Density Functional Theory Study on the Initial Step of the Permanganate Oxidation of Substituted Alkynes

Abstract

The initial step of the oxidation of alkynes by permanganate has been studied by DFT (B3LYP/6-311+G(d,p)) calculations. The influence of electron-withdrawing as well as electron-donating substituents was explored. The singlet and triplet potential energy hypersurfaces (PES) for a concerted [3 + 2] and a stepwise [2 + 2] mechanism were investigated. The crossing point between the singlet and triplet PES has been identified. The reaction proceeds via a [3 + 2] mechanism on the singlet PES with free energy activation barriers (ΔG298) between 18.4 and 28.9 kcal/mol, while the lowest lying identified state is a reactive triplet intermediate. Available experimental and computed data are in good agreement. For the [2 + 2] case a reaction on the triplet surface would be preferred over the singlet, but both pathways lay well above the [3 + 2] pathway.