Pinacol coupling is a reductive carbon–carbon bond-forming reaction in which two aldehyde or ketone carbonyl groups join to form a vicinal 1,2-diol—a compound containing two –OH groups on adjacent carbon atoms. ^[1,2]

It is called a coupling reaction because two carbonyl-derived units become joined through a new carbon–carbon bond. 

General Reaction

While the two carbonyl carbon atoms form a new C–C bond, each carbonyl oxygen ultimately becomes an –OH group: ^[1]

2 R¹R²C=O → R¹R²C(OH)–C(OH)R¹R²

Examples

1. Coupling of Acetone

Two molecules of acetone undergo reductive coupling:

2 CH₃COCH₃ → (CH₃)₂C(OH)–C(OH)(CH₃)₂

The product is 2,3-dimethylbutane-2,3-diol, commonly called pinacol.

2. Coupling of Benzaldehyde

Two molecules of benzaldehyde can undergo pinacol-type coupling to form:

2 C₆H₅CHO → C₆H₅–CH(OH)–CH(OH)–C₆H₅

The product is 1,2-diphenylethane-1,2-diol, commonly known as hydrobenzoin.

Because new stereogenic centers may be formed during coupling, more than one stereoisomer can sometimes be possible.

Mechanism

The mechanism of pinacol coupling can be understood through three main stages: formation of ketyl-type intermediates, carbon–carbon bond formation, and protonation. ^[1,3,4]

Step 1: Formation of Ketyl-Type Intermediates

The carbonyl compound first accepts a single electron from the reducing metal. This one-electron reduction produces a ketyl radical anion, or more generally in metal-mediated reactions, a ketyl-type intermediate.

The ketyl radical anion contains both a negative charge and an unpaired electron and can be represented by multiple resonance forms. In the diagram, one useful form shows the negative charge on oxygen and the radical on carbon.

In the magnesium-mediated mechanism shown here, Mg transfers electrons to two carbonyl molecules. The resulting ketyl-type fragments are coordinated to Mg²⁺ through their oxygen atoms.

Step 2: Formation of the New C–C Bond

Each ketyl-type intermediate has radical character at the carbon that was originally part of the C=O group. Two of these carbon centers join, forming a new C–C bond between the two carbon atoms that originally belonged to the carbonyl groups.

After the C–C bond forms, the two oxygen atoms are present as negatively charged alkoxide groups associated with the metal. This intermediate can be described as a metal-associated diolate.

Step 3: Protonation

During the final protonation work-up, the negatively charged oxygen atoms are protonated by the proton source, NH₄⁺. Protonation converts the metal-associated diolate into the vicinal diol. The final product therefore has an –OH group on each of two adjacent carbon atoms.