Electroreductive Dimerization of Coumarin and Coumarin Analogues at Carbon Cathodes

Abstract

Electrochemical reduction of coumarin (<b>1</b>), 6-methylcoumarin (<b>2</b>), 7-methylcoumarin (<b>3</b>), 7-methoxycoumarin (<b>4</b>), and 5,7-dimethoxycoumarin (<b>5</b>) at carbon cathodes in dimethylformamide containing 0.10 M tetra-<i>n</i>-butylammonium tetrafluoroborate has been investigated by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of <b>1</b>–<b>5</b> 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 <b>1</b>–<b>5</b> affords the corresponding hydrodimer as a mixture of <i>meso</i> and <i>dl</i> diastereomers. Although the <i>meso</i> form dominates, the <i>dl</i>-to-<i>meso</i> ratio varies, due to steric effects arising from substituents on the aromatic ring. Electroreduction of an equimolar mixture of <b>1</b> and <b>4</b> 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