Reactions bromine

The reactions of trialkylboranes with bromine and iodine are gready accelerated by bases. The use of sodium methoxide in methanol gives good yields of the corresponding alkyl bromides or iodides. AH three primary alkyl groups are utilized in the bromination reaction and only two in the iodination reaction. Secondary groups are less reactive and the yields are lower. Both Br and I reactions proceed with predominant inversion of configuration thus, for example, tri( X(9-2-norbomyl)borane yields >75% endo product (237,238). In contrast, the dark reaction of bromine with tri( X(9-2-norbomyl)borane yields cleanly X(9-2-norbomyl bromide (239). Consequentiy, the dark bromination complements the base-induced bromination. 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

The quantum efflciency of die inadiatioii of a hydrogen-chlorine mixture has the apparent value between 10" and 10 , but the coiTesponding value for the parallel bromine reaction is only about 10. Of course die real quantum efflciency can only reach the value of unity as an upper limit, and the large apparent value for the chlorine reaction indicates that once two chlorine atoms... 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

The reaction between hydrogen and bromine is the first step in the initiation reaction in which the chain carriers are formed. The thermal hydrogen bromine reaction begins with the initiation reaction ... 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

Current views (100) on the mechanism of bromination by NBS invoke the formation of molecular bromine and bromine atoms in low concentration, which subsequently act as the brominating agent. The bromination reaction was studied in detail in this laboratory under a variety of conditions using 93 (R = Ms) as a model. The product 94 (R = Ms) was indeed formed (42%) when NBS was substituted by 1.1 equivalents of bromine which was added at a slow rate to the reaction mixture. The yield was 68% when benzoyl peroxide was used as a catalyst. Using NBS alone or in the presence of reagents such as barium carbonate, pyridine, or s-trinitrobenzene, the yield was 60-70%. 

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

Draw resonance structures for the benzyl radical, C6H5CH2-, the intermediate produced in the NBS bromination reaction of toluene (Problem 10.27). 

Broadband-decoupled NMR, 451 Bromine, reaction with aldehydes, 846-848... 

Bromine, reaction with y-butyrolactone in presence of red phosphorus, 46, 22... 

Benzoxepin undergoes addition of bromine and chlorine to give l,2-dibromo-l,2-dihydro-3-benzoxepin (2 a) and 1,2-dichloro-l, 2-dihydro-3-benzoxepin (2b), respectively.93 The bromination reaction of dimethyl 3-benzoxepin-2,4-dicarboxylate takes the same course to give 2c.91... 

Bromine, reaction with furan in methanol to yield 2,5-dimethoxy-2,5-dihydrofuran, 40, 29... 

The temperature is controlled throughout the reaction by intermittently adding additional pieces of dry ice to the dry ice-acetone bath. Strict temperature control throughout the bromination reaction is important to obtain high regioselectivity and purity of the product as the side products cannot be removed with ease. 

In the second step, the hydrogen ion is pulled out of the ring by the HS04 ion acting as a Bronsted base. As in the bromination reaction, the restoration of the delocalization of the 7t-electrons facilitates the removal of the hydrogen ion. 

In our previous studies on chlorination of toluene we had found that solvent had an important effect on the selectivity. In particular, the use of diethyl ether as a cosolvent was advantageous for the production of a high proportion of the para-isomer (ref. 9). An experiment in which the amount of ether in a tetrachloromethane/diethyl ether solvent mixture was varied under otherwise identical reaction conditions (Ih reaction at 18°C with 1.04 molar equivalent of tert-butyl hypobromite) demonstrated that diethyl ether also had a marked influence on the selectivity of the bromination reaction (Fig. 6). There was also an effect on the yield of the reaction as performed under these standard conditions. As the... 

In connection with our continuing interest in the temperature bromination reactions (ref. 2) we have been interested in the bromination reactions of benzonorbornadiene 1 and benzobarrelene 2. 

In the course of studying the bromination reactions of the bicyclic systems we noticed that the reaction temperature has a dramatic influence on the product distribution. Increasing of the temperature gives non-rearranged reaction products (refs. 1,2). For this reason, we submitted 1 to high temperature bromination. To a solution of 1 in decalin at 150 C was added a hot solution of bromine in decalin in one portion. The colour of bromine disappeared immediately. After silica gel chromatography followed by fractional crystallization we isolated four products 2-6 in yields 8, 35, 37, and 9 % respectively. The structure of these compounds has been elucidated on the basis of spectral data by iH NMR and NMR experiments and by comparison with those reported in the literature. Symmetrical endo-c/5-isomer 6 has been observed for the first time. Studies concerning the mechanism of syn-addition show that the syn-adduct can arise either from direct... 

The effect of added [Br ] on the bromination reaction is such as to retard the rate. This phenomenon is attributable to a competing equilibrium,Br" + Br2 Br3 that decreases the available free [Br2]. In HOAc, the accepted value for the tribromide equilibrium constant is Kgq = 92 M-1 (ref. 11). Both Br2 and Br3 are brominating agents so that the rate of disappearance of total bromine, or [Br2]j, is... 

In conclusion, only Scheme 3 matches the kinetic and thermodynamic data, showing that the CTC s are essential intermediates of the bromination reaction. 

The second step of the bromination reaction in aprotic chlorinated solvents consists of the ionization of the CTC s, and leads to bromonium or bromocarbonium tribromide ion pairs. A direct evidence for the formation of bromonium-tribromide pairs is the isolation and X-ray structural characterization of the adamantylideneadamantane-bromonium tribromide species, obtained by Brown (ref. 13). 

Most of the olefins shown so far, for which reversibility of the bromonium ion formation had been demonstrated, are particular olefins, in which either steric bulk impedes the product forming step, or ring strain in the dibromide product retards this step. In order to check the general occurrence of the reversibility during the bromination reaction, a further approach, based on the cis-trans isomerization of stilbene derivatives during the bromination of the cis isomers, was devised. 

Recycle traditionally involves caustic scrubbing the gaseous HBr from bromination reactions (eqn. 3). The resulting sodium bromide solution is then treated with chlorine gas to generate bromine (eqn. 4). 

For the organic contaminants, the required bromine product quality wilt also be site specific. If the catalytic oxidation unit is dedicated to a single bromination process, phase separation and drying may be the only purification required. Contaminants in the recovered bromine which are either the starting materials or products of the original bromination reaction should not present a problem if present in bromine recycled to the bromination reactor. In this case, the catalytic reactor would be operated to minimize the formation of undesirable brominated byproducts. For example, if phenol is present in the waste HBr from a tribromo-phenol manufacturing process, minor tribromophenol contamination of the bromine recycled to the reactor should not be a problem. Similarly, fluorobenzene in bromine recycled to a fluorobenzene bromination process should not present a problem. 

The bromoketone 17 was prepared via bromination of 15 with PTAB in DME. Hydrogen bromide, formed during the reaction, reacted with DME to generate methyl bromide and 2-methoxyethanol, both of which could be easily removed from the reaction medium under vacuum. This method was more convenient than the bromination reaction in TH F because the resulting 4-bromobutanol by-product formed from THF was not volatile. The bromide 17 was used directly in the next reaction, partly because 17 is rather reactive with limited stability. 

Scheme 7.9 Optimized bromination reaction in the final route.

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