A Density Functional Theory Study on the Mechanism of the Permanganate Oxidation of Substituted Alkenes

Oxidation of substituted alkenes by permanganate follows a (3 + 2) cycloaddition mechanism. DFT calculations (Becke3LYP/6-31G(d)) strongly favor the (3 + 2) pathway against a (2 + 2) pathway that proceeds through a metallaoxetane. The difference in free activation energy between the two pathways is around 40 to 45 kcal/mol for the nine compounds covered. The results for the (3 + 2) cycloaddition mechanism are in agreement with experimentally observed kinetic data reported earlier. Symmetric transition states are calculated for alkenes with at least one CH₂ group between the double bond and the acid group, while all others are unsymmetrical due to the repulsion between the permanganate oxygens and the carboxylic oxygens. Steric bulk, introduced by the number and position of substituents at the double bond, has no significant effect on the activation energies. A higher level of theory (Becke3LYP/6-311G(d,p)) leads to a reduction of the rmsd between experimental and calculated values, but has little influence on the calculated geometries.