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.