Woodward cis-hydroxylation

The Prevost - Woodward -hydroxylation (also: Woodward cis -hydroxylation or Woodward variant) is a chemical reaction in which two hydroxyl groups can add to alkenes under mild chemical conditions. In contrast to Prevost hydroxylation both hydroxy groups are added to each other in this reaction, syn.

Named this reaction is, inter alia, by Robert Burns Woodward, who published this reaction in 1958 in the Journal of the American Chemical Society.

Mechanism

In the first step, the alkene reacted with elemental iodine to form a cyclic iodonium ion. This is then opened by the silver benzoate and produce the trans-1 ,2 -iodo carboxylate. By an intermolecular rearrangement, the iodine is eliminated and a cationic cyclic intermediate. In the presence of water, then, a hydroxide ion is added to the positive C- atom of the ester group. By rearrangement, the benzoate forms back and there is a hydroxyl group in the syn-position. In the last step, the ester group is hydrolyzed. Since the hydrolysis will not change the configuration of the molecule, the hydroxyl groups thus formed remain syn to each other.

By changing the starting materials ( other halogens, thallium (I) acetate, or benzoate ), the chemical yield is increased.

Application

The Prevost - Woodward -hydroxylation is well suited as an alternative to the usual dihydroxylation with osmium tetroxide or potassium permanganate, as opposed to those here, the sterically less favorable product is formed. It attacks here first the halogen at the sterically more favorable side of the alkene, then the benzoate is added to the sterically less favorable side.