Reaction dehalogenation

The most common leaving groups are sulfonate esters and halides. For the sake of convenience, the discussion of certain dehalogenation reactions is also included in this section even though they may not involve 8 2 type displacement. Benzylic alcohols are also known to be displaced by hydrides or deuterides, but there is no evidence for the application of these reactions to the steroid field. 

Stereochemistry of the halogenation-dehalogenation reaction was studied for 1,2,3,4-tetrabromodibenzodioxin, TBDD (Scheme 4). 

Totten LA, U Jans, AL Roberts (2001) Alkyl bromides as mechanistic probes of reductive dehalogenation reactions of vicinal stereoisomers with zerovalent metals. Environ Sci Technol 35 2268-2274. 

Scheme 6.176 Nitrocyclohexanol synthesis [332], an a,/ -dibromoester transformation [333], and a dehalogenation reaction [334].

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

It is well known that the comparatively inert Si-H bonds of triorganosilane R3SiH (R = alkyl, aryl) can be activated by transition metal complexes. Chatgililaloglu et al. have used Et3SiH in palladium-catalyzed dehalogenation reactions, which occur with the involvement of free radicals.245... 

As pointed out earlier a third class of B12-dependent enzymes, present in anaerobic microbes, carry out reductive dehalogenation reactions, which play an important role in the detoxification of chlorinated aliphatic and aromatic compounds, among which are many important man-made pollutants. The role of B12 in this class of enzymes is not clear— possibly by formation of an organocobalt adduct, as in the case of methyltransferases or alternatively by the corrinoid serving as an electron donor. 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

Polymer-supported metallohydride carbonyl dehalogenation reactions... 

An S Ar (nucleophilic substitution at aromatic carbon atom) mechanism has been proposed for these reactions. Both nonenzymatic and enzymatic reactions that proceed via this mechanism typically exhibit inverse solvent kinetic isotope effects. This observation is in agreement with the example above since the thiolate form of glutathione plays the role of the nucleophile role in dehalogenation reactions. Thus values of solvent kinetic isotope effects obtained for the C13S mutant, which catalyzes only the initial steps of these reactions, do not agree with this mechanism. Rather, the observed normal solvent isotope effect supports a mechanism in which step(s) that have either no solvent kinetic isotope effect at all, or an inverse effect, and which occur after the elimination step, are kinetically significant and diminish the observed solvent kinetic isotope effect. 

Reductive dechlorination of penclomedine was, indeed, observed with liver microsomes under anaerobic conditions, indicating that a composite sequence such as this should not be dismissed without the benefit of further investigations. Given that the dehalogenation reactions exemplified by tri-fluridine and penclomedine may be less uncommon than implied here, mechanistic studies designed to clarify these processes would be most welcome. 

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