Acyloin condensation

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. [cite journal
title = Action du sodium sur les éthers des acides monobasiques à fonction simple de la série grasse
author = Bouveault, L., and R. Loquin
journal = Comptes. Rendus.
volume = 140
issue =
pages = 1593–1595
year = 1905
url = http://gallica.bnf.fr/ark:/12148/bpt6k30949/f1689.table
doi = ] [cite journal
title = The Acyloin Condensation as a Cyclization Method
author = Finley, K. T.
journal = Chemical Reviews
volume = 64
issue = 64
pages = 573
year = 1964
url = http://pubs.acs.org/cgi-bin/abstract.cgi/chreay/1964/64/i05/f-pdf/f_cr60231a004.pdf
doi = 10.1021/cr60231a004
format = abstract ]

The reaction is most successful when "R" is aliphatic and inert. To achieve the condensation, chemists 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 [cite journal
title = Die Umsetzung von Carbonsäureestern mit Natrium in Gegenwart von Trimethylchlorsilan
author = Rühlmann K.
journal = Synthesis
volume = 1971
issue =
pages = 236–253
year = 1971
url = http://www.thieme-connect.com/ejournals/abstract/synthesis/doi/10.1055/s-1971-21707
doi =10.1055/s-1971-21707
format = abstract ] 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

ee also

*Benzoin condensation
*Bouveault-Blanc reduction
*Claisen condensation
*Dieckmann condensation


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