An experimental and theoretical study on the substituent effect of the permanganate oxidation of styrenes

Abstract

The kinetic data obtained for the cycloadditions of the permanganate ion to a series of styrene derivatives in dichloromethane solution in the presence of a quaternary ammonium ion were examined with two theoretical approaches, on the assumption that the reactions proceed via a concerted [3 + 2] mechanism. The semi-quantitative frontier molecular orbital analysis of the kinetic data shows a linear free energy relationship with better correlation than the Hammett plot with σ values when the point for p-NO₂ group is omitted. Further examination of the results of the FMO analysis reveals that the deviation of the point for p-nitrostyrene is attributed to the transition structure being more reactant-like than that of the other derivatives. The plot of log k₂vs. −ΔG^‡ calculated by the density functional theory (Becke3LYP) follows a straight line with the desired correlation for all the substituents. A marked tendency was observed for the MO calculations to underestimate the ΔG^‡ value for electron-withdrawing substituents when the calculation was carried out excluding the quaternary ammonium ion. This inconsistency was much improved by the calculations incorporating the quaternary ammonium ion. The actual values of ΔG^‡ obtained from the Eyring analysis are in good agreement with those calculated at the B3LYP/6-311+G(d,p)//B3LYP/LanL2DZ level.