Bromonium ions aromatic bromination

For stilbene bromination, a markedly non-linear structure-reactivity relationship is observed (Fig. 5). Detailed analysis of the kinetic effects of two substituents, X and Y, on each aromatic ring shows that the three pathways leading to the C+ and carbocations and to the bromonium ion can... 

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 electrophilic character of chlorine and bromine in their attack on olefins is confirmed by recent studies of substituent effects in styrenes and cinnamic acids [ 102]. The slope of the Hammett plot for chlorination = — 4.01) is consistent with a classical carbonium ion intermediate of type (i) (p. 91), but the smaller value [q — — 2.23) for bromination indicates less delocalisation of charge into the aromatic ring, and is consistent with the bromonium ion intermediate. 

The first possible reason is that the electrons in the alkynes are held more closely to the nucleus than is the case in alkenes, as evidenced by the difference in acidity of hydrogens that are attached to each type of carbon. The second possible reason is that in the reaction pathway of the alkyne, there is a possible hybridisation of the bromine atom in the cyclic bromonium species that gives rise to an anti-aromatic species and which is hence unstable. This would in turn result in a higher activation energy for this reaction pathway. However, probably the most important reason is that the cyclic bromonium ion that contains a carbon/carbon double bond is far more strained than the saturated version, and hence is more unstable, and so less readily formed. 

The pentacyclic core 63 was converted into codeine by a different approach from that of Taber (Scheme 14). Treatment of 63 with dibromohydantoin generated bromohydrin 64 stereoselectively. In this reaction, the reagent approaches from the less hindered (3-face and the resulting (3-cyclic bromonium ion was opened with water in trans-diaxial manner. Under the reaction conditions, the C-l position of aromatic ring was concomitantly brominated. Exposure of 64 to KOH/dioxane... 

Halogen atoms, usually chlorine or bromine, are introduced by the action of the elemental halogen on the aromatic hydrocarbon in the presence of a Lewis acid catalyst. The chlorinium or bromonium ion is the attacking electrophile. 

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