Hydroxylation Of Hydrocarbons Research Articles

Synthesis and Characterization of a Masked Terminal Nickel-Oxide Complex.

In exploring terminal nickel-oxo complexes, postulated to be the active oxidant in natural and non-natural oxidation reactions, we report the synthesis of the pseudo-trigonal bipyramidal NiII complexes (K)[NiII (LPh )(DMF)] (1[DMF]) and (NMe4 )2 [NiII (LPh )(OAc)] (1[OAc]) (LPh =2,2',2''-nitrilo-tris-(N-phenylacetamide); DMF=N,N-dimethylformamide; - OAc=acetate). Both complexes were characterized using NMR, FTIR, ESI-MS, and X-ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh )NiIII -O-H⋅⋅⋅OAc]2- , 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a NiII -phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI-MS, and X-ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo-wall remains standing, this complex represents an excellent example of a masked metal-oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo-wall.

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)-H Fluorination.

[FeII(NCCH3)(NTB)](OTf)2 (NTB = tris(2-benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ3-iodane (PhIF2) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally FeIV(F)2 oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F-(FeIII)2 product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Aromatic and aliphatic hydrocarbon hydroxylation via a formally NiIVO oxidant.

The reaction of (NMe4)2[NiII(LPh)(OAc)] (1[OAc], LPh = 2,2',2''-nitrilo-tris-(N-phenylacetamide); OAc = acetate) with 3-chloroperoxybenzoic acid (m-CPBA) resulted in the formation of a self-hydroxylated NiIII-phenolate complex, 2, where one of the phenyl groups of LPh underwent hydroxylation. 2 was characterised by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a previously characterised NiII-phenolate complex, 3. We postulate that self-hydroxylation was mediated by a formally NiIVO oxidant, formed from the reaction of 1[OAc] with m-CPBA, which undergoes electrophilic aromatic substitution to yield 2. This is supported by an analysis of the kinetic and thermodynamic properties of the reaction of 1[OAc] with m-CPBA. Addition of exogenous hydrocarbon substrates intercepted the self-hydroxylation process, producing hydroxylated products, providing further support for the formally NiIVO entity. This study demonstrates that the reaction between NiII salts and m-CPBA can lead to potent metal-based oxidants, in contrast to recent studies demonstrating carboxyl radical is a radical free-chain reaction initiator in NiII/m-CPBA hydrocarbon oxidation catalysis.

Visible-light-driven g-C3N4-doped Co catalyzed oxidation of benzylic hydroxylation of alkyl aromatic hydrocarbons

The functionalization of C(sp3)-H bonds of alkyl aromatic hydrocarbons is an important unit reaction in the petrochemical industry. In this work, a g-C3N4-doped Co photocatalyst was found, which can activate molecular oxygen to oxidize alkyl aromatic hydrocarbons benzylic C(sp3)-H bond hydroxylation at room temperature to prepare alcohol compounds. In the batch reaction system, the product can be obtained without additional free radical initiator and solvent, which is suitable for a variety of alkyl aromatic hydrocarbon substrates. In two different continuous flow strategies, no oxygen environment was required. 2-phenyl-2-propanol (BP) was prepared from cumene (CM) as a substrate, with the selectivity of BP can be increased from 89.3 % to 96.8 % and 97.7 %, respectively, and the reaction time can be shortened from 9 h to 100 min and 165 min. This work demonstrates the advantages of highly active carbon nitride-based photocatalysts in the field of green and sustainable chemistry.

Catalytic machinery of methane oxidation in particulate methane monooxygenase (pMMO)

In this focused review, we portray the recently reported 2.5 Å cyro-EM structure of the particulate methane monooxygenase (pMMO) from M. capsulatus (Bath). The structure of the functional holo-pMMO near atomic resolution has uncovered the sites of the copper cofactors including the location of the active site in the enzyme. The three coppers seen in the original X-ray crystal structures of the enzyme are now augmented by additional coppers in the transmembrane domain as well as in the water-exposed C-terminal subdomain of the PmoB subunit. The cryo-EM structure offers the first glimpse of the catalytic machinery capable of methane oxidation with high selectivity and efficiency. The findings are entirely consistent with the biochemical and biophysical findings previously reported in the literature, including the chemistry of hydrocarbon hydroxylation, regeneration of the catalyst for multiple turnovers, and the mechanism of aborting non-productive cycles to ensure kinetic competence.

Selective, Catalytic Oxidations of C–H Bonds in Polyethylenes Produce Functional Materials with Enhanced Adhesion

Summary The synthesis of functional and processable polyethylenes from simple vinyl monomers with controlled molecular weights and architectures has been a grand challenge of polymer chemistry. Post-polymerization modification of the homopolymers of ethylene is attractive for this purpose but has been hampered by the lack of efficient methods for the selective functionalization of C–H bonds in polyolefins. We report the selective, catalytic oxidation of C–H bonds in commodity polyethylenes with varying molecular weights and architectures. Remarkably, the functionalized materials, even at low levels of functionalization, exhibit physical properties that are absent in unmodified polyolefins, such as strong adhesion and the ability to be painted with common waterborne latex paint. Such observations indicate that selectively modified polyethylenes as described here may help to transform existing commodity plastics into more valuable and potentially more sustainable materials.

Metabolism and distribution of dibutyl phthalate in wheat grown on different soil types

Herein, we investigated the distribution of dibutyl phthalate (DBP) in the tissues of wheat grown on DBP-contaminated soils and determined the activity of detoxifying enzymes [cytochrome P450 (P450) and glutathione S-transferase (GST)] in these tissues at different growth stages of wheat. The content of DBP in wheat grown on all soil types increased with increasing DBP stress concentration, and the DBP content of each tissue decreased as wheat growth progressed. Under the same conditions, the DBP content of roots exceeded that of leaves and stems, and the DBP content of all tissues decreased in the order of brown soil > fluvo-aquic soil > cinnamon soil. The decrease of DBP content with growth and the content of mono-n-butyl dicarboxylate (MBP, main metabolite of DBP) followed the order of cinnamon soil > fluvo-aquic soil > brown soil; the latter parameter initially increased and then decreased as growth progressed. The changes in P450 content and GST activities in wheat tissues were highly consistent with the corresponding changes in DBP and MBP content, except for the DBP stress level of 40 mg kg−1 at the seedling stage. Based on the obtained results, it was speculated that P450 and GST were strongly involved in the metabolism of DBP in wheat: oxygen atoms were inserted into the aldehyde C–H bond of the species generated after hydrolysis of ester groups under the action of P450 to achieve hydrocarbon hydroxylation under mild conditions and generate MBP.

Expanding the substrate scope and reactivity of cytochrome P450 OleT

The efficient hydrogen peroxide-dependent hydroxylation and epoxidation of hydrocarbons is catalysed by a P450 fatty acid decarboxylase (OleT) active-site variant. The introduction of an acidic functionality in the protein framework circumvents the necessity for a carboxylate that is typically provided by the substrate for efficient H2O2 heterolysis. Spectroscopic and turnover studies show that the mutation eliminates the binding and metabolism of prototypical fatty acid substrates, but permits the oxidation of a broad range of inert hydrocarbon substrates.

Single Turnover Epoxidation of Propylene by α-Complexes (FeIII–O•)α on the Surface of FeZSM-5 Zeolite

Single turnover epoxidation of propylene by α-complexes (FeIII–O•)α on the surface of FeZSM-5 zeolite was studied in the temperature range from +25 °C to −60 °C. After extraction, the reaction products were identified by gas chromatography (GC), gas chromatography–mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR). The reaction C3H6 + (FeIII–O•)α was shown to proceed in a similar way as the oxidation of propylene by the enzyme methane mono-oxygenase sMMO, i.e., via the addition of active oxygen over C═C bonds to yield propylene oxide, not affecting a weakly bound allylic hydrogen. This result, together with the previous results on the hydroxylation of methane and other hydrocarbons, shows the (FeIII–O•)α to be the unique functional model of compound Q, the key intermediate of sMMO. In another aspect, the results relate to the low selectivity problem of the silver catalyst in propylene epoxidation and raise doubts about the presently accepted mechanism explaining an adverse effect of allylic hydrogen.

The DMDO Hydroxylation of Hydrocarbons via the Oxygen Rebound Mechanism.

Both DFT and G4 molecular orbital calculations have been employed to reexamine the mechanism of dimethyldioxirane (DMDO) oxidation of saturated hydrocarbons and the epoxidation of alkenes. The UM062X DFT functional provided the most accurate bond O-O dissociation energies for a series of typical peroxides. A diradicaloid process initiated by an O-O homolytic bond cleavage involving abstraction of hydrogen from the C-H bond followed by a final product forming "oxygen rebound" step best describes the DMDO oxidation of saturated hydrocarbons. In contrast, this study showed that the DMDO epoxidation of alkenes is a concerted process best described with the B3LYP DFT functional.

Review on High Valent FeVI (Ferrate): A Sustainable Green Oxidant in Organic Chemistry and Transformation of Pharmaceuticals

Iron is the most common metal by mass on earth and represents the basic element of industrial society. The usage of iron to synthesize consumer products and to remediate environment is an attractive approach. This perspective presents applications of the high-valent iron oxo compound FeVIO42– (ferrate) in sustainable organic synthesis and treatment technology. In synthesizing organic molecules, C–H bond activation and selectivity are two basic ingredients of efficiency, which are attainable by ferrate. Examples of hydroxylation of hydrocarbons and conversion of alcohols to aldehyde/ketone are presented. In addition, many other organic molecules of amines, aniline, phenolic, and thiol functionalities can be oxidized by ferrate. This oxidation chemistry of ferrate is expedient in transforming pharmaceuticals, micropollutants in bodies of water with implications for human and ecological health. A wide range of micropollutants, which are commonly found in drinking water resources and wastewater effluents, can...

Diesel-related hydrocarbons can dominate gas phase reactive carbon in megacities

Abstract. Hydrocarbons are key precursors to two priority air pollutants, ozone and particulate matter. Those with two to seven carbons have historically been straightforward to observe and have been successfully reduced in many developed cities through air quality policy interventions. Longer chain hydrocarbons released from diesel vehicles are not considered explicitly as part of air quality strategies and there are few direct measurements of their gaseous abundance in the atmosphere. This study describes the chemically comprehensive and continuous measurements of organic compounds in a developed megacity (London), which demonstrate that on a seasonal median basis, diesel-related hydrocarbons represent only 20–30 % of the total hydrocarbon mixing ratio but comprise more than 50 % of the atmospheric hydrocarbon mass and are a dominant local source of secondary organic aerosols. This study shows for the first time that 60 % of the winter primary hydrocarbon hydroxyl radical reactivity is from diesel-related hydrocarbons and using the maximum incremental reactivity scale, we predict that they contribute up to 50 % of the ozone production potential in London. Comparing real-world urban composition with regulatory emissions inventories in the UK and US highlights a previously unaccounted for, but very significant, under-reporting of diesel-related hydrocarbons; an underestimation of a factor ~4 for C9 species rising to a factor of over 70 for C12 during winter. These observations show that hydrocarbons from diesel vehicles can dominate gas phase reactive carbon in cities with high diesel fleet fractions. Future control of urban particulate matter and ozone in such locations requires a shift in policy focus onto gas phase hydrocarbons released from diesels as this vehicle type continues to displace gasoline world-wide.

Electrocatalytic Hydrocarbon Hydroxylation by Ethylbenzene Dehydrogenase from Aromatoleum aromaticum

We report the electrocatalytic activity of ethylbenzene dehydrogenase (EBDH) from the β-proteobacterium Aromatoleum aromaticum. EBDH is a complex 155 kDa heterotrimeric molybdenum/iron-sulfur/heme protein which catalyzes the enantioselective hydroxylation of nonactivated ethylbenzene to (S)-1-phenylethanol without molecular oxygen as cosubstrate. Furthermore, it oxidizes a wide range of other alkyl-substituted aromatic and heterocyclic compounds to their secondary alcohols. Hydroxymethylferrocenium (FM) is used as an artificial electron acceptor for EBDH in an electrochemically driven catalytic system. Electrocatalytic activity of EBDH is demonstrated with both its native substrate ethylbenzene and the related substrate p-ethylphenol. The catalytic system has been modeled by electrochemical simulation across a range of sweep rates and concentrations of each substrate, which provides new insights into the kinetics of the EBDH catalytic mechanism.

Monooxygenation of small hydrocarbons catalyzed by bacterial cytochrome p450s.

Cytochrome P450s (P450s) catalyze the NAD(P)H/O2-dependent monooxygenation of less reactive organic molecules under mild conditions. The catalytic activity of bacterial P450s is very high compared with P450s isolated from animals and plants, and the substrate specificity of bacterial P450s is also very high. Accordingly, their catalytic activities toward nonnative substrates are generally low especially toward small hydrocarbons. However, mutagenesis approaches have been very successful for engineering bacterial P450s for the hydroxylation of small hydrocarbons. On the other hand, "decoy" molecules, whose structures are very similar to natural substrates, can be used to trick the substrate recognition of bacterial P450s, allowing the P450s to catalyze oxidation reactions of nonnative substrates without any substitution of amino acid residues in the presence of decoy molecules. Thus, the hydroxylation of small hydrocarbons such as ethane, propane, butane and benzene can be catalyzed by P450BM3, a long-alkyl-chain hydroxylase, using substrate misrecognition of P450s induced by decoy molecules. Furthermore, a number of H2O2-dependent bacterial P450s can catalyze the peroxygenation of a variety of nonnative substrates through a simple substrate-misrecognition trick, in which catalytic activities and enantioselectivity are dependent on the structure of decoy molecules.

Oxidizing intermediates in P450 catalysis: a case for multiple oxidants.

Cytochrome P450 (P450 or CYP) catalysis involves the oxygenation of organic compounds via a series of catalytic intermediates, namely, the ferric-peroxo, ferric-hydroperoxo, Compound I (Cpd I) and FeIII-(H2O2) intermediates. Now that the structures of P450 enzymes have been well established, a major focus of current research in the P450 area has been unraveling the intimate details and activities of these reactive intermediates. The general consensus is that the Cpd I intermediate is the most reactive species in the reaction cycle, especially when the reaction involves hydrocarbon hydroxylation. Cpd I has recently been characterized experimentally. Other than Cpd I, there is a multitude of evidence, both experimental as well as theoretical, supporting the involvement of other intermediates in various types of oxidation reactions. The involvement of these multiple oxidants has been experimentally demonstrated using P450 active-site mutants in epoxidation, heteroatom oxidation and dealkylation reactions. In this chapter, we will review the P450 reaction cycle and each of the reactive intermediates to discuss their role in oxidation reactions.

Electron transfer control in soluble methane monooxygenase.

The hydroxylation or epoxidation of hydrocarbons by bacterial multicomponent monooxygenases (BMMs) requires the interplay of three or four protein components. How component protein interactions control catalysis, however, is not well understood. In particular, the binding sites of the reductase components on the surface of their cognate hydroxylases and the role(s) that the regulatory proteins play during intermolecular electron transfer leading to the hydroxylase reduction have been enigmatic. Here we determine the reductase binding site on the hydroxylase of a BMM enzyme, soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath). We present evidence that the ferredoxin domain of the reductase binds to the canyon region of the hydroxylase, previously determined to be the regulatory protein binding site as well. The latter thus inhibits reductase binding to the hydroxylase and, consequently, intermolecular electron transfer from the reductase to the hydroxylase diiron active site. The binding competition between the regulatory protein and the reductase may serve as a control mechanism for regulating electron transfer, and other BMM enzymes are likely to adopt the same mechanism.

Hydroxy Functionalization of Non‐Activated CH and CC Bonds: New Perspectives for the Synthesis of Alcohols through Biocatalytic Processes

New perspectives through enzymes: Recent breakthroughs have been achieved in the selective hydroxy functionalization of non-activated C-H and C=C bonds. Enzymes turned out to be suitable catalysts for the ω-hydroxylation of (substituted) alkanes and regioselective hydroxylation of aromatic hydrocarbons with atmospheric oxygen as the oxidant, and the asymmetric addition of water to non-activated alkenes.

Facile synthesis of Au/ZnO nanoparticles and their enhanced photocatalytic activity for hydroxylation of benzene

Au/ZnO nanocomposites have been prepared by a simple chemical method. For the first time, the nanocomposites were directly used as photocatalysts for hydroxylation of aromatic hydrocarbons under UV and visible light irradiation. The results show that the as-prepared photocatalysts display high photocatalytic activity for UV and visible catalytic hydroxylation of benzene. Without the assistance of any solvent or additive, high selectivity and high conversion efficiency were still obtained. Different photocatalytic mechanisms were proposed depending on whether excitation happens on ZnO semiconductor or on the surface plasmon band of Au. The former is Au nanoparticles act as electron buffer due to irradiation by UV light and ZnO nanoparticles as reactive sites for hydroxylation of benzene, the latter is that Au nanoparticles act as light harvesters and inject electrons into ZnO conduction band and as photocatalytic sites under visible light irradiation.

Hydroxylation of non-substituted polycyclic aromatic hydrocarbons by cytochrome P450 BM3 engineered by directed evolution

Chrysene and pyrene are known toxic compounds recalcitrant to biodegradation. Here directed evolution allowed us to identify two new mutants of cytochrome P450 BM3 that are able to hydroxylate both compounds. Random mutagenesis has been used to generate libraries of mutants of P450 BM3 active toward polycyclic aromatic hydrocarbons (PAHs) PAHs. After two rounds of error-prone PCR and backcross with parental DNA, three mutants were identified for improved activity toward pyrene and for the first time a new activity toward chrysene in comparison to the wild type enzyme. The mutants show higher affinity and coupling efficiency for chrysene with faster rates of product formation compared to the wild type. Furthermore, the mutants are able to hydroxylate chrysene in different positions, producing four metabolites, 1-, 3-, 4-, and 6-hydroxychrysene, and to hydroxylate pyrene to 1-hydroxypyrene.The majority of the mutation sites are found to be far from the active site, demonstrating the power of directed evolution in identifying mutations difficult to predict with a rational design approach.The different product profiles obtained for the different P450 BM3 mutants indicate that substrate orientation in the catalytic pocket of the protein can be modified by protein engineering. The mutants can be used for metabolic engineering for safe and cost-effective sustainable production of hydroxylated PAHs for industrial purposes as well as for the assessment of their carcinogenic activity in mammals.

Selective functionalisation of saturated C–H bonds with metalloporphyrin catalysts

The recent surge of interest in metal-catalysed C-H bond functionalisation reactions reflects the importance of such reactions in biomimetic studies and organic synthesis. This critical review focuses on metalloporphyrin-catalysed saturated C-H bond functionalisation reported since the year 2000, including C-O, C-N and C-C bond formation via hydroxylation, amination and carbenoid insertion, respectively, together with a brief description of previous achievements in this area. Among the metalloporphyrin-catalysed reactions highlighted herein are the hydroxylation of steroids, cycloalkanes and benzylic hydrocarbons; intermolecular amination of steroids, cycloalkanes and benzylic or allylic hydrocarbons; intramolecular amination of sulfamate esters and organic azides; intermolecular carbenoid insertion into benzylic, allylic or alkane C-H bonds; and intramolecular carbenoid C-H insertion of tosylhydrazones. These metalloporphyrin-catalysed saturated C-H bond functionalisation reactions feature high regio-, diastereo- or enantioselectivity and/or high product turnover numbers. Mechanistic studies suggest the involvement of metal-oxo, -imido (or nitrene), and -carbene porphyrin complexes in the reactions. The reactivity of such metal-ligand multiple bonded species towards saturated C-H bonds, including mechanistic studies through both experimental and theoretical means, is also discussed (244 references).