Halogenation and Dehalogenation Reactions

Halogen-containing compounds are not only produced by man, but also by Nature [1740-1742], A brominated indole derivative - Tyrian purple dye - was isolated from the mollusc Murex brandaris by the Phoenicians. Since that time, more than 1,000 halogenated natural products of various structural types have been isolated from sources such as bacteria, fungi, algae, higher plants, marine molluscs, insects, and mammals [1743], Whereas fluorinated and iodinated species are rather rare, chloro and bromo derivatives are found more often. The former are predominantly 

When REX2 fragments are used, the E-X bonds are again cleaved, but generally the R group is retained in the products, as seen in Eqs. (187) and (188).194 These reactions, to date, have most often yielded a mixture of saturated di- and trinuclear products. 

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STEVEN E. ROKITA, PhD, is Professor in the Department of Chemistry and Biochemistry at the University of Maryland. His research interests lie in sequence and conformation specihc reachons of nucleic acids, enzyme-mediated activation of substrates and coenzymes, halogenation and dehalogenation reactions in biology, and aromatic substitution and quinone methide generation in bioorganic chemistry. 

Halogenation and Dehalogenation Reactions. Usually, reactions of this type take place readily without utilizing catalysts. However, when selectivity of the desired product is low or it is necessary to run the reaetion at a lower temperature, the use of a catalyst is desirable. Supported copper and silver halides can be used for the halogenation of hydrocarbons. Hydrochlorination reactions can be carried out with mercury copper or zinc halides. 

Although halogenation and dehalogenation reactions can be catalyzed by enzymes, it is doubtful whether these reactions will be used widely, mainly because the corresponding conventional chemical methods are highly competitive. 

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

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... 

When halogenated thiophenes are irradiated in benzene as a solvent, both arylation and dehalogenation reactions are observed (Scheme 2) <1997H(45)1775>. Irradiation of 5-iodo-2-cyanothiophene 44 gave the dehalogenated... 

If 5-benzyl-3,7-diaryl-4,6-dihydro-l,2,5-triazepines (2Q2) are treated with bromine or iV-bromosuccinimide, a process involving consecutive halogenation and dehalogenation has been proposed. The reaction sequence (Scheme 116) has been postulated to pass through bicyclic intermediates (80AHC(27)241>. 

Halogenation and dehalogenation [36], Friedel-Crafts-type alkylation [37], 0-and iV-dealkylation [38], carboxylation [39], and decarboxylation [40], isomerization [41], acyloin [42], and aldol reactions [43]. Even Michael... 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

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... 

The 1,2-dehalogenation reaction is also successful with 1,1,2-trihalo- (56) or 1,1,2,2-tetrahalocyclopropanes (57) and, in the former case, provides a valuable entry to 1-halcyclopropenes which often undergo ready lithium-halogen exchange to produce synthetically valuable 1-lithiocyclopropenes. The elimination occurs extremely rapidly when X = Br even at - 95 °C. When X = Cl and Y = Br, the reaction occurs in a few minutes at a temperature of - 40 °C, whereas with X = Y = Cl, the reaction occurs at a reasonable rate only at ambient temperature111,112,84,87. 

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