Reactivity of 4-vinylphenol radical cations in solution: implications for the biosynthesis of lignans

Abstract

Nanosecond laser flash photolysis studies of the radical cation of 4-hydroxy-3-methoxystyrene show that the radical cation reacts with neutral 4-hydroxy-3-methoxystyrene and non-phenolic styrenes with rate constants that range from 1 × 10⁸ to 5 × 10⁸ M⁻¹ s⁻¹. Similar 4-vinylphenol radical cations such as the radical cations of isoeugenol and coniferyl alcohol display reduced reactivity, presumably due to the presence of β-alkyl substituents. Overall, the results show that the reactivity of 4-vinylphenol radical cations with neutral styrenes parallels the reactivity of non-phenolic styrene radical cations, which are known to undergo efficient radical cation mediated dimerization reactions to give lignan-like compounds. The possibility that the biosynthesis of some lignans may follow a radical cation mediated mechanism is discussed.