Bromine bond

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

FIGURE 6 13 Mechanism of bromohydrin formation from cyclopentene A bridged bromonium ion is formed and is attacked by a water molecule from the side opposite the carbon-bromine bond The bromine and the hydroxyl group are trans to each other in the product... 

The carbon-bromine bond is longer than the carbon-chlorine bond therefore although the charge e in the dipole moment expression p, = e d k smaller for the bromine than for the chlo nne compound the distance d is greater... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

Allylic bromides can also serve as progenitors for nucleophilic organochromium reagents. An elegant example is found in Still and Mobilio s synthesis of the 14-membered cembranoid asperdiol (4) (see Scheme 2).7 In the key step, reduction of the carbon-bromine bond in 2 with chromium(n) chloride in THF is attended by intramolecular carbonyl addition, affording a 4 1 mixture of cembranoid diastereoisomers in favor of the desired isomer 3. Reductive cleav-... 

The kinetics of bromination with the complex formed between bromine and dioxan have been examined using benzene (which is unattacked) as solvent311, and it is probably appropriate to regard this as a catalysed bromination in view of the effect of dioxan upon the polarity of the bromine-bromine bond. With anisole, phenetole, and isopropoxybenzene, third-order kinetics are obtained, viz. 

FIGURE 18.11 As a bromine molecule approaches a double bond in an alkene, the atom closer to the ethene molecule acquires a partial positive charge (the blue region). The computation that produced this image was carried out for the point at which the bromine molecule is so close to the double bond that a carbon-bromine bond is starting to form. 

The compounds 12,14 and 15 can be prepared from parent lysergic acid amides by bromination with NBS or PHT, but not the compounds of the type 12. The latter structure is very sensitive and under these circumstances only degradation products were obtained. Besides it was necessary to use much milder reagent, e.g. bromine bonded onto polymeric matrix with polyvinylpyrrolidone or polyvinylpyridine structure (29) ... 

Chiang, Y. Kresge, J. Zhu, Y. Flash photolytic generation and study of /j-quinone methide in aqueous solution. An estimate of rate and equilibrium constants for heterolysis of the carbon-bromine bond in p-hydroxybenzyl bromide. J. Am. Chem. Soc. 2002,124, 6349-6356. 

In the formation of bromohydrin, bromine bonds at the least substituted carbon (from nucleophilic attack by water), and the hydroxyl group bonds at the more substituted carbon (i.e., the carbon that accommodated more of the positive charge in the bromonium ion). 

Figure 8.B The carbon-bromine bond length (shown in angstroms) at the central carbon increases as less electron density from the bromine is needed to stabilize the positive charge. A lesser electron density contribution from bromine is needed because additional alkyl groups help stabilize the charge...

The principal intermolecular donor-acceptor interactions of this weakly bound complex are found to be of 7tcc-OBrBr form (3 x 0.20 kcalmol-1), as illustrated in Fig. 5.41(b). A prominent feature of Br2 (and other heavy halogens) is the nearly pure-p character of the bromine bonding hybrid, resulting in a conspicuous backside lobe on the OBrBr antibond (see Fig. 5.41(b)) that is effective in end-on complexation to the pi-donor face of benzene. The unusually small energy separation between donor and acceptor NBOs,... 

As the mobile ji electrons of the alkene approach the bromine molecule, the electrons of the bromine-bromine bond are drifted in the direction of that bromine which is more distant from the alkene..Thus the bromine molecule becomes polarized and a partial... 

These products obviously arose from metallation at unsubstituted ring positions in the starting tetrahalobenzene. To force metallation at the carbon-bromine bonds, they used 2,6-dibromo-3,5-difluoro-p-xylene which, with magnesium in THF gave mainly mono-adduct. With butyllithium, only the bis-adduct was obtained (15%). No mention was made of syn/anti isomers of the bis-adducts. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

Several trends emerge in these data (1) The reductive elimination of bromine is 6-13kJmol more facile than reductive elimination of chlorine in similar structures, which is consistent with weaker chalcogen-bromine bonds relative to chalcogen-chlorine bonds.(2) The reductive elimination of chlorine is accelerated by the presence of a chloride counterion as opposed to a less nucleophilic counterion such as hexafluorophosphate. (3) The rate of reductive elimination is accelerated by the presence of a more polar solvent (acetonitrile) relative to tetrachloroethane, which is consistent with development of charge in the rate-determining step. These observations suggest mechanisms for oxidative... 

Tertiary alkyl halides are easier to reduce than secondary alkyl halides, which are easier to reduce than primary alkyl halides. Carbon-iodine bonds are easier to reduce than carbon-bromine bonds, which are easier to reduce than carbon-chlorine bonds [1-3]. 

Reduction of 2-bromo-3-pentanone at mercury affords a mixture of 3-pentanone and l-hydroxy-3-pentanone, whereas electrolysis of Q ,Q -dibromoacetone in the presence of benzoate gives a mixture of products arising from both a carbon-bromine bond cleavage and an Sn2 displacement of bromide by benzoate [94]. In an acetic acid-acetate buffer, branched dibromo ketones, such as 2,4-dibromo-2,4-dimethyl-3-pentanone, are reduced to a-acetoxy ketones however, less highly substituted compounds, such as 4,6-dibromo-5-nonanone, undergo simple cleavage of both carbon-bromine bonds [95]. Other work dealing with the reduction of Q, Q -dibromoketones has been described [96]. 

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