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 ( E _pc = -1.58 V and E _pa = -1.47 V vs SHE) that is followed by an irreversible cathodic peak ( E _pc = -2.17 V vs SHE). When water (5.0 M) is introduced into the medium, the first peak for 1 becomes irreversible ( E _pc = -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(2 H ))-one) ( D2 ), obtained in significant amounts, that were characterized by means of ¹ H and ¹³ C 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.