Glaser coupling
In the Glaser coupling is a name reaction in organic chemistry, which was named after its discoverer, Carl Glaser. The reaction is used for the coupling of two terminal ( terminal ) alkynes to diynes. It is the oldest known coupling reaction of alkynes.
Overview reaction
Mechanism
The Glaser coupling takes place in an aqueous or alcoholic ammonia solution. As a catalyst of copper (I ) salts to be used. For this purpose, are, for example copper ( I) chloride, copper ( I) bromide or copper (I ) acetate. In the first step, a base ( classical, ammonia ) is used, which cleaves the C-H acidic proton of the alkyne. Since the mechanism is complex and not yet completely clear, the next step is running probably from this: The acetylide anion forms a copper complex with copper ( II ) cations. By oxidation of the copper (I ) salts to the oxidant is oxygen, the copper (II ) cations are produced. Transferred in the copper complex, the acetylide anions one electron on a copper ( II) cation. Thus, the acetylide anions are oxidized, reducing the copper (II) cations of copper (I ) cations. The result is a symmetrical diyne.
The Glaser coupling and the Eglinton coupling suitable for the synthesis of cyclic polyynes.
Versions
Hay coupling
The Hay- coupling reaction analogous to the Glaser coupling. Merely using TMEDA ( TEMED ) as a base, which contributes to increase the solubility in organic solvents and thus allowing the use of a wider range of solvents.
Eglinton coupling
The Eglinton coupling is related to the Glaser coupling closely. The difference is in the use of copper (II), - in place of copper (I) salt as an oxidizing agent. In addition, the copper salts are not required in catalytic but stoichiometric amounts. Since stöchimetrischem use of the copper salt of this must not be re-oxidized, oxygen is not required for the Eglinton coupling. As a copper source such as copper (II ) acetate is used as the base is used usually pyridine or another organic nitrogen base.
Cadiot - Chodkiewicz coupling
While the Glaser coupling allows only the coupling of two identical alkynes Cadiot - Chodkiewicz coupling allows the use of two different groups at the alkyne and allows the selective synthesis of unsymmetrical diynes.