Hydrogen Atom Abstraction Kinetics from Intramolecularly Hydrogen Bonded Ubiquinol-0 and Other (Poly)methoxy Phenols

Abstract

The effect of methoxy substitution on the abstraction of the phenolic hydrogen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cumyloxyl radicals has been investigated by laser flash photolysis. Also transition state calculations for methoxyl radical and 2-methoxyphenol have been carried out by a density functional theory (DFT) method. Hydrogen atom abstraction is surprisingly easy from intramolecularly hydrogen bonded methoxyphenols, in contrast to intermolecularly hydrogen bonded molecules. The kinetic solvent effect, investigated in six solvents with different hydrogen bond accepting properties, on the hydrogen atom abstraction reaction from o-methoxy phenols was shown to be smaller than for non-hydrogen bonded phenols, and is independent of further methoxy substitution. The high rate constant for hydrogen atom abstraction from ubiquinol-0 (2.8 × 109 M-1 s-1 in CCl4) and the small kinetic solvent effect make it a good antioxidant, even in a polar environment.