Benzilic acid rearrangement

The benzilic acid rearrangement is formally the 1,2-rearrangement of 1,2-diketones to form α-hydroxy–carboxylic acids using a base. This reaction receives its name from the reaction of benzil with potassium hydroxide to form benzilic acid. First performed by Justus von Liebig in 1838,^[1] it is the first reported example of a rearrangement reaction.^[2] It has become a classic reaction in organic synthesis and has been reviewed many times before.^[3][4][5] It can be viewed as an intramolecular redox reaction, as one carbon center is oxidized while the other is reduced.

The reaction has been shown to work in aromatic, semi-aromatic, aliphatic, and heterocyclic substrates. The reaction works best when the ketone functional groups have no adjacent enolizable protons, as this allows aldol condensation to compete. The reaction is formally a ring contraction when used on cyclic diketones. It has been found that aryl groups more readily migrate than alkyl groups, and that aryl groups with electron-withdrawing groups migrate the fastest.