Halogenation reactions alkenes

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

In agreement with the involvement of ionic intermediates for electrophilic halogenation of alkenes, an important role is also exerted by the solvent. Not only the reaction rate is strongly solvent-dependent, but also the stereochemical course of the addition process may be affected by the polarity of the medium. Solvent properties determine the reaction rate the overall kinetic order the nature of the products the stereochemistry of the products... 

When halogenation of alkenes is carried out in aqueous solvent, a vicinal halohydrm is obtained. The reaction is regioselective, and follows the Markovnikov rule. The halide adds to the less substituted carbon atom via a bridged halonium ion intermediate, and the hydroxyl adds to the more substituted carbon atom. The reaction mechanism is similar to the halogenation of alkenes, except that instead of the halide nucleophile, the water attacks as a nucleophile. 

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. 

When mechanisms involving radicals can be discounted, the main and general reaction pathway of addition of halogens to alkenes is shown in Scheme 10, where X is mainly Cl and Br69-70. 

Halogen addition is another example of a stereospecific reaction, in which different stereoisomers of the starting material give different stereoisomers of the product. Figure 8-4 shows additional examples of the anti addition of halogens to alkenes. 

The halogenation reaction proceeds in darkness and is reasonably considered as ionic. It has been shown that if chlorine gives primarily free radical addition on mono- and di-substituted alkenes, it gives ionic substitution products with tri- and tetra-substituted alkenes [73], A model compound study, together with NMR analysis of commercial chloro and bromobutyl samples, confirmed that the reaction on isoprenyl unit leads predominantly to the exomethylene-substituted structure A, and this is explained by steric hindrance due to the tetra-substituted carbon in f3-position which favors proton elimination rather than the nucleophilic attack of halide counter ion in the second phase of addition (Fig. 11, Table 1) [74,75]. 

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). 

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],... 

Selenium and tellurium reagents have been used for stereoselective halogenations of alkenes. For example, trans addition of benzeneselenenyl chloride to alkenes followed by the displacement of the seleno moie with chloride can lead to dr-1,2-dichlorides (equation 25). The addition of 2-naph-thyltellurium trichloride proceeds in an anti stereospecific manner (equation 26), whereas tellurium tetra-diloride gives a mixture of syn and anti adducts. The reaction of allyl esters with tellurium tetrachloride accompanies acyl migration to give the l-(trichloiottlluro)-3-chk>io adduct (54 equation 27). ... 

Table 9 contains some additional comparable reactions of acyclic, halogen-free alkenes with elemental fluorine. 

Reductive coupling of polyhaloalkanes with carbonyl compounds in the presence of Zn is used in the synthesis of various halogen-substituted alkenes. For example, the reaction of benzaldehyde with the organozinc reagent 4.101 gives the trifluoromethyl-substituted alkene 4.102 (Scheme 4.50). 

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