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

Org Biomol Chem. 2014 Sep 7;12(33):6393-8. doi: 10.1039/c4ob01207k.

Abstract

An electrochemical and theoretical analysis of a series of shikonin derivatives in aprotic media is presented. Results showed that the first electrochemical reduction signal is a reversible monoelectronic transfer, generating a stable semiquinone intermediate; the corresponding E(I)⁰ values were correlated with calculated values of electroaccepting power (ω(+)) and adiabatic electron affinities (A(Ad)), obtained with BH and HLYP/6-311++G(2d,2p) and considering the solvent effect, revealing the influence of intramolecular hydrogen bonding and the substituting group at position C-2 in the experimental reduction potential. For the second reduction step, esterified compounds isobutyryl and isovalerylshikonin presented a coupled chemical reaction following dianion formation. Analysis of the variation of the dimensionless cathodic peak potential values (ξ(p)) as a function of the scan rate (v) functions and complementary experiments in benzonitrile suggested that this process follows a dissociative electron transfer, in which the rate of heterogeneous electron transfer is slow (~0.2 cm s(-1)), and the rate constant of the chemical process is at least 10(5) larger.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Electrochemical Techniques*
  • Electron Transport
  • Esters / chemistry*
  • Molecular Structure
  • Naphthoquinones / chemical synthesis
  • Naphthoquinones / chemistry*
  • Quantum Theory*

Substances

  • Esters
  • Naphthoquinones
  • shikonin