Iron(II)/hydroperoxide (Fenton reagent)-induced activation of dioxygen for (A) the direct ketonization of methylenic carbon and (B) the dioxygenation of cis-stilbene.
Several iron complexes [FeII(PA)2 (PA = picolinate), FeII(bpy)2(2+), FeII(OPPh3)4(2+), FeII(MeCN)4(2+), (Cl8TPP)FeII(py)2 (Cl8TPP = tetrakis(2,6-dichlorophenyl)porphyrin), and FeIIICl3] in combination with R'OOH (R' = H, t-Bu) catalytically activate O2 to oxygenate hydrocarbons [e.g., c-C6H12-->c-C6H10(O) [9 O2 turnovers per FeII(PA)2 or FeII(bpy)2(2+), and 13 per (Cl8TPP)-FeII(py)2]; PhCH2CH3-->PhC(O)CH3 (up to 25 O2 turnovers per FeIILx); c-C6H10-->c-C6H8(O) (up to 9 O2 turnovers per FeIILx); PhCH(Me)2-->PhC(O)Me, Ph(Me)2COH, and Ph(Me)C = CH2 (up to 5 O2 turnovers per FeIILx); and cis-PhCH = CHPh-->2PhCH(O) (up to 2 O2 turnovers per FeLx)]. With large R'OOH/FeLx ratios spontaneous decomposition occurs to give free O2 that is incorporated into the substrates. The product profiles for the various FeIILx/R'OOH, O2/RH systems and their electrochemical characterization during steady-state turnover confirm that the first-formed intermediate is a one-to-one R'OOH/FeIILx adduct (e.g., [(PA)2-FeIIOOR' + pyH+] (1)) (Fenton reagent), which reacts with (a) excess FeII(PA)2 to give (PA)2FeIIIOR', (b) excess c-C6H12 to give (c-C6H11)py (kinetic isotope effect, [KIE] = kc-C6H12/kc-C6D12, 4.6 with t-BuOOH and 1.7 with HOOH), (c) excess R'OOH to give [(PA)2FeIV(OH)(OOR')] (3), then [(PA)2FeIV(O2)] (7) and O2, and (d) O2 to form an adduct, [(PA)2-FeIII(O2)(OOR') + pyH+] (5), that reacts with c-C6H12 to form c-C6H10(O), [K] = 8.2 (t-BuOOH) and 2.1 (HOOH). When PhCH2CH3 or c-C6H10 are the substrates (RH), 5 reacts to form [(PA)2FeIV(OH)(OOR)] (6), which in turn reacts with RH and O2 in a catalytic cycle to give PhC(O)Me or c-C6H8(O) [up to 7 O2 turnovers per iron with FeII(OPPh3)4(2+)]. Species 7 reacts with cis-PhCH = CHPh to give PhCH(O).
Kang
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Department of Chemistry, Texas A & M University, College Station 77843.
Redman
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Cepak
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Sawyer
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DT
D008401:Gas Chromatography-Mass Spectrometry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D010100:Oxygen / Q000737:chemistry*; D017382:Reactive Oxygen Species; D013237:Stereoisomerism; D013267:Stilbenes / Q000737:chemistry*
C045076:Fenton's reagent; D007659:Ketones; D017382:Reactive Oxygen Species; D013267:Stilbenes; D006861:Hydrogen Peroxide; D007501:Iron; D010100:Oxygen