Aromatic Oxidation Research Articles

Modelling studies of HOMs and their contributions to new particle formation and growth: comparison of boreal forest in Finland and a polluted environment in China

Abstract. Highly oxygenated multifunctional compounds (HOMs) play a key role in new particle formation (NPF), but their quantitative roles in different environments of the globe have not been well studied yet. Frequent NPF events were observed at two “flagship” stations under different environmental conditions, i.e. a remote boreal forest site (SMEAR II) in Finland and a suburban site (SORPES) in polluted eastern China. The averaged formation rate of 6 nm particles and the growth rate of 6–30 nm particles were 0.3 cm−3 s−1 and 4.5 nm h−1 at SMEAR II compared to 2.3 cm−3 s−1 and 8.7 nm h−1 at SORPES, respectively. To explore the differences of NPF at the two stations, the HOM concentrations and NPF events at two sites were simulated with the MALTE-BOX model, and their roles in NPF and particle growth in the two distinctly different environments are discussed. The model provides an acceptable agreement between the simulated and measured concentrations of sulfuric acid and HOMs at SMEAR II. The sulfuric acid and HOM organonitrate concentrations are significantly higher but other HOM monomers and dimers from monoterpene oxidation are lower at SORPES compared to SMEAR II. The model simulates the NPF events at SMEAR II with a good agreement but underestimates the growth of new particles at SORPES, indicating a dominant role of anthropogenic processes in the polluted environment. HOMs from monoterpene oxidation dominate the growth of ultrafine particles at SMEAR II while sulfuric acid and HOMs from aromatics oxidation play a more important role in particle growth. This study highlights the distinct roles of sulfuric acid and HOMs in NPF and particle growth in different environmental conditions and suggests the need for molecular-scale measurements in improving the understanding of NPF mechanisms in polluted areas like eastern China.

Synthesis of 4,5-dihydroisoxazole derivatives of maleopimaric acid by 1,3-dipolar cycloaddition reaction between its allyl derivatives and aromatic nitrile oxides

While searching for pharmacologically active compounds among 4,5-dihydrooxazole derivatives of maleopimaric acid, we studied for the first time the reaction between allyl-substituted maleopimaric acid derivatives and aromatic nitrile oxides that were generated from oximes by oxidation with sodium hypochlorite with or without ultrasonication of the biphasic system. The reaction occurred under mild conditions and regiospecifically provided 4,5-dihydroisoxazol-5-ylmethyl derivatives of maleopimaric acid in 77–99% yields.

Total Synthesis Establishes the Biosynthetic Pathway to the Naphterpin and Marinone Natural Products.

The naphterpins and marinones are naphthoquinone meroterpenoids with an unusual aromatic oxidation pattern that is biosynthesized from 1,3,6,8-tetrahydroxynaphthalene (THN). We propose that cryptic halogenation of THN derivatives by vanadium-dependent chloroperoxidase (VCPO) enzymes is key to this biosynthetic pathway, despite the absence of chlorine in these natural products. This speculation inspired a total synthesis to mimic the naphterpin/marinone biosynthetic pathway. In validation of this biogenetic hypothesis, two VCPOs were discovered that interconvert several of the proposed biosynthetic intermediates.

Total Synthesis Establishes the Biosynthetic Pathway to the Naphterpin and Marinone Natural Products

AbstractThe naphterpins and marinones are naphthoquinone meroterpenoids with an unusual aromatic oxidation pattern that is biosynthesized from 1,3,6,8‐tetrahydroxynaphthalene (THN). We propose that cryptic halogenation of THN derivatives by vanadium‐dependent chloroperoxidase (VCPO) enzymes is key to this biosynthetic pathway, despite the absence of chlorine in these natural products. This speculation inspired a total synthesis to mimic the naphterpin/marinone biosynthetic pathway. In validation of this biogenetic hypothesis, two VCPOs were discovered that interconvert several of the proposed biosynthetic intermediates.

Highly Efficient Aromatic C−H Oxidation with H2O2 in the Presence of Iron Complexes of the PDP Family

AbstractThe catalytic activity of a series of iron complexes of the PDP family (PDP=N,N′‐bis(2‐pyridylmethyl)‐2,2′‐bipyrrolidine) in the oxidation of aromatic substrates with H2O2 has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93 %), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)2], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron‐donating alkyl groups of the substrates: halogenbenzenes<benzene<monoalkylbenzenes<dialkylbenzenes, which, together with the results of competitive oxidation experiments and the observation of inverse primary kH/kD (0.90–0.92), agrees with the SEAr oxidation mechanism. Low‐temperature EPR studies have witnessed the presence of low‐spin (g1=2.071, g2=2.008, g3=1.960) perferryl intermediates, demonstrating direct reactivity toward benzene. The oxidation of m‐xylene in the presence of H218O has shown the same probability of 18O incorporation into the aromatic ring and the aliphatic moieties, which is indicative that both the aromatic and aliphatic oxidations are conducted by the same active species.

Effect of different carboxylic acids on the aromatic hydroxylation with H2O2 in the presence of an iron aminopyridine complex

In this contribution, the effect of the structure of the catalytic additive – carboxylic acid – on the catalytic performance of the iron based catalyst [(PDP)Fe(OTf)2], 2 (PDP = N,N′-bis(2-pyridylmethyl)-(S,S)-2,2′-bipyrrolidine) in the selective aromatic oxidation of alkylbenzenes with H2O2 is presented. Eight (linear and branched) carboxylic acids have been tested; in the presence of 2-ethylhexanoic acid, the system [(PDP)Fe(OTf)2]/RCOOH/H2O2 has demonstrated the highest substrate conversion and the highest selectivity for oxygen incorporation into the aromatic ring (up to > 99%) at the same time. Low-temperature EPR spectroscopic study of the system [(PDP)Fe(OTf)2]/2-ethylhexanoic acid/H2O2 witness the presence of the low-spin perferryl intermediate 2aEHA with small g-factor anisotropy (g1 = 2.069, g2 = 2.007, g3 = 1.963), which directly reacts with benzene at −80 °C with the rate constant k2 = 0.6 M−1s−1, and with toluene with k2 > 1 M−1s−1, thus giving evidence for its key role in the selective oxygenation of aromatic substrates.

Gold nanoparticle-functionalized niobium oxide perovskites as photocatalysts for visible light-induced aromatic alcohol oxidations

Spherical gold nanoparticles have been supported onto the surface of potassium niobium oxide perovskites, an underdeveloped class of semiconductor in photocatalytic organic transformations. The nanoparticle dopants of 9.5 nm in diameter and surface plasmon absorption at 530 nm are examined as possible visible light induced catalysts using alcohol photooxidation as the probe reaction. The nanomaterial-induced photooxidation of a series of aromatic alcohols is examined, in the absence of solvent, as a function of base, H2O2, and catalyst concentrations, as well as using multiple visible light sources. This experimental methodology affords extremely selective photooxidation to the carbonyl products (>99%) in as little as 2 h. Using the results obtained from the substitution of the aromatic alcohol, the proposed photocatalytic mechanism is suggested to rely heavily on plasmon-initiated electron transfer from the gold nanoparticle surface to the potassium niobium oxide perovskite and subsequent reductive decomposition of H2O2. This photodegradation step is proposed to favor the formation of ketyl radical species, a key intermediate in the visible light induced mechanism that undergoes both an electron and proton transfer to facilitate formation of the final, carbonyl products. Furthermore, the gold nanoparticle – potassium niobium oxide catalyst exhibits moderate reusability, highly desired in the realm of heterogeneous catalysis.

Montmorillonite-assisted synthesis of cobalt-nitrogen-doped carbon nanosheets for high-performance selective oxidation of alkyl aromatics

Two-dimensional carbon nanomaterials have been widely applied in the field of catalysis due to their unique structure and properties. In this work, the cobalt and nitrogen co-doped two-dimensional carbon nanomaterials with graphene-like and porous structure were synthesized via template method. Here, rhodamine B acted as the carbon source was intercalated into the CTAB-pillared montmorillonite. And the cobalt porphyrin was used as the cobalt and nitrogen precursors that made a great contribution to generate highly dispersed active sites due to their unique CoN4 structure. The characterization techniques such as HAADF-STEM, XPS, XRD, and Nitrogen adsorption-desorption were used to investigate the surface composites and structures of the CoNC catalysts. In addition, the as-prepared catalysts with relatively high specific surface area, large pore volume, and well-dispersed active sites were further evaluated for the selective oxidation of ethylbenzene using tert-butyl hydroperoxide as oxidant. The CoN/CsH catalyst synthesized via impregnation method presented an ethylbenzene conversion of 96% and selectivity of 99% to acetophenone, which was the highest catalytic activity among the investigated catalysts. The superior catalytic performance could be ascribed to the unique two-dimensional porous graphene-like structure, highly dispersed active sites, and the synergistic effect between the CoOx and CoNx.

Electrochemical degradation of norfloxacin using BDD and new Sb-doped SnO2 ceramic anodes in an electrochemical reactor in the presence and absence of a cation-exchange membrane

Abstract Electrochemical oxidation of Norfloxacin (NOR) in sodium sulphate media has been comparatively studied in an undivided and in a divided electrolytic cell both containing either a boron doped diamond (BDD) or a novel Sb-doped SnO2 ceramic anode under galvanostatic operation. The electro-oxidation was found to occur with first order kinetics mainly when using both anodes. The results showed the great oxidizing power of BDD in relation to the ceramic anode to convert NOR and all the intermediate accumulated into CO2. In the case of the BDD, although a 92% of TOC abatement was achieved, the complete mineralization was not possible probably due to the carboxylic acids still present in solution. On the contrary, for the ceramic electrode, which presented a maximum value of TOC removal of about 63%, the total mineralization of the aromatic oxidation intermediates was not reached under the experimental conditions. The use of a membrane divided cell showed positive aspects in terms of molecule degradation, degree of mineralization and current efficiency since prevents the intermediate products formed during the NOR oxidation process from being reduced on the cathode.

Direct Evaluation of the Reactivity of Nonheme Iron(V)–Oxo Intermediates toward Arenes

The reactivity of nonheme iron(V)–oxo intermediates toward aromatic C–H oxidation at −70 °C has been directly evaluated. The intermediates were generated upon the interaction of the ferric complex [(PDP*)FeIII(μ-OH)2FeIII(PDP*)](OTf)4 (4◊; PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine) with peracetic acid in the presence of acetic or 2-ethylhexanoic acid. The second-order rate constants (k2) for the reaction of substituted benzenes with iron–oxo intermediates [(PDP*)FeV═O(OC(O)R)]2+ at −70 °C were determined (R = CH3, 3-heptyl). For more electron rich arenes, much higher k2 values were observed, increasing in the order nitrobenzene < acetophenone < chlorobenzene < benzene < toluene, in accordance with the electrophilic aromatic substitution mechanism. The catalytic oxidation of mono- and dialkylbenzenes with H2O2 proceeded with good efficiency (up to 36.5 TN per Fe atom) and high selectivity toward aromatic oxidation products (up to 91%).

Aromatic sulfur compounds oxidation with H2O2 over fully coordinated and defect sites in Ti-beta zeolites: evaluation by density functional theory

The oxidative desulfurization mechanisms of aromatic sulfur compounds, including thiophene, benzothiophene, and dibenzothiophene, with H2O2 on fully coordinated and defective Ti sites on Ti-beta zeolites are investigated by means of density functional theory calculations. In this work, three current concerns are explored: First is the oxidation of sulfides into the sulfoxides and sulfones in the presence of H2O2; the second is the comparison between the sulfide oxidation mechanism on T5 and T2 defective sites in Ti-beta zeolites; and finally the comparison with the oxidation mechanism on fully coordinated Ti site on the Ti-beta zeolite. The model shows that T5 defect site in Ti-beta is the most catalytically active site for the oxidation of aromatic sulfides.

Regioselective synthesis of some isoxazolines and isoxazolidines bearing caprolactam moiety

ABSTRACTIsoxazoline and isoxazolidine series were obtained by the reactions of aromatic aldonitrones and nitrile oxides with N-vinyl caprolactam via regioselective 1,3-dipolar cycloadditions that yield only 5-caprolactam-substituted regioisomers in moderate yields. The cycloaddition reactions were optimized using different conditions. The utilized one for the transformation of nitrile oxides to isoxazoles is triethylamine at room temperature and the best protocol have been found for the transformation of nitrones to N-methyl isoxazolidines is silver acetate in refluxing xylene. The structural identification of target compounds was established by IR, nuclear magnetic resonance (NMR), high resolution mass spectrometry (HRMS) spectra, and X-ray diffraction analyses.

Investigation of applicability of Electro-Fenton method for the mineralization of naphthol blue black in water

In this study, mineralization and color removal performance of electro-Fenton method were examined in water containing naphthol blue black (NBB), a diazo dye. NBB was totally converted to intermediate species in a 15-min electrolysis at 60 mA, but complete de-colorization took 180 min. A very high oxidation rate constant ((3.35 ± 0.21) x 1010 M−1s−1) was obtained for NBB, showing its high reactivity towards hydroxyl radicals. A very high total organic carbon (TOC) removal value (45.23 mg L−1) was obtained in the first 60 min of the electro-Fenton treatment of an aqueous solution of NBB (0.25 mM) at 300 mA, indicating the mineralization efficiency of the electro-Fenton method. Mineralization current efficiency values obtained at 300 mA gradually decreased from 24.18% to 4.47% with the electrolysis time, indicating the presence of highly parasitic reactions. Gas chromatography-mass spectrometry analyses revealed that the cleavage of azo bonds of NBB led to formation of different aromatic and aliphatic oxidation intermediates. Ion chromatography analysis showed that ammonium, nitrate and sulfate were the mineralization end-products. The concentration of sulfate ion reached to its quantitative value at the 4th h of electrolysis. On the other hand, the total concentration of ammonium and nitrate ions reached to only 61% of the stoichiometric amount of initial nitrogen after a 7 h electrolysis. Finally, it can be said that the electro-Fenton method is a suitable and efficient method for the removal of NBB and its intermediates from water.

Formation of highly oxygenated organic molecules from aromatic compounds

Abstract. Anthropogenic volatile organic compounds (AVOCs) often dominate the urban atmosphere and consist to a large degree of aromatic hydrocarbons (ArHCs), such as benzene, toluene, xylenes, and trimethylbenzenes, e.g., from the handling and combustion of fuels. These compounds are important precursors for the formation of secondary organic aerosol. Here we show that the oxidation of aromatics with OH leads to a subsequent autoxidation chain reaction forming highly oxygenated molecules (HOMs) with an O : C ratio of up to 1.09. This is exemplified for five single-ring ArHCs (benzene, toluene, o-/m-/p-xylene, mesitylene (1,3,5-trimethylbenzene) and ethylbenzene), as well as two conjugated polycyclic ArHCs (naphthalene and biphenyl). We report the elemental composition of the HOMs and show the differences in the oxidation patterns of these ArHCs. A potential pathway for the formation of these HOMs from aromatics is presented and discussed. We hypothesize that AVOCs may contribute substantially to new particle formation events that have been detected in urban areas.

Design of Exceptionally Strong Organic Superbases Based on Aromatic Pnictogen Oxides: Computational DFT Analysis of the Oxygen Basicity in the Gas Phase and Acetonitrile Solution.

DFT B3LYP calculations convincingly showed that aromatic pnictogen oxides offer scaffolds suitable for tailoring powerful organic superbases exhibiting exceptional oxygen basicity in both the gas phase and polar aprotic acetonitrile solution. With their protonation enthalpies and pKa values, they surpass the basicity of classical proton sponges and related nitrogen bases. The most potent system is provided with two arsenic oxide moieties on the phenanthrene framework assisted by the two phosphazeno groups in the para-position to both basic centers. With its proton affinity PA = 300.5 kcal mol-1, the latter system breaks the gas-phase hyperbasicity threshold of 300 kcal mol-1, while its pKa = 54.8 promotes it as an unprecedented superbase in acetonitrile. The origin of such a dramatic basicity enhancement is traced to a fine interplay between (a) steric repulsions of the two negatively charged oxygens destabilizing a neutral base, (b) favorable intramolecular [O-H···O]- hydrogen bonding in conjugate acids, and (c) efficient cationic resonance upon protonation supported by the electron-donating substituents. Given the growing interest in highly basic compounds together with related basic catalysts and metal complexing agents, we hope that the results presented here will open a new avenue of research in these fields and direct attention toward utilizing aromatic pnictogen oxides in designing improved organic materials.

Selective aerobic oxidation of alkyl aromatics on Bi2MoO6 nanoplates decorated with Pt nanoparticles under visible light irradiation.

Pt/Bi2MoO6 nanoplates are efficient photocatalysts for the selective oxidation of saturated C-H bonds in alkyl aromatics under visible light illumination using O2 as an oxidant. This study opens a new window for direct C-H functionalization through the photocatalytic method based on cheap Bi2MoO6 semiconductor materials.

Methods for reducing low emissions from heating devices in single- family housing

By low emission, we mean the pollution of the lower troposphere from emitters not exceeding 50 m in height and most often located at a level of 10 m in height. Due to such emitters the altitude this pollution accumulates around is its place of emission. The basic source of low emissions in Poland are pollutants caused by vehicular traffic and during the process of burning low quality coal, and rubbish - in domestic furnaces. The reason is the lack of proper filtrtation in these devices. Most air pollutants are gaseous substances. The second group is dust. Contamination is different in physical and chemical terms. The most dangerous for people and the environment are: dioxins, polycyclic aromatic hydrocarbons (benzopyrene), carbon monoxide, sulfur dioxide, nitrogen oxides, heavy metals (eg cadmium, manganese, chromium), and hydrogen cyanide. The largest increase in suspended particulates during the heating season affects a majority of single-family housing estates, due to the lack of system heat. It is a fact that many users have antiquated coal furnaces in which low-quality coal and sometimes waste is burned, for example, colored press, rubber, plastics, and impregnated wood. Unfavorable weather conditions during the heating season intensify this phenomenon which is known as smog. This publication is aimed at presenting methods for reducing the emission of pollutants into the air due to the combustion of inadequate fuel in areas of low residential buildings. The solutions, based on the replacement of fifth-generation furnaces as well as a change of fuel, have been proposed.

An improved quantum mechanical model of P450-mediated aromatic oxidation

An improved quantum mechanical model of P450-mediated aromatic oxidation

Effect of Ru on the activity of Co3O4 catalysts for chlorinated aromatics oxidation

A Ru/Co3O4 catalyst with Ru–O–Co structure presented highly stable activity and selectivity in 1,2-DCB oxidation.

Thiol Reactivity toward Atomic Oxygen Generated during the Photodeoxygenation of Dibenzothiophene S-Oxide.

Aromatic heterocyclic oxides, such as dibenzothiophene S-oxide (DBTO), have been suggested to release ground state atomic oxygen [O(3P)] upon irradiation, and as such, they have been used to create a condensed phase reactivity profile for O(3P). However, thiols, which are highly reactive with O(3P) in the gas phase, were not previously investigated. An earlier study of O(3P) with proteins in solution indicated a preference for thiols. A further investigation of the apparent thiophilicity provided the subject for this study. DBTO was employed as a putative O(3P)-precursor. However, the effective rate of O(3P) formation was found to be dependent on reactant concentrations in certain cases. All reactants were found to increase the rate of deoxygenation to some extent, but in the presence of reactants containing an alcohol linked to a reactive functional group, deoxygenation occurred substantially more rapidly. The rate enhancement was quantified and attributed to the reaction of activated O atom within the solvent cage prior to escape into the bulk solution. Through competition experiments, the relative rate constants of O(3P) with thiols and other functional groups were found. A small preference for primary thiols was observed over other thiols, sulfides, and alkenes. A much larger preference was observed for thiols, sulfides, and alkenes over aromatic groups. In summary, DBTO was successfully used as an O(3P)-precursor, and the thiophilicity of O(3P) was confirmed and quantified.