Dechlorination, photosensitized

The dechlorinations of 4-chlorobiphenyl400,401 and of chlorobenzene402,403, ortho-, meta-and / ara-dichlorobenzene and 3,4-dichlorotoluene402 can be photosensitized by N,N-dimethylaniline. As with alkylamines, the reaction involves electron transfer but in this case it is not the halobenzene but the dimethylaniline (DMA) that is brought into an excited state. The reaction, supposed to proceed via a exciplex (Y), follows the steps indicated by equations 104-106. 

The photochemical dechlorination of 4-chlorobiphenyl in the presence of anthracene and triethylamine is supposed to proceed via two electron transfer steps, first from triethylamine to excited anthracene and then from the anthracene radical anion to the aryl halide410. This type of mechanism is also operative in the dechlorination of 4-chlorobiphenyl, 4,4 -dichlorobiphenyl, 1,3,5-trichlorobenzene, 1,2,3,5-tetrachloroben-zene and pentachlorobenzene photosensitized by visible dyes (protoporphyrin IX, acriflavin, rose bengal, zinc protoporphyrin, methylene green) in the presence of triethylamine411. The various steps of the mechanism are summarized in equations 112-115. A mechanistic alternative is that the excited photosensitizer transfers an electron directly to the aryl chloride and becomes regenerated by electron transfer from the amine. 

The photodechlorination of 2,2/,3,3/,6,6/-hexachlorobiphenyl and of three commercial mixtures of polychlorinated biphenyls solubilized in an aqueous solution of poly(sodium styrenesulphonate-co-2-vinylnaphthalene) was studied with the use of solar-simulated radiation426,427. The reaction was found to be photosensitized by the naphthalene antenna units present in the copolymer. Exciplex formation and generation of radical anions lead to dechlorination. 

There are only few examples that are initiated or facilitated by the excited singlet of anilines. As mentioned earlier, this could be attributed to its weak absorption at long wavelength that limits the choice of substrates as ground-state acceptors. Previously, photosensitized dechlorination of chlorobenzenes by A,A-dimethylaniline (lb) was found to be efficient ( = 0.2 - 0.6)242,243. In accord with the fluorescence quenching behavior of anilines by chlorobenzenes244, the dechlorination process occurs by electron transfer from the excited lb to the chlorobenzene, followed by fast rupture of the C—Cl bond. 

The process has also been used to attach photosensitizers to silica.54 The product in Structure 5.16 was made by treating aminopropylsilica with 2-hydroxy-l-naphthoic acid. It was more effective in the photochemical dechlorination of polychlorinated biphenyls than a homogeneous... 

Nowakowska, M., Sustar, E., and GuiUet, J. E., Studies of the antenna effect in polymer molecules. 23. Photosensitized dechlorination of 2,2, 4,4, 6,6 -hexachlorobiphenyl solubilized in an aqueous solution of poly(sodium styrenesulfonic acid-co-2-vinylnaphthalene),/. Am. Chem. Soc., 113, 253, 1991. 

Galadi, A., Bitar, H., Chanon, M., and JuUiard, M., Photosensitized reductive dechlorination of chloroaromatic pesticides, Chemosphere, 30,1655-1669,1995. 

---