Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide

Abstract

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)FeIV(O)(CH3CN)]2+ (2; cyclam=1,4,8,11‐tetraazacyclotetradecane), is reported in the reactions of [(cyclam)FeII]2+ with aqueous hydrogen peroxide (H2O2) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo‐ and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)FeII]2+ and H2O2. In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high‐yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.

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

“…To date the supporting ligand influence on the properties of Fe­(IV) oxo complexes, predominantly studied in organic solvents, show that trans donors, not unexpectedly, have a significant influence on the spectroscopic properties and reactivity. , The cis position as part of a chelating ligand to the oxo ligand has been more scantily probed. Notably, it was recently found that (in organic solvent) using a N3O macrocycle analogue of cyclam through replacement of one of the tertiary amine donors with a neutral ether oxygen atom enhances the rate of HAT reactions by 5 orders of magnitude . In the only SAR study to be carried out in water, Chartarojsiri et al have shown that an increase in ν FeO correlates with an increase in HAT and OAT rates within a series of iron­(IV) oxo complexes supported by pentadentate pentapyridyl ligands, where the para substituent on the pyridyl donor trans to the oxo group is varied (H, CH 3 , CF 3 ).…”

“…Notably, it was recently found that (in organic solvent) using a N3O macrocycle analogue of cyclam through replacement of one of the tertiary amine donors with a neutral ether oxygen atom enhances the rate of HAT reactions by 5 orders of magnitude. 50 In the only SAR study to be carried out in water, Chartarojsiri et al 51 have shown that an increase in ν FeO correlates with an increase in HAT and OAT rates within a series of iron(IV) oxo complexes supported by pentadentate pentapyridyl ligands, where the para substituent on the pyridyl donor trans to the oxo group is varied (H, CH 3 , CF 3 ).…”

Low‐Coordinated Iron(II) Siloxide Complexes – Structural Diversity and Reactivity Towards O2 and Oxygen Atom Transfer Reagents

“…Indeed, the oxygenation of cyclohexene can take two possible reaction pathways. Either epoxidation takes place, that is, cyclohexene is directly oxidized to cyclohexene oxide, which is the usual pathway observed for Fe IV =O species, [22] or a radical chain reaction is initiated. In the latter, the reactive intermediate resulting from the reaction between the iron(II) complex and dioxygen abstracts an H atom resulting in the formation of a cyclohexenyl radical, which reacts to 2-cyclohexene-1-hydroperoxide (CÀ OOH).…”

“…To test for in-situ oxygenation reactivity, different molecules such as triphenylphosphine, thioanisole and cyclohexene, where employed that are known to function as acceptors for O atoms derived from Fe IV =O oxidants. [13,[21][22][23] However, even after using inert solvents like 1,2-fluorobenzene and hexafluorobenzene, to exclude a side reaction with the solvent, no oxidized compounds such as triphenylphosphine oxide, methylphenyl sulfoxide or cyclohexene oxide could be detected. This is consistent with the aforementioned assumption, that the reactive intermediate immediately reacts with a ligand phenyl group in close proximity, prohibiting a reaction with a substrate molecule.…”

Unmasking Steps in Intramolecular Aromatic Hydroxylation by a Synthetic Nonheme Oxoiron(IV) Complex

“…However,t his step is often not fast enough, thus allowing the substrate radical to escape from the solvent cage and be intercepted by O 2 . [8] To date,t he reactivity of synthetic Fe IV =Os pecies with respect to two-electron transfer has mainly been exemplified by oxygen atom transfer (OAT)r eactions to phosphines or thioethers,which are substrates that readily form corresponding oxides.M uch less common are examples of more intriguing reactions such as olefin epoxidation, [9] where the Oatom is transferred to aC = Cb ond, or aromatic hydroxylation, where Oatom transfer results in the functionalization of the aromatic ring. [10] With respect to the latter,noF e IV =O species characterized to date has been reported to convert benzene directly to phenol.…”