20.15:
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.
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.
Suggested Reading
- Loudon, M., & Parise, J. (2016). Organic Chemistry. New York, NY: Macmillan Publishers, 851-852.
- Klein, D. (2017). Organic Chemistry. New Jersey, NJ: Wiley, 462.