Dehalogenation, of aryl halides

Scheme 20 Proposed mechanism for dehalogenation of aryl halides...

Oxidative addition of C2 - H bonds of imidazolium salts to low valent metals was first observed by Nolan and coworkers in 2001, who proposed a NHC - Pd - H intermediate in the catalytic cycle of the dehalogenation of aryl halides with Pd(dba)2 in the presence of imidazolium salts [154]. More direct evidence of this process was described by Crabtree and coworkers two years later [155]. The reaction between a pyridine-imidazolium salt and Pd2(dba)3 afforded the preparation of bis-NHC - Pd(II) complexes by C2 - H oxidative addition (Scheme 40). The presumed Pd - H intermediates were not detected. The authors proposed a mechanism via two successive C - H oxidative additions followed by reductive elimination of H2 [ 155]. 

C-H Bond-Forming Reactions the Dehalogenation of Aryl Halides.263... 

Even though the Jourdan-Ullmann condensation has been known for more than a century, the mechanism is still not quite clear. On the one hand, the reaction may proceed via a free-radical mechanism, pertaining to the reductive dehalogenation of aryl halides and the acceleration of the reaction rate by ultraviolet irradiation, as outlined in Scheme 1. On the other hand, the reaction may involve halonium and proceed as a simple aromatic nucleophilic substitution, as displayed in Scheme 2. However, for the reaction of 6>-halobenzoic acid, it is believed that the copper ion coordinates with both carboxyl and... 

Viciu, M.S., Grasa, G.A., Nolan, S.P., Catalytic dehalogenation of aryl halides mediated by a palladium/imidazolium salt system, Organometallics 2001, 20 3607-3612. 

The dehalogenation of aryl halides was efficiently performed in refluxing THF using a catalytic combination composed of Ni /IMes/i-PrONa. IMes was the most effective NHC, compared to IPr, SIPr, SIMes, ITol, SITol, or ItBu, for the dehalogenation of functionalized aryl chlorides, bromides, iodides and polyhalogenated (hetero)aromatic compounds [eqn (10.2)]. 

The reductive dehalogenation of aryl halide by triethylsilane with catalytic amount of palladium chloride under microwave irradiation is a simple, efficient and selective method. The silicon hydrides are nontoxic and stable and hence, these are effective reducing agents. 

The dehalogenation of aryl halides is accelerated by aliphatic sulfides, and an electron transfer mechanism is implicated by the incorporation of deuterium in the reduction product when the reaction is carried out in the presence of D2O. 

These reactions are relevant to this review in that they may be photoinduced, but as radical chain processes many of them can be initiated in other ways. Radical chain dehalogenation of aryl halides has been reviewed previously.The chain propagation sequence for the deiodination of aryl iodides with CH5O-/CH3OH is shown in Scheme 1. The species (Arl) is very short lived and deiodinates efficiently. Reactivity diminishes in the order Arl > ArBr > ArCl and is enhanced in cases where there is relief of steric strain. Photochemical dehalogenations of aryl halides with AIH4 and BH4 have been proposed to follow similar radical chain mechanisms. Evidence for electron transfer from BH4 to ArCl has been present by Freeman and Ramnath. ... 

The dehalogenation of aryl halides can be carried out in stoichiometric or catalytic reactions in the presence of bases. Reducing agents as LiAlH or NaBH are used in the stoichiometric reactions [2]. The reaction can be carried out in the presence of homogeneous and heterogeneous catalyst under condition of transfer hydrogenation... 

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