8.12 : Ossidazione degli alcheni: diidrossilazione anti con perossiacidi

Anti dihydroxylation is another oxidative addition reaction that proceeds with the anti addition of two hydroxyl groups across the alkene double bond.

Here, an alkene is treated with a peroxycarboxylic acid, which is a strong oxidant, to form an epoxide: a highly strained three-membered ring with an oxygen and two carbons.

Epoxide formation is followed by ring-opening of the epoxide in the presence of an aqueous acid to give a trans diol.

The mechanism begins with a concerted nucleophilic attack by the alkene π bond on the electrophilic oxygen of peroxyacetic acid, breaking the O–O bond and forming a new C=O bond, leading to a cyclic transition state.

In the first step, the transfer of the oxygen atom occurs to the same face of the alkene double bond, forming an epoxide with syn stereochemistry.

The second step of the reaction involves an acid-catalyzed ring-opening of the epoxide. First, a proton transfer to the oxygen atom forms a protonated epoxide.

The ring strain in epoxides makes them reactive towards nucleophilic attack at one of the electrophilic carbons, causing the ring to open, thereby relieving the strain.

A backside nucleophilic attack by water opens the ring, forming an oxonium ion. This is consistent with an S_N2 process, resulting in a stereochemical inversion at the reaction center.

Steric factors dominate at primary or secondary carbon centers, favoring an attack at the less-substituted carbon. With a tertiary carbon, electronic effects dominate, favoring attack at the more-substituted carbon.

Lastly, water, acting as a base, deprotonates the oxonium ion to yield a trans diol as the final product.