Electrochemical and theoretical analysis of the reactivity of shikonin derivatives: dissociative electron transfer in esterified compounds

Naphthazarin-Polycyclic Conjugated Hydrocarbons and Iptycenes

“…42 The computed transition energies in both cases are slightly lower than the experimental values (Table S1 and Electrochemical analysis of these compounds reveals two quasireversible waves, consistent with literature studies (see Figure S114 and Table S5). 43,44 [M] -) Susceptibility to hydrolysis was consistent with previous reports in the literature. 28,29 Presumably due to moisture present in the air, hydrolysis was observed for BF 2 -1a to occur in CDCl 3 over 1-2 hours, precluding 13 C NMR analysis.…”

“…Electrochemical analysis of these compounds reveals two quasireversible waves, consistent with literature studies (see Figures S114 and Table S5). 43,44 Upon analysis, some of the compounds synthesized in this study showed interesting fluorescent properties (Table 1 and Figure 3b). The unoxidized cycloaddition adducts (compounds 6a−d, 8a−d, and 10) showed strong blue fluorescence with high quantum yields (Φ F = 0.58−0.91).…”

Insights into dissociative electron transfer in esterified shikonin semiquinones by in situ ESR/UV-Vis spectroelectrochemistry

“…Previous analysis of the variation of peak potential values for the second reduction process as a function of the scan rate for isovalerylshikonin was recently reported: 13 experimental data evidenced that a stepwise DET takes place as the obtained slope was approx. 40 mV dec À1 (thus a 4 0.5), and the partial deconvolution of the reversible oxidation signal at the highest scan rate employed (100 V s À1 ) was achieved (Peak IIa, Fig.…”

“…In a recent study, this behaviour was explained by considering a dissociative electron transfer mechanism, leading to the elimination of the ester function, which was associated with the electron-withdrawing character of this moiety within the neutral species. 13 In situ spectroelectrochemical-Electron Spin Resonance (ESR) experiments were performed to characterize the semiquinone species formed during the first electron transfer (Fig. 1 peak Ic); this was achieved by applying potential values after peak Ic (E = À1.01 V (shikonin) and À0.97 V (isovalerylshikonin) vs. E 0 (Fc/Fc + )) (Fig.…”

“…10,11 In terms of Electron Transfer (ET) chemistry, the formation of intermediates is key to explain their enhanced biological activity: once the quinone function is reduced, a semiquinone radical anion is generated; depending on the reactivity of this intermediate, it can undergo further reduction (generating a dianion species), disproportionate, or form a quinone methide whose presence is favoured if a good leaving group is located in the b-position, thus facilitating reductive cleavage. 7,12 In a recent electrochemical analysis of a series of shikonin and some esterified derivatives, results showed that the presence of the ester function leads to a remarkable difference in the voltammetric signal during the reduction of the corresponding semiquinone to generate a dianion, mainly associated with the electron-withdrawing effect of the ester function, 13 and the analysis suggested the presence of a Dissociative Electron Transfer (DET) process, once the dianion species is generated. In the same previous study, the nature of the bond breaking reaction remained to be considered; also, reaction intermediates were not characterized to provide more insights into the DET process taking place.…”