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Concept
Alkynylation
Résumé
In organic chemistry, alkynylation is an addition reaction in which a terminal alkyne () is added to a carbonyl group () to form an α-alkynyl alcohol ().
When the acetylide is formed from acetylene (), the reaction gives an α-ethynyl alcohol. This process is often referred to as ethynylation. Such processes often involve metal acetylide intermediates.
The principal reaction of interest involves the addition of the acetylene () to a ketone () or aldehyde ():
RR'C=O + HC#CR'' -> RR'C(OH)C#CR''
The reaction proceeds with retention of the triple bond. For aldehydes and unsymmetrical ketones, the product is chiral, hence there is interest in asymmetric variants. These reactions invariably involve metal-acetylide intermediates.
This reaction was discovered by chemist John Ulric Nef in 1899 while experimenting with reactions of elemental sodium, phenylacetylene, and acetophenone. For this reason, the reaction is sometimes referred to as Nef synthesis. Sometimes this reaction is erroneously called the Nef reaction, a name more often used to describe a different reaction (see Nef reaction). Chemist Walter Reppe coined the term ethynylation during his work with acetylene and carbonyl compounds.
In the following reaction (scheme 1), the alkyne proton of ethyl propiolate is deprotonated by n-butyllithium at -78 °C to form lithium ethyl propiolate to which cyclopentanone is added forming a lithium alkoxide. Acetic acid is added to remove lithium and liberate the free alcohol.
Several modifications of alkynylation reactions are known:
In the Arens–van Dorp synthesis the compound ethoxyacetylene is converted to a Grignard reagent and reacted with a ketone, the reaction product is a propargyl alcohol.
The Isler modification is a modification of Arens–Van Dorp Synthesis where ethoxyacetylene is replaced by β-chlorovinyl ether and lithium amide.
Alkynylations, including the asymmetric variety, have been developed as metal-catalyzed reactions. Various catalytic additions of alkynes to electrophiles in water have also been developed.
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