Alkenes, mixed halogenation

Numerous reagents may react with alkenes to yield mixed 1,2-dihalogenated compounds. Interhalogens prepared on mixing two different halides or their stoichiometric equivalents are used in mixed halogenations. 

The mixed halogens ICl, IBr, and BrCl also add to alkenes. White and Robertson found third-order kinetics for the reaction and determined the relative reactivities to be BrCl > ICl > Br2 > IBr > l2- The l ge reactivity of BrCl means that addition of BrCl can occur when alkenes react with mixtures of Br2 and Cl2. The regioselectivity of addition of mixed halogens was reported by Ingold and Smith. Addition of ICl to propene gave 69% of 2-chloro-l-iodo-propane (30) and 31% of l-chloro-2-iodopropane (31), and addition to styrene gave more than 95% of l-chloro-2-iodo-l-phenylethane. ... 

Tertiary carbocations may be conveniently prepared in such media from alkyl halides, alcohols, and alkenes. Secondary cations can be observed at low temperatures, but they rearrange readily to more stable tertiary ions. For such cases, special techniques of mixing the reactants, e.g. cocondensation on a cold surface ( molecular beam technique ), have been developed43. Attempts to prepare primary ions in the same manner have not been successful. Methyl and ethyl fluorides exchange halogen but do not generate observable concentrations of cations. All other simple... 

Reactions of alkenes with Br2 or CI2 are generally carried out by mixing the alkene and the halogen in an inert solvent, such as dichloromethane (CH2CI2), that readily... 

Insertion of alkynes can also be used to form a carbon-carbon bond at the same time as the carbon-zirconium bond. This process, carbometallation, is useful for the stereospecific construction of trisubstituted alkenes (Scheme 5.63). The active reagent is formed by mixing zirconocene dichloride and trimethylaluminium. Both metals are required for the reaction to proceed. The vinyl organometallic 5.221 produced may be subjected to protonolysis or halogenation. An application with iodination can be found in Scheme 8.121. 

When CI2 (or Br2) adds to an alkene, the final product of the reaction is a vicinal dichloride (or dibromide). Vicinal means that the two bromines are on adjacent carbons (vicinus is the Latin word for near ). Reactions of alkenes with Br2 or CI2 are generally carried out by mixing the alkene and the halogen in an inert solvent, such as dichloromethane (CH2CI2), which readily dissolves both reactants, but does not participate in the reaction. 

Zweifel s group has developed two complementary syntheses of 1,1-dihalo-alkenes this permits mixed dihaloalkenes to be prepared. Thus, the hydro-boration of 1-halo-l-alkynes (117) followed by oxidation gives the borinic alkenes (118), which are converted into the bromohaloalkenes (119) stereo-specifically. Alternatively, tra 5-halogenation of ( )-haloalkenylsilanes (120) followed by a/iti-desilicohalogenation gives the ( )-dihaloalkenes (121) [similarly. 

In ehmination reactions, a similar comparison of the stereochemistry of starting materials and products can indicate syn, anti, or mixed processes. One method for changing the stereochemistry of a double bond includes halogenation to the dihalide followed by heating with sodium iodide to affect first an Sj 2 reaction and then anti-elimination, illustrated for one iodination product in Equation 4.5 [7]. Kinetic data support this sequence of reaction steps leading to the trans alkene as the only product. 

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