Bromonium ion from

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

The formation of a bromohydrin via the addition of hyprobromous acid to olefins complements the above mentioned route. This reagent adds to the double bond in a tm/w-diaxial manner, the addition being initiated by the attack of a positive bromonium ion from the less hindered side. The... 

Display the lowest-unoccupied molecular orbital (LUMO) for cyclohexyl bromonium ion. From which side will the Br attack Will this lead to formation of cis-1,2-dibromo-cyclohexane or 1,2-dibromocyclohexane Is this... 

For a long time, it was considered that the formation of a bromonium ion from olefin and bromine is irreversible, i.e. the product-forming step, a cation-anion reaction, is very fast compared with the preceding ionization step. There was no means of checking this assumption since the usual methods—kinetic effects of salts with common and non-common ions—used in reversible carbocation-forming heterolysis (Raber et al., 1974) could not be applied in bromination, where the presence of bromide ions leads to a reacting species, the electrophilic tribromide ion. Unusual bromide ion effects in the bromination of tri-t-butylethylene (Dubois and Loizos, 1972) and a-acetoxycholestene (Calvet et al, 1983) have been interpreted in terms of return, but cannot be considered as conclusive. 

Kinetic investigations83 into the bromination of methylidene adamantane (31) and allyl-benzene (32) reveals the importance of the assistance of the solvent to the rate-limiting ionization to form bromonium ion from the CT complex. 

Also from this study the difference in the rate of bromination of alkenes and alkynes is quite evident. Thus the bimolecular coefficient (k2) for styrene is 2 x 103 times that for phenylacetylene and k2 for 3-hexene is 1-4 x 105 times the value for 3-hexyne. Since the first two compounds are believed to react via carbonium ions and the latter two via bromonium ions there seems to be an extra factor of 102 in the stability of bromonium ions from alkenes relative to those from alkynes. 

The generation of an electrophilic chloronium or bromonium ion from chlorine or bromine, by heterolytic fission in an ionizing solvent such as... 

Reactions which may at first glance appear to involve ylides may not in fact do so. For example, a bromomalonic ester reacted with a variety of aldehydes in the presence of tributylarsine to form alkylidene- or arylidene-malonic esters, but no base was required and it was suggested that reaction proceeded not via an ylide but via an arsonium salt formed by extraction of a bromonium ion from the malonic ester The malonate anion... 

Consistent with the mechanism proposed, Strating et al isolated a bromonium ion from addition of bromine to the hindered alkene 43 in fact the hindrance was such that the second stage, which would normally give the vicinal dibromoalkane product, did not occur. For an X-ray crystallographic structure determination of this bromonium ion, see Slebocka-Tilk et al4... 

These reactions are characteristically stereospecific. The formation of a bromonium ion from attack of bromine on a pi bond was the first example of this path (Section 8.3.6). 

The trans product results from water attacking the bromonium ion from the face opposite the bromine. Equal amounts of the two enantiomers result from the equal probability that water will attack either C-1 or C-2. 

As in the previous example, dehydration in ring C, in this case by means of phosphorus oxychloride in pyridine, gives the A(9)l 1-olefin, 14-1 (Scheme 7.14). Treatment of the resulting intermediate with A-bromoacetamide, or some other source of electrophilic bromide ion, in water (in effect HOBr) leads to the formation of the bromohydrin 14-2. The stereochemistry is dictated by the initial formation of a bridged a-bromonium ion from attack on the more open underside of the molecule. Axial opening by hydroxide then gives the observed bromohydrin (14-2). 

Fig. 5.2. X-ray crystal structure of the bromonium ion from adamantylideneadamantane. Reproduced from J. Am. Chem. Soc., 107, 4504 (1985), by permission of the American Chemical Society.

Another aspect of the mechanism is the reversibility of formation of the bromonium ion. Reversibility has been demonstrated for highly hindered alkenes, and attributed to a relatively slow rate of nucleophilic capture. However, even the bromonium ion from cyclohexene appears to be able to release Br2 on reaction with Br. The bromonium ion can be generated by neighboring-group participation by solvolysis of frfln -2-bromocyclohexyl triflate. If cyclopentene, which is more reactive than cyclohexene, is included in the reaction mixture, bromination products from cyclopentene are formed. This indicates that free Br2 is generated by reversal of bromonium ion formation. Other examples of reversible bromonium ion formation have been found. " ... 

The computed structure of bromonium ions from alkenes such as 2-methylpropene are highly dependent on the computational method used and inclusion of correlation is essential. CISD/DZV calculations gave the following structural characteristics. 

MP2/6-311G(4,/ ) calculations favored open carbocations for the ions derived from cyclohexene. On the other hand, the bridged bromonium ion from cyclopentene was found to be stable relative to the open cation. 

This result is in qualitative agreement with an NMR study under stable ion conditions that found that the bromonium ion from cyclopentene could be detected, but not the one from cyclohexene. Broadly speaking, the computational results agree with the F < Cl < Br order in terms of bridging, but seem to underestimate the stability of the bridged ions, at least as compared to solution behavior. 

Reversibility of bromonium ion formation has been observed directly in the regeneration of adamantylideneadamantane from the bromonium ion salt (Figure 9.12). Further evidence for reversibility is the observation that the bromonium ion from adamantylideneadamantane can transfer Br to cyclohexene in CH2CI2 solution. Since a sterically hindered bromonium ion can transfer Br, it seems reasonable that bromonium ions that are not sterically hindered should also be capable of transferring Br. Therefore, reversibility of bromonium ion formation could be a general process. Reversibility may... 

By comparison, nucleophilic susceptibility was found to be 0.46 for each of the two carbon atoms in the bromonium ion formed from ethene. These results are consistent with the tendency for the bromonium ion from isobutene to react with nucleophiles at the more highly substituted position, but the calculated structures implicitly represent gas phase species. Structures in solution may be affected by solvent or counterions. 

These results are consistent with a model in which the bromonium ion from a symmetrically substituted alkene behaves more like a bromonium ion and less like a carbocation. That is, with a s)rmmetrical ion there is relatively little solvent incorporation, and solvent is relatively indiscriminate in attacking the two carbon atoms of the bromonium ion. On the other hand, there is more solvent incorporation product formed from the more unsymmetrical ions, and the solvent is more likely to add to the more highly substituted carbon atom than to the less highly substituted carbon atom. This behavior suggests that solvent is attracted to the center of greater partial positive charge. ... 

Imagine that diatomic bromine reacts with cis-2-butene such that the bromonium ion has a bromine on the left of the C=C unit, as in 47. Subsequent attack of the bromonium ion from the face opposite the bromine atom will generate the (2i ,3i ) diastereomer (i.e., 44B), which is the enantiomer of 44A generated from bromonium ion 46, in which bromine was on the right. This analysis could easily be done with the alkene drawn in a way that bromine attacks from top and bottom (see 41). In other words, the bromine may add to either face of the alkene with equal facility. Because the reaction of bromine and an alkene may occur from either side of the C=C with equal facility, a mixture of... 

Scheme 7.21. A representation of the addition of radiolabeled bromine (indicated as Br2> to 3-bromo-l-propene (allyl bromide, H2C=CHCH2Br). It is important to note that the initially formed bromonium ion is capable of being opened by internal (backside) attack from the bromine atom on the neighboring carbon. This is an example of the process called neighboring group participation (this chapter, Rearrangement). In this particular case, the newly formed bromonium ion is, except for the position of the label ( Br), the mirror image of the bromonium ion from which it was generated. Each of these bromonium ions is capable of being attacked by exogenous labeled bromide anion. Thus, depending on the specific bromonium ion, the final relationship of the two isotopically unique bromine atoms in the product (1,2,3-tribromopropane) will be either 1,2 or 1,3.

In the bromonium ion, there is the full complement of six electrons for the three O bonds binding the two carbons and the bromine. The bromonium ion is a normal compound even though it may look odd at first. To see this clearly, imagine forming the bromonium ion from an open cation. 

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