Halogenation, electrophilic alkenes

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... 

More than six decades after the postulation of the cyclic bromonium ion intermediate in electrophilic bromination of alkenes important, new findings are still emerging.567 Updated general treatments of the halogenation of alkenes became available.568,569... 

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. 

Calculated equilibrium geometries, bond lengths, and charge densities have been compared for halonium ions (1-5 X = Cl or Br) derived from the addition of halogen electrophiles to fluoro-substituted terminal alkenes (R = alkyl or perfluoroalkyl with... 

Conceptually similar a,a -annulations of ketone enamines have also been carried out with electrophilic alkenes containing a reactive allylic halogen, such as ethyl a-bromo-methylacrylate or dimethyl y-bromomesaconate318-320 (Scheme 156). An elegant one-pot synthesis of the adamantane ring system involving sequential double Stork and Dieckmann reactions has been developed by Stetter and Thomas321 (Scheme 157). Reaction of the bis-enamine 154 with ethyl a-bromomethylacrylate leads to the pentacy-clic system 155322 (Scheme 158). 

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],... 

Many electrophilic alkenes and stabilized a-halo carbanions (derived from a-halogenated ketones, esters, nitriles or sulphones) are reported to be applicable in this condensation (equation 53). Usually the reaction was carried out in the presence of sodium... 

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 ... 

Step 1 Make a new bond between a nucleophile and an electrophile and simultaneously break a bond to give stable molecules or ions. The reason for the extremely rapid hydrolysis of the sulfur mustards is neighboring group participation by sulfur in the ionization of the carbon-chlorine bond to form a cyclic sulfonium ion. This is the rate-determining step of the reaction although it is the slowest step, it is much faster than reaction of a typical primary chloroalkane with water. At this point, you should review halogenation of alkenes (Sections 6.3D and 6.3F) and compare the cyclic halonium ions formed there with the cyclic sulfonium ion formed here. 

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