Reversible dehalogenation reaction

Figure 5. Reversible dehalogenation reaction active bleaching agent is OCI (Reproduced with permission from reference 33. Copyright 1994 AATCC.)...

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

In the following we will try to illustrate these general points by discussing two specific types of redox reactions the reduction of aromatic nitro groups (Eq. 14-9) and the reductive dehalogenation of polyhalogenated Cr and C2-compounds (Eqs. 14-6 to 14-8). These two cases represent two very different types of reactions. In the first case, the transfer of the first electron is reversible, whereas in the second case, it is typically irreversible and involves the breaking of a bond. In the latter case, therefore, one speaks of a dissociative electron transfer. Furthermore, compounds... 

Transformation of many compounds appears to be direct, without detectable chlorinated intermediates. In some cases, direct transformation occurs in parallel with other reactions, such as the formation of radicals or sequential dehalogenation. Some alkanes (1,1,2,2-TCA) appears to be dehalogenated sequentially, while others (DBCP) first undergo elimination to form an alkene, followed by complete dehalogenation and hydrogenation. Hydrogenation of PAHs is reversible products form equilibrium ratios. 

Recently it was described that the halohydrin dehalogenases can be employed for the enantioselective formation of a new carbon-carbon bond. Their natural task is the dehalogenation of halohydrins, however this reaction can be reversed. When utilizing cyanide instead of a halide as the nucleophile in the reverse reaction the desired /ttiydroxynitrile is formed with good to excellent enantioselectivity (Scheme 5.34) [60]. 

Thus, it was shown that the tautomerization may be of importance in the thermal dehalogenation of chloroaromatics14. The formation of diarylamines in this reaction can be also explained by the presence of aniline tautomers in the reaction mixtures14. One of the well-known types of aniline tautomerism is the enol-imine-keto-amine equilibrium. In effect, it is a reversible conversion of Schiff bases into enamines that is frequently employed for the synthesis of photochromic compounds, as well as of mesomorphic... 

Reagent for Polarity Reversal Catalysis in Radical Reactions. Polarity reversal catalysis (PRC) has been established by Roberts in free-radical chemistry as an efficient alternative to the use of stannanes (e.g., tri-/>butylstannane) and their associated toxicity and purification problems. Silyl radicals can be a valid alternative to tin radicals for one of the most common radical reactions, that is, radical dehalogenation, but silanes, contrary to stannanes, cannot sustain an effective radical chain reaction, due to the stronger Si-H bond. 

Bromination of the olefin (617) gives the dibromide (618) with a rearranged carbon skeleton. The reaction may be reversed. Treatment of (618) with silver bromide gives the dibromide (619) with the original carbon skeleton, and dehalogenation of (619) with sodium and tetraphenylethylene regenerates the hydrocarbon. 

Elimination reactions can be regarded as the reverse of addition reactions. One substrate is converted into at least two molecules, with dehydrogenation, dehydration, dehalogenation, and dehydrohalogenation reactions being of highest technical... 

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