Metal acetylide

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The general structure of a metal acetylide

A metal acetylide is an alkyne that has had a proton (hydrogen) removed from the terminal end by a metal such as sodium or an organolithium. So, for example, the alkyne CH₃C≡CH could be deprotonated to form the acetylide ion CH₃C≡C⁻. Once it has been deprotonated it becomes a strong nucleophile. Some acetylides, eg. silver acetylide or copper acetylide, are explosives.

Acetylide ions are very useful in organic chemistry reactions in combining carbon chains, particularly addition and substitution reactions. One type of reaction displayed by acetylides are addition reactions with ketones to form tertiary alcohols. In the reaction in 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.

Scheme 1. Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.

Scheme 1. Reaction of ethyl propiolate with n-butyllithium to form the lithium acetylide.

Coupling reactions of alkynes like the Sonogashira coupling, the Cadiot-Chodkiewicz coupling, the Glaser coupling and the Eglinton coupling often have metal acetylides as intermediates.

Several modifications of the reaction with carbonyls 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 .
• In the Isler modification ethoxyacetylene is replaced by beta-chlorovinyl ether and lithium amide.
• In the Favorskii-Babayan synthesis ketones and acetylenic compounds react in presence of alkali .

Formation of acetylides poses a risk in handling of gaseous acetylene in presence of metals such as mercury, silver or copper, or alloys with their high content (brass, bronze, silver solder).