Aromatic halogen compounds reductive dehalogenation

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. 

A six-electron reduction mechanism involving repeated steps of electron transfer, protonation, and dehydration has been proposed (Fig. 8) [24]. A conventional reduction of a halogenated aromatic nitro compound with metals in acidic media is often accompanied by dehalogenation [147]. The semiconductor-mediated reduction of the compoimds does not cause dehalogenation. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

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