Bromonium ion mechanism

The behaviour of the tram-3-bromide 38 closely resembled that of its cyclopentyl analogue 32. Thus with silver oxide only the cis-2-bromo-[3.2.1]peroxide 40 expected for a SN2 ring closure was obtained, and although some 40 was also formed in the reaction of 38 with silver trifluoroacetate, the predominant (90 %) bicyclic peroxide obtained was 41, i.e. the [3.2.1]peroxide available via a bromonium ion mechanism. The behaviour of the tran.v-4-bromide 39 was very revealing, for it did not react with silver oxide and 41 was the only bicyclic peroxide formed with silver trifluoroacetate. 

These results support the existence of a bromonium ion pathway for dioxabicyclization of 3,4-dibromoeyeloalkyl hydroperoxides and confirm a dependence of mechanism upon choice of silver salt. More significantly, from a synthetic viewpoint the results also indicate that both SN2 and bromonium ion mechanisms are... 

There are readily observable stereochemical consequences that serve as a test of the bromonium ion mechanism ... 

The regiochemistry of the bromination (Scheme 20) of a,/l-unsaturated ketones101 (in methanol) by bromine is affected by the presence of A-bromosuccinimide, which also depresses the overall rate of bromination. A possible explanation of the observed behaviour is the presence of a complex (50) involving the C=C double bond of olefin, TV-bromosuccinimide and bromine, as shown in Scheme 21. Probably, NBS acts as a hydrogen bromide scavenger, thereby causing a change in the bromination mechanism from an acid-catalysed pathway to a bromonium ion mechanism. 

Anti stereochemistry results from the bromonium ion mechanism. When a nucleophile attacks a halonium ion, it must do so from the back side, in a manner similar to the SN2 displacement. This back-side attack assures anti stereochemistry of addition. 

Checking that you understand the bromonium ion mechanism with an external nucleophile. 

Now, how does the bromonium ion mechanism account for a/i/i-addition Using models, let us first consider addition of bromine to ciJ-2-butene (Fig. 7.6). 

While the bromonium ion mechanism is typical for isolated alkenes, the greater stability of the resonance-stabilized carbocation will make it the lower energy intermediate for conjugated systems. 

An intermediate that cannot be observed spectroscopically might be trapped by added reagents. In Chapter 5 we discussed the use of spin trap reagents to capture transient radicals for analysis by EPR spectrometry. Another example can be seen in the studies that led to the bromonium ion mechanism for the addition of bromine to frans-2-butene (14) to produce meso-2,3-dibromobutane (15, equation 6.7). Adding a nucleophile such as methanol to the reaction mixture led to a product incorporating the nucleophile (16) as shown in equation 6.8, which suggested that a cation may be an intermediate in the reaction. Additional evidence is necessary to determine the structure of the intermediate (i.e., bromonium ion or bromine-substituted carbocation), but that is a matter of detail once the existence of an intermediate of some kind is established. 

FIGURE 10.16 The bromonium ion mechanism demands that the products be formed by anti addition, as they are. This stereochemical experiment uses cyclohexene to show that the bromine and solvent molecule become attached from different sides of the ring through the anti addition enforced by the presence of the bromonium ion. 

The fact that rearrangements characteristic of carbocation intermediates are not observed in bromination supports a bromonium ion mechanism as does the isolation and characterization of a stable bromonium ion. Rearrangements, however, are sometimes observed in chlorine addition. 

Mechanism 6.6 describes the bromonium ion mechanism for the reaction of cyclopen-tene with bromine. 

Addition of bromine has been the subject of a number of studies. When a neutral salt of maleic or fumaric acid (A = COO ) is brominated in water, the meso derivative is the predominant product. Bell and Pring also report that the meso product is obtained on bromination of the diethyl esters of fumaric and maleic acids. Terry and Eicheberger observed 78% meso product from disodium maleate, a cis addition product which is unexpected. The prevalent bromonium ion mechanism proposed by Roberts and Kim-ball could easily explain the trans addition as follows ... 

Thus the mechanism for electrophilic addition of Bi2 to ethylene as presented m Figure 6 12 IS characterized by the direct formation of a cyclic bromonium ion as its... 

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... 

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... 

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... 

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... 

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. 

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 simplified mechanism shown in Scheme 2 focuses attention on the ionic intermediate which is presented in the usual form of a bromonium ion although it is now well known that it can be an open p-bromocarbocation (ref. 4). 

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... 

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. ... 

Markovnikov s rule is also usually followed where bromonium ions or other three-membered rings are intermediates. This means that in these cases attack by W must resemble the SnI rather than the Sn2 mechanism (see p. 461), though the overall stereospecific anti addition in these reactions means that the nucleophilic substitution step is taking place with inversion of configuration. 

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

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 mechanism of this reaction involves formation of a bromonium ion, followed by attack to open it up ... 

The mechanism of this reaction will have three steps (1) formation of bromonium ion, (2) attack by water, and (3) deprotonation ... 

The possibility that 34 and 35 were formed via 33 was eliminated and hence it must be concluded that, in contradistinction to the reaction with 2,3-dibromocyclo-pentyl hydroperoxide 38), the Ag02CCF3-induced dioxabicyclization of 3,4-dibromo-cyclopentyl hydroperoxide involves preferential displacement of the ciy-3-bromine. It seems highly probable that this process is assisted by the vicinal bromine, i.e. that the frans-bromonium ion 36 is an intermediate. Failure to observe the analogous mechanism with 2,3-dibromocyclopentyl hydroperoxide presumably reflects the disfavoured nature of the mode of ring closure needed in the corresponding species 37. 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

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