Kinetic study of the reaction of thiophene-tocopherols with peroxyl radicals enlightenings the role of O˙⋯S noncovalent interactions in H-atom transfer.

Abstract

Three new α-tocopherol thiophene derivatives were efficiently synthesized, characterized and used for the first time as chain-breaking antioxidants for the inhibition of the autoxidation of reference oxidizable substrates. The rate constant of the reaction with alkylperoxyl (ROO˙) radicals and the stoichiometry of radical trapping (n) for the thiophene-tocopherol compounds were determined by measuring the oxygen consumption during the autoxidation of styrene or isopropylbenzene, using a differential pressure transducer. The measurement of the reaction with ROO˙ radicals in an apolar solvent at 30 °C showed inhibition rate constants (k_inh) in the order of 10⁴ M^-1 s^-1. To rationalise the kinetic results, the effect of the thiophene ring on the H-atom donation by O-H groups of the functionalized tocopherols was investigated by theoretical calculations. The importance of noncovalent interactions (including an unusual O˙⋯S bond) for the stability of the conformers has been shown, and the O-H bond dissociation enthalpy (BDE(OH)) of these derivatives was determined. Finally, the photophysical properties of these new compounds were investigated to understand if the addition of thiophene groups changes the absorption or emission spectra of the tocopherol skeleton for their possible application as luminescent molecular probes.