Synthesis and physico-chemical properties of nitrocaffeic acids

Abstract

The synthesis and spectroscopic properties of the three isomers of nitrocaffeic acid are described. The three pKa s of each isomer were measured by UV–visible spectroscopy. The comparison of the UV–visible spectra of nitrocaffeic acids and those obtained from the reaction of caffeic acid with reactive nitrogen species led to the conclusion that the nitration of caffeic acid with acidic nitrite does not significantly occur and confirmed the absence of nitration when caffeic acid reacts with peroxynitrite. Attempts to obtain free radical species from nitrocaffeic acids by classical methods showed a different reactivity to that of nitroaromatics and catechols. Nitrocaffeic acids do not autoxidize under aqueous basic conditions and are insensitive to t-BuOK or O2−· (two reactants known for their capabilities to oxidize catechols and reduce nitroaromatics). Nitroaromatic anion radicals may be obtained using sodium borohydride as reductant and are particularly stable under an uncontrolled atmosphere. Copyright © 2000 John Wiley & Sons, Ltd.