Bromonium ions, as intermediates

We believe that this difference in chemistry arises because of the role of bromonium ions as intermediates in the ISB-DMB case. It should be noted, however, that addition, and not substitution, is the major reaction between tri-or tetra-substituted olefins and bromine. But the relative heats of formation of bromonium ions are sensitive to the degree of methyl substitution, as are the rates of bromination. We conclude that the introduction of the second methyl group at the olefinic bond in the ISB-DMB copolymer leads to a dramatic change in reaction mechanism. That in the isoprene... 

We used a similar approach in aprotic solvents and produced bromonium-bromide type intermediates by reacting stilbene bromohydrins with hydrogen bromide using a vigorous stream of the gaseous reagent to strip the Br2 generated by attack of Br to Br+ of the bromonium ion, as shown is Scheme 6 for erythro-2-bromo-1,2-diphenylethanol. 

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... 

The electrophilic bromonium ion adds to the diene at the position which yields the most stable cationic intermediate and the stereochemical relation of the Br and the MeO group in the product is always trans when the diene system is cyclic. The fact that 1,2-addition takes place selectively but 1,4-addition does not occur is explained by the formation of the bridged bromonium ion as the intermediate. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

Oxymercuration involves electrophilic addition to the carbon-carbon double bond, with the mercuric ion acting as electrophile. The absence of rearrangement and the high degree of stereospecificity (typically anti)—in the oxymercuration step—argues against an open carbonium ion as intermediate. Instead, it has been proposed, there is formed a cyclic mercurinium ion analogous to the bromonium... 

You can think of the bromonium Ion as a carbocatlon that has been stabilized by Interaction with a nearby bromine atom. You have seen a similar effect with oxygen—this oxonlum Ion was an Intermediate, for example, In the SnI substitution of MOM chloride on p. 000 of Chapter 17. 

In this case there is an initial acid/base reaction in which the hydroxyl oxygen is protonated, and this in turn activates the carbon to which the oxygen is attached. Then, there is an internal attack by the bromine atom, which forms a bridged bromonium ion as an intermediate. This may then be attacked by the incoming bromide ion at either end to give the desired products. It is the formation of the bridged bromonium ion, and the stereochemical demands thereby placed on any further attack, that explains the stereochemistry of the products. 

The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord with the picture of the intermediate bromonium ion as a weakly bridged species. 

As in the case of hydrogen halide additions, this attack should lead to anti addition. Alternatively, one can account for the observed third-order kinetics and retain the bromonium ion as a key intermediate by postulating that the bromonium ion is in equilibrium with such a complex ... 

Chiral bromohydrin derivatives reacted under acidic conditions with very high stereoselectivity (essentially stereospecific). This points to a mechanism involving a chiral bromonium ion as the reactive intermediate. The protonation of chloroethane by the carborane superacid H(CHBuClii) proceeds via a shared-proton intermediate that decays by HCl loss to form the carbocation-like ethyl carborane. This reacts with a second EtCl to form the Et2Cl+ cation. The transannular electrophilic bromination of a polycyclic system with two C=C in close proximity was studied by computational methods. The initial bromonium was found to rearrange into more stable carbocations through reaction with the nearby carbon—carbon double bond. 

The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord with the picture of the intermediate bromonium ion as a weakly bridged species. If the C-Br bonds were of equal strength, preferential attack at the least hindered carbon would be anticipated. The addition of bromide salts to the reaction mixture... 

Low-temperature experiments have since confirmed the existence of bromonium ions as the intermediate in electrophilic additions of Br2 to alkenes, including additions to wmcyclic alkenes such as the one in Model 1. 

In summary, it appears friat bromination usually involves a charge-transfer complex which collapses to an ion-pair intermediate. The cation can be a carbocation, as in the case of styrenes, or a bromonium ioiL The complex can evidently also be captured by bromide ion when it is present in sufficiently high concentration. 

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. 

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