20.28: Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview
Wilhelm Rudolph Fittig discovered the pinacol coupling reaction in 1859. It is a radical dimerization reaction and involves the reductive coupling of aldehydes or ketones in the presence of hydrocarbon solvent to yield vicinal diols.
The radical reaction is initiated by a single electron transfer from metals like sodium and magnesium to a spin-paired molecule like aldehydes or ketones to generate a ketyl—a radical anion. The ketyl has a radical character on the carbon atom and a charge on the oxygen atom. Its resonance form has a radical positioned on the oxygen atom and a charge on the carbon atom.
The behavior of ketyl is greatly influenced by the solvent in which the reaction is carried out.
Protic solvents, like ethanol, protonate the ketyl. This is followed by a second electron transfer from metal to give an alkoxide anion. Further, the acidification of an alkoxide anion forms alcohol. Here, the metal dissolves with the reaction, acting as a source of free electrons.
In the presence of aprotic solvents, like benzene or ether, ketyl radical anions do not undergo protonation due to the absence of protons. This promotes ketyl dimerization and forms a part of the famous pinacol coupling reaction.
Interestingly, metals like magnesium, aluminum, or sodium, favor the reaction by forming covalent metal-oxygen bonds, which coordinate with several ketyl radicals, reacting rapidly to form a diol. Notably, the electrostatic repulsions between the negative charges of the ketyls do not hamper dimerization. In summary, protic solvents favor the formation of alcohol, while aprotic solvents favor vicinal diols.