Cyclic bromonium ion

Such a carbocation however has been demonstrated to be less stable than an alterna tive structure called a cyclic bromonium ion, m which the positive charge resides on bromine not carbon... 

Thus the mechanism for electrophilic addition of Bi2 to ethylene as presented m Figure 6 12 IS characterized by the direct formation of a cyclic bromonium ion as its... 

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 idea that a cyclic bromonium ion was an intermediate was a novel concept when It was first proposed Much additional evidence including the isolation of a sta ble cyclic bromonium ion has been obtained since then to support it Similarly cyclic... 

The highly hindered alkene adamantylideneadamantane forms a bromonium ion which crystallizes as a tribromide salt. An X-ray crystal structure (Fig. 6.1) has confirmed the cyclic nature of the bromonium ion species. This particular bromonium ion does not react further because of extreme steric hindrance to back-side proach by bromide ion. 

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. 

Acid-catalyzed epoxide opening takes place by protonation of the epoxide to increase its reactivity, followed by nucleophilic addition of water. This nucleophilic addition is analogous to the final step of alkene bromination, in which a cyclic bromonium ion is opened by a nucleophile (Section 7.2). That is,... 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

Yet another intermediate, a cyclic bromonium ion, 15, is involved in the electrophilic addition of bromine to 3-hexyne, 1 -hexyne (32,33), and other alkylacetylenes (44). In these reactions, only trans dibromides have been... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

This neighbouring group participation by bromine (cf. p. 93) does not of course prove that addition to alkenes proceeds via cyclic bromonium ions, but it does mean that such species are no longer merely ad hoc assumptions, and to that extent are correspondingly more plausible as intermediates. 

In attempting to add Br2 to the highly unusual alkene (13), it has proved possible actually to isolate the cyclic bromonium ion (14) ... 

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) ... 

There is just the possibility that in adding bromine to butadiene the 1,4-adduct might be obtained via an unstrained, five-membered cyclic bromonium ion (70). This would lead, on nucleophilic cleavage by Bre, to the cis 1,4-dibromide (71) ... 

The cyclic bromonium ion was first used to explain the stereochemistry and course of the bromination of olefins and later proposed as an intermediate in the displacement reaction 232 288 Y... 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

The electrophilic bromonium ion adds to the diene at the position which yields the most stable cationic intermediate and the stereochemical relation of the Br and the MeO group in the product is always trans when the diene system is cyclic. The fact that 1,2-addition takes place selectively but 1,4-addition does not occur is explained by the formation of the bridged bromonium ion as the intermediate. 

Chapter 18 by C. Chiappe focuses on the mechanism of bromination of alkenes, exploring the role of solvent on the formation of cyclic bromonium ion versus P-bromocarbemium ion, as key intermediates. In Chapter 19, H. P. A. Mercier et al. discuss the utility of a novel class of noble-gas onium salts as oxidants for generation and isolation of various trihalomethyl cation salts. 

Fig. 6.10. El mass spectrum of 1-bromo-octane. The product of the a-cleavage, m/z 91, 93 is of minor intensity, whereas the cyclic bromonium ion, m/z 135 and 137, dominates the spectrum. Spectmm used by permission of NIST. NIST 2002.

Initial attack will always take place on a terminal carbon atom of the conjugated system, otherwise the carbocationic intermediate (64), that is stabilised by delocalisation, would not be obtained. It is because of this stabilisation that a carbocation intermediate is formed rather than a cyclic bromonium ion (cf. 66). Completion of overall addition by nucleophilic attack of Br on (64) can then take place at C2 [1,2-addition, (a) (68)] or C4 [1,4-addition, (f ) (69)] ... 

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