The Prévost reaction is an organic reaction used to convert an alkene into a vicinal 1,2-diol with anti stereochemistry. A vicinal diol contains two hydroxyl (–OH) groups attached to neighboring carbon atoms. ^[1–4]

French chemist Charles Prévost discovered the reaction and published it in 1933. 

General Reaction

In the classical Prévost reaction, an alkene reacts with iodine (I₂) and a silver(I) carboxylate, commonly silver benzoate (PhCO₂Ag), under anhydrous conditions. ^[1,2]

The reaction does not add two –OH groups directly to the alkene. Instead, it first produces a vicinal dicarboxylate, such as a dibenzoate. Hydrolysis of the ester groups then gives the corresponding anti-1,2-diol. 

In an anti addition, the two oxygen-containing groups are introduced from opposite faces of the original carbon–carbon double bond.

For example, cyclohexene undergoes the Prévost reaction to ultimately form trans-1,2-cyclohexanediol.

Mechanism

The mechanism can be understood in six main steps. ^[1–4]

Step 1: Formation of the Reactive Iodine–Benzoate Species

Silver benzoate reacts rapidly with iodine (I₂) to form a highly reactive iodine–benzoate intermediate. During this process, silver ions help remove iodide as insoluble silver iodide (AgI). The reactive iodine species then provides an electrophilic iodine atom, which can accept electron density from the alkene in the next step.

Step 2: Formation of the Cyclic Iodonium Ion

The alkene attacks the electrophilic iodine species. The carbon–carbon double bond is converted into a three-membered cyclic iodonium ion. This positively charged intermediate is highly reactive toward nucleophiles.

Step 3: Ring Opening by Benzoate

A benzoate ion (PhCOO^–) attacks one of the carbon atoms of the cyclic iodonium ion from the side opposite the iodine bridge. This backside attack is an SN2-type ring-opening reaction and produces a β-iodo benzoate ester, in which iodine and the benzoate group are positioned on opposite faces.

Step 4: Formation of the Cyclic Oxonium Ion

Silver ion helps remove the remaining iodine by forming insoluble AgI. As iodine leaves, the carbonyl oxygen of the benzoate group already attached to the molecule attacks the neighboring carbon from within the same molecule. This intramolecular attack closes the structure into a cyclic oxonium (acyloxonium) ion. This type of assistance by a nearby group is called neighboring-group participation.

Step 5: Attack by a Second Benzoate Ion

A second benzoate ion attacks one of the carbon atoms of the cyclic oxonium ion from the backside. This SN2-type attack opens the ring and forms the vicinal dibenzoate. The two oxygen-containing groups end up on opposite faces of the molecule, giving the anti arrangement.

Step 6: Hydrolysis

Finally, hydrolysis of the dibenzoate ester, for example under aqueous basic conditions, converts the two benzoate ester groups into hydroxyl (–OH) groups. The final product is an anti vicinal diol.

Woodward Modification of Prévost Reaction

The stereochemical outcome of this reaction depends strongly on whether water is present during the reaction.

The Prévost reaction is carried out under anhydrous conditions and ultimately produces an anti-diol upon hydrolysis. If water is present, however, it can intercept the cyclic intermediate. This changes the reaction pathway and leads to the closely related Woodward modification, which gives overall syn addition.

Thus, the key difference is the reaction medium: the anhydrous Prévost reaction gives overall anti-addition, whereas the Woodward modification in the presence of water gives overall syn-addition.