Bromonium ions reversibly formed

In protic solvents there is evidence that bromonium ions are formed reversibly when they are highly congested. In the absence of crowding, no experimental data support return but none exclude it either. When bromination intermediates are /f-bromocarbocations, it is highly improbable that they are formed reversibly, since the bromine atom bears no charge. 

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. " ... 

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. 

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. 

A preliminary indication that bromonium ions could be formed reversibly was provided by the reaction of adamantylideneadamantane (p. 249) leading to a highly stable bromonium-tribromide ion pair that readily releases bromine and the initial alkene (Strating et al, 1969). However, the first evidence for possible return came from the acetolysis of 2-bromocyclohexyl-brosylate in the presence of bromide ions. It was shown (Brown et al, 1984) that the cyclohexylbromonium ion intermediate is able to release bromine. The drastic reaction conditions (high temperature, long duration and high bromide concentrations) cast some doubt on the generality of this observation. 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

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. ... 

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. 

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. 

Furthermore, in the addition of bromine to 44 (in 1,2-dichloroethane, chloroform or carbon tetrachloride, see Scheme 19) to obtain the trans dibromo derivative99 (47), which is found to be in two main conformers100, the formation of the bromonium ion (46) from 5//-dibenz[ >,/]azepine-5-carbonyl chloride (44) is a reversible step. The formation of the bromonium ion (46) follows the association of reagents in the CT complex (45). 49 reacts with hydrogen bromide through the protonated 48, forming" both the dibromo derivative 47 and the olefin 44. The bromonium ion (46) is the probable intermediate... 

Large inverse DKIE have been observed400 in the reaction402 of 370 and 371 with Br2 in AcOH and MeOH at 25 °C in the presence of LiBr, and explained by a pronounced steric DKIE401-403 on the partitioning of a reversibly formed bromonium ion 372. There is less compression of the erido C-L groups in the rate-limiting TS for electrophilic addition to 371 (D20 species) than to 370 (H20 species). 

This suggests that the cis bromide is the kinetic product and the more stable trans compound is the thermodynamic product, formed by reversible loss of bromide and reformation of the bromonium ion. 

Since formation of the cyclic bromonium ion is known to be reversible (ref. 43), other double bonds elsewhere in the molecule, which are incapable of forming an intermediate comparable to III, should be unable to compete with the acetal oxygen. 

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