Establishing the (3 + 2) Mechanism for the Permanganate Oxidation of Alkenes by Theory and Kinetic Isotope Effects.

Permanganate hydroxylation follows the (3 + 2) cycloaddition mechanism established earlier for osmium tetroxide. Density functional theory (Becke3LYP/6-31G) predicts that the concerted (3 + 2) pathway is favored by about 40 kcal/mol relative to a stepwise process involving a (2 + 2) cycloaddition. The activation energy for the (3 + 2) transition state is calculated to be only 9.2 kcal/mol. The energetics are remarkably similar to the results for osmylation, where the activation energies are calculated to be 3.2 kcal/mol for the (3 + 2) and 44.1 kcal/mol for the (2 + 2) transition state. Calculated kinetic isotope effects for a substituted case are in good agreement with experiment.