Samide M J, Peters D G
Department of Chemistry, Indiana University, Bloomington 47405, USA.
J Pharm Biomed Anal. 1999 Feb;19(1-2):193-203. doi: 10.1016/s0731-7085(98)00055-7.
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.
我们研究了在含有四氟硼酸四甲基铵的乙腈中，在玻碳阴极上电生成的钴（I）salen对3-氯-2,4-戊二酮的催化双电子还原反应。当钴（I）salen在-0.65 V（该电位比钴（II）salen可逆单电子还原的峰电位负30 mV）上电生成时，3-氯-2,4-戊二酮的碳-氯键被催化裂解形成2,4-戊二酮-3-酸根离子；该阴离子可被偶然存在的水或特意添加的质子供体质子化生成2,4-戊二酮，或者该阴离子可与碘乙烷捕获生成3-乙基-2,4-戊二酮。然而，当钴（I）salen在-0.40 V（此时钴（I）salen的生成速率相对较小）上电生成时，上述催化双电子裂解产生的2,4-戊二酮-3-酸根离子可使未还原的起始原料去质子化，形成3-氯-2,4-戊二酮-3-酸根离子和2,4-戊二酮。在进一步的工作中，我们发现2,4-戊二酮-3-酸根离子可直接被氧化形成相应的自由基，该自由基偶联生成3,4-二乙酰基-2,5-己二酮。化学制备的2,4-戊二酮-3-酸根离子与电生成的钴（III）salen反应生成二氧代钴（III）salen物种，该物种经历单电子还原以释放钴（II）salen和二酸根离子。此外，钴（II）salen与分子氧反应生成钴（III）salen和超氧阴离子，后者将3-氯-2,4-戊二酮还原形成氯离子、2,4-戊二酮-3-基自由基和分子氧。
