Reaction of 2-Alkyl-3-bromo-1,4-naphthoquinone with Amines; A Novel Demethylation Reaction

Abstract

Derivatives of 1,4-naphthoquinone are utilized as anti-fungal, antibacterial, and potential anticancer pharmaceuticals, which often show unexpected reactivities. Here we study the reactivity of 2-bromo-3-methyl-1,4-naphthoquinone toward primary and secondary amines. In the reaction with primary amines in methanol, contrary to our expectations, demethylation of 2-bromo-3-methyl-1,4-naphthoquinone generates 2-alkylamino-3-bromo-1,4-naphthoquinone as the major product. Furthermore, secondary amines such as N-methylethylamine gave 2-methylamino-3-bromo-1,4-naphthoquinone, presumably as the secondary reaction product. Unexpectedly, the reactions of 2-bromo-3-ethyl-1,4-naphthoquinone and 2-bromo-3-isopropyl-1,4-naphthoquinone with primary, secondary, and tertiary amines yield very different results. In these reactions, substitution occurs for the bromine. We believe this difference in reactivity is due to the varying acidity of the ??-protons on the three 2-alkyl-3-bromo-1,4-naphthoquinones. The scope and limitations of the reaction and reaction mechanism are studied.