Halogenation intermediates

Halogenated intermediates, dibromoneopentyl glycol [3296-90-0] (DBNPG), and alkoxylated derivatives of tetrabromobisphenol A are used extensively in flame-retardant apphcations. Similar properties can be derived from halogenated dibasic acids, chlorendic anhydride [115-27-5] (CAN), and tetrabromophthahc anhydride [632-79-1] (TBPA). Processes can be used to produce brominated products by the in situ bromination of polymers derived from tetrahydrophthahc anhydride. 

Flame-Retardant Resins. Flame-retardant resins are formulated to conform to fire safety specifications developed for constmction as well as marine and electrical appHcations. Resins produced from halogenated intermediates (Table 5) are usually processed at lower temperatures (180°C) to prevent excessive discoloration. Dibromoneopentyl glycol [3296-90-0] (DBNPG) also requires glass-lined equipment due to its corrosive nature. Tetrabromophthahc anhydride (TBPA) and chlorendic anhydride (8) are formulated with ethylene glycols to maximize fiame-retardant properties reaction cycle times are about 12 h. Resins are also produced commercially by the in situ bromination of polyester resins derived from tetrahydrophthahc anhydride... 

Halogenated intermediates based on chlorendic anhydride and alkoxylated, brominated bisphenol are quite stable and are used extensively in flame-retarded high temperature compositions, but brominated aUcychcs, such as dibromotetrahydrophthahc resin, are rapidly dehydrohalogenated at lower temperatures. 

In the biosynthetic schemes proposed for some halogenated natural products, positive halogen intermediates are attacked by electrons from the n bond of an alkene or alkyne in an addition reaction. 

In conclusion, reductive dehalogenation with Pd catalysts offers a number of advantages it can treat a wide variety of compounds, including mixtures it generally results in simple alkanes, with few halogenated intermediates, if any and it is extremely rapid, which allows small, in-well reactors. As more studies are conducted, the applicability to a broad range of conditions will be tested and will provide opportunities to better understand the process. This will facilitate optimization of catalyst parameters and column operation for the most effective remediation under a variety of field conditions. 

Libby RD, Thomas JA, Kaiser LW et al (1982) Chloroperoxidase halogenation reactions. Chemical versus enzymic halogenating intermediates. J Biol Chem 257 5030-5037... 

Phenols can be converted to quinones using halogens generated in situ from hydrogen peroxide and hydrogen halide, sometimes with a catalyst. These reactions involve halogenated intermediates (see Section 10). 

Heme haloperoxidases can also use peroxide and halide ions to halogenate an activated (benzylic/allylic) carbon. The halide is first oxidized to an active halogenating intermediate (Fig. 10.4, pathway (2)). The substrate is halogenated in the next step. The overall reaction is... 

Various procedures have been developed for the production of oximes from nitroparaffins. Direct reduction with zinc dust and acetic acid has been proposed, but the yields are poor because of the simultaneous formation of amines. A synthesis for cyclohexanone oxime has been demonstrated which involves the formation and selective hydrogenation of 1-chloro-l-nitrocyclohexane. The halogenated intermediate is prepared in quantitative yield by chlorination of the sodium salt of acz -nitrocyclo-hexane, and subsequent hydrogenation is performed in an 80% yield over palladium-on-charcoal, ... 

This suggestion was attacked by Abel who preferred positive halogen intermediate species (Cr and CIO2 ) in general . ... 

The reaction of oxazolines (331) derived from L-serine with diethylphosphite leads to a mixture of racemic a- and p-phosphono alanines (332) and (333). This new reaction proceeds without the use of any halogenated intermediate, and offers a simple route for various phosphonoamino acids bearing suitable protecting groups (Scheme 89). " ... 

The dihalogenated starting material (A) loses HX to the base (B) to form the mono-halogenated intermediate (C), which subsequently loses HX to the base to produce the desired final product (D). Over-addition of base causes polymerization and loss of the batch. 

This reaction pattern involving nucleophilic capture of an initial halogenation intermediate is also observed in indoles and forms the basis for a convenient method of converting indoles to bromooxindoles. Reaction of indole with NBS in aqueous /-butanol gives 3,3-dibromooxindole (Scheme 113) <64JOC1206, 89JCS(P1)2009>. 

Organoselenide-catalyzed oxidative chlorination reactions included halolactonization, a-chlorination of ketones, and aUyhc chlorination. The ability of selenium to undergo reversible 2e oxidation-reduction chemistry facilitates halogenation through selenium-bound halogen intermediates. ... 

Lithium or sodium dissolving in tetrahydrofuran-t-butanol can accomplish efficient reductive dechlorination of chlorine compounds. This synthetic method has been important in obtaining hydrocarbons via halogenated intermediates. Important examples are conversion of dichlorocyclopropanes (prepared by dichlorocarbene... 

The biosynthesis of certain halogenated marine natural products is intriguing. Some of their halogens appear to have been introduced as electrophiles rather than as Lewis bases or nucleophiles, which is their character when they are solutes in seawater. But how do marine organisms transform nucleophilic halide anions into electrophilic species for incorporation into their metabolites It happens that many marine organisms have enzymes called haloperoxidases that convert nucleophilic iodide, bromide, or chloride anions into electrophilic species that react like F, Br, or CF. In the biosynthetic schemes proposed for some halogenated natural products, positive halogen intermediates are attacked by electrons from the 77 bond of an alkene or alkyne in an addition reaction. 

The major brominated flame retardant used with epoxy resins is TBBA. This is a reactive halogenated intermediate incorporated during resin preparation. However, other bromine containing species plus phosphorus containing monomers or curing agents may be reacted with the main epoxy resin eomponents during polymerisation. The latter may be alkyl or aryl phosphates. The best appear to be phenyl phosphate derivatives. 

Haloperoxidases have been shown to transform alkenes by a formal addition of hypohalous acid to produce halohydrins. The reaction mechanism of enzymatic halogenatitHi has been debated for some time and it is now accepted that it proceeds via a halonium intermediate [1770, 1771], similar to the chemical formation of halohydrins (Scheme 2.228). The former species is derived from hypohalous acid or molecular halogen, which is in turn produced by the enzyme via oxidation of halide [1772]. In support of this, a HOCl-adduct of Fe -protoporphyrin IX was identified as a direct enzyme-halogen intermediate involved in chloroperoxidase-catalyzed halogenaticHi [1773]. 

Halogenated intermediates, dibromoneopentyl glycol [3296-90-0] (DBNPG), and alkoxylated derivatives of tetrabromobisphenol A are used extensively in flame-retardant applications. Similar properties can be derived from halogenated... 

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