Olefin Cis-Dihydroxylation with Bio-Inspired Iron Catalysts. Evidence for an Fe^II/Fe^IV Catalytic Cycle

Abstract

Iron(II) complexes of a series of N-acylated dipyridin-2-ylmethylamine ligands (R-DPAH) have been investigated as catalysts for the cis-dihydroxylation of olefins to model the action of Rieske dioxygenases that catalyze arene cis-dihydroxylation. The Rieske dioxygenases have a mononuclear iron active site coordinated to a 2-histidine-1-carboxylate facial triad motif. The R-DPAH ligands are designed to provide a facial N,N,O-ligand set that mimics the enzyme active site. The iron(II) complexes of the R-DPAH ligands activate H(2)O(2) to effect the oxidation of olefin substrates into cis-diol products. As much as 90% of the H(2)O(2) oxidant is converted into cis-diol, but a large excess of olefin is required to achieve the high conversion efficiency. Reactivity and mechanistic comparisons with the previously characterized Fe(TPA)/H(2)O(2) catalyst/oxidant combination (TPA = tris(pyridin-2-ylmethyl)amine) lead us to postulate an Fe(II)/Fe(IV) redox cycle for the Fe(R-DPAH) catalysts in which an Fe(IV)(OH)(2) oxidant carries out the cis-hydroxylation of olefins. This hypothesis is supported by three sets of observations: (a) the absence of a lag phase in the conversion of the H(2)O(2) oxidant into a cis-diol product, thereby excluding the prior oxidation of the Fe(II) catalyst to an Fe(III) derivative as established for the Fe(TPA) catalyst; (b) the incorporation of H(2)(18)O into the cis-diol product, thereby requiring O-O bond cleavage to occur prior to cis-diol formation; and (c) the formation of cis-diol as the major product of cyclohexene oxidation, rather than the epoxide or allylic alcohol products more commonly observed in metal-catalyzed oxidations of cyclohexene, implicating an oxidant less prone to oxo transfer or H-atom abstraction.

A DFT Study of the cis-Dihydroxylation of Nitroaromatic Compounds Catalyzed by Nitrobenzene Dioxygenase

“…25 18 O-labeled products may be attributed to rapid intermolecular exchange between HO–Fe V =O and H 2 18 O prior to substrate oxidation. 74,75 Therefore, we probed possible dioxygenation pathways starting from a direct attack of HO–Fe V =O on the substrate (Figure 4b).…”

Mimicry of the 2‐His‐1‐Carboxylate Facial Triad Using Bulky N,N,O‐Ligands: Non‐Heme Iron Complexes Featuring a Single Facial Ligand and Easily Exchangeable Co‐Ligands

“…[19] Que et al furthermore published the catalytic activity of a series of complexes of the [Fe(R-DPAH) 2 ] 2+ family (Figure 1), which have a strong selectivity towards the formation of diols with TONs up to 9.4 for styrene. [22,24] For these complexes, the active species is proposed to have a 1:1 iron/ligand ratio; however, no hard evidence for this is provided. Our group previously reported the activity of [Fe(PyProMe)Cl 2 ] in the epoxidation of olefins with TONs up to 50, but the flexible structure of this ligand allows isomerization to meridional coordination, which makes the structure of the active species uncertain.…”

“…[21] Furthermore, the R-DPAH ligand family developed by Que and co-workers and the polydentate pyridyl ligand from Bauer et al resulted in bis-ligand complexes (Figure 1). [24,32] By mimicking the active site of these iron enzymes more strictly, our group aims to develop new catalysts that allow further insights to be gained regarding the reactivity and the mechanism of operation of facial triad enzymes. To this end, we have previously reported the development of the tripodal 3,3-bis(alkylimidazol-2-yl)propionate (BAIP) N,N,O-ligand BMIP nPr ( Figure 1).…”

Oxidative Decarboxylation of Benzilic Acid by a Biomimetic Iron(II) Complex: Evidence for an Iron(IV)–Oxo–Hydroxo Oxidant from O2

“…[14][15][16][17] For the enzymes, the accumulated mechanistic evidence favors an iron(V)-oxo-hydroxo oxidant, [14][15][16] although the corresponding iron(IV)-oxo-hydroxo species has also been suggested. [17] Indirect evidence for a high-valent iron-oxo-hydroxo oxidant has also been obtained from studies of biomimetic nonheme iron complexes that catalyze alkene cis-dihydroxylation using hydrogen peroxide as oxidant; [18][19][20][21] an iron(V)-oxo-hydroxo oxidant is proposed for iron catalysts with tetradentate N 4 ligands, [19][20][21] while a dihydroxoiron(IV) oxidant is postulated for iron catalysts with facial N,N,O ligands. [18] In summary, we have reported a biomimetic iron(II) complex (1) which reacts with O 2 to oxidatively cleave benzilic acid to benzophenone.…”

“…[10] The fate of the atomic constituents of dioxygen was determined by 18 [10,11] This hydroxylation reaction can be inhibited by the presence of different reagents that can intercept the putative high-spin Fe IV =O oxidant formed in the oxidative decarboxylation of the iron(II)-benzoylformate complex 2.…”