Brominating reagents

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... 

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. 

However, the former reaction requires very low temperatures and very dilute conditions and the latter uses an unstable and explosive brominating reagent. 

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. 

Typical flow sheet of bromination with polymeric bromine reagent ... 

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. 

N-Bromosuccinimide purchased from Arapahoe Chemical Company was used without purification. If the purity of the N-bromosuccinimide is in doubt, it should be titrated before use by the standard iodide-thiosulfate method and purified, if necessary, by recrystallization from 10 times its weight of water. Solutions of N-bromosuccinimide in dimethyl sulfoxide cannot be stored, since the solvent is oxidized by the brominating reagent. 

In general, cyclization can be expected in compounds having the potential for formation of five- or six-membered rings. In addition to the more typical bromination reagents, such as those listed in Table 4.2, the combination of trimethylsilyl bromide, a tertiary amine, and DMSO can effect bromolactonization. 

Analogous cyclization reactions are induced by brominating reagents but they tend to be less selective than the iodocyclizations.83 The bromonium ion intermediates are much more reactive and less selective. 

Add 4 pi of the N-bromosuccinimide solution to the DNA solution (makes an 8mM final concentration of brominating reagent). Mix well. 

DesilylbrominationThis reaction was first used by Fleming et al. (8, 196 11, 75) in connection with protection of enones, but it is also useful for synthesis of chiral 5-alkylcyclohexenones. Thus reaction of (R)-(-)-l with lithium dialkyl-cuprates gives the trans-adduct 2 as the only product. Of several bromination reagents, only CuBr2 in DMF is useful for conversion of 2 to optically active 3. 

Benzyl salicylate, 22 16 25 184 physical properties of, 22 13t Benzyl sulfone pyrolysis, 21 141 Benzyltrimethylammonium tribromide, bromination reagent, 4 344 Bepadin, molecular formula and structure, 5 97t, 118t Bepridil, 5 104... 

Al-Bromosuccinimide (NBS), 13 116 brominated butyl rubber from, 4 434 bromination reagent, 4 344 Bromothymol blue, 4 362t Bromotrichloroethane, 6 269 Bromotrifluoromethane, 4 348, 348t Bromous acid, 4 333... 

Conversion of secondary and tertiary alcohols into the corresponding chlorides has been shown to proceed efficiently when the powdered solid alcohol is exposed to HCl in a desiccator [28]. Solid-state bromination of anilines and phenols with gaseous bromine and solid bromination reagents has been shown to proceed with higher yields than in solution [29]. 

Addition of X2 (Bt2 and CI2) to alkenes gives vicinaZ-dihalides. This reaction is used as a test for unsaturation (n bonds), because the red colour of the bromine reagent disappears when an alkene or aUcyne is present. For example, when ethylene is treated with Br2 in CCI4 in the dark at room temperatures, the red colour of Br2 disappears rapidly, forming 1,2-dibro-moethane, a colourless product. 

The methods for the preparation of bromotrimethylsilane include the treatment of hexamethyldisiloxane with bromine reagents such as phosphorus tribromide,1,2 2,2,2-tribromo-l, 3,2 /5-benzodioxaphosphole (catechyl phosphorus tribromide),2 and aluminium tribromide,3 the yields being in the range from 73 to 87%. A further method4 shows that bromotrimethylsilane can be obtained from aminosilanes and hydrobromic acid, but the yield (55%) is lower than in the former methods. Other methods involve the use of expensive reagents, such as hexamethyldisilane, l,4-bis(trimethylsilyl)-2, 5-cyclohexadiene, and 1,4-dihydro-l, 4-bis(trimethylsilyl)naphthalene.5 Finally, some alternative procedures have been developed for its in situ preparation.5... 

Caution. Owing to the lachrymatory properties of bromine reagent and halosilane product, this reaction should be carried out in a well-ventilated hood. 

Other highly regioselective brominating reagents are chlorotrimethylsilane/lith-ium bromide and hexamethyldisilane/pyridinium perbromide.88 An outline mechanism of the former reaction is formulated below and a preparative example is given in Expt 5.56. 

Side-chain halogenation is illustrated also by the bromination of p-nitrotoluene (Expt 6.28). The radical mechanism with molecular bromine is similar to that of chlorination above N-bromosuccinimide has also found use as a side-chain brominating reagent and its application is described in Expts 6.119 and 6.152. 

Lemieux and Thiem developed the iodine glycosylation.16 You need an I+-reagent e. g. Y-iodosuccinimide (l-iodo-2,5-pyrrolidinedione, NIS) (47) or I(coll)2 (iodonium di-.sym-collidine perchlorate) (48). The mechanism of this step with the bromine reagent of 48 was evaluated by Brown. 1... 

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