Radical Oxidation of Allylic and Benzylic Alcohols

Allylic and benzylic alcohols undergo selective oxidation by activated manganese (IV) dioxide via a radical intermediate.

Primary allylic alcohols give aldehydes, and secondary allylic alcohols form ketones.

Activated MnO2 is prepared by an oxidation-reduction reaction when a Mn⁺² salt, such as manganese sulfate, reacts with potassium permanganate.

The high selectivity of MnO₂ toward the allylic and benzylic alcohols can be understood from the reaction mechanism.

The hydroxyl group of alcohol initially adds to MnO₂ to form an ester.

Next, the Mn(IV) accepts an electron, and a hydrogen atom is transferred from the allylic or benzylic carbon to the oxygen attached to manganese.

This reduces the Mn(IV) to Mn(III), giving an allylic or benzylic carbon radical, which is resonance-stabilized.

Finally, further electron rearrangement reduces Mn(III)  to Mn(II) along with the formation of an aldehyde or ketone product.

Notably, during the radical oxidation, the non-benzylic alcohol remains unaffected.