Electrochemical behavior of 3-chloro-2,4-pentanedione in the presence of cobalt salen.
We have studied the catalytic two-electron reduction of 3-chloro-2,4-pentanedione by cobalt(I) salen electrogenerated at a glassy carbon cathode in acetonitrile containing tetramethylammonium tetrafluoroborate. When cobalt(I) salen is electrogenerated at -0.65 V (a potential that is 30 mV more negative than the peak potential for the reversible one-electron reduction of cobalt(II) salen), the carbon-chlorine bond of 3-chloro-2,4-pentanedione is catalytically cleaved to form 2,4-pentanedion-3-ate; this anion can be protonated either by adventitious water or by a deliberately added proton donor to produce 2,4-pentanedione, or the anion can be trapped with iodoethane to give 3-ethyl-2,4-pentanedione. However, when cobalt(I) salen is electrogenerated at -0.40 V (a potential at which the rate of generation of cobalt(I) salen is relatively small), the 2,4-pentanedion-3-ate, resulting from the catalytic two-electron cleavage described above, can deprotonate unreduced starting material to form 3-chloro-2,4-pentanedion-3-ate and 2,4-pentanedione. In further work, we have found that 2,4-pentanedion-3-ate can be oxidized directly to form the corresponding radical which couples to yield 3,4-diacetyl-2,5-hexanedione. Chemically produced 2,4-pentanedion-3-ate reacts with electrogenerated cobalt(III) salen to give a dionylcobalt(III) salen species which undergoes a one-electron reduction to liberate cobalt(II) salen and the dionate. In addition, cobalt(II) salen reacts with molecular oxygen to give cobalt(III) salen and superoxide, and the latter reduces 3-chloro-2,4-pentanedione to form chloride ion, the 2,4-pentanedion-3-yl radical, and molecular oxygen.
Samide
M J
MJ
Department of Chemistry, Indiana University, Bloomington 47405, USA.
Peters
D G
DG
D000097:Acetonitriles / Q000737:chemistry; D001881:Borates / Q000737:chemistry; D002384:Catalysis; D002614:Chelating Agents / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D004563:Electrochemistry; D004566:Electrodes; D004572:Electrolysis; D005029:Ethylenediamines / Q000737:chemistry*; D008401:Gas Chromatography-Mass Spectrometry; D007202:Indicators and Reagents; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D010422:Pentanones / Q000032:analysis*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013481:Superoxides / Q000737:chemistry
C405663:3-chloro-2,4-pentanedione; D000097:Acetonitriles; D001881:Borates; D002614:Chelating Agents; D005029:Ethylenediamines; D007202:Indicators and Reagents; D010422:Pentanones; D000644:Quaternary Ammonium Compounds; C405664:tetramethylammonium tetrafluoroborate; D013481:Superoxides; D003035:Cobalt; C011452:disalicylaldehyde ethylenediamine; D010100:Oxygen