Photodehalogenation

A cydic allene generated by photodehalogenation of chlorocarbanion 175 reacted with furan to yield the endo adduct 176 [143],... 

If a potential electron acceptor is built into the anion itself, electron redistribution within the excited state may represent an pseudointramolecular electron transfer. For example, in the photodehalogenation of halosubstituted diphenyl-indenyl anions, eq. 84 (257),... 

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. 

The relative reactivities of photodehalogenation of the three dibromobenzenes, the three bromotoluenes, the three bromoanisoles, the three bromophenols and the three iodo-toluenes in hexane have been determined by irradiation under otherwise identical conditions at X > 290 nm and at a > 230 nm377. With the short-wavelength radiation, the dibromobenzenes and the bromoanisoles react fastest, having almost equal reactivity. The bromotoluenes react about three times as slow and the iodotoluenes are somewhat less reactive than their bromo analogues. The bromophenols are very unreactive. At X > 290 nm, the highest yields are observed with the iodotoluenes, as expected on the basis of their absorption spectra. 

Quantum yields have been determined of photodehalogenation in aliphatic hydrocarbons of derivatives of chlorobenzene, C1C6H4R, with R = Cl, CH3, F, CF3, OCH3, CN, CH2CN and CH2OH at ortho, meta and para positions382. The quantum yields of the... 

The photodehalogenation of 2-(4-halophenyl)benzoxazoles has been investigated394,395, particularly because derivatives of these compounds have therapeutic applications which... 

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 rate of photodehalogenation of 3-chlorobiphenyl, 4-chlorobiphenyl, 4,4 -dichloro-biphenyl and 2,2, 5,5 -tetrachlorobiphenyl in acetonitrile/water is enhanced when the compounds are irradiated in the presence of sodium borohydride429. A mechanism is proposed involving direct attack of hydride on the photoexcited biphenyls. Photodehalogenation ofpara-chlorotoluene is also strongly stimulated by sodium borohydride,... 

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. 

A photochemical route using phase-transfer catalysts for the debromination of dibromostilbene has been reported. The related photodehalogenation of 1,2-dihalides in methanol or triethylamine results in the formation of alkenes.A study of the photochemical behaviour of the polychloro-compound (31) has been reported. ... 

In view of the many reports which apptear each year on photodehalogenation of aryl halides, it is noteworthy that photochemistry is also used to synthesize such arenes. Thus Williams and co-workers describe the photobromination of benzene and halogenobenzenes to give polyhalogenocyclohexanes which themselves are photodecomposed to bromobenzene, dibromobenzenes, and bromochlorobenzenes. The same workers also report that the photobromination of biphenyl in carbon tetrachloride gives the 2-, 3-, and 4-bromoarenes in respective yields of 24, 27, and 49%. 

Direct and indirect photodehalogenations can involve free radicals (R ) that react mainly with NOM to produce the reduced hydrocarbon (eq 8) or with oxygen to produce various oxygenated products (eq 9). 

Halogeno-alkenes.- The photochemical behaviour of AJdrm (30a) is complex. A reinvestigation of the irradiation of this cyclodiene insecticide in hexane has isolated two new photo products (30b) and (31). Irradiation in acetone also provides surprises and the two new adducts (32) and (33) have been identified. The influence of triethylamine on photodehalogenation of cyclodiene insecticides has also been evaluated. Irradiation appears to bring about selective dehalogenation at C-11 in (30a) yielding a 9 1 mixture of the products (34a) and (34b). This stereoselective loss of chlorine is also seen with Dieldrin (35) and Endrin (36). ... 

Photodehalogenation (photoreduction) replacement of the halogen atom by hydrogen in the presence of hydrogen-atom donors is an interesting alternative to Raney nickel-catalysed reduction. Irradiation of 2-bromophenol (482) in propan-2-ol, for example, leads to phenol in >90% chemical yield via a radical mechanism (Scheme 6.234).1341... 

Explain the following concepts and keywords photoinduced radical chain mechanism photonitrosylation nucleophilic photosubstitution photodehalogenation. 

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,... 

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