Alkynes alkynylide alkylation

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. 

Lithium or sodium alkynylides derived from 1-alkynes react with primary alkyl halides in the presence of HMPA to give disubstituted alkynes. This method is limited to primary alkyl halides that are not branched at the (3-position. Secondary and tertiary alkyl halides tend to undergo dehydrohalogenation (E2-elimination). 

Tandem chain extension at both the 1-alkyne and propargylic positions of 1-alkynes by sequential (1) alkylation and (2) hydroxy-alkylation (1,2-addition) of 1,3-dilithiated alkynes provides an attractive, one-pot preparation of functionally disubstituted alkynes. The required 1,3-dilithiated species are obtained by lithiation of 1-alkynes at both the 1- and 3- positions with two equivalents of n-BuLi in the presence of TMEDA (tetramethylethylenediamine). The two-stage chain extension depicted below for propyne involves initial alkylation of the dilithiated propyne with the less reactive electrophile, n-BuBr, at the more nucleophilic propargylic position. Subsequent hydroxyalkylation at the less nucleophilic alkynylide position with the more electrophilic formaldehyde furnishes the a-hydroxyalkyne. ... 

Corey-Fuchs reaction is a two step reaction converting an aldehyde to an alkyne by one-carbon homologation of the aldehyde. The Wittig-like reaction of aldehyde 1 and dibromocarbene forms dibromoalkene 2. The treatment of dibromoalkene 2 with two equivalent of -BuLi form a lithium alkynylide, quenched by eletrophiles, such as proton, CO2, aldehydes, ketones, and alkyl halides to form alkyne 3. 

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