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In organic chemistry, alkynylation is an addition reaction in which a terminal alkyne (−C≡CH) is added to a carbonyl group (C=O) to form an α-alkynyl alcohol (R 2 C(−OH)−C≡C−R). [1] [2] When the acetylide is formed from acetylene (HC≡CH), the reaction gives an α-ethynyl alcohol. This process is often referred to as ethynylation.
A metal acetylide is formed in situ when an alkyne is treated with a strong bases such as a hydroxide or an alkoxide: [3] HC≡CH + KOH ⇌ HC≡CK + H 2 O; RR'C=O + HC≡CK ⇌ RR'C(OK)C≡CH; The metal acetylide then reacts with an aldehyde or ketone to form a propargyl alcohol.
Multiple carbon–carbon bond are replaced by carbonyl (C=O) groups, such as aldehydes, ketones, and carboxylic acids. The reaction is predominantly applied to alkenes, but alkynes and azo compounds are also susceptible to cleavage. The outcome of the reaction depends on the type of multiple bond being oxidized and the work-up conditions. [1]
In broad strokes, the mechanism of the reaction is believed to first involve a Mannich-like addition of the alkynylmetal species into the iminium ion formed by condensation of the aldehyde and the secondary amine. This first part of the process is a so-called A 3 coupling reaction (A 3 stands for aldehyde-alkyne-amine
The imine can isomerise and the alkyne group is placed at the other available nitrogen alpha position. [11] [12] [13] This reaction requires a copper catalyst. The redox A 3 coupling has the same product outcome but the reactants are again an aldehyde, an amine and an alkyne as in the regular A 3 coupling. [11] [14] [15] [16]
The overall combined transformation of an aldehyde to an alkyne by this method is named after its developers, American chemists Elias James Corey and Philip L. Fuchs. The Corey–Fuchs reaction By suitable choice of base, it is often possible to stop the reaction at the 1-bromoalkyne, a useful functional group for further transformation.
The Seyferth–Gilbert homologation is a chemical reaction of an aryl ketone 1 (or aldehyde) with dimethyl (diazomethyl)phosphonate 2 and potassium tert-butoxide to give substituted alkynes 3. [ 1 ] [ 2 ] Dimethyl (diazomethyl)phosphonate 2 is often called the Seyferth–Gilbert reagent .
The alkyne zipper reaction requires a strong base, which can be generated from the reaction of potassium hydride and a diamine: [3] [1] Alkyne zipper reaction. The potassium 3-aminopropylamide deprotonates the less-substituted methylene adjacent to the alkyne group. [3] [1] Example mechanism for alkyne zipper reaction.