Bromination reagents for

COBr2 has recently been shown to be a useful general brominating reagent for the preparation of d- and f-block bromides and oxide bromides.Thus, when V2O5 is heated with an excess of COBr2 in a sealed Car-ius tube at 125°C for 10 days, a quantitative yield of VOBr2 is obtained by a reaction that is driven thermodynamically by... 

The usual aromatic bromination are performed by free bromine in the presence of a catalyst, most often iron. However, liquid bromine is not easy to handle because of its volatile and toxic character. On the other hand, alumina-supported copper(II) bromide can be treated easily and safely as a solid brominating reagent for aromatic compounds. The advantages of this procedure using the solid reagent are simple workups, mild conditions, and higher selectivities. Products can be isolated in good yield by simple filtration and solvent evaporation, and no extraction steps are required. 

Recently, several selective bromination reagents for reactive aromatic amines have been developed, for example, 2,4,4,6-tetrabromocyclohexa-2,5-dienone (35), iV-bromosuccinimide-dimethylformamide (36), and hexabro-mocyclopentadiene (37). Although molecular bromine is too reactive to perform selective bromination (mono- versus polybromination), the combined used of bromine and zeolites X and Y has been reported to be applicable to the selective bromination of halobenzenes and alkylbenzenes (38). This zeolite method, however, was not successful in the selective bromination of highly active aromatic compounds. Bromine preadsorbed on zeolite 5A (Ca type) was found to monobrominate aniline in carbon tetrachloride with excellent regioselectivity (91-93% para selectivity) in the presence of organic base, pyridine or 2,6-lutidine (Table XII) (39). The preadsorption of bromine on zeolite 5A is necessary for selective bromination, because the inverse procedure of adding bromine to aniline that had been adsorbed on zeolite beforehand caused a nonselective reaction. 

Goswami P, Baruah A, Das B. Design and application of 2,2-dibromodimedone as organic brominating reagent for asymmetric bromination of 1,3-dicarbonyl compounds and ketones catalysed by chiral amino acids. Adv. Synth. Catal. 2009 351(10) 1483-1487. 

A -Bromosuccinimide (prepared by the action of bromine on succinimide at o in the presence of sodium hydroxide) is a valuable specific reagent for brominating olefines in the a-methylene position to the double bond without simultaneously adding bromine to this bond. For example, if A -bromosuccini-mide is represented by (C4H40t)NBr —... 

Mono-substitution products of primary amines cannot easUy be prepared by direct action of the appropriate reagent for example, bromination of aniline yields largely the 2 4 6-tribomo derivative and nitration results in much oxidation. If, however, the amino group is protected as in acetanilide, smooth substitution occurs. Thus with bromine, />-bromoacetanilide is the main product the small quantity of the ortlio isomeride simultaneously formed can be easily eliminated by crystallisation. Hydrolysis of p-bromoacetanilide gives/ -bromoaniline ... 

N Bromosuccimmide will be seen again as a reagent for selective bromination in Sec tion 11 12... 

Specific reagents for titrations with bromine or bromate... 

Pyridine hydrobromide perbromide (pyridinium bromide perbromide) [39416-48-3] M 319.9, m 130 (dec), 132-134 (dec). It is a very good brominating agent - liberating one mol. of Br2. Purified by recrystn from glacial acetic acid (33g from lOOmL of AcOH). [Fieser and Fieser Reagents for Organic Chemistry Vol 1 967 1967.]... 

Detection and result The chromatogram was freed from mobile phase (heated to 110°C for 30 min) and then exposed to bromine vapor for 1 h in a chamber, after blowing off excess bromine from the layer it was immersed for 1 s in the reagent solution. On drying in air dibutyltin dilaurate hRf 25 — 30), dibutyltin dichloride (kR( 25 — 30), dioctyltin oxide (hR( 40), tributyltin oxide (hRf 80), tributyltin chloride (hRf 80) and tetrabutyltin (hRf 85-90) produced persistent blue zones on a yellow ochre background (Fig. 1). 

Halogenation (Section 11.12) Free-radical halo-genation of alkylbenzenes is highly selective for substitution at the benzylic position. In the example shown, elemental bromine was used. Alternatively, A/-bromosuccinimide is a convenient reagent for benzylic bromination. 

Not only were the reaction rates for bromination by bromine and by hypobromous acid very similar, but the corresponding activation energies (determined over a 20 °C range) were between 11.8 and 12.6 (for Br2) and 12.5 and 12.7 (for HOBr). Thus all this kinetic data is consistent with the rapid formation of an intermediate which is identical for both brominating reagents, and from which the slow loss of a proton subsequently occurs. 

As described above, it was shown that A,iV-dibromomethylamine was effective for orr/io-dibromination of phenol (ref. 7). We also carried out a bromination using NBB as A-bromoamine analogue. One molar amount of NBB did not give 2-bromophenol selectively, but gave a mixture of o/t/io-monobromophenol and 2,6-dibromophenol, and a considerable amount of phenol was recovered. On the other hand, 2,6-dibromophenol was obtained in an 81.7 % yield when two molar amounts of NBB were used. These results suggested that 7V-bromoamines were the best reagents for orf/io-bromination of phenol. However N-bromoamines were very unstable and decomposed explosively in less than a day at room temperature. 

The selection of appropriate polymeric bromine reagent according to the polymeric structure (step of crosslinking, porosity, step of dilution with styrene, granulation, type of heterocyclic ring incorporated, numbers of N) is very important. From production point of view the use of polymeric reagents reduces the costs for solvents and working hours. 

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