Pinacol coupling reaction

The pinacol coupling of a chemical reaction in the organic chemistry field.

It was first described 140 years ago by Wilhelm Rudolph Fittig for the synthesis of pinacols. By this reaction, ketones or aldehydes are coupled (eg magnesium) under formation of a CC bond to give 1,2- diols by reaction with metals. Such alcohols are versatile intermediates in organic synthesis.

Reaction mechanism

In the reaction a one-electron transfer from the metal takes place at the carbonyl compound (in the example acetone). Due to this reduction, a Ketylradikalanion forms. Combining two such radical anions to form a dianion, obtained after subsequent acidic hydrolysis of the vicinal diol:

The coupling can result in the formation of two adjacent stereocenters, if instead an unsymmetrical ketone or acetone, an aldehyde is used (excluding formaldehyde) as starting material.

If the reductive coupling of two carbonyl compounds lead to 1,2- diols, or of further deoxygenation, depends on the oxygen affinity of the metal used. In the event of deoxygenation is formed from the pinacolate then an alkene.

The McMurry reaction is also suitable for Pinakolsynthese and, depending on reaction conditions to give very high yields.