Molecular mechanics study of hindered phenols used as antioxidants

Abstract

The influence of 2,6-substituents on the properties and reactions of hindered phenols (1) and peroxides (2) has been studied by computer-assisted molecular modelling and molecular mechanics and the results of the calculations have been compared with our experimental observations. The minimum-energy configurations of the 2,6-disubstituted peroxides, (R₂C₆H₃O)₂(R = Me, Et, Prⁱ, Bu^t), were evaluated in order to assess the repulsion across the peroxy group between the substituents in the phenyl rings, and hence the feasibility of the dimerisation of the corresponding phenoxy radicals.

Molecular mechanics calculations were performed with the MM2 program of Allinger with a variety of forcefield parameters of the C—O—O—C linkage. In the minimised structure for bis(2,6-dimethylphenyl)peroxide there were no steric repulsions greater than 0.1 kcal mol^–1 between methyl groups regardless of the values of the C(ar)—C(ar)—O—O and C(ar)—O—O—C(ar) torsion angles. However, the steric energy of the di-ethyl, -isopropyl, and -isobutyl compounds was dependent upon the values of these torsion angles with preferred C(ar)—C(ar)—O—O angles closer to 90 than 180 °. The minimum energies increased in the order Me < Et < Prⁱ Bu^t. The steric energy of the t-butyl derivative was much the highest (47 kcal mol^–1 compared with less than 12 kcal mol^–1 for the other three). Owing to the steric effect of the t-butyl groups, the t-butyl peroxide is unstable and the HO group of the corresponding bis(t-butyl)phenol is protected and hence unreactive. These conclusions are consistent with experimental results which indicate the absence of hydrogen bonding and the lack of reactivity of the HO group of 2,6-di(t-butyl)phenol.