Dehalogenation

Mechanistic and thermodynamic concepts

Removal of a halogen atom from an organohalide generates a radical. Such reactions are difficult to achieve and, when they can be achieved, these processes often lead to complicated mixtures. When a pair of halides are mutually adjacent (vicinal), their removal is favored. Such reactions give alkenes in the case of vicinal alkyl dihalides: :

Most desirable from the perspective of remediation are dehalogenations by hydrogenolysis, i.e. the replacement of a bond by a bond. Such reactions are amenable to catalysis: :

The rate of dehalogenation depends on the strength of the bond between the carbon and halogen atom. The bond dissociation energies of carbon-halogen bonds are described as: (234 kJ/mol), (293 kJ/mol), (351 kJ/mol), and (452 kJ/mol). Thus, for the same structures the bond dissociation rate for dehalogenation will be: F secondary tertiary halides.

Applications

Main article: Reductive dechlorination

Since organochlorine compounds are the most abundant organohalides, most dehalogenations entail manipulation of C-Cl bonds.

Organic halides belong to a class of organic compounds that contain carbon-halogen bond. In 1832, scientist named Justus von Liebig synthesized the first organic halide (charcoal) via chlorination of ethanol. Since then, organohalides have gained a lot of attention. Organohalides are commonly used as pesticides, biodegradables, soil fumigants, refrigerants, chemical reagents – solvents, and polymers. It has been classified as pollutant despite of their wide use in various applications. Due to which, dehalogenation is a key reaction to convert toxic organohalides to less hazardous products.

Organic synthesis

Of some interest in organic synthesis, electropositive metals react with many organic halides in a metal-halogen exchange: : The resulting organometallic compound is susceptible to hydrolysis: : Heavily studied examples are found in organolithium chemistry and organomagnesium chemistry. Some illustrative cases follow.

Lithium-halogen exchange is essentially irrelevant to remediation, but the method is useful for fine chemical synthesis. Sodium metal has been used for dehalogenation process. Removal of halogen atom from arene-halides in the presence of Grignard agent and water for the formation of new compound is known as Grignard degradation. Dehalogenation using Grignard reagents is a two steps hydrodehalogenation process. The reaction begins with the formation of alkyl/arene-magnesium-halogen compound, followed by addition of proton source to form dehalogenated product. Egorov and his co-workers have reported dehalogenation of benzyl halides using atomic magnesium in 3P state at 600 °C. Toluene and bi-benzyls were produced as the product of the reaction. Morrison and his co-workers also reported dehalogenation of organic halides by flash vacuum pyrolysis using magnesium.

With transition metal complexes

Many low-valent and electron-rich transition metals effect stoichiometric dehalogenation. The reaction achieves practical interest in the context of organic synthesis, e.g. Cu-promoted Ullmann coupling.

The reaction is mainly conducted as stoichiometrically. Some metalloenzymes Vitamin B12 and coenzyme F430 are capable of dehalogenations catalytically. Of great interest are hydrodehalogenations, especially for chlorinated precursors: :

References

References

1. (2004). "Anaerobic Microbial Dehalogenation". Annual Review of Microbiology.
2. J. C. Sauer. (1956). "1,1-Dichloro-2,2-Difluoroethylene". Organic Syntheses.
3. (1991). "Comprehensive Organic Synthesis – Selectivity, Strategy and Efficiency in Modern Organic Chemistry". Elsevier.
4. Klein, U. Experiments, models, paper tools: Cultures of organic chemistry in the nineteenth century, Stanford university press: California, 2003, 191-193
5. (2009). "Palladium catalyzed-dehalogenation of aryl chlorides and bromides using phosphite ligands". J. Organomet. Chem..
6. Ware, G.; Gunther, F. Reviews of environmental contamination and toxicology, Springer-Verlag: New York, 1998, 155, 1-67.
7. Ramón, D.; Yus, M. Masked lithium bishomoenolates: Useful intermediates in organic synthesis, J. Org. Chem. 1991, 56, 3825-3831.
8. Guijarro, A.; Ramón, D.; Yus, M. Naphthalene-catalysed lithiation of functionalized chloroarenes: regioselective preparation and reactivity of functionalized lithioarenes, Tetrahedron, 1993, 49, 469-482.
9. Yus, M.; Ramón, D. Arene-catalysed lithiation reactions with lithium at low temperature, Chem. Comm. 1991, 398-400.
10. Hawari, J. Regioselectivity of dechlorination: reductive dechlorination of polychlorobiphenyls by polymethylhydrosiloxane-alkali metal. J. Organomet. Chem. 1992, 437, 91-98.
11. Mackenzie, K.; Kopinke, F.-D. Debromination of duroplastic flame-retarded polymers. Chemosphere, 1996, 33, 2423-2428.
12. Tarakanova, A.; Anisimov, A.; Egorov, A. Low-temperature dehalogenation of benzyl halides with atomic magnesium in the 3P state. Russian Chemical Bulletin, 1999, 48, 147-151.
13. Aitken, R.; Hodgson, P; Oyewale, A.’ Morrison, J. Dehalogenation of organic halides by flash vacuum pyrolysis over magnesium: a versatile synthetic method. Chem. Commun. 1997, 1163-1164.
14. Grushin, V.; Alper, H. Activation of otherwise unreactive C-Cl bonds. Top. Organomet. Chem. 1999, 3, 193-226.
15. (2015). "Vitamin B12catalysed reactions". Chemical Society Reviews.
16. (2002). "Metal-Mediated Reductive Hydrodehalogenation of Organic Halides". Chemical Reviews.