Catalytic Alkane Hydroxylation Research Articles

Mechanistic studies on catalytic alkane oxidation by Murahashi’s O2/copper(II)/aldehyde system

Mechanistic studies on catalytic alkane hydroxylation by the Murahashi’s O2/copper(II)/aldehyde system have been conducted to show that the autoxidation of aldehyde (RCHO) by O2 generating acyl radical intermediate (RC(O)•) is...

μ-Oxo-bridged diiron(iii) complexes of tripodal 4N ligands as catalysts for alkane hydroxylation reaction using m-CPBA as an oxidant: substrate vs. self hydroxylation.

A series of non-heme μ-oxo-bridged dinuclear iron(iii) complexes of the type [Fe2(μ-O)(L1–L6)2Cl2]Cl21–6 have been isolated and their catalytic activity towards oxidative transformation of alkanes into alcohols has been studied using m-choloroperbenzoic acid (m-CPBA) as an oxidant. All the complexes were characterized by CHN, electrochemical, and UV-visible spectroscopic techniques. The molecular structures of 2 and 5 have been determined successfully by single crystal X-ray diffraction analysis and both possesses octahedral coordination geometry and each iron atom is coordinated by four nitrogen atoms of the 4N ligand and a bridging oxygen. The sixth position of each octahedron is coordinated by a chloride ion. The (μ-oxo)diiron(iii) core is linear in 2 (Fe–O–Fe, 180.0°), whereas it is non-linear (Fe–O–Fe, 161°) in 5. All the diiron(iii) complexes show quasi-reversible one electron transfer in the cyclic voltammagram and catalyze the hydroxylation of alkanes like cyclohexane, adamantane with m-CPBA as an oxidant. In acetonitrile solution, adding excess m-CPBA to the diiron(iii) complex 2 without chloride ions leads to intramolecular hydroxylation reaction of the oxidant. Interestingly, 2 catalyzes alkane hydroxylation in the presence of chloride ions, but intramolecular hydroxylation in the absence of chloride ions. The observed selectivity for cyclohexane (A/K, 5–7) and adamantane (3°/2°, 9–18) suggests the involvement of high-valent iron–oxo species rather than freely diffusing radicals in the catalytic reaction. Moreover, 4 oxidizes (A/K, 7) cyclohexane very efficiently up to 513 TON while 5 oxidizes adamantane with good selectivity (3°/2°, 18) using m-CPBA as an oxidant. The electronic effects of ligand donors dictate the efficiency and selectivity of catalytic hydroxylation of alkanes.

Enhancing catalytic alkane hydroxylation by tuning the outer coordination sphere in a heme-containing metal-organic framework.

Catalytic heme active sites of enzymes are sequestered by the protein superstructure and are regulated by precisely defined outer coordination spheres. Here, we emulate these protective functions in the porphyrinic metal-organic framework PCN-224 by post-synthetic acetylation and subsequent hydroxylation of the Zr6 nodes. A suite of physical methods demonstrates that both transformations preserve framework structure, crystallinity, and porosity without modifying the inner coordination spheres of the iron sites. Single-crystal X-ray analyses establish that acetylation replaces the mixture of formate, benzoate, aqua, and terminal hydroxo ligands at the Zr6 nodes with acetate ligands, and hydroxylation affords nodes with seven-coordinate, hydroxo-terminated Zr4+ ions. The chemical influence of these reactions is probed with heme-catalyzed cyclohexane hydroxylation as a model reaction. By virtue of passivated reactive sites at the Zr6 nodes, the acetylated framework oxidizes cyclohexane with a yield of 68(8)%, 2.6-fold higher than in the hydroxylated framework, and an alcohol/ketone ratio of 5.6(3).

High-Spin Mn(V)-Oxo Intermediate in Nonheme Manganese Complex-Catalyzed Alkane Hydroxylation Reaction: Experimental and Theoretical Approach.

Mononuclear nonheme manganese complexes are highly efficient catalysts in the catalytic oxidation of hydrocarbons by hydrogen peroxide in the presence of carboxylic acids. Although high-valent Mn(V)-oxo complexes have been proposed as the active oxidants that afford high regio-, stereo-, and enantioselectivities in the catalytic oxidation reactions, the importance of the spin state (e.g., S = 0 or 1) of the proposed Mn(V)-oxo species is an area that requires further study. In the present study, we have theoretically demonstrated that a mononuclear nonheme Mn(V)-oxo species with an S = 1 ground spin state is the active oxidant that effects the stereo- and enantioselective alkane hydroxylation reaction; it is noted that synthetic octahedral Mn(V)-oxo complexes, characterized spectroscopically and/or structurally, possess an S = 0 spin state and are sluggish oxidants. In an experimental approach, we have investigated the catalytic hydroxylation of alkanes by a mononuclear nonheme Mn(II) complex, [(S-PMB)MnII]2+, and H2O2 in the presence of carboxylic acids; alcohol is the major product with high stereo- and enantioselectivities. A synthetic Mn(IV)-oxo complex, [(S-PMB)MnIV(O)]2+, is inactive in C-H bond activation reactions, ruling out the Mn(IV)-oxo species as an active oxidant. DFT calculations have shown that a Mn(V)-oxo species with an S = 1 spin state, [(S-PMB)MnV(O)(OAc)]2+, is highly reactive and capable of oxygenating the C-H bond via oxygen rebound mechanism; we propose that the triplet spin state of the Mn(V)-oxo species results from the consequence of breaking the equatorial symmetry due to the binding of an equatorial oxygen from an acetate ligand. Thus, the present study reports that, different from the previously reported S = 0 Mn(V)-oxo species, Mn(V)-oxo species with a triplet ground spin state are highly reactive oxidants that are responsible for the regio-, stereo-, and enantioselectivities in the catalytic hydroxylation of alkanes by mononuclear nonheme manganese complexes and terminal oxidants.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation.

A series of Ni(ii) complexes of the types [Ni(L)(CH3CN)2](BPh4)21-3, 5 and [Ni(L4)](BPh4)24, where L = N,N'-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N'-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N,N'-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L3), N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethylenediamine (L5) and L4 = N,N'-bis((1-methyl-1H-imidazole-2-yl)methyl)-1,4-diazepane, have been isolated and characterized. The complex cations of 1 and 4 possess, respectively, distorted octahedral and low-spin square planar coordination geometries in which nickel(ii) is meridionally coordinated to all four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with the ethylenediamine backbone coordinates in the cis-β mode in [Ni(L5)(CH3CN)2]2+5, but in the cis-α mode in [Ni(L5)(H2O)2]2+. Also, they illustrate the role of ligand donor atom type, diazacyclo backbone and steric hindrance to coordination of pyridyl nitrogen in conferring novel coordination geometries on Ni(ii). All these complexes catalyse the oxidation of cyclohexane in the presence of m-CPBA as the oxidant up to 600 turnover numbers (TON) with relatively good alcohol selectivity (A/K, 5.6-7.2). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with high bond selectivity (3°/2°, 8.7-11.7). The incorporation of methyl substituent(s) on one (2) or both (3) of the pyridyl rings and the replacement of both the pyridylmethyl arms in 1 by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with the cis-β mode of coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and product/bond selectivity.

Non-heme μ-Oxo- and bis(μ-carboxylato)-bridged diiron(iii) complexes of a 3N ligand as catalysts for alkane hydroxylation: stereoelectronic factors of carboxylate bridges determine the catalytic efficiency.

A series of non-heme (μ-oxo)bis(μ-dicarboxylato)-bridged diiron(iii) complexes, [Fe2(O)(OOCH)2(L)2](2+)1, [Fe2(O)(OAc)2(L)2](2+)2, [Fe2(O)(Me3AcO)2(L)2](2+)3, [Fe2(O)(OBz)2(L)2](2+)4, [Fe2(O)(Ph2AcO)2(L)2](2+)5 and [Fe2(O)(Ph3AcO)3(L)2](2+)6, where L = N,N-dimethyl-N'-(pyrid-2-ylmethyl)ethylenediamine, OAc(-) = acetate, Me3AcO(-) = trimethylacetate, OBz(-) = benzoate, Ph2AcO(-) = diphenylacetate and Ph3AcO(-) = triphenylacetate, have been isolated and characterized using elemental analysis and spectral and electrochemical techniques. They have been studied as catalysts for the selective oxidation of alkanes using m-chloroperbenzoic acid (m-CPBA) as the oxidant. Complexes 2, 3, and 4 possess a distorted bioctahedral geometry in which each iron atom is coordinated to an oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-carboxylate bridge and three nitrogen atoms of the 3N ligand. In an acetonitrile/dichloromethane solvent mixture all the complexes display a d-d band characteristic of the triply bridged diiron(iii) core, revealing that they retain their identity in solution. Upon replacing electron-donating substituents on the bridging carboxylates by electron-withdrawing ones the E1/2 value of the one-electron Fe(III)Fe(III)→ Fe(III)Fe(II) reduction becomes less negative. On adding one equivalent of Et3N to a mixture of one equivalent of the complex and an excess of m-CPBA in the acetonitrile/dichloromethane solvent mixture an intense absorption band (λmax, 680-720 nm) appears, which corresponds to the formation of a mixture of complex species. All the complexes act as efficient catalysts for the hydroxylation of cyclohexane with 380-500 total turnover numbers and good alcohol selectivity (A/K, 6.0-10.1). Adamantane is selectively oxidized to 1-adamantanol and 2-adamantanol (3°/2°, 12.9-17.1) along with a small amount of 2-adamantanone (total TON, 381-476), and interestingly, the sterically demanding trimethylacetate bridge around the diiron(iii) centre leads to high 3°/2° bond selectivity; on the other hand, the sterically demanding triphenylacetate bridge gives a lower 3°/2° bond selectivity. A remarkable linear correlation between the pKa of the bridging carboxylate and TON for both cyclohexane and adamantane oxidation is observed, illustrating the highest catalytic activity for 3 with strongly electron-releasing trimethylacetate bridges.

Synthesis, characterization, cis-ligand substitution and catalytic alkane hydroxylation by mononuclear nickel(II) complexes stabilized with tetradentate tripodal ligands

The synthesis and spectroscopic characterization of the mononuclear complexes [Ni(bqenH2)(H2O)2](ClO4)21 and [Ni(bqenMe2)(H2O)2](ClO4)22 (where bqenH2=N,N′-bis(8-quinolyl)ethane-1,2-diamine and bqenMe2=N,N′-dimethyl-N,N′-bis(8-quinolyl)ethane-1,2-diamine) is reported. The bqenMe2 ligand was prepared by a simple modification to the earlier procedure. The reaction of 1 and 2 with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy) resulted in the formation of [Ni(bqenH2)(phen)](ClO4)23, [Ni(bqenMe2)(phen)](ClO4)2 CH3CN 4, [Ni(bqenH2)(bpy)](ClO4)25, and [Ni(bqenMe2)(bpy)](ClO4)26. The redox properties of 1–6 are reported. The crystal structures of 3 and 4 consist of distorted octahedral [Ni(bqenH2)(phen)]2+ and [Ni(bqenMe2)(phen)]2+ cations which are stabilized by N–H⋯O and C–H⋯O interactions. Compounds 1 and 2 afforded hydroxylation of alkanes with high alcohol to ketone ratio in the presence of m-CPBA oxidant.

A non-heme cationic Fe(iii)-complex intercalated in montmorillonite K-10: synthesis, characterization and catalytic alkane hydroxylation with H2O2 at room temperature

An efficient catalyst for highly selective hydroxylation of alkanes with environmentally benign H2O2 at room temperature has been designed by the intercalation of a non-heme iron(iii) complex into smectite montmorillonite K-10.

ChemInform Abstract: Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry.

AbstractThe hydroxylation of cycloalkanes (I) and (III) proceeds smoothly in the presence of an Ir—Cp* catalyst to give the product cycloalkanols (II) and (IV), resp., with retention of stereoselectivity.

Cp* Iridium Complexes Give Catalytic Alkane Hydroxylation with Retention of Stereochemistry

A series of Cp*Ir complexes can catalyze C-H oxidation, with ceric ammonium nitrate as the terminal oxidant and water as the source of oxygen. Remarkably the hydroxylation of cis-decalin and 1,4-dimethylcyclohexane proceeds with retention of stereochemistry. With H(2)O(18), cis-decalin oxidation gave (18)O incorporation into the product cis-decalol.

Catalytic Alkane Hydroxylation Reaction with Nickel(II) Complexes Supported by Di- and Triphenol Ligands

Abstract NiII-complexes supported by the tetradentate ligands containing two or three 2,4-di-tert-butylphenol groups were synthesized and structurally characterized. The NiII-complex of the diphenol ligand has been found to act as a very efficient turnover catalyst in the alkane hydroxylation reaction with m-chloroperbenzoic acid (m-CPBA).

Biomimetic alkane hydroxylation by cobalt(iii) porphyrin complex and m-chloroperbenzoic acid

The catalytic hydroxylation of alkanes by an electron-deficient cobalt(III) porphyrin complex and m-chloroperbenzoic acid yielded alcohols as major products with a high kH/kD value, >99% retention of stereochemistry, and a high regioselectivity; a high-valent cobalt–oxo porphyrin complex was suggested as a reactive hydroxylating intermediate.

Syntheses of mixed-ligand ruthenium(II) complexes with a terpyridine or a tris (pyrazolyl) methane and a bidentate ligand: their application for catalytic hydroxylation of alkanes

The new mixed-ligand chlororuthenium(II) complexes [RuCl(L) (tpy)]Cl n and [RuCl(L) (tpm)]Cl n (L = bpy, dmpa or dmgly; n = 0 or 1) were prepared, and the catalytic activity for alkane oxidation using m-chloroperbenzoic acid was examined. Alcohol was the main product: oxidation of adamantane, cyclooctane or ethylbenzene gave hydroxylated products, 1-adamantanol (66%), cyclooctanol (22%) or 1-phenylethanol (48%), respectively.

ChemInform Abstract: Catalytic Hydroxylation of Alkanes by Immobilized Mononuclear Iron Carboxylate.

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Catalytic hydroxylation of alkanes by immobilized mononuclear iron carboxylate

Mononuclear iron carboxylate complex immobilized and isolated on a modified silica surface (1) catalyzes oxidation of hexane to a mixture of hexan-1-ol, -2-ol and -3-ol without ketone formation in the presence of mercaptan, acetic acid, triphenylphosphine and O2 at ambient conditions.

Catalytic hydroxylation of alkanes by immobilized mononuclear iron carboxylate

Catalytic hydroxylation of alkanes by immobilized mononuclear iron carboxylate

New ruthenium complexes for the catalytic alkane hydroxylation

New ruthenium complexes for the catalytic alkane hydroxylation