Add time:08/07/2019         Source:sciencedirect.com

Cyclic voltammograms for reduction of 4-(bromomethyl)-2-oxo-2H-chromen-7-yl acetate (or 7-acetoxy-4-bromomethylcoumarin) (1) at carbon cathodes in either dimethylformamide (DMF) containing tetramethylammonium tetrafluoroborate (TMABF4) or acetonitrile (CH3CN) containing TMABF4 show four irreversible cathodic peaks, the one at least negative potential being due to scission of the carbon–bromine bond. Bulk electrolysis of 1 at a reticulated vitreous carbon electrode held at potentials slightly more negative than that of the first cathodic peak afford two products: (i) a monomer, 4-methyl-2-oxo-2H-chromen-7-yl acetate (2), formed in yields of 18–22% in DMF–TMABF4 and 23–24% in CH3CN–TMABF4 and (ii) a dimer, 4,4′-(ethane-1,2-diyl)bis(2-oxo-2H-chromene-7,4-diyl) diacetate (3), obtained in yields of 75–80% and 72–77%, respectively, in DMF–TMABF4 and CH3CN–TMABF4. Reduction of 1 was found to conform to a classic EC mechanism; the first-order rate constant for cleavage of the carbon–bromine bond of the electrogenerated radical–anion is 2.7 × 105 s−1 in DMF–TMABF4 and 2.5 × 105 s−1 in CH3CN–TMABF4. For both solvent–electrolyte systems, fast-scan cyclic voltammetry carried out with carbon-fiber ultramicroelectrodes shows that the electron-transfer process associated with formation of the radical–anion begins to exhibit reversibility at scan rates greater than 5000 V s−1.

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