Benzoin addition
The benzoin condensation (often, but incorrectly as benzoin condensation called) is the addition of mostly aromatic aldehydes , particularly benzaldehyde under cyanide - catalysis to aromatic α-hydroxy ketones ( benzoin ). The mechanism was proposed by Arthur Lapworth back in 1903 . The reaction is shown using the example of the reaction of two molecules of benzaldehyde to form benzoin.
mechanism
The mechanism of the reaction is also explained using the example above:
In a first step, the cyanide ion reverses the polarity of the carbonyl carbon on the benzaldehyde ( 1 ). The addition of the cyanide ion to the carbonyl group creates the anion of the corresponding cyanohydrin . From this, the tautomeric carbanion 2 is formed by transferring the proton from carbon to oxygen , which now nucleophilically attacks another benzaldehyde. With an intramolecular proton transfer and cleavage of the cyanide from molecule 3 , the product benzoin ( 4 ) is formed. The reaction is reversible. The cyanide is a very specific catalyst . Due to its –I effect, it facilitates the migration of the proton from the carbon to the oxygen atom and thus enables the nucleophilic addition of the second aldehyde molecule.
The two aldehyde molecules have different roles. The aldehyde that gives up its proton is called the donor, and the one that accepts it is called the acceptor. Some aldehydes can only act as a donor (e.g. 4-dimethylaminobenzaldehyde ), others can take on both roles (e.g. benzaldehyde). This enables the targeted synthesis of some unsymmetrical benzoins.
The reaction can also be extended to aliphatic aldehydes and mixtures of aliphatic and (hetero) aromatic aldehydes by using certain thiazolium salts in a basic medium. The mechanism is essentially analogous to the above.
The α-hydroxyketones obtained in these reactions are also referred to as acyloins and are important starting materials for further syntheses, in particular of heterocyclic compounds.
The addition can also take place on α, β-unsaturated ketones (e.g. methyl vinyl ketone ). This variant is known as the Stetter reaction .
The coenzyme thiamine which occurs in the organism and is responsible for the synthesis of acyloins also contains such a thiazolium salt.
Individual evidence
- ^ H. Beyer, W. Walter, W. Francke: Textbook of organic chemistry . 23rd edition, S. Hirzel Verlag, Stuttgart, Leipzig 1998, ISBN 978-3-7776-0808-2 .