Theory and Defination :
The Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones.The homocoupling of benzaldehyde gives the parent benzoin.The first methods were only suitable for the conversion of aromatic aldehydes.
The modifications of the benzoin condensation include the use of acylsilanes as well as imine derivatives in place of one aldehyde partner.
The modifications of the benzoin condensation include the use of acylsilanes as well as imine derivatives in place of one aldehyde partner.
General Reaction :
e.g.
Mechanism :
Addition of the cyanide ion to create a cyanohydrin effects an umpolung of the normal carbonyl charge affinity, and the electrophilic aldehyde carbon becomes nucleophilic after deprotonation: A thiazolium salt may also be used as the catalyst in this reaction (we will see Stetter Reaction in upcoming posts ).
A strong base is now able to deprotonate at the former carbonyl C-atom:
A second equivalent of aldehyde reacts with this carbanion; elimination of the catalyst regenerates the carbonyl compound at the end of the reaction:.
Application and Example :
In biochemistry, the coenzyme thiamine is responsible for biosynthesis of acyloin-like compounds. This coenzyme also contains a thiazolium moiety, which on deprotonation becomes a nucleophilic carbene. In one study, a custom-designed N-heterocyclic carbene (NHC, the framework is related to thiazolium salts) was found to facilitate anenantioselective intramolecular benzoin condensation.
This finding was confirmed in
another study with a slightly modified NHC using DBU as the base instead of
potassium tert-butoxide.