Bromonium ion intermediate

Step 1 Reaction of ethylene and bromine to form a bromonium ion intermediate... 

Step 2 of the mechanism m Figure 6 12 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion For reasons that will be explained m Chapter 8 reactions of this type normally take place via a transition state m which the nude ophile approaches carbon from the side opposite the bond that is to be broken Recall mg that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only trans 1 2 dibromocyclopentane m accordance with the experimental observations... 

The positive bromine which leads to bromonium ion intermediates is softer and also has unshared electron pairs which can permit a total of four electrons to participate in the bridged bromonium ion intermediate. This would be expected to lead to a more strongly bridged and more stable species than is possible in the case of the proton. The bromonium ion can be represented as having two covalent bonds to bromine and is electrophilic but not electron-deficient. 

In order to explain the observed product, a cyclic bromonium ion intermediate has been proposed. 

We saw in the previous section that when Br2 reacts with an alkene, th cyclic bromonium ion intermediate reacts with the only nucleophile presen Br- ion. If the reaction is carried out in the presence of an additional nuclec phile, however, the intermediate bromonium ion can be intercepted by th added nucleophile and diverted to a different product. In the presence of watei for instance, water competes with Br- ion as nucleophile and reacts with th bromonium ion intermediate to yield a broinohydrin. The net effect is additioi of HO-Br to the alkene by the pathway shown in Figure 7.1. 

Reaction of the alkene with Br2 yields a bromonium ion intermediate, as previously discussed. 

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. 

Epoxides, though uncharged, have a formal resemblance to cyclic bromonium ion intermediates (cf. p. 180), but unlike them are stable and may readily be isolated. They do, however, undergo nucleophilic attack under either acid- or base-catalysed conditions to yield the 1,2-diol. In either case attack by the nucleophile on a carbon atom will be on the side opposite to the oxygen bridge in (49) such attack on the epoxide will involve inversion of configuration (cf. p. 94) ... 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

More than six decades after the postulation of the cyclic bromonium ion intermediate in electrophilic bromination of alkenes important, new findings are still emerging.567 Updated general treatments of the halogenation of alkenes became available.568,569... 

In the above discussion of stereoselectivity the mechanisms of various reactions have been used to rationalize why some are stereoselective and some are not. Thus the bromination of olefins proceeds via a bridged bromonium ion intermediate and gives only trails addition across the double bond [reactions (6.2) and (6.3)]. In contrast, the addition of HBr across a double bond gives a carboca-tion intermediate that does not maintain the facial integrity of the olefin and is thus much less stereoselective [reaction (6.1)]. In these examples the mechanism of the reaction is used to explain and understand the diastereoselectivity that is observed. There are many other examples (usually in textbooks) where the mechanism of a reaction is used to rationalize the stereoselectivity of the process. To do this requires that the mechanism be known with certainty. 

The efficient azidobromination of cycloalkenes [Eq. (5.342)] and open-chain alkenes to give /3-bromoalkyl azides with iV-bromosuccinimide and azidotrimethylsi-lane is catalyzed by Nafion-H.919 Terminal alkenes and alkenes with bulky substituents do not react, whereas 2,3-trimethylbutene-2 reacts without catalysis. The stereochemistry of the process suggests the involvement of a bromonium ion intermediate. 

For instance, starting with a 2,3-di-O-isopropylidene-a-D-mannopyranoside 52, an attack by dibromomethyl-methyl ether (DBE) yields the 2,6-dideoxy-2-bromo-3-0-formyl-a-D-glucopyranosyl bromide 53. In the presence of a silver salt, predominantly P-glycosides like 54 are formed [21-23], Little is known about the mechanistic aspects, however, the stereoselectivity favors a 1,2-bromonium ion intermediate. 

The cr,/3-unsaturated amides 396 (where R = aryl or heteroaryl) cyclized in the presence of sodium acetate and iV-bromosuccinimide, presumably through the bromonium ion intermediate 397, to furnish N-unsubstituted... 

Figure 3.33). In the bromonium ion intermediate the backside attack by the H20 molecule does not take place at the hindered neopentyl center according to the rules for Sn2 reactivity (Section 2.4.4). 

In the transition state (marked %) the peroxyacid prefers to be well away from R, even if R is only a methyl group. The selectivity is 76 24 with methyl. The opposite stereoselectivity can be achieved by bromination in water. The bromonium ion intermediate is formed stereoselectively on the less hindered side and the water is forced to attack stereospecifically in an Sj j2 reaction from the more hindered side. 

Treatment of propene or styrene with N6S m aqueous dimethyl MiHbxide ivea primarily a single broiiKdiydrin. Uae SpartanView to examine the bromonium ion intermediates involved in thexe reactions. How do the C-6r bond disUnces relate to the structure of the major pmdoeV in each case ... 

Stereochemlttry of the addition of Br to ch- >buCen . A racemic mfxiure ot and producla it formed becautr attack of Br on both [Pg.358]

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