The reversible formation of bromonium ions

Slebocka-Tilk H, Ball RG, Brown RS (1985) The question of reversible formation of bromonium ions during the course of electrophilic bromination of olefins. 2. The crystal and molecular structure of the bromonium ion adamantylideneadamantane. J Am Chem Soc 107 4504-4508... 

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. 

In the mechanism preferred by the authors, the observed KIE is the product of the EIE for the reversible formation of the bromonium ion and the KIE for the rate-determining formation of the /3-bromocarbocation (Scheme 3). Because the steric crowding of the C-2, C-2, C-3 and C-3 endo-hydrogens in the bromonium ion would be relieved in going to the /3-bromocarbocation intermediate, one would expect the secondary deuterium KIE for the k2 step of the reaction to be normal, i.e. >1.00. If this is the case, the EIE for the formation of the bromonium ion must be significantly more inverse than the KIE for the k step of the reaction, i.e. the KIE for the formation of the... 

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 reversibility of the formation of bromonium ion is a process comparable to the formation of /5-bromo carbocations. In fact, the carbocation formation may be solvent-assisted81 as reported in reaction 8, which parallels reaction 4. 

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... 

Reversible formation of ionic intermediates in halogenated solvents has been suggested to be due to the weakly nucleophilic character of the counteranion, the tribromide ion, which should dissociate into nucleophilic bromide and free bromine before reacting with the bromonium ion (refs. 11,25,26). In order to check this hypothesis the product distribution of the c/s-stilbene bromination in chloroform was investigated (ref. 27). In the latter solvent the formation constant of Br3 is considerably lower than in DCE, Kf = 2.77 (0.13) x 10 against > 2 x 107 M 1. (ref. 28). As a consequence, at 10 3 M [Br2] relevant amounts of bromide ions are present as counteranion of the bromonium intermediate. Nevertheless, the same trend for the isomerization of cis- to rran -stilbene, as well as an increase of... 

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... 

The full paper on the reactions of iodine(l) thiocyanate and 5a-androst-2-ene and other alkenes has appeared." An investigation into the stereochemistry of the addition of BrCl to cholesterol revealed that the ratio of a /8 attack was 4.5 1, which was higher than that for other electrophilic additions. It was suggested that the reaction involved rapid reversible formation of the bromonium ions followed by attack by chloride ion and that the opening of the 5/8,6/3-bromonium ion was slow relative to that of the 5a,6a-bromonium ion owing to steric interactions with the la-, 3a-, 7a-, and 9a-axial hydrogen atoms. 

Although kinetic control typically favors the trans diaxial product, the trans diequatorial product is commonly thermodynamically preferred. If the epoxide can be regenerated due to reversibility under particular reaction conditions, a small amount of competing twist boat opening can in principle allow eventual product equilibration. An analogy is found in the classical steroid diaxial to diequatorial dibromide interconversion that is believed to occur via reversible formation of a bromonium ion intermediate that slowly drains to the thermodynamically favored product through the kinetically disfavored antiperiplanar twist boat intermediate. ... 

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. " ... 

Various pathways for the electrophilic addition to a glycal followed by nucleophilic opening of the resulting halonium ion can account for the different product distributions obtained from these reactions [74]. Irreversible formation of the halonium ion and subsequent nucleophilic displacement leads to the (B-g/wco compound, while reversible formation of the halonium ion followed by slow nucleophilic trans diaxial opening leads to the a-manno compound. Common T sources used to synthesise a-manno species include V-iodosuccinimide (NIS), and iodonium di-iym-collidine perchlorate (IDCP). An iodonium source is particularly favoured over a bromonium source, as this more readily allows further subsequent functionalisation of the product often via radical chemistry. The yields of... 

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... 

Another aspect of the mechanism is the reversibility of formation of the bromonium ion. Reversibility has been demonstrated for highly hindered alkenes. This can be... 

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 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. ... 

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