Debromination, catalytic

Radiolabeled folate provides a powerful tool for folate bioavaHabiUty studies in animals and for diagnostic procedures in humans. Deuteration at the 3- and 5-positions of the central benzene ring of foHc acid (31) was accompHshed by catalytic debromination (47,48) or acid-cataly2ed exchange reaction (49). Alternatively, deuterium-labeled fohc acid (32) was prepared by condensing pteroic acid with commercially available labeled glutamic acid (50). 

Catalytic hydrogenation of 4-bromo-l //-3-benzazepin-2-amine (13) in the absence of base (see Section 3.2.1.5.3.) is accompanied by debromination to give the 4,5-dihydro-l//-3-benzazepin-2-amine (14) which is isolated as the hydrobromide.15 41... 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

Several other oxidation reactions of selenoxides and telluroxides are summarized in Fig. 23. Ley, Barton, and co-workers discovered that di-4-methoxyphenyltellur-oxide (54) could be used catalytically as an oxidant in the presence of 1,2-dibromotetrachloroethane. After reduction of the telluroxide to the telluride, the di-4-methoxyphenyltelluride (24) debrominated the 1,2-dibromotetrachloroethane to give the tellurium(IV) dibromide, which was hydrolyzed in situ to give the telluroxide 54. This process was used to oxidize phosphines to phosphine oxides and... 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

Method B. This is an improved modification of method A, effecting the debromination in a catalytic cycle by means of potassium bisulphite and catalytic amounts of diaryl tel-luride. 

Methods and Since elemental tellurium is separated during the reaction, the debromination can be performed using only catalytic amounts of tellurium, the reagent being continuously regenerated at the expense of excess Rongalite (method E) and (Et0)2P(0)Na (method E). 

Considering that diaryl tellurides debrominate vic-dibromides, forming diarylteUurium dibromides (see Section 4.1.12.1), which in tnm can be easily hydrolysed to the corresponding tellnroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Dibromobenzo[6]thiophene and its 6-carboxylic acid undergo bisdehalogenation on treatment with sodium hydrazide and hydrazine 515 and on catalytic reduction in alkaline solution,77 respectively. Bromonitrobenzo[6]thiophenes readily undergo debromination on treatment with copper and quinoline,412 copper bronze and benzoic acid,84 or tin and hydrochloric acid.516... 

Fully as expected from the inspection of molecular models, the generation of the oxetane ring has the effect of heightening the structural curvature in compounds such as 85 and 86. Reagent approach from the exterior should materialize under kinetically controlled conditions. These features are reflected in the catalytic hydrogenation of 86, which results in saturation of the double bond from the (3 surface along with reductive debromination to furnish 87. The lesson learned here is that the oxetane ring should be released or not formed at all if an opportunity to approach C9 from the a direction has any chance to take place. 

Debromination. wc-Dibromides are converted into alkenes by reaction with zinc dust and a catalytic amount of TiCl4 in THF at 0°. Isolated yields are generally in the range 80-90%. 

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

Debrominatton of ic-dibromides. v/r-Dibromides are reduced to alkenes by treatment with Na,S or NaSH in benzene/HiO in the presence of catalytic amounts of meth-yltrioctylammonium chloride (Adogen 464). The debromination occurs by stereoselective / t/-climination. The phase-transfer catalyst is not required with water-soluble substrates. Example ... 

Chiral methyl chiral lactic acid (5). This labeled molecule, useful for study of stereospecificity of enzymic reactions, has been prepared in a way that allows for synthesis of all 12 possible isomers. One key step is the stereospecific debromination of 1, accomplished by conversion to the vinyl-palladium cr-complex 2 followed by cleavage with CF3COOT to give the tritium-labeled 3. The next step is the catalytic deuteration of 3, accomplished with a rhodium(I) catalyst complexed with the ligands norbornadiene and (R)-l,2-bis(diphenylphosphino)propane. This reaction gives 4 with an optical purity of 81%. The product is hydolyzed to 5, which is obtained optically pure by cr3rstallization. 

T. Bhaskar, J. Kaneko, A. Muto, Y. Sakata, E. Jakab, T. Matsui, and M. A. Uddin, Effect of poly(ethylene terephthalate) on the pyrolysis of brominated flame retardant containing high impact polystyrene and catalytic debromination of the liquid products, J. Anal. Appl. Pyro., 71, 765-777 (2004). 

Catalytic hydrogenation, especially the use of active Raney nickel with or without gaseous hydrogen, has been used as a method for removing halide substituents an example is the debromination of ibjS-bromo-iya-hydroxypregnan-ao-ones [g8]. No detailed mechanistic description of such reactions has been given. It is probable, in common with catalytic reduction of C=C and C=0 double bonds, that the reaction involves bond dissociation and combination with hydrogen atoms attached to the catalyst surface, represented schematically by ... 

Dehalogenation, dehalohydrination. See also yS-Chloroethyl ethyl ether. The debromination of cholesterol dibromide with zinc dust requires only a catalytic amount of acetic acid and once the reaction has started it proceeds with vigor, like... 

Dehalogenation, dehydrohalogenation. An early procedure for debromination of 5a,6/8-dibromocholestane-3-one without isomerization of the nonconjugated ketone initially formed involved reaction with zinc dust in hot ethanol, but the reaction proceeds better in ether containing a catalytic amount of acetic acid (see above ). 

A simple and efficient method for the debromination of vzc-dibromidcs to ( )-alkenes utilized the Sm-TMSC1-H20 catalytic system (Scheme 8.19). For example, franj-stilbene was produced from 1,2-dibromo-l,2-diphenylethane within 5 h in good yield at room temperature. The benzylic vz odibromides similarly gave the corresponding ( )-alkenes in a high yield. In the case of 1,2-dibromocyclohexane, a longer reaction time was needed to obtain cyclohexene. This is perhaps because the radical or anion intermediate of an aliphatic... 

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