Alkylation of alkynes

Alkylation of Alkynes. Organic halides can alkylate acetylenes in the presence of Lewis acids. In most cases, however, the products are more reactive than the starting acetylenes. This and the ready polymerization of acetylenes under the reaction conditions result in the formation of substantial amounts of byproducts. Allyl, benzyl, and tert-alkyl halides giving stable carbocations under mild conditions are the best reagents to add to acetylenes.44 56 

The alkylation of phenylacetylene with ferf-butyl chloride, benzyl chloride, and diphenylmethyl chloride follows an AdE2 mechanism57-59 with the involvement of the open vinyl cationic intermediate 17  

The anti stereoselectivity originates from the preferential attack of the chloride ion on 17 from the less hindered side. Stereoselectivity, therefore, is determined by the relative effect of the two p substituents. Alkyl-substituted alkynes show decreased stereoselectivity compared with that of aryl-substituted acetylenes.59 60 

Now we will see how to combine smaller structural units, one of which already has a carbon—carbon triple bond, to build an alkyne having a more complex structure. 

The attachment of an alkyl group to a selected molecular structure is called alkylatioii. We will encounter this method of forming carbon—carbon bonds repeatedly in later chapters dealing with other functional groups. Alkylation is one of the fundamental reactions used to construct complex structures. 

The alkylation reaction considered here replaces the hydrogen atom of a terminal alkyne by an alkyl group derived from an alkyl hahde. In the first step, the alkynide ion forms. Sodium amide in ammonia is a strong base that quantitatively abstracts protons from C—H bonds of sp-hybridized carbon atoms. 

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Alkylation of alkynes. The reaction of alkynes with carbonyl compounds in lolucne-aqueous NaOH is markedly accelerated by catalytic amounts of tetra-n-butylammonium bromide. 

The only functional group is the triple bond, and we shall want to use the chemistry of alkynes to show us where to disconnect. You know that alkylation of alkynes is a reliable reaction, so a sensible disconnection is next to the triple bond. 

The most utilized method for alkylation of alkynes is via alkynylide anions. Their reaction with electrophilic reagents provides access to both terminal and internal alkynes as well as to functionally substituted alkynes. The higher electronegativity of carbon in the sp-hybridization state imparts relatively greater acidity to acetylene and 1-alkynes (pATa 24-26), so that bases such as alkyllithiums, lithium dialkylamides. 

Alkylation of alkynes. Ethynylsilanes are alkylated at the unsubstituted terminus in the presence of Sm and Smia. 

Chapter 17 Aldehydes and Ketones Nucleophilic Addition to the Carbonyl Croup (b) Alkylation of alkyne... 

Nickel-Catalyzed Alkylation of Alkynes with Alkyl Halides, Nickel-Catalyzed Sonogashira Reaction... 

Bis(isopinocampheyl)borane has been used in the enantioselective preparation of alkynes by the reaction sequence for alkylation of alkynes (see page 452). 

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