Dehalogenation chemoselective

Another example from the Ciba-Geigy group concerns the chemoselective reductive dehalogenation of 2,2,4-trichlorobutanal to 2,4-dichlorobutanol (Eqn. (4)), a herbicide intermediate (Bader et al., 1996). 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

In 2007 the scope of the trialkylborane/water system was extended to the dehalogenation of alkyl iodides and the chemoselective deoxygenation of secondary alcohols in the presence of alkyl and aryl halides [86]. The rate constants for the hydrogen-atom transfer from this reagent to secondary radicals (Scheme 37) are substantially lower than those of the Ti(III) aqua-complex [78, 87]. 

Sodium borohydride reduction offers a significant advantage in synthetic applications. The method allows the reductive removal of halides selectively without affecting other functional groups, such as ester, carboxylic acid, nitrile and sulfone. A typical chemoselective dehalogenation is illustrated in Scheme 19. ... 

LnCpa and Cp2LnCl complexes initiate the dehalogenation of aiyl and vinyl halides by NaH. Chemoselectivity is observed in the reduction of m-bromochloro-benzene and p-iodochlorobenzene to chlorobenzene [192], The asymmetric reduction of methylphenyl glyoxylate to methyl mandelate by NADH models is catalyzed by chiral lanthanide )6-diketonates [193],... 

Two competitive pathways have been discovered for the reaction of phosphites with haloacetonitriles (335). The addition of phosphites to the nitrile bond afforded N-unprotected iminophosphonates (336), while reductive dehalogenation of perfluoro(chloro) acetonitriles yielded dihaloacetonitriles (337) and corresponding halophosphates (338) (Scheme 95). The direction and chemoselectivity of the reactions were controlled by the nature and quantity of halogen atoms in the starting nitrile (335) and by the nature of the phosphites. ... 

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