Bromonium ions, cyclic, addition reactions with alkenes forming

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. 

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 ... 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

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. 

Anti additions to alkenes typically result in a stepwise mechanism formation of a cationic cyclic intermediate such as a bromonium ion, followed by backside attack by a nucleophile to open up the ring. Such is the mechanism for the anti addition of Br2. The bromination reaction results in trans bromines since the second bromine (as Br ) has to come in from the opposite face as the first bromine (as Br+) in order to do an Sn2 attack on the bromonium ion. Other mechanisms involving the bromonium ion include reaction of an aikene with Br2/H20 (adds -Br and -OH anti) and Br2/ROH (adds -Br and -OR flnfi)-Treatment of an aikene with a peroxyacid in water forms an epoxide that undergoes a ring opening in situ to give a trans diol product. Trans stereoselectivity is also seen in the dissolving metal reduction of alkynes. 

---

See also in sourсe #XX -- [ , , , , , ]

---