Selective oxygenation of unactivated C-H bonds by dioxygen via the autocatalytic formation of oxoiron(v) species

Highly regioselective oxidation of C-H bonds in water using hydrogen peroxide by a cytochrome P450 mimicking iron complex

Cytochrome P450, one of nature's oxidative workhorses, catalyzes the oxidation of C-H bonds in complex biological settings. Extensive research has been conducted over the past five decades to develop a fully functional mimic that activates O2 or H2O2 in water to oxidize strong C-H bonds. We report the first example of a synthetic iron complex that functionally mimics cytochrome P450 in 100% water using H2O2 as the oxidant. This iron complex, in which one methyl group is replaced with a phenyl group in either wing of the macrocycle, oxidized unactivated C-H bonds in small organic molecules with very high selectivity in water (pH 8.5). Several substrates (34 examples) that contained arenes, heteroaromatics, and polar functional groups were oxidized with predictable selectivity and stereoretention with moderate to high yields (50-90%), low catalyst loadings (1-4 mol%) and a small excess of H2O2 (2-3 equiv.) in water. Mechanistic studies indicated the oxoiron(v) to be the active intermediate in water and displayed unprecedented selectivity towards 3° C-H bonds. Under single-turnover conditions, the reactivity of this oxoiron(v) intermediate in water was found to be around 300 fold higher than that in CH3CN, thus implying the role water plays in enzymatic systems.

Use of Singlet Oxygen in the Generation of a Mononuclear Nonheme Iron(IV)-Oxo Complex

Nonheme iron(III)-superoxo intermediates are generated in the activation of dioxygen (O₂) by nonheme iron(II) complexes and then converted to iron(IV)-oxo species by reacting with hydrogen donor substrates with relatively weak C-H bonds. If singlet oxygen (¹O₂) with ca. 1 eV higher energy than the ground state triplet oxygen (³O₂) is employed, iron(IV)-oxo complexes can be synthesized using hydrogen donor substrates with much stronger C-H bonds. However, ¹O₂ has never been used in generating iron(IV)-oxo complexes. Herein, we report that a nonheme iron(IV)-oxo species, [Fe^IV(O)(TMC)]²⁺ (TMC = tetramethylcyclam), is generated using ¹O₂, which is produced with boron subphthalocyanine chloride (SubPc) as a photosensitizer, and hydrogen donor substrates with relatively strong C-H bonds, such as toluene (BDE = 89.5 kcal mol^-1), via electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂, which is energetically more favorable by 0.98 eV, as compared with electron transfer from [Fe^II(TMC)]²⁺ to ³O₂. Electron transfer from [Fe^II(TMC)]²⁺ to ¹O₂ produces an iron(III)-superoxo complex, [Fe^III(O₂)(TMC)]²⁺, followed by abstracting a hydrogen atom from toluene by [Fe^III(O₂)(TMC)]²⁺ to form an iron(III)-hydroperoxo complex, [Fe^III(OOH)(TMC)]²⁺, that is further converted to the [Fe^IV(O)(TMC)]²⁺ species. Thus, the present study reports the first example of generating a mononuclear nonheme iron(IV)-oxo complex with the use of singlet oxygen, instead of triplet oxygen, and a hydrogen atom donor with relatively strong C-H bonds. Detailed mechanistic aspects, such as the detection of ¹O₂ emission, the quenching by [Fe^II(TMC)]²⁺, and the quantum yields, have also been discussed to provide valuable mechanistic insights into understanding nonheme iron-oxo chemistry.

Visible-Light-Driven Selective Air-Oxygenation of C-H Bond via CeCl3 Catalysis in Water

Visible-light-induced C-H aerobic oxidation is an important chemical transformation that can be applied for the synthesis of aromatic ketones. High-cost catalysts and toxic solvents were generally needed in the present methodologies. Here, an efficient aqueous C-H aerobic oxidation protocol was reported. Through CeCl₃ -mediated photocatalysis, a series of aromatic ketones were produced in moderate to excellent yields. With air as the oxidant, this reaction could be performed under mild conditions in water and demonstrated high activity and functional group tolerance. This method is economical, highly efficient, and environmentally friendly, and it will provide inspiration for the development of aqueous photochemical synthesis reactions.

Heterogeneous C-H Functionalization in Water via Porous Covalent Organic Framework Nanofilms: A Case of Catalytic Sphere Transmutation

Heterogeneous catalysis in water has not been explored beyond certain advantages such as recyclability and recovery of the catalysts from the reaction medium. Moreover, poor yield, extremely low selectivity, and active catalytic site deactivation further underrate the heterogeneous catalysis in water. Considering these facts, we have designed and synthesized solution-dispersible porous covalent organic framework (COF) nanospheres. We have used their distinctive morphology and dispersibility to functionalize unactivated C-H bonds of alkanes heterogeneously with high catalytic yield (42-99%) and enhanced regio- and stereoselectivity (3°:2° = 105:1 for adamantane). Further, the fabrication of catalyst-immobilized COF nanofilms via covalent self-assembly of catalytic COF nanospheres for the first time has become the key toward converting the catalytically inactive homogeneous catalysts into active and effective heterogeneous catalysts operating in water. This unique covalent self-assembly occurs through the protrusion of the fibers at the interface of two nanospheres, transmuting the catalytic spheres into films without any leaching of catalyst molecules. The catalyst-immobilized porous COF nanofilms' chemical functionality and hydrophobic environment stabilize the high-valent transient active oxoiron(V) intermediate in water and restricts the active catalytic site's deactivation. These COF nanofilms functionalize the unactivated C-H bonds in water with a high catalytic yield (45-99%) and with a high degree of selectivity (cis:trans = 155:1; 3°:2° = 257:1, for cis-1,2-dimethylcyclohexane). To establish this approach's "practical implementation", we conducted the catalysis inflow (TON = 424 ± 5) using catalyst-immobilized COF nanofilms fabricated on a macroporous polymeric support.

Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)Fe^III]^- (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH₃COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O₂ by one-electron reductants. An iron(V)-oxo complex, [(TAML)Fe^V(O)]^- (2), was produced by oxygenation of 1 with O₂ via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me₁₀Fc) and octamethylferrocene (Me₈Fc) reduced 2 to 1 by two electrons in the presence of CH₃COOH to produce decamethylferrocenium cation (Me₁₀Fc⁺) and octamethylferrocenium cation (Me₈Fc⁺), respectively. Then, 1 was oxygenated by O₂ to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br₂Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O₂ occurred.

Oxoiron(v) mediated selective electrochemical oxygenation of unactivated C-H and C[double bond, length as m-dash]C bonds using water as the oxygen source

An efficient electrochemical method for the selective oxidation of C–H bonds of unactivated alkanes (BDE ≤97 kcal mol⁻¹) and CC bonds of alkenes using a biomimetic iron complex, [(bTAML)Fe^III-OH₂]⁻, as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes is reported. The O-atom of water remains the source of O-incorporation in the product formed after oxidation. The products formed upon oxidation of C–H bonds display very high regioselectivity (75 : 1, 3° : 2° for adamantane) and stereo-retention (RC ∼99% for cyclohexane derivatives). The substrate scope includes natural products such as cedryl acetate and ambroxide. For alkenes, epoxides were obtained as the sole product. Mechanistic studies show the involvement of a high-valent oxoiron(v) species, [(bTAML)Fe^V(O)]⁻ formed via PCET (overall 2H⁺/2e⁻) from [(bTAML)Fe^III-OH₂]⁻ in CPE at 0.80 V (vs. Ag/AgNO₃). Moreover, electrokinetic studies for the oxidation of C–H bonds indicate a second-order reaction with the C–H abstraction by oxoiron(v) being the rate-determining step.

A biomimetic iron complex-mediated selective and efficient electrochemical oxygenation of unactivated C–H bonds and CC bonds using water as an O-atom source.