Acyloin condensation
Acyloin condensation is a reductive coupling of two carboxylic esters using metallic sodium to yield an α-hydroxyketone, also known as an acyloin.[1][2]
The reaction is most successful when R is aliphatic and inert. To achieve the condensation, the chemist should employ aprotic solvents with a high boiling point, such as benzene and toluene. (Usage of protic solvents results in the Bouveault-Blanc reduction.) Depending on ring size and steric properties, but independent from high dilution, the acyloin condensation of diesters favours intramolecular cyclisation over intermolecular polymerisation.
Variations
Rühlmann-method
The method according to Rühlmann[3] employs trimethylchlorosilane as a trapping reagent; by this, competing reactions are efficiently subdued. Generally, yields increase considerably. The hydrolytic cleavage of the silylether gives the acyloin. To achieve a mild cleavage methanol can be used in several cases.
Usually toluene, dioxane, tetrahydrofuran or acyclic dialkylethers are employed as solvents. Advantageously also N-methyl-morpholine has been used. It allowed in some cases a successful reaction, in which otherwise the reaction failed in less polar media.
References
- ^ Bouveault, L., and R. Loquin (1905). "Action du sodium sur les éthers des acides monobasiques à fonction simple de la série grasse". Comptes. Rendus. 140: 1593–1595.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Finley, K. T. (1964). "The Acyloin Condensation as a Cyclization Method" (abstract). Chemical Reviews. 64 (64): 573. doi:10.1021/cr60231a004.
- ^ Rühlmann K. (1971). "Die Umsetzung von Carbonsäureestern mit Natrium in Gegenwart von Trimethylchlorsilan" (abstract). Synthesis. 1971: 236–253. doi:10.1055/s-1971-21707.