Electrophilic Addition of Halogens to Alkenes

For electrophilic additions of halogens to alkenes, not only is the reaction rate strongly solvent-dependent [79-81] [cf. Eq. (5-29) in Section 5.3.2), but the stereochemical course may also be affected by the polarity of the medium [79, 386-388], For example, the stereoselectivity of bromine addition to cis- and trans -stilbene according to Eq. (5-140) has been found to be solvent-dependent, as shown in Table 5-23 [79, 386],... 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

Interactions between the C=C double bond and halogens deserve particular attention. The importance of these interactions arises from the fact they are the first step of the reactions of addition of halogens to alkenes (and, in general, of the electrophilic addition to the carbon-carbon double bond68), which is a very useful functionalization reaction of the alkene double bond. 

A strongly solvent-dependent electrophilic reaction is the addition of halogens to alkenes [79-81] and alkynes [81a]. In a rapid equilibrium, a loose transitory EPD/EPA complex (1 1) between halogen and alkene is formed [512]. This is followed by the ratedetermining step, which involves an SNl-hke unimolecular ionization to form a halo-nium intermediate which can be either symmetrical or unsymmetrical. This then reacts with a nucleophile Nu to give the products cf. Eq. (5-29). 

We can see from these examples that, if the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon. As a consequence, the halogen ends np at the more snbstituted carbon. This phenomenon, referred to as the Markovnikov rule, can be explained by what we know about the mechanism of electrophile additions of protons to alkenes. The key is the relative stability of the resulting carbocation intermediates. 

The electrophile-induced cyclization of heteroatom nucleophiles onto an adjacent alkene function is a common strategy in heterocycle synthesis (319,320) and has been extended to electrophile-assisted nitrone generation (Scheme 1.62). The formation of a cyclic cationic species 296 from the reaction of an electrophile (E ), such as a halogen, with an alkene is well known and can be used to N-alkylate an oxime and so generate a nitrone (297). Thus, electrophile-promoted oxime-alkene reactions can occur at room temperature rather than under thermolysis as is common with 1,3-APT reactions. The induction of the addition of oximes to alkenes has been performed in an intramolecular sense with A-bromosuccinimide (NBS) (321-323), A-iodosuccinimide (NIS) (321), h (321,322), and ICl (321) for subsequent cycloaddition reactions of the cyclic nitrones with alkenes and alkynes. 

We shall give particular attention here to the addition of bromine to alkenes because this reaction is carried out very conveniently in the laboratory and illustrates a number of important points about electrophilic addition reactions. Much of what follows applies to addition of the other halogens, except fluorine. 

This is an example of electrophilic addition of Cl to an alkene. The mechanism of this reaction involves the following steps. In the first step, the ethylene reacts with chlorine to form the cyclic ethylene chloronium ion (intermediate) and chloride ion. Note that in this cyclic intermediate, the chlorine has a positive charge. This step is followed by the nucleophilic attack by chloride ion on the chloronium ion. The reaction is enhanced by electron-donating substituents such as alkyl groups on the carbon-carbon double bond, since such groups can further stabilize the formation of the transition state which results in the formation of the chloronium ion. Halogen addition is usually an anti addition process. 

The absence of rearrangement and the anti stereochemistry of the addition product must be accommodated in a proposed mechanism for the addition of a halogen to an alkene involving a carbocation intermediate. The first step of the reaction mechanism is the electrophilic addition of bromine to the Jt bond to give a three-membered ring called a tydic bromonium ion. 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

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