Acyloin condensation

In organic chemistry, acyloin condensation is the term used to describe the condensation of two esters to form α-hydroxycarbonyl compounds ( acyloins ) by reacting them with sodium in toluene or xylene under heat . The reaction is one of the radical reactions.

Overview reaction

In the acyloin condensation, two esters react to form an acyloin:

The radicals R ¹ and R ² are organyl radicals . The green bond in acyloin is the newly made connection between the two esters.

mechanism

The mechanism of the acyloin condensation is explained using the example above.

First, the ester 1 reacts with sodium to form the radical anion 2 , which dimerizes with a further radical anion to form the dianion 3 . By splitting off two alcoholate residues (R ² O ^- ), a diketone 4 is formed , which is reduced to the dianion 5 with excess sodium . This dianion is then hydrolyzed to α-hydroxyketone 6 , the acyloin, in aqueous work-up .

Rühlmann variant

To drastically improve the yield, trimethylsilyl chloride (TMSCl) can be added, which serves as a trapping reagent for the basic alcoholates. Competitive reactions are thus suppressed.

{\ displaystyle \ mathrm {(CH_ {3}) _ {3} SiCl + NaOR \ longrightarrow (CH_ {3}) _ {3} Si {-} OR + NaCl}}

The acyloins are obtained by hydrolyzing the silyl ethers .

Intramolecular variant

The intramolecular variant ( Prelog-Stoll cyclization ) is also possible from starting materials with two ester groups. Corresponding ring systems can be obtained from suitable dicarboxylic acid diesters via this method - depending on the chain length of the starting material. The yields for the cyclic variant vary in terms of chain length - some systems form more easily than others. The formation of larger rings (10 or 12 members) using the Ziegler-Ruggli dilution principle is of particular preparative importance . The presence of double or triple bonds in the carbon system does not generally interfere with the reaction.

Catenanes

Catenanes
Using the inner molecular variant, catenanes can also be synthesized.

literature

Hans Beyer , Wolfgang Walter : Textbook of organic chemistry. S. Hirzel Verlag, Stuttgart / Leipzig 1998, 23rd revised. and updated edition, ISBN 3-7776-0808-4 .

Individual evidence

^ ^A ^b Z. Wang: Comprehensive Organic Name Reactions and Reagents, 3 Volume Set. John Wiley & Sons, Hoboken, New Jersey 2009, ISBN 978-0-471-70450-8 , p. 13.
↑ L. Kürti , B. Czakó: Stratigic Applications of Named Reactions in Organic Synthesis. Elsevier Academic Press, Amsterdam 2005, ISBN 978-0-12-429785-2 , pp. 4-5.
↑ K. Rühlmann: The reaction of carboxylic acid esters with sodium in the presence of trimethylchlorosilane . In: Synthesis . No. 5 , 1971, p. 236-253 , doi : 10.1055 / s-1971-21707 .

Web links

Commons : Acyloin Condensation - Collection of Images, Videos and Audio Files

[Wang_XXX-1] A ^b Z. Wang: Comprehensive Organic Name Reactions and Reagents, 3 Volume Set. John Wiley & Sons, Hoboken, New Jersey 2009, ISBN 978-0-471-70450-8 , p. 13.

[Kürti_XXX-2] L. Kürti , B. Czakó: Stratigic Applications of Named Reactions in Organic Synthesis. Elsevier Academic Press, Amsterdam 2005, ISBN 978-0-12-429785-2 , pp. 4-5.

[3] K. Rühlmann: The reaction of carboxylic acid esters with sodium in the presence of trimethylchlorosilane . In: Synthesis . No. 5 , 1971, p. 236-253 , doi : 10.1055 / s-1971-21707 .