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.