Pinacol coupling reaction

The first step in the reaction mechanism is a one-electron reduction of the carbonyl group by a reducing agent —such as magnesium— to a ketyl radical anion species.

With magnesium as an electron donor, the initial reaction product is a 5-membered cyclic compound with the two oxygen atoms coordinated to the oxidized Mg2+ ion.

[2] Although an active metal reduction, modern pinacol reactions tolerate protic substrates and solvents; it is sometimes performed in water.

In another system with benzaldehyde, Montmorillonite K-10]] and zinc chloride in aqueous THF under ultrasound the reaction time is reduced to 3 hours (composition 55:45).

[6] On the other hand, certain tartaric acid derivatives can be obtained with high diastereoselectivity in a system of samarium(II) iodide and HMPA.

[7] A titanium-catalyzed photocatalytic approach was also developed: the use of catalytic titanocene dichloride in the presence of a red-absorbing organic dye as the photosensitizer, and Hantzsch ester as the terminal reducing agent, enabled the homocoupling reactions of a wide variety of aromatic aldehydes in trifluorotoluene under orange-light irradiation, with high yields and diastereoselectivities (more than 20:1 dl:meso).