Addition Involving Symmetrically Bridged Cationic Intermediates

Addition Involving Symmetrically Bridged Cationic Intermediates 

In reactions with HX, alkenes behave more or less as Brpnsted-Lowry bases. Alkenes can also function as Lewis bases in reactions with electrophilic halogens. Symmetrical reagents such as bromine or chlorine are not usually polarized, but when they are brought into proximity with a n bond, a dipole is induced in the X—X bond [C=C- X+X ]. This type of structure is sometimes called a charge transfer complex. It is important to note that this occurs only with reagents such as diatomic chlorine, bromine, or iodine that are polarizable. Molecules that are not polarizable, such as diatomic hydrogen, do not react in this way, and actually require 

The diastereoselectivity that arises from the intermediacy of a halonium ion has been exploited in a new type of reaction that produces lactone products. When 322 was treated with iodine and sodium bicarbonate, the initially formed iodonium ion (322) was opened by the carboxylate anion, generated by the reaction of bicarbonate with the acid moiety.Backside displacement of the iodonium ion led to the trans-relationship between the lactone ring and the iodide moiety in iodolactone 324. This reaction is commonly referred to as iodolactonization.251 This reaction can be extended to include bromolactonization and chlorolactonization. 

Chapter 2. Acids, Bases, Functional Group Exchanges 

Another addition reaction involves reaction of alkenes with aqueous solutions of bromine or chlorine rather than the anhydrous conditions noted above. The reaction can proceed by formation of a cation under aqueous 

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C. Addition Involving Symmetrically Bridged Cationic Intermediates... 

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