Alkynes reagents, electrophilic

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Alkynes are electron-rich reagents. The triple bond acts as a nucleophile, and attacks the electrophile. Therefore, alkynes undergo electrophilic... 

Just as alkene chemistry is the chemistry of the carbon-carbon double bond, so alkyne chemistry is the chemistry of the carbon-carbon triple bond. Like alkenes, alkynes undergo electrophilic addition, and for the same reason availability of the loosely held tt electrons. For reasons that are not understood, the carbon-carbon triple bond is less reactive than the carbon-carbon double bond toward electrophilic reagents. 

The TT-electrons in both alkenes and alkynes provide a Lewis-base site for interaction with electrophilic reagents, which are Lewis acids, so one of the typical reactions of alkenes and alkynes is electrophilic addition as illustrated in Equations 11.2 and 11.3. Such reactions of alkynes can be stopped after the addition of one equivalent of a reagent, but the use of excess reagent leads to the formation of saturated products in which a second equivalent has been added. Some aspects of the chemistry of alk)mes will be explored in the experiments in this chapter. 

Alkynes undergo electrophilic addition reactions. The same reagents that add to alkenes also add to alkynes. 

Alkynes react with many of the same electrophilic reagents that add to the carbon-carbon double bond of alkenes Hydrogen halides for example add to alkynes to form alkenyl halides... 

Alkynes react with electrophilic selenium reagents such as phenylselenenyl tosylate.155 The reaction occurs with anti stereoselectivity. Aryl-substituted alkynes are regioselective, but alkyl-substituted alkynes are not. 

Mixed copper-zinc reagents also react with alkynes to give alkenylcopper species that can undergo subsequent electrophilic substitution. 

Such reactions involve the addition of organometallic reagent to unsaturated systems like alkynes, alkenes, allenes, or related structures in order to create a new C-C bond and C-M bond at the same time. The C-M bond can then be functionalized by an electrophile to generate a wide range of products. 

Scheme 3.16. Silylcupration of alkynes with (t-BuPh2Si)2CuLi-LiCN and electrophilic trapping of the vinylcuprate reagent [69].

In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The jt electrons in these systems provide regions of high electron density, and electrophilic reactions feature as... 

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