The acyloin condensation usually involves the reductive di-merisation of a carboxylic ester, although acid chlorides and the anhydrides have been used. The reducing agent is in an alkali metal and the product is an ene-diolate. Two gram-atoms of metal are required for each mole of ester with the concomitant formation of a mole of akoxide and one-half mole of the ene-diolate. Oxidation of acyloins to diketoes can be accomplished by a variety of reagents. Acyloins can be reduced to ketones by various modifications of the Clemmenson technique. Under mild conditions the ketone will dominate. This chapter presents a complete picture of both the linear and cyclic acyloin condensation with particular emphasis on developments since 1960. The discussion is closely limited to the acyloin condensation and its modifications.
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.
Since, the reaction proceeds in the presence of metallic Sodium, a radical reaction happens.
The metallic Sodium donates its electron to the carboxyl cardon to give an ionic complex. Now, with the loss of alkoxy groups from (2) produces the 1,2-diketone. Further, reduction gives Sodium salt of enediol. Finally, addition of acid yields 1,2-diol which tautamerizes to ACYLOIN as shown in step-4.
As, the miniscule trace of Oxygen can reduce the yield, the whole reaction is carried out in Oxygen free Nitrogen.
Application : Preparation of cyclic acyloins :–
This condensation has been used for preparation cyclic acyloins. Long chain dicarboxylic esters have been converted to large ring compounds without the use of dilution technique. It is used best for closing rings of ten members or more.
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.
Mechanisms:
The mechanism of the following reaction is not well understood yet it has been assumed that the reaction went through by a DIKETONE intermediate as these diketone has been isolated in small amounts as by-product.Since, the reaction proceeds in the presence of metallic Sodium, a radical reaction happens.
The metallic Sodium donates its electron to the carboxyl cardon to give an ionic complex. Now, with the loss of alkoxy groups from (2) produces the 1,2-diketone. Further, reduction gives Sodium salt of enediol. Finally, addition of acid yields 1,2-diol which tautamerizes to ACYLOIN as shown in step-4.
As, the miniscule trace of Oxygen can reduce the yield, the whole reaction is carried out in Oxygen free Nitrogen.
Application : Preparation of cyclic acyloins :–
This condensation has been used for preparation cyclic acyloins. Long chain dicarboxylic esters have been converted to large ring compounds without the use of dilution technique. It is used best for closing rings of ten members or more.
Preparation of CATENANE :–
CATENANE is a very interesting & unique compound with interlocking rings. It is formed when acyloin condensation was employed for ring closure with esters of 34-carbon di-carboxylic acids.
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