Halogenation dehalogenation

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

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

Scheme 4. Isomeric specific Reaction Pattern of the Halogenation-dehalogenation of 1,2,3,4-Tetrabromodibenzodioxin on the Surface of Fly-Ash. 

It was suggested that halogenation-dehalogenation with diaryltellurium derivatives is an equilibrium process since, at identical concentrations, debromination of erythro-l, 2-dibromo-l,2-diphenylethane with diaryItellurides andbromination of trani-stilbene with dibromodiarylteUurides gives identical product mixtures. 

Zier B, Lenoir D, Lahaniatis ES, et al. 1991. Surface catalyzed halogenation-dehalogenation reactions of aromatic bromine compounds adsorbed on fly ash. Chemosphere. 22(12) 1121-1129. 

While there are no examples of the formation of fluorinated unsaturated aldehydes by de-halogenation, dehalogenation is the preferred method for the synthesis of reactive fluorinated ketenes. Examples of dehalogenations giving unsaturated fluorinated ketones and fluorinated ketenes are listed in Table 9. 

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. 

Mercury can be used to dehalogenate acyl halides to ketenes when a halogen IS present next to the COCl group [67] (equation 35)... 

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." ... 

In 1980 Sonogashira reported a convenient synthesis of ethynylarenes - the Pd-catalyzed cross-coupfing of bromo- or iodoarenes with trimethylsilylacetylene followed by protiodesilylation in basic solution [15]. Prior to this discovery, formation of terminal acetylenes required manipulation of a preformed, two-carbon side chain via methods that include halogenation/dehydrohalogenation of vinyl- and acetylarenes, dehalogenation of /1,/1-dihaloalkenes, and the Vils-meier procedure [ 14]. With the ready availability of trialkylsilylacetylenes, the two-step Sonogashira sequence has become the cornerstone reaction for the construction of virtually all ethynylated arenes used in PAM and PDM synthesis (vide infra). 

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