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Benzoin Condensation

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The benzoin condenzation is the coupling of two aldehyde molecule to give the $\alpha$-hydroxyketone product. In its most classical form, the homocoupling of benzaldehyde gives the parent benzoin. Contemporary modifications of the benzoin condensation include the use of acylsilanes as well as imine derivatives in place of one aldehyde partner.

A wide variety of heteroazolium salts in the presence of bases that are capable to deprotonate at the corresponding carbon atom and yield N-heterocyclic carbene have been reported as a pre-catalysts for the benzoin condensation.

N-heterocyclic Carbene

In 1966, Sheehan and Hunneman reported an asymmetric benzoin condensation afforded benzoin in 22% under the catalyst of the chiral thiazolium salt. Later on a series of chiral thiazonium salts have been tested for the reaction with varied enantioselectives achieved.

Mechanism

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The mechanism of benzoin condensation involves the following steps.
  1. Benzoin condensation is specifically catalyzed by cyanide ions.
  2. Because of its electron withdrawing properties the cyano group assists in the removal of the proton from the carbon atom which is gained by the negatively charged oxygen atom to form the cyanohydrin anion.
  3. It is the reversible reaction and the formation of the anion is the key step.
  4. The anion then makes a nucleophilic attack on a second aldehyde molecule.
  5. The proton transfer in subsequent steps and loss of the cyanide ion results in the benzoin.

Mechanism of Benzoin Condensation

Applications

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Anisaldehyde and p-tolualdehyde give the corresponding $\alpha$-hydroxyketones. Similarly furfural gives furoin.

alpha-Hydroxyketones

Another application is benzoin condensation of p-formyl styrene gives the corresponding product.

p-Formyl Styrene