Esters undergo pinacol-type coupling: the acyloin reaction

You’ve seen examples of pinacol and McMurry reactions of ketones and aldehydes. What about esters? You would expect the ketyl radical anion to form from an ester in the same way, and then to undergo radical dimerization, and this is indeed what happens.

The product of the dimerization looks very much like a tetrahedral intermediate in a car bonyl addition–elimination reaction, and it collapses to give a 1,2-diketone.

The diketone is, however, still reducible—in fact, 1,2-diketones are more reactive towards electrophiles and reducing agents than ketones because their π* is lower in energy and straight away two electron transfers take place to form a molecule, which we could term an enediolate. On quenching the reaction with acid, this dianion is protonated twice to give the enol of an α-hydroxy-ketone, and it is this α-hydroxy-ketone that is the final product of the acyloin reaction. The yield in this example is a quite respectable 70%. However, in many other cases, this usefulness of the acyloin reaction is hampered by the formation of by-products that arise because of the reactivity of the enediolate dianion. It is, of course, quite nucleophilic, and is likely to be formed in the presence of the highly electrophilic diketone. It is also basic, and often catalyses a competing Claisen condensation of the esters being reduced.

The solution to these problems is to add trimethylsilyl chloride to the reaction mixture. The silyl chloride silylates the enediolate as it is formed, and the product of the acyloin reaction becomes a bis-silyl ether.

These silyl ethers are rarely wanted as final products, and they can easily be hydrolysed to α-hydroxyketones with aqueous acid. This improved version makes four-membered rings efficiently.

It’s not by accident that these two examples of the acyloin reaction show the formation of cyclic compounds. It is a particularly powerful method of making carbocyclic rings from four-membered upwards: the energy to be gained by pairing up the two electrons in the radical–radical reaction step more than compensates for the strain that may be generated in forming the ring.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct