Reduction of Haloaromatics

Benzylic halides are reduced very easily using complex hydrides. In a-chloroethylbenzene lithium aluminium deuteride replaced the benzylic chlorine by deuterium with inversion of configuration (optical purity 79%) [537]. Borane replaced chlorine and bromine in chloro- and bromodiphenylme-thane, chlorine in chlorotriphenylmethane and bromine in benzyl bromide by hydrogen in 90-96% yields. Benzyl chloride, however, was not reduced [5iSj. Benzylic chlorine and bromine in a jy/n-triazine derivative were hydrogeno-lyzed by sodium iodide in acetic acid in 55% and 89% yields, respectively [5i9]. 

The difference in the reactivity of benzylic versus aromatic halogens makes it possible to reduce the former ones preferentially. Lithium aluminum hydride replaced only the benzylic bromine by hydrogen in 2-bromomethyl-3-chloro-naphthalene (yield 75%) [540]. Sodium borohydride in diglyme reduces, as a rule, benzylic halides but not aromatic halides (except for some iodo derivatives) [505, 541]. Lithium aluminum hydride hydrogenolyzes benzyl halides and aryl bromides and iodides. Aryl chlorides and especially fluorides are quite resistant [540,542], However, in polyfluorinated aromatics, because of the very low electron density of the ring, even fluorine was replaced by hydrogen using lithium aluminum hydride [543]. 

Bromobenzene was reduced also with sodium bis 2-methoxy- 

An interesting case is the hydrogenolysis of aryl chlorides, bromides and iodides in respective yields of 72%, 72% and 82% on irradiation in isopropyl alcohol [310]. 

Replacement of halogen in compounds with functional groups is discussed in the appropriate chapters. 

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Figure 6.20 Selective reduction of haloaromatic ketones in the multiphasic system using Pd/C.

Nitroaromatics can be obtained, though only in a low to moderate yield, in the photolysis of haloaromatics bearing electron-donor substituents [180-183]. Haloanilines underwent conversion to nitroanilines upon irradiation in a 4% sodium nitrite aqueous solution. Better yields were obtained with chloroderivatives especially when the halogen was present in the para position with respect to the amino group. The same held for halophenols (p-nitrophenol was isolated in 45% yield), whereas a negligible amount of photoreplacement of the halo group took place in the case of p-chloroanisole (see Scheme 4.34). Noteworthy, carrying out the reaction in aqueous media avoided further reduction of the nitrite ion to ammonia observed in an alcoholic environment [180-183]. 

Aryltriazenes can also be decomposed by hydrogen fluoride in organic solution after extraction from their aqueous mother phase. In this case, hydrogen fluoride can be used in small excess but the nature of the solvent is crucial for example, tetrahydrofuran gives complex mixtures, dichloromethane promotes radical reactions (dimerizations, reductions) and acetic acid favors triazene decomposition before fluorination. Aromatic and haloaromatic compounds seem to be the best solvents.283 Such a technique, especially suited for the rapid introduction of an 18F atom, has been employed to produce [ 8F]haloperidol (3), the specific receptors of which have been localized in the brain by positron emission transaxial tomography.298... 

Photolysis of Related HaloaromatIcs. Many other classes of hazardous materials may be susceptible to In situ photochemical treatment. Polybromoblphenyls (PBB) photodecompose more readily than PCB due to the weaker C-Br bonds. The major reaction Is reductive debromlnatlon but quaterphenyls also are formed In low yield (30). Ortho-bromines are cleaved preferentially for pure Isomers but apparently not In commercial PBB mixtures (29). 

Enhancement of PCB photodegradation also is observed in the presence of t r i f luoroace t i c acid (40), presumably via the protonated intermediates (ArClH ). Dienes, such as 1,3-cyclohexadiene, accelerate haloaromatic photoreactions (41.) and modify chloronaphthalene photochemistry by enhancing reductive dechlorination and suppressing dimerization (42). The mechanism is believed to involve exciplex formation with the diene or protonation by the olefin (43). 

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