Electrophilic addition cyclic bromonium ions

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... 

Yet another intermediate, a cyclic bromonium ion, 15, is involved in the electrophilic addition of bromine to 3-hexyne, 1 -hexyne (32,33), and other alkylacetylenes (44). In these reactions, only trans dibromides have been... 

From alkylacetylenes trans dibromide is formed in large or quantitative yields and no detectable amounts of cis or solvent-incorporated diadducts were observed. The indications are that in this case the electrophilic addition occurs via a cyclic bromonium ion (47) and that this is probably more stable as an intermediate than the linear alkyl vinyl oation. From 47 only trans addition is expected to occur. 

The bromonium ion postulate, made more than 60 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. More recently, strong evidence supporting the mechanism has come from the work of George Olah, who has prepared and studied stable solutions of cyclic bromonium ions in liquid SO-2. There s no question that bromonium ions exist. 

This analysis of the simple addition of an electrophilic bromine molecule to a symmetrical alkene or alkyne has highlighted many points. First, there is the induction of a temporary dipole of the soft electrophile by the n electrons of the carbon/carbon double bond. Second, there is the heterolytic fission of the bromine molecule, and the subsequent formation of the cyclic bromonium ion. Third, this cyclic intermediate places certain restrictions on the potential line of attack for the second reagent, and so controls the structural and stereochemical consequences for the product. 

In the first step of the oxymercuration mechanism, the electrophilic mercury of mercuric acetate adds to the double bond. (Two of mercury s 5d electrons are shown.) Because carbocation rearrangements do not occur, we can conclude that the product of the addition reaction is a cyclic mercurinium ion rather than a carbocation. The reaction is analogous to the addition of Br2 to an alkene to form a cyclic bromonium ion. 

The mechanism for the addition of oxygen to a double bond to form an epoxide is analogous to the mechanism described in Section 4.7 for the addition of bromine to a double bond to form a cyclic bromonium ion. In one case the electrophile is oxygen, and in the other it is bromine. So the reaction of an alkene with a peroxyacid, like the reaction of an alkene with Br2, is an electrophilic addition reaction. 

The elemental halogens are important electrophiles. An important and familiar example is the electrophilic addition of bromine to carbon-carbon double bonds. As mentioned in Section 1.12, the first product of electrophilic addition is a cyclic bromonium ion. 

The reaction of bromine with (E)-stilbene (47) to give meso-stilbene dibromide (48) as the major product (Eq. 10.21) is another example of an electrophilic addition reaction of alkenes. The addition of bromine to many alkenes is a stereospecific reaction that proceeds by anti addition to the double bond. However, the addition of bromine to 47 is not stereospecific because small amounts of dl-stilbene dibromide (49) are also formed in this reaction. The formation of wreso-stilbene dibromide presumably occurs via the nucleophilic attack of bromide on the intermediate cyclic bromonium ion, 50. The possible interconversion of 50 and the acyclic carbocation 51 (Eq. 10.22) is one possible way to account for the presence of dl-stilbene dibromide in the product. 

The bromine molecule (Br2) is normally symmetrical. However, as it approaches the nucleophilic and electron-rich tt bond of the alkene, it becomes polarized by induction and can then function as the electrophile in an addition reaction. The result is the generation of a cyclic bromonium ion ... 

In contrast, a cyclic bromonium ion is postulated in alkylacetylene bromina-tion with the isolation of tran -dibromides from hex-3-ynes andhex-l-ynes. A kinetic and product study of the reaction of diphenylacetylenes with triphenylaluminium is interpreted as monomeric aluminium attacking the acetylene electrophilically in the rate-determining step (71) c/j-Addition products are obtained. Vinylalanes, formed by addition of aluminium... 

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. 

To account for the stereospecificity of bromine addition to alkenes, it has been suggested that in the initial electrophilic attack of bromine a cyclic intermediate is formed that has bromine bonded to both carbons of the double bond. Such a bridged ion is called a bromonium ion because the bromine formally carries the positive charge ... 

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... 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

Larger electrophiles, especially ones with lone pairs (e.g., Br + from Br2), add to give positively charged three-membered rings (bromonium ion, chloronium ion. etc.). Nucleophiles react with these ions in much the same way that they open the rings of the cyclic alkyloxonium ions of Chapter 9. Addition occurs at the most highly substituted carbon (Markovnikov orientation) in an anti manner. 

The next two are examples of additions of electrophiles that form bridged cationic intermediates a cychc bromonium ion in the first case (Section 12-5) and a cyclic mercurinium ion in the second (Section 12-7). Additions therefore proceed stereospecifically anti, because the cychc ion can be attacked only from the direction opposite the location of the bridging electrophile (Figure 12-3). With Br2, identical atoms add to both alkene carbons, so regiochemistry is not a consideration. In oxymer-curation, the nucleophile is water, and it wiU add to the most substituted alkene carbon, because the latter is tertiary and possesses the greatest partial positive charge. Stereochemistry must be considered because addition of the electrophile occurs with equal probability from the top and from the bottom and stereocenters are created. So we have... 

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