The Prévost Reaction allows the synthesis of anti-diols from alkenes by the addition of iodine followed by nucleophilic displacement with benzoate in the absence of water. Hydrolysis of the intermediate diester gives the desired diol.
The Woodward Modification of the Prévost Reaction gives syn-diols.
Mechanism of the Prevost Reaction
The initial addition of iodine leads to a cyclic iodonium ion, which is opened through nucleophilic substitution by benzoate anion:
A neighbouring-group participation mechanism prevents the immediate nucleophilic substitution of iodine by a second equivalent of benzoate that would lead to a syn-substituted product. Instead, a cyclic benzoxonium ion intermediate is formed:
Opening of this intermediate by a second addition of benzoate gives the anti-substituted dibenzoate:
Hydrolysis then delivers the diol.
In the Woodward-Modification, added water decomposes the above benzoxonium intermediate directly to a syn-substituted diol.
The use of expensive silver salts, the requirement for a stoichiometric amount of molecular halogen, and the formation of a relatively large amount of organic and inorganic wastes are definite drawbacks to this reaction. Sudalai recently reported on catalytic versions of both the Prévost Reaction and the Woodward-Modification.
The proper choice of stoichiometric oxidant allows either syn- or anti-selective dihydroxylations. NaIO4 as the oxidizing agent generates H2O as a side product of the oxidation and therefore enables the Woodward Reaction to take place.
High-valent iodine reagents are still relatively expensive, and the identification of a less costly stoichiometric oxidant would be a significant improvement for this process.