20.15 : Radical Oxidation of Allylic and Benzylic Alcohols

Activated manganese(IV) oxide can selectively oxidize allylic and benzylic alcohols via a radical intermediate mechanism. Primary allylic alcohols are oxidized to aldehydes, while secondary allylic alcohols yield ketones. The redox reaction of potassium permanganate with an Mn(II) salt such as manganese sulfate (under either alkaline or acidic conditions), followed by thorough drying, yields the oxidizing agent: activated MnO₂. While MnO₂ is insoluble in the solvents used for the reaction, the oxidation of allylic and benzylic alcohols occurs on the MnO₂ surface. Because water competes with alcohol for the active sites on the MnO₂, it must be removed by drying.

Radical oxidation by MnO₂ is selective, and the reagent does not affect non-benzylic or non-allylic alcohols. Initially, the hydroxyl group of alcohol 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. Thus Mn(IV) is reduced to Mn(III), producing an allylic or benzylic carbon radical, which is resonance-stabilized. The increased stability of this benzylic/allylic radical intermediate, compared to the stability of the intermediate formed by non-benzylic or non-allylic alcohols, increases the rate of this step and promotes selectivity. Further electron rearrangement results in the reduction of Mn(III) to Mn(II) and yields the aldehyde or ketone.