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A metal acetylide is an alkyne for which the terminal proton (hydrogen) has been replaced by a metal such as sodium or an organolithium.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.[1]
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 [2] is converted to a Grignard reagent and reacted with a ketone, the reaction product is a propargyl alcohol [3].
- 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 [4].
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).
References
- ^ Synthesis of alkyl 4-hydroxy-2-alkynoates M. Mark Midland, Alfonso Tramontano, John R. Cable J. Org. Chem.; 1980; 45(1); 28-29. Abstract
- ^ Organic Syntheses, Coll. Vol. 4, p.404 (1963); Vol. 34, p.46 (1954). Link
- ^ van Dorp and Arens, Nature, 160, 189 (1947).
- ^ Favorskii-Babayan Synthesis
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