Chemo- and stereoselective CH oxidations and epoxidations/cis-dihydroxylations with H2O2, catalyzed by non-heme iron and manganese complexes

Increase of Direct C-C Coupling Reaction Yield by Identifying Structural and Electronic Properties of High-Spin Iron Tetra-azamacrocyclic Complexes

Macrocyclic ligands have been explored extensively as scaffolds for transition metal catalysts for oxygen and hydrogen atom transfer reactions. C-C reactions facilitated using earth abundant metals bound to macrocyclic ligands have not been well-understood but could be a green alternative to replacing the current expensive and toxic precious metal systems most commonly used for these processes. Therefore, the yields from direct Suzuki-Miyaura C-C coupling of phenylboronic acid and pyrrole to produce 2-phenylpyrrole facilitated by eight high-spin iron complexes ([Fe3+L1(Cl)2]+, [Fe3+L4(Cl)2]+, [Fe2+L5(Cl)]+, [Fe2+L6(Cl)2], [Fe3+L7(Cl)2]+, [Fe3+L8(Cl)2]+, [Fe2+L9(Cl)]+, and [Fe2+L10(Cl)]+) were compared to identify the effect of structural and electronic properties on catalytic efficiency. Specifically, catalyst complexes were compared to evaluate the effect of five properties on catalyst reaction yields: (1) the coordination requirements of the catalyst, (2) redox half-potential of each complex, (3) topological constraint/rigidity, (4) N atom modification(s) increasing oxidative stability of the complex, and (5) geometric parameters. The need for two labile cis-coordination sites was confirmed based on a 42% decrease in catalytic reaction yield observed when complexes containing pentadentate ligands were used in place of complexes with tetradentate ligands. A strong correlation between iron(III/II) redox potential and catalytic reaction yields was also observed, with [Fe2+L6(Cl)2] providing the highest yield (81%, -405 mV). A Lorentzian fitting of redox potential versus yields predicts that these catalysts can undergo more fine-tuning to further increase yields. Interestingly, the remaining properties explored did not show a direct, strong relationship to catalytic reaction yields. Altogether, these results show that modifications to the ligand scaffold using fundamental concepts of inorganic coordination chemistry can be used to control the catalytic activity of macrocyclic iron complexes by controlling redox chemistry of the iron center. Furthermore, the data provide direction for the design of improved catalysts for this reaction and strategies to understand the impact of a ligand scaffold on catalytic activity of other reactions.

Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity

Oxo-iron(V) species have been implicated in the catalytic cycle of the Rieske dioxygenase. Their synthetic analog, [Fe^V(O)(OC(O)CH₃)(PyNMe₃)]²⁺ (1, PyNMe₃ = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-trimethyl), derived from the O-O bond cleavage of its acetylperoxo iron(III) precursor, has been shown experimentally to perform regio- and stereoselective C-H and C═C bond functionalization. However, its structure-activity relation is poorly understood. Herein we present a detailed electronic-structure and spectroscopic analysis of complex 1 along with well-characterized oxo-iron(V) complexes, [Fe^V(O)(TAML)]^- (2, TAML = tetraamido macrocyclic ligand), [Fe^V(O)(TMC)(NC(O)CH₃)]⁺ (4, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [Fe^V(O)(TMC)(NC(OH)CH₃)]²⁺ (4-H⁺), using wave function-based multireference complete active-space self-consistent field calculations. Our results reveal that the x/ y anisotropy of the ⁵⁷Fe A-matrix is not a reliable spectroscopic marker to identify oxo-iron(V) species and that the drastically different A _x and A _y values determined for complexes 1, 4, and 4-H⁺ have distinctive origins compared to complex 2, a genuine oxo-iron(V) species. Complex 1, in fact, has a dominant character of [Fe^IV(O···OC(O)CH₃)^2-•]²⁺, i.e., an S_Fe = 1 iron(IV) center antiferromagnetically coupled to an O-O σ* radical, where the O-O bond has not been completely broken. Complex 4 is best described as a triplet ferryl unit that strongly interacts with the trans acetylimidyl radical in an antiferromagnetic fashion, [Fe^IV(O)(^•N═C(O^-)CH₃)]⁺. Complex 4-H⁺ features a similar electronic structure, [Fe^IV(O)(^•N═C(OH)CH₃)]²⁺. Owing to the remaining approximate half σ-bond in the O-O moiety, complex 1 can arrange two electron-accepting orbitals (α σ*_O-O and β Fe-d _xz) in such a way that both orbitals can simultaneously interact with the doubly occupied electron-donating orbitals (σ_C-H or π_C-C). Hence, complex 1 can promote a concerted yet asynchronous two-electron oxidation of the C-H and C═C bonds, which nicely explains the stereospecificity observed for complex 1 and the related species.

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.

Arylruthenium(III) Porphyrin-Catalyzed C-H Oxidation and Epoxidation at Room Temperature and [RuV(Por)(O)(Ph)] Intermediate by Spectroscopic Analysis and Density Functional Theory Calculations

The development of highly active and selective metal catalysts for efficient oxidation of hydrocarbons and identification of the reactive intermediates in the oxidation catalysis are long-standing challenges. In the rapid hydrocarbon oxidation catalyzed by ruthenium(IV) and -(III) porphyrins, the putative Ru(V)-oxo intermediates remain elusive. Herein we report that arylruthenium(III) porphyrins are highly active catalysts for hydrocarbon oxidation. Using catalyst [Ru^III(TDCPP)(Ph)(OEt₂)] (H₂TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin), the oxidation of C-H bonds of various hydrocarbons with oxidant m-CPBA at room temperature gave alcohols/ketones in up to 99% yield within 1 h; use of [ ⁿBu₄N]IO₄ as a mild alternative oxidant avoided formation of lactone from cyclic ketone in C-H oxidation, and the catalytic epoxidation with up to 99% yield and high selectivity (no aldehydes as side product) was accomplished within 5 min. UV-vis, electrospray ionization-mass spectrometry, resonance Raman, electron paramagnetic resonance, and kinetic measurements and density functional theory calculations lend evidence for the formation of Ru(V)-oxo intermediate [Ru^V(TDCPP)(O)(Ph)].

Aliphatic C-H Bond Oxidation with Hydrogen Peroxide Catalyzed by Manganese Complexes: Directing Selectivity through Torsional Effects

Substituted N-cyclohexyl amides undergo aliphatic C-H bond oxidation with H₂O₂ catalyzed by manganese complexes. The reactions are directed by torsional effects leading to site-selective oxidation of cis-1,4-, trans-1,3-, and cis-1,2-cyclohexanediamides. The corresponding diastereoisomers are unreactive under the same conditions. Competitive oxidation of cis- trans mixtures of 4-substituted N-cyclohexylamides leads to quantitative conversion of the cis-isomers, allowing isolation and successive conversion of the trans-isomers into densely functionalized oxidation products with excellent site selectivity and good enantioselectivity.

cis-Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations

We report the first examples of ruthenium complexes cis-[(N₄)Ru^IIICl₂]⁺ and cis-[(N₄)Ru^II(OH₂)₂]²⁺ supported by chiral tetradentate amine ligands (N₄), together with a high-valent cis-dioxo complex cis-[(N₄)Ru^VI(O)₂]²⁺ supported by the chiral N₄ ligand mcp (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)Ru^IIICl₂](ClO₄) (1a), cis-[(Me₂mcp)Ru^IIICl₂]ClO₄ (2a) and cis-[(pdp)Ru^IIICl₂](ClO₄) (3a) (Me₂mcp = N,N'-dimethyl-N,N'-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1'-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)Ru^III(O₂CCF₃)₂]ClO₄ (1b) and cis-[(pdp)Ru^III(O₃SCF₃)₂]CF₃SO₃ (3c'). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp)Ru^VI(O)₂]²⁺ and cis-[(pdp)Ru^VI(O)₂]²⁺ complexes are strong oxidants with E° = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with D_O-H = 90.1-90.8 kcal mol^-1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH₄)₂[Ce^IV(NO₃)₆] (CAN) in aqueous medium afforded cis-[(mcp)Ru^VI(O)₂](ClO₄)₂ (1e) or cis-[((R,R)-mcp)Ru^VI(O)₂](ClO₄)₂ (1e*), respectively, a strong oxidant with E(Ru^VI/V) = 0.78 V (vs. Ag/AgNO₃) in acetonitrile solution. Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e* in ^t BuOH/H₂O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp)Ru^II(OH₂)₂]²⁺ (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.

Engineering a polyoxometalate-based metal organic framework with more exposed active edge sites of Ag for visible light-driven selective oxidation of cis-cyclooctene

The polyoxometalate-based MOF with exposed active edge sites of Ag was designed for visible light-driven selective oxidation of cis-cyclooctene.

A computational study of the Fenton reaction in different pH ranges

The mechanism of the Fenton reaction is pH dependent and four distinct reactive species have been identified and found to display quite different oxidation reactivities.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.

Synthesis, characterization, and reactivity of a chiral Fe(iv)–oxo complex bearing an l-proline-derived aminopyridine ligand

A chiral nonheme Fe(iv)–oxo complex was synthesized and spectroscopically characterized. Its reactivity in C–H activation as well as in asymmetric sulfoxidation and C–H hydroxylation was investigated.

Equilibrating (L)FeIII-OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts Using H2O2 and AcOH

Inspired by the remarkable chemistry of the family of Rieske oxygenase enzymes, nonheme iron complexes of tetradentate N4 ligands have been developed to catalyze hydrocarbon oxidation reactions using H2O2 in the presence of added carboxylic acids. The observation that the stereo- and enantioselectivity of the oxidation products can be modulated by the electronic and steric properties of the acid implicates an oxidizing species that incorporates the carboxylate moiety. Frozen solutions of these catalytic mixtures generally exhibit EPR signals arising from two S = 1/2 intermediates, a highly anisotropic g2.7 subset (gmax = 2.58 to 2.78 and Δg = 0.85-1.2) that we assign to an FeIII-OOAc species and a less anisotropic g2.07 subset (g = 2.07, 2.01, and 1.96 and Δg ≈ 0.11) we associate with an FeV(O)(OAc) species. Kinetic studies on the reactions of iron complexes supported by the TPA (tris(pyridyl-2-methyl)amine) ligand family with H2O2/AcOH or AcOOH at -40 °C reveal the formation of a visible chromophore at 460 nm, which persists in a steady state phase and then decays exponentially upon depletion of the peroxo oxidant with a rate constant that is substrate independent. Remarkably, the duration of this steady state phase can be modulated by the nature of the substrate and its concentration, which is a rarely observed phenomenon. A numerical simulation of this behavior as a function of substrate type and concentration affords a kinetic model in which the two S = 1/2 intermediates exist in a dynamic equilibrium that is modulated by the electronic properties of the supporting ligands. This notion is supported by EPR studies of the reaction mixtures. Importantly, these studies unambiguously show that the g2.07 species, and not the g2.7 species, is responsible for substrate oxidation in the (L)FeII/H2O2/AcOH catalytic system. Instead the g2.7 species appears to be off-pathway and serves as a reservoir for the g2.07 species. These findings will be helpful not only for the design of regio- and stereospecific nonheme iron oxidation catalysts but also for providing insight into the mechanisms of the remarkably versatile oxidants formed by nature's most potent oxygenases.

Manganese complex-catalyzed oxidation and oxidative kinetic resolution of secondary alcohols by hydrogen peroxide

The highly efficient catalytic oxidation and oxidative kinetic resolution (OKR) of secondary alcohols has been achieved using a synthetic manganese catalyst with low loading and hydrogen peroxide as an environmentally benign oxidant in the presence of a small amount of sulfuric acid as an additive. The product yields were high (up to 93%) for alcohol oxidation and the enantioselectivity was excellent (>90% ee) for the OKR of secondary alcohols. Mechanistic studies revealed that alcohol oxidation occurs via hydrogen atom (H-atom) abstraction from an α-CH bond of the alcohol substrate and a two-electron process by an electrophilic Mn-oxo species. Density functional theory calculations revealed the difference in reaction energy barriers for H-atom abstraction from the α-CH bonds of R- and S-enantiomers by a chiral high-valent manganese-oxo complex, supporting the experimental result from the OKR of secondary alcohols.

Selective C-H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C-H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using mCPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C-H oxidation. Mechanistic studies, which include correlation of the product formed with the Fe^V(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C-H bonds by Fe^V(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of ¹⁸O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of mCPBA and O₂ by Fe-bTAML, leading to formation of the Fe^V(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation.

Following a Chemical Reaction on the Millisecond Time Scale by Simultaneous X-ray and UV/Vis Spectroscopy

An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of Fe^II into two subsequent Fe^III species and the latter involving the more complex Fe^II-Fe^III-Fe^IV-Fe^III sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.