Journal of Electroanalytical Chemistry, Vol.654, No.1-2, 44-51, 2011
Reduction of 4-(bromomethyl)-2-oxo-2H-chromen-7-yl acetate at carbon cathodes in dimethylformamide and acetonitrile: Lifetime of the electrogenerated radical-anion
Cyclic voltammograms for reduction of 4-(bromomethyl)-2-oxo-2H-chromen-7-yl acetate (or 7-acetoxy4-bromomethylcoumarin) (1) at carbon cathodes in either dimethylformamide (DMF) containing tetramethylammonium tetrafluoroborate (TMABF(4)) or acetonitrile (CH(3)CN) containing TMABF(4) 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-TMABF(4) and 23-24% in CH(3)CN-TMABF(4) 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-TMABF(4) and CH(3)CN-TMABF(4). 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 x 10(5) s(-1) in DMF-TMABF(4) and 2.5 x 10(5) s(-1) in CH(3)CN-TMABF(4). 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). (C) 2011 Elsevier B.V. All rights reserved.