Mechanism of oxidation of 1-benzyl-1,4-dihydronicotinamide by the biologically active p-benzoquinone derivative, avarone, in a cationic micellar medium

Reactivity and Biological Activity of the Marine Sesquiterpene Hydroquinone Avarol and Related Compounds from Sponges of the Order Dictyoceratida

“…The results clearly show that in presence of cationic micelles (cetyltrimethylammonium bromide, CTAB), depending on concentrations up to critical micellar concentration (CMC, 1.5×10 -3 M), a rate increase for compounds 2, 20, 33, 34 ( Figure 9) was recorded, while in presence of anionic micelles (sodium dodecyl sulfate, SDS) a slight rate decrease takes place; in neutral micelles (Tween), essentially no change in rate, relative to those in EtOH/water, was noticed. Based on these results and supported by the established stability of the quinone radical anion/CTA + complex [79] with a stability constant of 1.4x10 -2 at pH 8.1, the following stepwise electron transfer reaction pathway, involving quinone anion radical formation, is proposed: The lower reactivity of quinones 35-38 can be explained by the less lipophilic character of these compounds (less solubilized in the micellar pseudophase). In another study the e-p-e mechanism was supported by the fact that long-chain N-alkyl-1,4-dihydronicotinamides, compared to less lipophilic short-chain analogues, show a significant increase in the reaction rates of avarone reduction in protic solvents.…”

“…This effect is a consequence of the stabilization of the intermediate radical-ion pair by automicellization [80]. A more precise description of the nature of the intermediate in avarone reduction was obtained by cyclic and rotating disc electrode voltammetry in micellar systems [79]. In presence of the cationic micellar agent CTAB, two well-defined peaks in avarone reduction can be observed in cyclic voltammograms, confirming one-electron reduction, whereby the intermediate avarone radical-anion was stabilized by the cationic micellar medium.…”