Bromonium ions characterization

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... 

Very hindered alkenes form bromonium ions that are resistant to nucleophilic attack. In this very hindered case, the bromonium ion is sufficiently stable to be characterized by X-ray crystallography. 

Some bromonium ions have even been isolated and fully characterized. This is possible when steric hindrance impedes nucleophilic attack. For example, the structure shown in the margin has been characterized by x-ray crystallography. 

The fact that rearrangements characteristic of carbocation intermediates are not observed in bromination supports a bromonium ion mechanism as does the isolation and characterization of a stable bromonium ion. Rearrangements, however, are sometimes observed in chlorine addition. 

Olah and coworkras further characterized Reed s silicenium ion 3 as a silylated bromonium ion [exp. R(Si—Br) = 2.479 A]. Despite all the criticism and theoretical evidence, both Lambat and Reed defended their own interpretation of structures 2 and 3 as being chanical entities with predominant silicenium ion character . 

The second step of the bromination reaction in aprotic chlorinated solvents consists of the ionization of the CTC s, and leads to bromonium or bromocarbonium tribromide ion pairs. A direct evidence for the formation of bromonium-tribromide pairs is the isolation and X-ray structural characterization of the adamantylideneadamantane-bromonium tribromide species, obtained by Brown (ref. 13). 

In contrast, in protic solvents and at low bromine concentration, the addition process is characterized by a second order rate law (first order in bromine), Scheme 2, path b. In this case, due to the ability of the solvent to provide a specific electrophilic solvation to the leaving bromide ion, the reaction occurs via an SN1 -like unimolecular ionization of the 1 1 it complex to form a bromonium or P-bromocarbenium bromide ion pair. It is worth noting that protic solvents can also give nucleophilic assistance, depending on their specific solvent properties. 

One of the possible ways to stabilize the amine-halonium complexes is to increase the basicity of the amine, bearing in mind that an appropriate one must also not have easily removable P-hydrogens which will lead to oxidation of the amine and formation of an imine. Quinuclidine (pKa of quinuclidinium ion is 11.3 (55)) is 105-106-fold more basic than the pyridines and both the bromonium (10 (36)) and iodonium (11 (57)) BF4 salts have been made and characterized by X-ray crystallography. Interestingly, although the reaction must generally occur as outlined in Figure 7, neither of these ions shows any observable reaction... 

Stable bromonium and iodonium ions 69-71 of Ad=Ad and bicyclo[3.3.1]nonylidenebi-cyclo[3.3.1]nonane have been characterized by X-ray diffraction125. The data have... 

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