Benzilic acid rearrangement

The benzylic acid rearrangement is a rearrangement reaction of 1,2- diketones to α -hydroxy carboxylic acids in the presence of strong bases such as potassium hydroxide. It is closely related to the Cannizzaro reaction and the benzidine rearrangement, and was first used in 1838 by Justus Liebig.

Reaction

The benzilic as namesake of the reaction is caused by a benzylic acid rearrangement of benzil ( dibenzoyl ).

This diketone reaction is similar to other rearrangements: the corresponding keto aldehyde ( an alkyl group is replaced by hydrogen) rearranges in a Cannizzaro reaction, the 1,2-diol reacts according to the pinacol rearrangement.

Reaction mechanism

The following reaction is typical of 1,2- rearrangements. Normally at this rearrangement carbocations are involved, but here come carbanions to reaction. This long-known reaction mechanism was confirmed by in silico studies and is executed in the following. A hydroxide anion engages one of the keto groups in a nucleophilic addition of 1 and 2 forms a hydroxide anion in the next step is a rotation about the central axis C-C to obtain the 3 conformers. The radicals R are cis to one another. This is necessary so that the migratory rest can attack the second carbonyl group in a concerted reaction and the resulting hydroxy group may revert to the first carbonyl group. This step is similar to a nucleophilic acyl substitution.

The carboxylic acid in an intermediate step 4 is less basic than the hydroxide anion. Therefore, the proton migrates to the intermediate 5, this in turn can be protonated by acid and the final product is formed α -hydroxy carboxylic acid 6

Case of cyclic 1,2- diketones it comes to the ring strain.

The benzilic as namesake of the reaction is caused by a benzylic acid rearrangement of benzil ( dibenzoyl ).

Variants

A variant of the rearrangement occurs in the synthesis of various steroids. In the D - homo rearrangement of steroids a cyclopentane ring under base exposure is extended to the cyclohexane ring.