Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview

Pinacol coupling is a radical dimerization reaction, and involves the reductive coupling of aldehydes or ketones in a hydrocarbon solvent to give vicinal diols.

A single electron transfer from a metal to a spin-paired molecule like ketone initiates the reaction and forms a ketyl —a radical anion. The ketyl can be represented with the radical on carbon or oxygen and the charge on the other.

Here, metals like sodium and magnesium act as a source of electrons.

The fate of the ketyl depends on the solvent used for the reaction.

In protic solvents, like ethanol, the ketyl gets protonated, followed by a second electron transfer to give an alkoxide anion, which upon acidification, forms alcohol.

However, in aprotic solvents, like benzene, the absence of protons allows the accumulation of ketyl, leading to its dimerization into a diol. This forms the basis of pinacol coupling.

Notably, the electrostatic repulsion between the negative charges does not impede the dimerization because metals like magnesium coordinate to ketyls by forming covalent metal–oxygen bonds, enabling them to react rapidly.