Catalytic Alkyl Hydroperoxide and Acyl Hydroperoxide Disproportionation by a Nonheme Iron Complex

Abstract

Alkyl hydroperoxides are commonly used as terminal oxidants because they are generally acknowledged to be stable towards disproportionation compared with H2O2. We show that alkylperoxide disproportionation is effectively catalyzed by the [Fe(tpena)] 2+ (tpena = N,N,N'tris(2-pyridylmethyl)ethylendiamine-N'-acetate). A peroxidase type mechanism, in other words, involvement of iron(IV)oxo species, is consistent with the rates and product distribution. Accordingly, O2 and tert-butanol and cumyl alcohol are concurrently produced for substrates tertbutyl hydroperoxide and cumene hydroperoxide respectively, in the presence of [Fe(tpena)] 2+ with O2 yields of 88 % and 44 % respectively. Rate constants for initial O2 production ([Fe] 0.005 mol%) were measured to 3.66(6) mMs-1 and 0.29(3) mMs-1 , respectively. Participating in the mechanism are spectroscopically detectable (UV-vis, EPR, resonance Raman) transient alkyl-and acylperoxide adducts, [Fe III OOR(tpenaH)] 2+ (R = C(CH3)3, C(CH3)2Ph, C(O)PhCl; T½ = 30s (5 C), 20s 2 (5 C), 1s (-30C)) with their common decay product [Fe IV O(tpenaH)] 2+. Concurrently organic radicals proposed to be ROO • were detected by EPR spectroscopy. A lower yield of O2 at 23 % with an initial rate of 0.10(3) mMs-1 for the disproportionation of m-chloroperoxybenzoic acid is readily explained by catalyst inhibition by coordination of the product m-chlorobenzoic acid. Oxidative decomposition of the alkyl groups by a unimolecular β-scission pathway, favoured for cumene hydroperoxide, competes with ROOH disproportionation. Despite the fact that the catalytic disproportionation is effective, external C-H substrates-when they are present in excess of ROOHcan be targeted and catalytically and selectively oxidized by ROOH using [Fe(tpena)] 2+ as the catalyst.

cis Donor Influence on O–O Bond Lability in Iron(III) Hydroperoxo Complexes: Oxidation Catalysis and Ligand Transformation

“…Given that the background • OH radical concentration will be the same in all reactions, and presuming that the iron hydroperoxides are not active as the direct oxidants, 60,61 then it can be concluded that [Fe IV O(Htpena)] 2+ is the most potent iron(IV) oxo species in this series and is consistent with the oxidation of cyclohexanol in water using [Fe IV O(Htpena)] 2+ generated by electroactivation where no • OH radicals are present. 50 69 We can conclude from all the observations that the latent iron(IV)oxo complex of tpena is more rapidly available, in higher concentrations, in comparison with the tpen-and tpenOsupported systems in the presence of H 2 O 2 .…”

Engineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C–H Oxidation by a cis Donor and Variation of the Second Coordination Sphere

“…Rate constants of 1.2 × 10 −3 and 4 × 10 −4 M −1 s −1 were deduced respectively for the decay of [Fe IV O(tpena)] + at pH 6−7 and [Fe IV O(Htpena)] 2+ pH 3−4 in buffered water. 60 This was ascribed to slow water oxidation. While our work shows that [Fe IV O(Htpena)] 2+ is the most potent aqueous iron(IV) oxo complex for C−H oxidation in the present series, slow background water oxidation might marginally compromise this activity, when C−H substrates are limited.…”

Biomimetic Aerobic Epoxidation of Alkenes Catalyzed by Cobalt Porphyrin under Ambient Conditions in the Presence of Sunflower Seeds Oil as a Co-Substrate

“…Hence, LAOOH would readily be decomposed by CoPor into LAO· through O–O bond homolytic cleavage. , With respect to the strongest relative abundance of LAOH in the presence of CoPor (Figure B), LAOOH probably converted into LAOH via LAO· through H-abstraction. In addition, LAO· would likely to be converted into the corresponding fatty aldehyde through homolytic β-scission. − …”