8.12: Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
Diols are compounds with two hydroxyl groups. In addition to syn dihydroxylation, diols can also be synthesized through the process of anti dihydroxylation. The process involves treating an alkene with a peroxycarboxylic acid to form an epoxide. Epoxides are highly strained three-membered rings with oxygen and two carbons occupying the corners of an equilateral triangle. This step is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol. Peroxycarboxylic acids are strong oxidizing agents and analogous to carboxylic acids. However, they have an extra oxygen atom between the carbonyl group and the hydrogen atom. Commonly used organic peracids include meta-chloroperoxybenzoic acid and peroxyacetic acid.
The mechanism begins with a concerted nucleophilic attack by the alkene π bond on the electrophilic oxygen of the peroxy acid, breaking the oxygen–oxygen bond and forming a new carbon–oxygen double bond, leading to a cyclic transition state.
The second step of the reaction involves an acid-catalyzed ring-opening of the epoxide to finally form a trans diol.
Overall, the regiochemistry of the reaction is governed by a combination of steric and electronic factors. In epoxides with a primary and secondary carbon, steric factors dominate, favoring an attack at the less substituted carbon. With a tertiary carbon, electronic effects dominate, favoring attack at the more substituted carbon.