Bromonium ions reactions

As BC can attack either carbon atom of the bromonium ion, reaction with Z-but-2-ene produces a 1 1 mixture of enantiomers (only the 2R,3R isomer is shown above). For -but-2-ene, the attack of Be at either carbon atom of the bromonium ion produces the same compound. This compound has a plane of symmetry and hence is an achiral meso compound. 

In summary, it appears that bromination usually involves a complex that collapses to an ion pair intermediate. The ionization generates charge separation and is assisted by solvent, acids, or a second molecule of bromine. The cation can be a P-carbocation, as in the case of styrenes, or a bromonium ion. Reactions that proceed through bromonium ions are stereospecific anti additions. Reactions that proceed through open carbocations can be syn selective or nonstereospecific. 

The second mechanism in the box shows how 2-butene reacts at the bottom face to yield an intermediate bromonium ion that is chiral. (Reaction at the other face would produce the enantiomeric bromonium ion.) Reaction of this chiral bromonium ion (or its enantiomer) with a bromide ion either by path (a) or by path (b) yields the same achiral product, wefo-2,3-dibromobutane. 

Step 1 Reaction of ethylene and bromine to form a bromonium ion intermediate... 

Step 2 IS the conversion of the bromonium ion to 1 2 dibromoethane by reaction with bromide ion (Br )... 

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... 

Step 2 of the mechanism m Figure 6 12 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion For reasons that will be explained m Chapter 8 reactions of this type normally take place via a transition state m which the nude ophile approaches carbon from the side opposite the bond that is to be broken Recall mg that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only trans 1 2 dibromocyclopentane m accordance with the experimental observations... 

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. 

In analogy with the peracid attack on steroidal double bonds, the formation of the bromonium ion, e.g., (81a), occurs from the less hindered side (usually the a-side of the steroid nucleus) to give in the case of the olefin (81) the 9a-bromo-l l -ol (82). Base treatment of (82) provides the 9 5,1 l S-oxide (83). Similarly, reaction of 17/3-hydroxyestr-5(10)-en-3-one (9) with A -bromosuccinimide-perchloric acid followed by treatment with sodium hydroxide and sodium borohydride furnishes the 3, 17 5-dihydroxy-5a,l0a-oxirane. As mentioned previously, epoxidation of (9) with MPA gives the 5, 10 -oxirane. °... 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

We saw in the previous section that when Br2 reacts with an alkene, th cyclic bromonium ion intermediate reacts with the only nucleophile presen Br- ion. If the reaction is carried out in the presence of an additional nuclec phile, however, the intermediate bromonium ion can be intercepted by th added nucleophile and diverted to a different product. In the presence of watei for instance, water competes with Br- ion as nucleophile and reacts with th bromonium ion intermediate to yield a broinohydrin. The net effect is additioi of HO-Br to the alkene by the pathway shown in Figure 7.1. 

Mechanism of bromohydrin formation by reaction of an alkene with Br2 in the presence of water. Water acts as a nucleophile to react with the intermediate bromonium ion. 

Reaction of the alkene with Br2 yields a bromonium ion intermediate, as previously discussed. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

The driving force behind this remarkable reaction in which the sulphone participates intramolecularly in a 5-exo-tet ring closure process96 is that the geometry of the intermediate bromonium ion is nicely set up for the attack by the sulphone oxygen. Probably, the process is assisted by the ready cleavage of the sulphur to the t-butyl bond, with the concomitant formation of t-butyl bromide. 

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... 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

The Markovnikov regioselectivity of the gem-alkenes is associated with a chemoselectivity. in favour of methanol attack, significantly greater than that observed for the other alkenes. If no sodium bromide is added to the reaction medium, no dibromide is observed for this series. Therefore, these alkenes behave as highly conjugated olefins, as regards their regio- and chemo-selectivity. In other words, the bromination intermediates of gem-alkenes resemble P-bromocarbocations, rather than bromonium ions. Theoretical calculations (ref. 8) but not kinetic data (ref. 14) support this conclusion. 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

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... 

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