Bromonium ion formation

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

MeBr is a strong poison only with Et2 All coinitiator. Since Et2 All forms the least nucleophilic counterion, Et2AHXe, it is expected to produce a relatively free carbenium ion, facilitating bromonium ion formation by interaction with MeBr solvent. With more nucleophilic counteranions, like Me3 AlXe or Et2 AlXf (X = Cl, Br), bromonium ion formation is more difficult and poisoning is modest. Evidently, the less stable bromonium ions form only with weakly nucleophilic counterions. MeCl is the weakest poison or may be inert, since chloronium ions are highly unstable. 

We passed then to a particular olefin, adamantylideneadamantane, whose reaction with Br2 had been shown to stop at the stage of bromonium ion formation because of steric hindrance to backside nucleophilic attack. An UV-Vis spectrophotometric study (ref. 10) has shown that the complicated equilibrium reported in Scheme 4 is immediately established on mixing the olefin and Br2 in DCE. Equilibrium (1) could be isolated by working at low Br2 and ten to hundred fold higher olefin concentrations. A Scott plot followed by a NLLSQ refinement of the data gave a Kf = 2.89 x 10 (4.0) M-l. It is worth noting that conductimetric measurements showed the non-ionic nature of the 1 1 adduct, consistent with a CTC intermediate, but not with a bromonium-bromide species. 

A kinetic evidence for reversibility of bromonium ion formation has been obtained in the reaction of tetraisobutylethylene and its Dg labeled derivative with Br2 in acetic acid (ref. 9). Owing to steric effects, the first formed bromonium ion cannot undergo backside attack to give the dibromide, but looses a proton to yield... 

Most of the olefins shown so far, for which reversibility of the bromonium ion formation had been demonstrated, are particular olefins, in which either steric bulk impedes the product forming step, or ring strain in the dibromide product retards this step. In order to check the general occurrence of the reversibility during the bromination reaction, a further approach, based on the cis-trans isomerization of stilbene derivatives during the bromination of the cis isomers, was devised. 

Finally, every kind of calculation shows that there is no substantial charge on the bromine atom of bromonium ions (Cioslowski et ai, 1990), in agreement with the conclusions from nmr spectra. This result is relevant to the possible reversibility of bromonium ion formation, as discussed later. 

Fig. 10 Free energy profiles of bromination (A) when bromonium ion formation is irreversible and (B) when it is reversible..

In conclusion, the reversibility of bromonium ion formation is at present inferred from particular experiments only nothing allows us to conclude that this mechanistic feature is general. However, when nucleophilic trapping... 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

Basically, the formation of the bromonium ion may be considered either a reversible or irreversible process. In spite of some evidence of reversibility of the formation of bromonium ion, in 1990, Ruasse affirmed that, in protic solvents, the ionic intermediate is formed irreversibly 81. Now, there is a body of evidence supporting the reversibility of the bromonium ion formation in bromination of olefins. 

Bromonium ion formation (reversible or irreversible) may be assisted by the presence of a nucleophilic solvent, as shown in 30 of reaction 4 involving alcohols as solvent. 

The bromonium ion may be generated by other methods than direct bromination solvolytic reaction of trans-2-bromo-[(4-bromophenyl)sulphonyl]cyclohexane (36) (and cyclopentane) forms (reaction 7) the bromonium ion94 (37). If Br- is present in the reaction mixture, the generation of Br2 (and olefin) is observed (Scheme 14). This confirms the reversibility of the bromonium ion formation in the usual bromination pathway. When other olefin scavengers are present, a formal Br+ transfer is observed94. This may occur without the formation of Bit. 

Usually, the attack of the nucleophile on the bromonium ion is a fast process. On the other hand, kinetic investigations100 on the bromination shown in Scheme 19 indicate that bromonium ion formation (i.e. the ionization of the CT complex 45) cannot be the RDS. The apparent activation energy for the overall bromination (and the experimental reaction order in bromine, which changes by changing the temperature) confirms the neutralization of the bromonium ion to form the product (47) in a step limiting the observed rate of the overall process. 

The process of bromonium ion formation is related to rearrangement of the carbon skeleton of carbocations. A nearby atom or group may relieve the electron deficiency of another atom through induction, resonance, or, as in bromonium ion formation, by actually carrying the electrons to where they are needed. 

Transition state for bromonium ion formation from an alkene and bromine... 

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. " ... 

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