The role of intramolecular hydrogen bonding in the electrochemical behavior of hydroxy-quinones and in semiquinone stability

Abstract

O estudo de um grupo selecionado de e -hidroxiquinonas ([ ] 2-hidroxi-1,4-naftoquinona 2HNQ, [ ] 5-hidroxi-1,4-naftoquinona 5HNQ and [ , ]5,8-dihidroxi-1,4-naftoquinona DHNQ) mostrou que ambos os tipos de funcionalidades diferem consideravelmente em termos de reatividade eletroquímica e química, e que essas diferenças também se manifestam na comparação de compostos do mesmo grupo. Os resultados provaram que a energia necessária para reduzir eletroquimicamente as hidroxoquinonas estudadas está relacionada com a estabilidade intramolecular das ligações de hidrogênio (IHBs) e com as reações de acoplamento que ocorrem durante a redução eletroquímica, tal como a seqüência de auto-protonação em 2HNQ. A análise da cinética de transferência de elétron associada com a redução monoeletronica que ocorre para 5HNQ e DHNQ mostraram que os efeitos IHB não afetam a cinética do primeiro processo de redução, mas induzem a grandes mudanças na cinética de segunda transferência eletrônica. Isso sugere que as semiquinonas são quimicamente diferentes dos compostos estudados, o que foi comprovado pela análise da estrutura eletrônica das semiquinonas por espectroscopia ESR. Essa análise parece relacionada com a perda de simetria no radical gerado eletronicamente e induz a valores ks de menor velocidade para a segunda redução de transferência de 5HNQ, comparada com DHNQ.The study of a selected group of and -hydroxyquinones ([ ] 2-hydroxy-1,4-naphthoquinone 2HNQ, [ ] 5-hydroxy-1,4-naphthoquinone 5HNQ and [ , ]5,8-dihydroxy-1,4-naphthoquinone DHNQ) showed that both type of functionalities differ considerably in terms of electrochemical and chemical reactivity, and these differences are also manifest even upon comparison between compounds of the same group. The results proved that the energy needed to electrochemically reduce the studied hydroxyquinones is related both to the stability of intramolecular hydrogen bonds (IHBs) and coupled chemical reactions occurring during their electrochemical reduction such as self-protonation sequences in 2HNQ. The analysis of the electron transfer kinetics associated with the monoelectronic reductions occurring for 5HNQ and DHNQ showed that IHB effects do not affect the kinetics of the first reduction process, but induce great changes in the second electron transfer kinetics. This suggests that semiquinone species are chemically different for the studied compounds, as was corroborated by the analysis of the electronic structure of semiquinone species by ESR spectroscopy. This analysis seems to be related to the loss of symmetry in the electrogenerated radical and induces a lower rate ks value for the second reduction transfer of 5HNQ, compared with DHNQ.

Spectroelectrochemistry of the redox activation of anti-cancer drug mitoxantrone

“…Comparison with literature data on 5,8-dihydroxy-1,4-naphtoquinone and quinizarin (1,4-dihydroxi-9,10-anthraquinone) anion radicals [18,19] shows an increase of the hfs constants of the protons adjacent to the OH groups in the OHsubstituted ring from 0.046 to 0.240, for dihydroxy-1,4-naphtoquinone, all four protons becoming equivalent, and from 0.100 to 0.190 mT for quinizarin. Therefore, the preferential delocalization of the odd electron on the OH-substituted ring a in the case of the anion radical of mitoxantrone does not seem unreasonable.…”

“…Assignment of the splitting constants obtained by simulation to the different positions in the molecule is only tentative and was performed by comparison with literature data [7,18,19] and with the calculated spin density pattern. Although the PM3 and AM1 methods are not especially adequate for spin density calculations, we think that the results can give some indication about the delocalization of the odd electron on the different positions of the molecule.…”

Effects of the molecular structure on the electrochemical properties of naturally occurring α-hydroxyquinones. An electrochemical and ESR study

“…Due to the distance between each participating atoms, the interaction with this hydrogen atom can be related to the lowest hyperfine coupling constant observed experimentally (0.06 G, Table 2). The magnitude of the experimental hyperfine coupling constant for this hydrogen bond is lower than the observed for hydrogen atoms participating in such interactions for the radical anions electrogenerated from b-hydroxynaphthoquinones (of about 0.5 G [40]), which indicates the low energy of this interaction. Based on the spin density isosurface (Fig.…”

“…This group can be therefore responsible of the experimental coupling constant of 1.86 G. This result suggests that the spin density in this radical dianion structure is highly located at the ring adjacent to the quinone moiety. Another interesting feature is that the distance between the hydroxyl function located at position C-7 and the Oxygen atom at C-14 is 2.4 Å , which is lower that the sum of the Van der Waals radii of the involved atoms (1.2-1.45 Å for H and 1.5 Å for O [38]), and suggests the formation of an intramolecular hydrogen bond [39,40]. Due to the distance between each participating atoms, the interaction with this hydrogen atom can be related to the lowest hyperfine coupling constant observed experimentally (0.06 G, Table 2).…”

The Synthesis, Characterization, and Electrochemical Investigation of Novel Thio- and Alkoxy-Substituted Benzoquinone Derivatives

“…17,18 The electrochemical behaviors of compounds 1(p-chloranil), 8, 16, 18, and 20 have been studied in DMF using TBAP 0.10 M as supporting electrolyte at 100 mVs −1 . The p-chloranil (1) is reduced in two steps producing anion radical (semiquinone, Q − ) and the dianion (Q 2− ).…”