Halogenation of alkynes

See also page 410, Section 2, for additional preparations of vinylic halides. 

AgN03 ICN (on RC—CMgBr) Cul or Znlj, (Me3SiO)2 n-BuLi/ZnI2, (MejSiO  

Review Comprehensive Organic Synthesis, Eds. B. M. Trost and I. Fleming, Pergamon, Oxford (1991), Vol 4, Part 1.7, p 269 

The monosubstituted intermediates do not seem to be effectively bridged, since syn addition predominates. A very short-lived vinyl cation appears to be the best description of the intermediate in this case.  

The stereochemistry of bromination is usually anti for alkyl-substituted alkynes. A series of substituted arylalkynes has been examined in dichloroethane. As with alkenes, a TT-complex intermediate was observable. The A// for formation of the complex with 1-phenylpropyne is about —3.0kcal/mol. The overall kinetics are third order, as for an Ad S mechanism. The rate-determining step is the reaction of Br2 with the TT complex to form a vinyl cation, and both syn and anti addition products are formed. 

For the aryl-substituted alkynes, the reaction stereochemistry is sensitive to the aryl substitution. With EWG substituents (NO2, CN) the reaction becomes stereospecif-ically anti and the same is true for 2-hexyne, reflecting the diminished stability of the vinyl cation in these cases. Aryl-substituted alkynes can be shifted toward anti addition by including a bromide salt in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by a bromide ion. This intermediate is presumably the complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following equations. 

This scheme shows an alkyne-bromine complex as an intermediate in all alkyne brominations, which is analogous to the case of alkenes. The complex may dissociate to a vinyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latter is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are nonstereospecific. 

As for alkenes, the alkyne-Btj complex can be intercepted by nucleophilic solvent. Alkynes react with -Bu4N Br3 in methanol to give a mixture of dimethoxydi-bromo and A-dibromo products. A key aspect of this reaction is the high reactivity of the methoxybromo intermediate, which is more reactive than the starting material. Evidently, the dibromo intermediate is unreactive toward Br 

In the previous chapter, we saw that alkenes will react with Bt2 or CI2 to produce a dihalide. In much the same way, alkynes are also observed to undergo halogenation. The one major difference is that alkynes have two it bonds rather than one and can, therefore, add two equivalents of the halogen to form a tetrahahde  

In some cases, it is possible to add just one equivalent of halogen to produce a dihaUde. Such a reaction generally proceeds via an anti addition (just as we saw with alkenes), producing the E isomer as the major product  

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Halogenation of alkenyl organometallic compounds Addition of hydrogen halides to triple bonds Halogenation of alkynes or allenes Addition of alkyl halides to triple bonds Addition of acyl halides to triple bonds... 

Halogenation of alkynes or allenes 5-33 Addition of alkyl halides to triple bonds... 

Scheme 3.15 Highly (>98%) stereoselective routes to types lll-VIII alkenyl halides by halogenations of alkynes [88-94].

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