μ-Oxo diferric complexes as oxidation catalysts with hydrogen peroxide and their potential in asymmetric oxidation

An enantioselective oxidative coupling reaction of 2-naphthol derivatives catalyzed by chiral diphosphine oxide-iron(ii) complexes

An enantioselective oxidative coupling of 2-naphthol derivatives is developed with the use of chiral Fe(ii)-diphosphine oxide complexes. Optically active 1,1-bi-2-naphthol derivatives can be synthesized in high yields when a 2 : 1 complex of (S)-xylyl-iPrO-BIPHEP-oxide and Fe(OTf)₂ is used in the presence of t-butyl hydroperoxide as an oxidant. The non-linear effect, X-ray crystal structure and ESI-MS suggest that a 2 : 1 complex of (S)-xylyl-iPrO-BIPHEP-oxide and Fe(OTf)₂ is a pre-catalyst for a Fe(iii)/Fe(iv) redox cycle.

Formation, Characterization, and Reactivity of a Nonheme Oxoiron(IV) Complex Derived from the Chiral Pentadentate Ligand asN4Py

The chiral pentadentate low-spin (S = 1) oxoiron(IV) complex [Fe^IV(O)(asN4Py)]²⁺ (2) was synthesized and spectroscopically characterized. Its formation kinetics, reactivity, and (enantio)selectivity in an oxygen-atom-transfer reaction was investigated in detail and compared to a similar pentadentate ligand-containing system.

In situ-generated chiral iron complex as efficient catalyst for enantioselective sulfoxidation using aqueous H2O2 as oxidant

This study represents the rare combination of non-toxic Fe based catalyst/H₂O₂ as an efficient catalytic protocol for asymmetric sulfoxidation reaction.

Functional models of α-keto acid dependent nonheme iron oxygenases: synthesis and reactivity of biomimetic iron(II) benzoylformate complexes supported by a 2,9-dimethyl-1,10-phenanthroline ligand

Two biomimetic iron(II) benzoylformate complexes, [LFe(II)(BF)(2)] (2) and [LFe(II)(NO(3))(BF)] (3) (L is 2,9-dimethyl-1,10-phenanthroline and BF is monoanionic benzoylformate), have been synthesized from an iron(II)-dichloro complex [LFe(II)Cl(2)] (1). All the iron(II) complexes have been structurally and spectroscopically characterized. The iron(II) center in 2 is coordinated by a bidentate NN ligand (2,9-dimethyl-1,10-phenanthroline) and two monoanionic benzoylformates to form a distorted octahedral coordination geometry. One of the benzoylformates binds to the iron in 2 via both carboxylate oxygens but the other one binds in a chelating bidentate fashion via one carboxylate oxygen and the keto oxygen. On the other hand, the iron(II) center in 3 is ligated by one NN ligand, one bidentate nitrate, and one monoanionic chelating benzoylformate. Both iron(II) benzoylformate complexes exhibit the facial NNO donor environment in their solid-state structures. Complexes 2 and 3 are stable in noncoordinating solvents under an inert atmosphere, but react with dioxygen under ambient conditions to undergo oxidative decarboxylation of benzoylformate to benzoate in high yields. Evidence for the formation of an iron(IV)-oxo intermediate upon oxidative decarboxylation of benzoylformate was obtained by interception and labeling experiments. The iron(II) benzoylformate complexes represent the functional models of α-keto acid dependent oxygenases.

Iron-catalyzed asymmetric aerobic oxidation: oxidative coupling of 2-naphthols

Fe(salan) complexes were found to be efficient catalysts for the asymmetric aerobic oxidative coupling of 2-naphthol derivatives. This reaction can be carried out in air at 60 degrees C with high enantioselectivity up to 97% ee. This is the first report for asymmetric aerobic oxidation using molecular oxygen in air in the absence of additives.

Iron-Catalyzed Oxidation of Thioethers by Iodosylarenes: Stereoselectivity and Reaction Mechanism

Catalytic properties of a series of iron(III)-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) and related complexes in asymmetric sulfoxidation reactions, with iodosylarenes as terminal oxidants, have been explored. These catalysts have been found to efficiently catalyze oxidation of alkyl aryl sulfides to sulfoxides with high chemoselectivity (up to 100 %) and moderate-to-high enantioselectivity (up to 84 % with isopropylthiobenzene and iodosylmesitylene), the TON (TON=turnover number) approaching 500. The influence of the ligand (electronic and steric effects of the substituents), oxidant, and substrate structures on the oxidation stereoselectivity has been investigated systematically. The structure of the reactive intermediates (complexes of the type [Fe(III)(ArIO)(salen)] and the reaction mechanism have been revealed by both mechanistic studies with different iodosylarenes and direct in situ (1)H NMR observation of the formation of the reactive species and its reaction with the substrate.

Investigations on the iron-catalyzed asymmetric sulfide oxidation

The development of an enantioselective sulfide oxidation involving a chiral iron catalyst and aqueous hydrogen peroxide as oxidant is described. In the presence of a simple carboxylic acid, or a carboxylate salt, the reaction affords sulfoxides with remarkable enantioselectivities (up to 96 % ee) in moderate to good yields. The influence of the structure of the additive on the reaction outcome is reported. In the sulfoxide-to-sulfone oxidation a kinetic resolution (with s = 4.8) occurs, which, however, plays only a negligible role in the overall enantioselective process. Furthermore, a positive nonlinear relationship between the ee of the product and that of the catalyst has been found. On the basis of these observations, a possible catalyst structure is proposed.

Simple iron catalyst for terminal alkene epoxidation

[reaction: see text] A mu-oxo-iron(III) dimer, [((phen)(2)(H(2)O)Fe(III))(2)(mu-O)](ClO(4))(4), is an efficient epoxidation catalyst for a wide range of alkenes, including terminal alkenes, using peracetic acid as the oxidant. Low catalyst loadings, in situ catalyst preparation from common reagents, fast reaction times (<5 min at 0 degrees C), and enhanced reaction performance at high substrate concentrations combine to create a temporally and synthetically efficient procedure for alkene epoxidation.

Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III) [bond] salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [O [double bond] Fe(IV)(salen)](*+) as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis-Menten kinetics correlate well with Hammett sigma constants, giving reaction constant (rho) values in the range of -0.65 to -1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by substituted oxo(salen)iron complexes also correlate with Hammett sigma constants, giving positive rho values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step.

Reactivity of hydroperoxide bound to a mononuclear non-heme iron site

The first isolation and spectroscopic characterization of the mononuclear hydroperoxo-iron(III) complex [Fe(H(2)bppa)(OOH)](2+) (2) and the stoichiometric oxidation of substrates by the mononuclear iron-oxo intermediate generated by its decomposition have been described. The purple species 2 obtained from reaction of [Fe(H(2)bppa)(HCOO)](ClO(4))(2) with H(2)O(2) in acetone at -50 degrees C gave characteristic UV-vis (lambda(max) = 568 nm, epsilon = 1200 M(-1) cm(-1)), ESR (g = 7.54, 5.78, and 4.25, S = (5)/(2)), and ESI mass spectra (m/z 288.5 corresponding to the ion, [Fe(bppa)(OOH)](2+)), which revealed that 2 is a high-spin mononuclear iron(III) complex with a hydroperoxide in an end-on fashion. The resonance Raman spectrum of 2 in d(6)-acetone revealed two intense bands at 621 and 830 cm(-1), which shifted to 599 and 813 cm(-1), respectively, when reacted with (18)O-labeled H(2)O(2). Reactions of the isolated (bppa)Fe(III)-OOH (2) with various substrates (single turnover oxidations) exhibited that the iron-oxo intermediate generated by decomposition of 2 is a nucleophilic species formulated as [(H(2)bppa)Fe(III)-O*].

H2 O2 -Dependent Fe-Catalyzed Oxidations: Control of the Active Species

Manipulation of the coordination sphere of an Fe^II ion can be used to tune the balance between different catalytic pathways for oxidation (OH^. versus iron-based oxidant; see scheme). This reinvestigation of Fenton chemistry uses the iron complex shown as a mechanistic probe.

Mild and efficient flavin-catalyzed H2O2 oxidations

Based on a previously discovered method for amine oxidations using flavins as catalysts and hydrogen peroxide as oxidant, a comparative kinetic study using NMR spectroscopy was undertaken with a series of flavins for amine and thioether oxidations. Included in this series is the newly prepared 7,8-difluoro-1,3-dimethyl-5-ethyl-5,10-dihydroalloxazine. This study shows that flavins, which bear electron-donating groups on the aromatic ring and/or the N-10 position, are less active and are deactivated during the course of the reaction. Moreover, flavins that are alkylated at the N-1 position instead of the N-10 position and having either no substituents or electron-withdrawing groups on the aromatic ring, remain the most active and stable.

Enantioselective Sulfoxidation as a Probe for a Metal-Based Mechanism in H(2)O(2)-Dependent Oxidations Catalyzed by a Diiron Complex

The catalytic properties of the diiron complex 1, Fe(2)OL(4)(H(2)O)(2)(ClO(4))(4) with L = (-)-4,5-pinenebipyridine, a chiral bipyridine derivative, have been investigated. Complex 1 efficiently catalyzes the oxidation of aryl sulfides to the corresponding sulfoxides by hydrogen peroxide, with yields ranging from 45 to 90% based on the oxidant. Furthermore the reactions were enantioselective and produced a mixture of sulfoxide enantiomers with significant enantiomeric excesses. The largest ee value (40%) was found in the case of p-bromophenyl methyl sulfide. Optimal ee's were obtained in polar solvents and at low temperature (below 0 degrees C), when the excess of the oxidant was limited. The observation of (i) a saturation kinetics with respect to both sulfide and H(2)O(2) concentrations, (ii) a linear Hammett correlation of the initial V(max) values with sigma(p) values, for a series of p-substituted aryl methyl sulfides, (iii) iron-peroxo complexes, characterized by light absorption and Raman resonance spectroscopies, during reaction of complex 1 with H(2)O(2), and (iv) a saturation kinetics with respect to sulfide during oxidation of sulfide into sulfoxide by the iron-peroxo complexes led us to propose that the iron-peroxo moiety is the actual oxygen atom donor to the substrate, thus explaining the enantioselective control of the catalytic reaction. These data demonstrate that oxidations by non heme diiron complexes can proceed through metal-based pathways and can thus be made stereoselective.