The bimolecular reductive coupling of carboxylic esters by reaction with metallic sodium in an inert solvent under reflux gives an α-hydroxyketone, which is known as an acyloin. This reaction is favoured when R is an alkyl. With longer alkyl chains, higher boiling solvents can be used. The intramolecular version of this reaction has been used extensively to close rings of different sizes, e.g. paracyclophanes or catenanes.
If the reaction is carried out in the presence of a proton donor, such as alcohol, simple reduction of the ester to the alcohol takes place (Bouveault-Blanc Reduction).
The Benzoin Condensation produces similar products, although with aromatic substituents and under different conditions.
When the acyloin condensation is carried out in the presence of chlorotrimethylsilane, the enediolate intermediate is trapped as the bis-silyl derivative. This can be isolated and subsequently is hydrolysed under acidic condition to the acyloin, which gives a better overall yield.
Mechanism of Acyloin Condensation
Switching Regioselectivity in Crossed Acyloin Condensations between Aromatic Aldehydes and Acetaldehyde by Altering N-Heterocyclic Carbene Catalysts
M. Y. Jin, S. M. Kim, H. Han, D. H. Ryu, J. W. Yang, Org. Lett., 2011, 13, 880-883.
N-Heterocyclic Carbene Catalyzed Highly Chemoselective Intermolecular Crossed Acyloin Condensation of Aromatic Aldehydes with Trifluoroacetaldehyde Ethyl Hemiacetal
B. T. Ramanjaneyulu, S. Mahesh, R. V. Anand, Org. Lett., 2015, 17, 6-9.