Electrosynthesis of a Biaurone by Controlled Dimerization of Flavone: Mechanistic Insight and Large-Scale Application

The electrochemistry of flavone (1) has been carefully
investigated at glassy carbon cathodes in dimethylformamide containing
0.10 M tetra-n-butylammonium tetrafluoroborate as
supporting electrolyte. In this medium, a cyclic voltammogram for
a reduction of 1 exhibits a reversible cathodic process
(Epc = −1.58 V and Epa = −1.47 V vs SHE) that is followed by an irreversible
cathodic peak (Epc = −2.17 V vs
SHE). When water (5.0 M) is introduced into the medium, the first
peak for 1 becomes irreversible (Epc = −1.56 V vs SHE), and the second (irreversible)
peak shifts to −2.07 V vs SHE. Bulk electrolyses of 1 at −1.60 V vs SHE afford flavanone, 2′-hydroxychalcone,
2′-hydroxy-3-phenylpropionate, and two new compounds, namely
(Z)-1,6-bis­(2-hydroxyphenyl)-3,4-diphenylhex-3-ene-1,6-dione
(D1) and (Z)-2,2′-(1,2-diphenylethene-1,2-bis­(benzofuran-3­(2H))-one) (D2), obtained in significant amounts,
that were characterized by means of 1H and 13C NMR spectrometry as well as single-crystal X-ray diffraction. Along
with the above findings, we have proposed a mechanism for the electroreduction
of 1, which has been further corroborated by our quantum
mechanical study.