogenation of a-halo ketones (X = Cl, Br) in alcohols have been investigated by Tomioka et al. In their interesting work on photosolvolysis of a-halo ketones, they proposed that the homolysis of CX bonds produces a... 

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