Bridged bromonium ion

Intramolecular reactions usually dominate intermolecular addition for favorable ring sizes. Semiempirical (AMI) calculations found the intramolecular TS favorable to a comparable intermolecular reaction.68 (See Figure 4.1) The intramolecular TS, which is nearly 4 kcal/mol more stable, is quite productlike with a C—O bond distance of 1.6 A, and a bond order of 0.62. The bromonium ion bridging is unsymmetrical and fairly weak. The bond parameters for the intra- and intermolecular TSs are quite similar. 

A common feature of the compounds that give extensive syn addition is the presence of at least one phenyl substituent on the double bond. The presence of a phenyl substituent diminishes the strength of bromonium ion bridging by stabilizing the cationic center. A weakly bridged structure in equilibrium with an open benzylic cation can account for the loss in stereospecificity. 

The electrophile in oxymercuration reactions, " HgX or Hg, is a soft acid and strongly polarizing. It polarizes the -electron distribution of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. A three-center two-electron bond implies weaker bridging in the mercurinium ion than in the three-center four-electron bond of a bromonium ion. Bridging in the mercurinium ion is much more pronounced than is the case for bridging by hydrogen. 

FIGURE 6 13 Mechanism of bromohydrin formation from cyclopentene A bridged bromonium ion is formed and is attacked by a water molecule from the side opposite the carbon-bromine bond The bromine and the hydroxyl group are trans to each other in the product... 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

Both proton loss and rearrangement reflect the greater positive charge at carbon in a chloronium ion than in a bromonium ion because of the weaker bridging by chlorine. 

The positive bromine which leads to bromonium ion intermediates is softer and also has unshared electron pairs which can permit a total of four electrons to participate in the bridged bromonium ion intermediate. This would be expected to lead to a more strongly bridged and more stable species than is possible in the case of the proton. The bromonium ion can be represented as having two covalent bonds to bromine and is electrophilic but not electron-deficient. 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br- to alkenes at 77 K. The ESR spectra of the resulting species were consistent with bridged structures. ... 

Let s look at the mechanism to understand why. In the first step, we form a bridged intermediate, called a bromonium ion ... 

You will notice that the bromonium ion has the bridge coming out toward you (on wedges), but we did not say at the time that it could also have formed with the bridge going away from you (on dashes) ... 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

The bridging by bromine prevents rotation about the remaining bond and back-side nucleophilic opening of the bromonium ion by bromide ion leads to the observed anti addition. Direct evidence for the existence of bromonium ions has been obtained from NMR measurements.31 A bromonium ion salt (with Br3 as the counterion) has been isolated from the reaction of bromine with the very hindered alkene adamantylide-neadamantane.32... 

A bridged intermediate exactly analogous to a bromonium ion cannot be formed as H has no electron pair available, but it may be that in some cases a n complex (21) is the intermediate. We shall, however, normally write the intermediate as a carbocation, and it is the relative stability of possible, alternative, carbocations (e.g. 23 and 24) that determines the overall orientation of addition, e.g. in the addition of HBr to propene (22) under polar conditions ... 

Epoxides, though uncharged, have a formal resemblance to cyclic bromonium ion intermediates (cf. p. 180), but unlike them are stable and may readily be isolated. They do, however, undergo nucleophilic attack under either acid- or base-catalysed conditions to yield the 1,2-diol. In either case attack by the nucleophile on a carbon atom will be on the side opposite to the oxygen bridge in (49) such attack on the epoxide will involve inversion of configuration (cf. p. 94) ... 

The ionic intermediates bridged bromonium ions or open /3-bromocarbocations 220... 

Early interest of theoretical chemists in bromonium ions was focused on two aspects are they bridged or open and do they resemble n- or c-complexes ... 

Old semi-empirical (Bach and Henneike, 1970) and recent ab initio (Hamilton and Schaefer, 1990, 1991) calculations are in complete agreement with experimental data as regards the structure of the parent ethylene-bromonium ion. Its most stable structure is definitely symmetrically bridged. Early ab initio results showed that the bridged form [7] is more stable than the 2-bromoethylcation [8] and the 1-bromoethylcation [9] by 1-4 and... 

The relative magnitude of the kinetic effects of two substituents, Rt and R2, on the C0 and carbon atoms of the double bond (Scheme 7) is taken as a measure of the symmetry of the charge development and therefore of bromine bridging in the bromocations. It is assumed that in a bromonium ion the effects of Rx and R2 must be similar, whereas for a /J-bromocarbocation, C+, the effect of Rj must be significantly greater than that of R2. Consequently, the substituent effects are analysed in terms of a multipathway scheme (Scheme 7) where open carbocations and the bridged ion are formed via discrete pathways with rate constants kx, kf and kBr respectively (Ruasse and Dubois, 1974). The rate constant k in (4) is therefore the sum of these three... 

Fig. 3 Minimum energy profiles for the opening of bromonium ions (Galland et al., 1990). They are not double-well curves, i.e. bridged and open structures are not in equilibrium, whatever the number of methyl substituents.

Bromination can exhibit stereo-, regio- and chemo-selectivity when the reaction is carried out in the presence of nucleophiles (solvent or added salt). When the ionic intermediate is a bromonium ion, a stereospecific but non-regioselective reaction is expected. In contrast, for an open bromo-carbocation, the products should be formed regioselectively but not stereo-specifically. These considerations were understood very early since, in fact, Roberts and Kimball (1937) suggested bridged ions as bromination inter-... 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

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