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-pentan edione by cobalt(I) salen electrogenerated at a glassy carbon cathode in ac etonitrile containing tetramethylammonium tetrafluoroborate. When cobalt(I) salen is electrogenerated at -0.65 V (a potential that is 30 mV more negat ive than the peak potential for the reversible one-electron reduction of co balt(II) salen), the carbon-chlorine bond of 3-chloro-2,4-pentanedione is c atalytically cleaved to form 2,4-pentanedion-3-ate; this anion can be proto nated 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 electrogen erated 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-2,3-pentanedion-3-ate and 2,4-pentanedio ne. In further work, we have found that 2,4-pentanedion-3-ate can be oxidiz ed directly to form the corresponding radical which couples to yield 3,3-di acetyl-2,5-hexanedione. Chemically produced 2,4-pentanedion-3-ate reacts wi th electrogenerated cobalt(III) salen to give a dionylcobalt(III) salen spe cies 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, an d molecular oxygen. (C) 1999 Elsevier Science B.V. All rights reserved.