Bromonium ion structure

In nonpolar solvents, the observed rate is frequently found to be described as a sum of the first two terms in the general expression. The second-order term is interpreted as the collapse of an alkene-halogen complex to an ion pair, which then goes on to product. The cationic intermediate can have the bromonium ion structure. 

As in the case of hydrogen halide additions, this attack should lead to anti addition. The initial complex could resemble a bromonium ion structurally. One can account... 

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... 

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. 

Fig. 6.1. Cfystal structure of bromonium ion from aiJamantyli-deneadamantane. (Reproduced from Ref. 39 permission of the Ameriean Chemieal Society.)...

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br- to alkenes at 77 K. The ESR spectra of the resulting species were consistent with bridged structures. ... 

Which group will end up on the more substituted carbon Br or OH In other words, does water attack the more substituted carbon or the less substituted carbon To answer this question, we need to look at the structure of the bromonium ion more... 

Compared to a bromonium ion, the C-S bonds are stronger and the TS for nucleophilic addition is reached later. This is especially true for the sulfurane structures. Steric interactions that influence access by the nucleophile are a more important factor in determining the direction of addition. For reactions involving phenylsulfenyl chloride or methylsulfenyl chloride, the intermediate is a fairly stable species and ease of approach by the nucleophile is the major factor in determining the direction of ring opening. In these cases, the product has the anti-Markovnikov orientation.62... 

Data on molecular structure of bromonium ions are sometimes extrapolated from that of the tribromide-adamantylideneadamantane bromonium ion pair [6] (Slebocka-Tilk et ai, 1985), the only stable ionic bromination intermediate that can be isolated and whose crystal structure has been determined. Since the first observation by Strating et al. (1969), it has been established that bromine addition to adamantylideneadamantane [5] in... 

Old semi-empirical (Bach and Henneike, 1970) and recent ab initio (Hamilton and Schaefer, 1990, 1991) calculations are in complete agreement with experimental data as regards the structure of the parent ethylene-bromonium ion. Its most stable structure is definitely symmetrically bridged. Early ab initio results showed that the bridged form [7] is more stable than the 2-bromoethylcation [8] and the 1-bromoethylcation [9] by 1-4 and... 

Fig. 3 Minimum energy profiles for the opening of bromonium ions (Galland et al., 1990). They are not double-well curves, i.e. bridged and open structures are not in equilibrium, whatever the number of methyl substituents.

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... 

Fig. 5 Reactivity-structure relationship for the bromination of monosubstituted stilbenes (data from Ruasse and Dubois, 1972). The curvature shows the X-dependence of the competition between carbocation and bromonium ion pathways.

No product has as yet been isolated that is believed to have the bromonium ion ring intact, but some closely related ionic compounds are known. The latter compounds could have the bromonium and iodonium structure but their colors and the probable stability of the alternative carbonium, ion make it unlikely.282... 

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 ... 

A common feature of the compounds that give extensive syn addition is the presence of at least one phenyl substituent on the double bond. The presence of a phenyl substituent diminishes the strength of bromonium ion bridging by stabilizing the cationic center. A weakly bridged structure in equilibrium with an open benzylic cation can account for the loss in stereospecificity. 

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