Bromonium ions, cyclic, addition

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... 

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... 

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,... 

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. 

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. 

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)] ... 

The open carbocation is replaced by a cyclic bridged ion having Br partially bonded to each C (bromonium ion). In this way the stereochemical differences of the starting materials are retained in the intermediate. In the second step, the nucleophile attacks the side opposite the bridging group to yield the anti addition product. 

To account for the stereospecificity of bromine addition to alkenes, it has been suggested that in the initial electrophilic attack of bromine a cyclic intermediate is formed that has bromine bonded to both carbons of the double bond. Such a bridged ion is called a bromonium ion because the bromine formally carries the positive charge ... 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

The addition of bromine to (R) -4-chlorocyclohex- 1-ene yields initially two diastereomeric cyclic bromonium ions, which then undergo a ring opening reaction with bromide ion. The attack of bromide ion is an Sn2 process and the reaction is stereospecific with inversion. Consequently, the bromo substituents in both diastereomeric products will have a trans relationship. 

From alkylacetylenes trans dibromide is formed in large or quantitative yields and no detectable amounts of cis or solvent-incorporated diadducts were observed. The indications are that in this case the electrophilic addition occurs via a cyclic bromonium ion (47) and that this is probably more stable as an intermediate than the linear alkyl vinyl oation. From 47 only trans addition is expected to occur. 

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