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.

Electroreductive dimerization of coumarin and coumarin analogues at carbon cathodes.

Electrochemical reduction of coumarin (1), 6-methylcoumarin (2), 7-methylcoumarin (3), 7-methoxycoumarin (4), and 5,7-dimethoxycoumarin (5) at carbon cathodes in dimethylformamide containing 0.10 M tetra-n-butylammonium tetrafluoroborate has been investigated by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1-5 exhibit two irreversible cathodic peaks: (a) the first peak arises from one-electron reduction of the coumarin to form a radical-anion intermediate, which is protonated by the medium to give a neutral radical; (b) although most of this radical undergoes self-coupling to yield a hydrodimer, reduction of the remaining radical (ultimately to produce a dihydrocoumarin) causes the second cathodic peak. At a potential corresponding to the first voltammetric peak, bulk electrolysis of 1-5 affords the corresponding hydrodimer as a mixture of meso and dl diastereomers. Although the meso form dominates, the dl-to-meso ratio varies, due to steric effects arising from substituents on the aromatic ring. Electroreduction of an equimolar mixture of 1 and 4 gives, along with the anticipated symmetrical hydrodimers, an unsymmetrical product derived from the two coumarins. A mechanistic scheme involving both radical-anion and radical intermediates is proposed to account for the formation of the various products.