A Study of an Insight into the Reactivity of 1,4-Naphthoquinone Derivatives
MetadataShow full item record
Understanding the reactivity of 1,4-naphthoquinones is interesting in two different aspects; as a redox agent and as a conjugated-carbonyl electrophile. In biological systems, they function as a redox component that is important in electron transfer processes. As a conjugated carbonyl, 1,4-naphthoquinones react with a nucleophile, initiated by Michael addition. Here we measured the redox potential of various 1,4-naphthoquinone derivatives. As judged from our results on the reactivity of 1,4-naphthoquinones with alkylamines, we could generalize that for all 1,4-naphthoquinones whose redox potential is higher than -720 mV, an alkylamine replaces a substituent at the 2-/3-position such as a methoxy group that potentially serves as a leaving group, whereas an alkyl group such as the methyl group won?t be. For example, 2-bromo-3-methoxy-1,4-napthoquinone yields 2-bromo-3-methylamino-1,4-naphthoquinone in its reaction with methylamine in methanol. In this reaction, interestingly, the methoxy group, instead of the bromine atom, is exclusively replaced with methylamine. We understand the observed behavior because the substitution of methoxy group with an alkylamine is not via Michael addition-elimination of a conjugated carbonyl system but via electron transfer, dictated by the redox potential of 1,4-naphthoquinones as an electron acceptor. When the reduction potential of 1,4-naphthoquinones is more positive, or 1,4-naphthoquinone derivatives are stronger oxidizing agents, for example, 2,3-dibromo-1,4-naphthoquinone, alkylamines easily substitute one bromine atom, forming 2-alkylamino-3-bromo-1,4-naphthoquinone. In order to understand the reactivity of 1,4-naphthoquinones further, then, it is interesting to study the reactivity of 1,4-naphthoquinones whose redox potential is in the range of 2,3-dibromo-1,4-naphthoquinone but don?t carry a potential leaving group. We are currently synthesizing 2-acyl-1,4-napthoquinone starting with 1,4-naphthoquinone in four steps: reductive acylation, Fries rearrangement, hydrolysis, and oxidation.