And bromonium ions

The NMR characteristics of the bromonium ion triflate prove to be interesting in that the proton NMR spectrum of a 1 1 mixture of parent olefin and bromonium ion at room temperature is an average of those of the two isolated species. This suggests that there is a rapid site exchange of the Br+ between the ion and olefin. The phenomenon can be most simply studied by NMR spectroscopy. An approximately 100 mM solution of the ion in CD2C12 held at - 80°C was treated with small aliquots of Ad=Ad, the NMR spectrum was monitored after each addition. At zero added Ad=Ad, the spectrum of the ion consists of a seven line pattern indicative of a species (eqn. 3, 5a) having two perpendicular planes of symmetry that pass, respectively, through the three heavy atoms of the bromonium... 

Quantitative information about the equilibrium between olefin and Br2 on the one hand and CTC s and bromonium ion species on the other (Scheme 4) has been obtained by the already mentioned UV-Vis spectrophotometric study of the adamantylideneadamantane Br2 system (ref. 10). The spectrophotometric UV-Vis data of a large set of solutions of different reagents concentrations have been used to dissect, using a program based on NLLSQ fitting procedures, the complex spectra in those of the single species present at the equilibrium, as shown in Figure 5. 

These results support the existence of a bromonium ion pathway for dioxabicyclization of 3,4-dibromoeyeloalkyl hydroperoxides and confirm a dependence of mechanism upon choice of silver salt. More significantly, from a synthetic viewpoint the results also indicate that both SN2 and bromonium ion mechanisms are... 

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.

A stereo specific addition of BufOI to /1-methylstyrene was observed in the presence of BF3, yielding Markovnikov products. This result contrasts with the non-stereospecific addition of Bu OCl and Bu OBr. It has been suggested that the bridging in the intermediate chloronium and bromonium ion derived from PhCH=CHMe is not as symmetrical as in the iodonium ion. Consequently, charge develops on the benzylic carbon in the first two cases, and rotation occurs about the C—C bond190. By contrast, a radical mechanism is assumed in the absence of BF3 as anti-Markovnikov products are formed (both in the dark and upon UV irradiation)190. 

The PM3 and ab initio calculations have been employed to compare mercuronium and bromonium ions 55 and 52111. Experimental comparison of the mechanism of the oxymercuration and bromination has also been made (see the section on bromination)142. 

Dimethyl-chloronium and -bromonium ions give similar methylation results and are quite reactive even at temperatures as low as —50°C. The dimethyliodonium ion is less reactive and alkylates benzene and toluene in SO2CIF solution only when allowed to react (if necessary under pressure) at or above 0°C. 

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

Y amabe, S. Mina to, T. Inagaki, S. /. Chem. Soc. Chem. Commun. 1988,532. The same calculations identified three-centered transition structures for the addition of chlorine and bromine such transition structures should lead to chloronium and bromonium ions, respectively. 

One key difference Cyclopropanes do not ring-open easily like epoxides and bromonium ions.)... 

FIGURE 11.13 S, j2 ring opening of epoxides and bromonium ions. 

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