Nonheme iron catalyst mimics heme-dependent haloperoxidase for efficient bromination and oxidation

The [Fe]/H2O2 oxidation system has found wide applications in chemistry and biology. Halogenation with this [Fe]/H2O2 oxidation protocol and halide (X-) in the biological system is well established with the identification of heme-iron-dependent haloperoxidases. However, mimicking such halogenation process is rarely explored for practical use in organic synthesis. Here, we report the development of a nonheme iron catalyst that mimics the heme-iron-dependent haloperoxidases to catalyze the generation of HOBr from H2O2/Br- with high efficiency. We discovered that a tridentate terpyridine (TPY) ligand designed for Fenton chemistry was optimal for FeBr3 to form a stable nonheme iron catalyst [Fe(TPY)Br3], which catalyzed arene bromination, Hunsdiecker-type decarboxylative bromination, bromolactonization, and oxidation of sulfides and thiols. Mechanistic studies revealed that Fenton chemistry ([Fe]/H2O2) might operate to generate hydroxyl radical (HO•), which oxidize bromide ion [Br-] into reactive HOBr. This nonheme iron catalyst represents a biomimetic model for heme-iron-dependent haloperoxidases with potential applications in organic synthesis, drug discovery, and biology.

Insight into the chemoselective aromatic vs. side-chain hydroxylation of alkylaromatics with H2O2 catalyzed by a non-heme imine-based iron complex

Side-chain/ring oxygenated product ratio increases upon decreasing the benzylic bond dissociation energy in the oxidation of alkylaromatics with H₂O₂ catalyzed by an imine-based iron complex.

Increasing the steric hindrance around the catalytic core of a self-assembled imine-based non-heme iron catalyst for C-H oxidation

Sterically hindered imine-based non-heme complexes 4 and 5 rapidly self-assemble in acetonitrile at 25 °C, when the corresponding building blocks are added in solution in the proper ratios. Such complexes are investigated as catalysts for the H2O2 oxidation of a series of substrates in order to ascertain the role and the importance of the ligand steric hindrance on the action of the catalytic core 1, previously shown to be an efficient catalyst for aliphatic and aromatic C-H bond oxidation. The study reveals a modest dependence of the output of the oxidation reactions on the presence of bulky substituents in the backbone of the catalyst, both in terms of activity and selectivity. This result supports a previously hypothesized catalytic mechanism, which is based on the hemi-lability of the metal complex. In the active form of the catalyst, one of the pyridine arms temporarily leaves the iron centre, freeing up a lot of room for the access of the substrate.

Iron-Catalyzed Oxidation of 1-Phenylethanol and Glycerol With Hydrogen Peroxide in Water Medium: Effect of the Nitrogen Ligand on Catalytic Activity and Selectivity

The iron(II) complexes [Fe(bpy)₃](OTf)₂ (bpy = 2,2'-bipyridine; OTf = CF₃SO₃) (1) and [Fe(bpydeg)₃](OTf)₂ (bpydeg = N⁴,N⁴-bis(2-(2-methoxyethoxy)ethyl) [2,2'-bipyridine]-4,4'-dicarboxamide) (2), the latter being a newly synthesized ligand, were employed as catalyst precursors for the oxidation of 1-phenylethanol with hydrogen peroxide in water, using either microwave or conventional heating. With the same oxidant and medium the oxidation of glycerol was also explored in the presence of 1 and 2, as well as of two similar iron(II) complexes bearing tridentate ligands, i.e., [Fe(terpy)₂](OTf)₂ (terpy = 2, 6-di(2-pyridyl)pyridine) (3) and [Fe(bpa)₂](OTf)₂ (bpa = bis(2-pyridinylmethyl)amine) (4): in most reactions the major product formed was formic acid, although with careful tuning of the experimental conditions significant amounts of dihydroxyacetone were obtained. Addition of heterocyclic amino acids (e.g., picolinic acid) increased the reaction yields of most catalytic reactions. The effect of such additives on the evolution of the catalyst precursors was studied by spectroscopic (NMR, UV-visible) and ESI-MS techniques.

Efficient and selective oxidation of alcohols to carbonyl compounds at room temperature by a ruthenium complex catalyst and hydrogen peroxide

An efficient system comprising a ruthenium complex and hydrogen peroxide was developed for the oxidation of various primary and secondary alcohols at room temperature.

Pyrazine dicarboxylate-bridged arsenotungstate: synthesis, characterization, and catalytic activities in epoxidation of olefins and oxidation of alcohols

The catalytic properties and polyanionic structure of pyrazine dicarboxylate-bridged arsenotungstate are reported.

Catalytic oxidation of alcohols with novel non-heme N4-tetradentate manganese(ii) complexes

We report the preparation and characterisation of a series of novel non-heme N4-tetradentate Mn(OTf)2 complexes of the type, [(L)MnOTf2], where L = R,R and S,S enantiomers of BPMCN, its 6-methyl and 6-bromo derivatives as well as the novel ligand BMIMCN (BPMCN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-(R,R/S,S)-1,2-diaminocyclohexane, BMIMCN = N,N'-dimethyl-N,N'-bis(1-methyl-2-imidazolemethyl)-(R,R/S,S)-1,2-diaminocyclohexane). Solid state structural analysis of the BMIMCN-ligated Mn-triflate complexes (R,R-C4 and S,S-C4) revealed opposite helicity but identical metal site accessibility. This feature was exploited in the catalytic oxidation of primary and secondary alcohols, with hydrogen peroxide as oxidant and acetic acid as co-catalyst. Complexes R,R-C4 and S,S-C4 displayed the highest activity in benzyl alcohol oxidation, attributed to the electron-donating property of the BMIMCN ligand. Complex S,S-C4, displayed high activity for a variety of primary alcohol substrates, but the reaction suffered from reduced selectivity and side-reactions due to the presence of acetic acid. In contrast, secondary alcohol substrates could be oxidised to the corresponding ketone products in excellent isolated yields under mild reaction conditions and short reaction times.

Oxidative functionalization of aliphatic and aromatic amino acid derivatives with H2O2 catalyzed by a nonheme imine based iron complex

The oxidation of a series of N-acetyl amino acid methyl esters with H₂O₂ catalyzed by a very simple iminopyridine iron(ii) complex 1 easily obtainable in situ by self-assembly of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)₂ was investigated. Oxidation of protected aliphatic amino acids occurs at the α-C–H bond exclusively (N-AcAlaOMe) or in competition with the side-chain functionalization (N-AcValOMe and N-AcLeuOMe). N-AcProOMe is smoothly and cleanly oxidized with high regioselectivity affording exclusively C-5 oxidation products. Remarkably, complex 1 is also able to catalyze the oxidation of the aromatic N-AcPheOMe. A marked preference for the aromatic ring hydroxylation over Cα–H and benzylic C–H oxidation was observed, leading to the clean formation of tyrosine and its phenolic isomers.

Amino acid derivatives are oxidized by the 1/H₂O₂ system. A marked preference for the aromatic over Cα–H and benzylic C–H oxidation is observed with phenylalanine.

Chemoselective hydrogen peroxide oxidation of primary alcohols to aldehydes by a water-soluble and reusable iron(iii) catalyst in pure water at room temperature

A novel, water-soluble and reusable FeCl₃ catalyst showed high catalytic activity and chemoselectivity in the H₂O₂-oxidation of primary alcohols into aldehydes.

Direct hydroxylation of benzene and aromatics with H2O2 catalyzed by a self-assembled iron complex: evidence for a metal-based mechanism

An imine-based catalyst easily obtained by self-assembly of cheap and commercially available starting materials selectively catalyzes the hydroxylation of aromatic compounds.