Radical Chain Mechanism Research Articles

Radical Germylzincation of α-Heteroatom-Substituted Alkynes.

The regio- and stereoselective addition of germanium and zinc across the C-C triple bond of nitrogen-, sulfur-, oxygen-, and phosphorus-substituted terminal and internal alkynes is achieved by reaction with a combination of R3GeH and Et2Zn. Diagnostic experiments support a radical-chain mechanism and the β-zincated vinylgermanes that show exceptional stability are characterized by NMR spectroscopy and X-ray crystallography. The unique feature of this new radical germylzincation reaction is that the C(sp2)-Zn bond formed remains available for subsequent in situ Cu(I)- or Pd(0)-mediated C-C or C-heteroatom bond formation with retention of the double bond geometry. These protocols offer modular access to elaborated tri- and tetrasubstituted vinylgermanes decorated with heteroatom substituents β to germanium that are useful for the preparation of stereodefined alkenes.

Propylene glycol oxidation with hydrogen peroxide over Zr-containing metal-organic framework UiO-66

Zirconium-based metal–organic framework UiO-66 catalyzes oxidation of propylene glycol (PG) using hydrogen peroxide as green oxidant. Hydroxyacetone (HA) is the main oxidation product, while the main side product is acetic acid (AcA). The nature of the solvent drastically affects PG adsorption, oxidant utilization efficiency and product yields. The best catalytic performance (85% selectivity towards HA at ca. 10% PG conversion) was achieved with water–acetonitrile (3/7 (v/v)) mixture as a solvent. Additives of radical chain scavengers produce a rate-inhibiting effect, suggesting radical chain mechanism of the oxidation process. The PG oxidation over UiO-66 proceeds without leaching of the active metal into solution, and the catalysis has a truly heterogeneous nature. The catalyst can be recycled without significant loss of activity and selectivity and retains its structure during at least five reuses.

Electrolytic Cross-Coupling of Arenediazonium Salts and Heteroarenes

Abstract The cathodic reduction-induced cross-coupling of arenediazonium salts and heteroarenes was employed to prepare heterobiaryl derivatives. This reaction is attractive because it allows the direct arylation of an aromatic C-H bond in a heteroarene derivative. A radical chain mechanism appears to be involved.

Photoredox Generated Vinyl Radicals: Synthesis of Bisindoles and β-Carbolines.

A photoredox catalyzed approach enabling use of alkynes as surrogate of 2-oxoaldehydes/1,2-diones is reported. The method overcomes the difficulty associated with application of unsubstituted aliphatic α-oxoaldehydes, which has hitherto limited their general use. Indoles, tryptamine, and tryptophan methyl ester participated in the reaction to give a variety of α-oxo based analogues. Quantum yield investigations support a radical chain mechanism.

The ultrasound thermal cracking for the tar-sand bitumen

The influence of ultrasonic irradiation on the tar-sand bitumen in the process of thermal cracking with an inert atmosphere was investigated thoroughly. The product distribution and coke characteristic produced by the conventional thermal cracking (CTC) and ultrasound thermal cracking (UTC) were invested at the following condition: ultrasound frequency 20 kHz, ultrasonic power 2000 W, reaction time 2 h, reaction temperature from 400 to 440 °C. The result of the liquid products distribution indicated that UTC can significantly increase gasoline yield and diesel yield, and dramatically reduce VGO (Vacuum Gas Oil) yield and residuum (greater than 500 °C) yield. The analysis of gas products showed that there were no significant differences for the gas distribution between the two reactions (methods), indicating that reaction of UTC still conformed to a radical chain mechanism, but the ratio of olefin/paraffin was greatly reduced in the process of UTC, which was attributed to the hydrogen transfer reaction promoted by ultrasound. The result of the analysis by SEM, FT-IR, Raman, XRD and Zeta potential demonstrated that there was a significant difference for the morphology of cokes produced by the two methods. Mesocarbon microbeads (MCMB) was discovered in the process of UTC, which should be due to that the polymerization of the free macro-radicals produced from PAHs (Polycyclic Aromatic Hydrocarbons) promoted by ultrasonic cavitation. In addition, it can be inferred that the viscosity of the second liquid phase was reduced by ultrasonic mechanical function through the breakage of the stack of asphaltene molecules cross-linked by van der Waals force. According to the mesophase theory, the ultrasound irradiation promoted the formation of the second liquid phase, extended its existence time and reduced its viscosity, resulting in the formation of MCMB controlled by the surface tension during the process of UTC.

Kinetic Study of the Pyrolysis and Oxidation of Guaiacol.

Guaiacol or 2-methoxy phenol is one of the main primary tars produced during lignin pyrolysis. Tar conversion in the gas phase influences the production of gaseous and condensable products, and is also responsible for PAH and soot formation during biomass and bio-oil gasification or combustion. Guaiacol pyrolysis and oxidation under stoichiometric conditions were studied in a jet stirred reactor between 623 and 923 K for a residence time of 2 s and under a pressure of 800 Torr (106.7 kPa). Speciation was obtained thanks to online gas chromatography using flame ionization detection and mass spectrometry and allowed the quantification of 22 species in pyrolysis and 42 species in oxidation. Decomposition of guaiacol starts at 650 K, and a conversion degree of 50% is obtained at about 785 K in pyrolysis and 765 K in oxidation. The main products of reaction are pyrocatechol o-HOC6H4OH, o-hydroxybenzaldehyde, methylcatechols, and light products, such as methane, carbon monoxide, ethylene, and hydrogen. A detailed kinetic model based on a combustion model for light aromatics and anisole has been extended to guaiacol. Thermochemical data of guaiacol and main products were calculated theoretically at the CBS-QB3 level of theory. The model predicts well the conversion of guaiacol and the formation of the main products. Guaiacol decomposes mainly through a unimolecular O-C bond breaking to hydroxy phenoxy and methyl radicals in both pyrolysis and oxidation, but H atom abstractions are also of importance in the low temperature range of the study. The unimolecular mechanism leads mainly to pyrocatechol and methylcatechols, whereas the chain radical mechanism is responsible for the formation of hydroxybenzaldehyde. As for anisole but in a much lower extent, an early formation of benzene and soot precursors is observed.

Thermal Decomposition of Triethylindium

Thermal decomposition of triethylindium has been studied under static conditions. The temperature dependence of the thermal decomposition rate is described by the equation log k = 8.1 ± 0.4–(156 600 ± 5000)/2.3 RT. The composition of gaseous thermolysis products has been determined. The obtained data suggest radical chain mechanism of the process.

Acyl Radicals from Terminal Alkynes: Photoredox-Catalyzed Acylation of Heteroarenes.

A photoredox-mediated acylation reaction of electron deficient heteroarenes with terminal alkynes is reported. The method relies on oxidative cleavage of phenylacetylenes for generation of acyl radicals as a key enabling feature. The reaction is regioselective with broad substrate scope. Quantum yield investigations support a radical chain mechanism.

NATURAL CLINOPTILOLITE ANCHORED CHLORIDES OF 3D METALS IN THE REACTION OF LOW-TEMPERATURE SULFUR DIOXIDE OXIDATION WITH AIR OXYGEN

Such adsorbents as natural aluminosilicates and activated coal and such chemisorbents as transition metal oxides are commonly used for cleaning gas emissions from sulfur dioxide. The main disadvantage of the latters except for manganese oxides is that high temperatures (up to 800 °C) are needed for the reaction proceeding. However sulfur dioxide can be oxidized to H 2 SO 4 in conformity with complicated mechanisms in aqueous solutions of coordination compounds of copper(II,III), iron(II,III), manganese(II) and cobalt(II) at ambient temperature. Three types of mechanisms of interaction between sulfur dioxide and air oxygen can be considered as most probable: the chain-radical mechanism with single-electron transfer, the non-radical mechanism with two-electron transfer, and the combined mechanism having some elements of two above-mentioned mechanisms. We studied the reaction of sulfur dioxide oxidation by air oxygen over natural clinoptilolite anchored chlorides of 3d metals (MCl2/NCLI where М = Cu2+, Co2+, Mn2+, and Cd2+). It has been found that the amounts of sulfur dioxide reacted (Qexp) at all CMCl2 are far more, than Qexp in the case of pure N-CLI. That is 3d metal chlorides can affect the reaction of sulfur dioxide oxidation with air oxygen. However the mechanisms of their action depend on the nature of the metal ion and its concentration. This phenomenon can be seen from the values of Qexp, tMPC (a time of protective action, i.e. a period of time from experiment beginning up to the moment when C fSO2 achieves its maximum permissible concentration (MPC), 10 mg/m3), and t1/2 (a half-conversion time, i.e.a period of time from experiment beginning up to the moment when C fSO2b ecame equal to 0.5C in SO2 ) . For instance, at CMCl2 = 2.9×10-5 mol/g, Qexp, tMPC and t1/2 decrease in the order Mn(ІІ)> Cu(ІІ) > Cd(ІІ) > Co(ІІ), whereas at CMCl2 = 11.7 ∙10-5 mol/g, the order changes as follows: Cu(ІІ) > Co(ІІ) > Cd(ІІ) > Mn(ІІ). Thus, comparing the results of our experiments and the data for resolved 3d metal chlorides reported in literature, a certain similarity can be traced.

The free radical chain mechanism of the initial stages of crude oil oxidation in term of SARA fractions

The formation and decomposition of hydro-peroxides are the key stages of combustion. These stages strongly depend on the several factors accelerating or slowing this process. The aim of this work is to estimate experimentally which oil components act as inhibitors of initial stages of oxidation and which accelerate the process. The next aim is to explore the process of adsorption of oil components on the grain of rock, which turned to be also a key process in the low temperature oxidation.The work includes experimental part where differential scanning calorimeter (PDSC) experiments with pure saturates, mixtures of saturates and aromatic oil fractions and mixtures of saturates, aromatic fractions and rock samples are considered. Effects of inhibition and acceleration of the initial oxidation stages are explored.

Molecular Oxygen Mediated Radical Dicarbofunctionalization of Olefin with Aldehyde.

Herein we present a metal- and peroxide-free method for the synthesis of sterically hindered amides bearing an all-carbon quaternary stereogenic center at the α-position via a one-pot alkylation/aryl-migration/desulfonylation radical cascade. The novel process is developed employing readily available and inexpensive aldehyde as an alkyl radical precursor and O2 as the sole oxidant. The method features a broad substrate scope, operational simplicity, convenient reagents, and scalability. A radical chain mechanism that is initiated by O2 is proposed for the process.

Isotopic constraints on heterogeneous sulfate production in Beijing haze

Abstract. Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42−)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM2.5 concentrations ranged from 16 to 323 µg m−3 with a mean of (141 ± 88 (1σ)) µg m−3, with SO42− representing 8–25 % of PM2.5 mass. The observed Δ17O(SO42−) varied from 0.1 to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰. Δ17O(SO42−) increased with PM2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM2.5 ≥ 75 µg m−3) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III–V, heterogeneous sulfate production (Phet) was estimated to contribute 41–54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggested S(IV) ( = SO2 ⚫ H2O + HSO3− + SO32−) oxidation by H2O2 in aerosol water accounted for 5–13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42−). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21–22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42−), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 via a radical chain mechanism, contributed the remaining 66–73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2. Our local atmospheric conditions-based calculations suggest sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O2 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.

Redox-Active Cobalt(II/III) Metal-Organic Framework for Selective Oxidation of Cyclohexene.

We report herein a new cobalt(II/III) mixed-valence metal-organic framework formulated as [CoIICo2III(μ3-O)(bdc)3(tpt)]·guest 1, where bdc = benzene-1,4-dicarboxylate and tpt = 2,4,6-tri(4-pyridinyl)-1,3,5-triazine, which can be used as a redox-active heterogeneous catalyst for selective oxidation of cyclohexene on the allylic position without destroying the adjacent double bond. Two oxidants were chosen to demonstrate this result. For using tert-butyl hydroperoxide, the conversion rate is 63% and only allylic oxidation products ( tert-butyl-2-cyclohexenyl-1-peroxide, 86%; 2-cyclohexen-1-one, 14%) are found, whereas if using O2 as oxidant, a total conversion of 38% is achieved and also only the allylic oxidation products (cyclohexenyl hydroperoxide, 72%; 2-cyclohexen-1-one, 20%; and cyclohex-2-en-1-ol, 8%) are found. The absence of any adduct on the double bond may be due to the unique radical chain mechanism triggered by the mixed-valent [CoIICo2III(μ3-O)] centers.

Insight into a highly efficient electrolysis-ozone process for N,N-dimethylacetamide degradation: Quantitative analysis of the role of catalytic ozonation, fenton-like and peroxone reactions

A highly efficient electrolysis catalyzed ozone (ECO) process was developed for N,N-dimethylacetamide (DMAC) degradation. The pseudo-first-order rate constants (kobs) of DMAC degradation by ECO process were 1.73–19.09 times greater than those by ozonation and electrolysis processes in a wide pH range of 3.0–10.0. Interestingly, we found O2•− could be generated from ozone decomposition by a radical chain mechanism instead of monovalent reduction of O2 in ECO system at the initial pH of 3.0. Subsequently, the H2O2 derived from O2•− could participate in Fenton-like and peroxone reactions with the released Fe2+ from iron anode and the aerated O3, respectively. Therefore, the extraordinary DMAC removal efficiency was mainly caused by the more generation of •OH through the multiple reactions of homogeneous catalytic ozonation, Fenton-like and peroxone in ECO system. Importantly, the roles of involved reactions in ECO system at various initial pH were quantitatively evaluated according to a series of trapping experiments. The results reveal that the solution pH could significantly affect the contributions of various reactions and convert the reaction mechanisms of multiple reactions in ECO system. Finally, the degradation intermediates were detected to propose a possible DMAC oxidation pathway in the ECO system. This work provides a deep insight into the quantitative analysis of the role of multiple oxidation reactions mechanism and the design of efficient electrochemical advanced oxidation technology for recalcitrant organic pollutant removal.

(Invited) Multielectron Oxygen Reduction in Photocatalytic Organics Decomposition By Nano/Micrometer-Sized Hierarchical Structured Bismuth Tungstate Particles

It has been reported that hydrothermally prepared flake ball (FB)-shaped bismuth tungstate (Bi2WO6: BWO) particles exhibit photocatalytic activity for oxidative decomposition of organic compounds, which is comparable to that of a commercial titania photocatalyst, while the activity for methanol dehydrogenation is almost negligible even in the presence of loaded with platinum [1]. These facts suggest that two (or four)-electron transfer to the surface-adsorbed oxygen (O2), requiring relatively lower, i.e., more anodic potential of electrons in photocatalyst particles, proceeds on photoirradiated FB-BWO, but there has been reported no evidence for this multielectron-transfer mechanism. In the present study, light-intensity dependence of the rate of acetic-acid decomposition was studied to obtain kinetic evidence for the multielectron transfer. FB-BWO, spherical assembly of BWO flakes, was prepared following the previous report [1]. As-prepared FB, its ball-milled samples (L and H) and their 773 K-calcined samples (500FB, 500L and 500H) were used for decomposition of acetic acid in aerobic aqueous suspensions under monochromatic photoirradiation by (A) a diffraction grating-type illuminator (Jasco CRM-FD; max. 10 mW) or (B) a 365-nm UV-LED (max. 320 mW). It was found that order (n) of light-intensity dependences at four wavelengths (irradiation A) calculated by assuming; R = a × I n (R: rate, a: constant and I: light intensity). Except for the samples L and H at 380 and 410 nm, BWO showed almost first-order light-intensity dependences at wavelengths between 320 and 410 nm. With the higher intensity irradiation (irradiation B), the order (n) was changed (decreased) to ca. 0.5 for FB and L at a folding point (Fig. 1). As has been reported previously, photocatalytic acetic-acid decomposition proceeds through radical-chain mechanism with an alkyl peroxy radical as a chain carrier when titania was used as a photocatalyst, and the observed order of light-intensity dependence was ca. 0.5 [2]. The above-mentioned first-order light-intensity dependence for the most BWO samples can be interpreted by combination of second-order dependence for the accumulation of two electrons to reduce O2 and 0.5th-order dependence owing to the radical-chain mechanism. Difference in folding points of plots between FB and L was observed; a folding point for FB was appreciably lower than that of L. One of the possible reasons for this difference is that the probability of the second-photon absorption by one photon-absorbed FB particle within its lifetime is higher than that of L particle owing to larger volume of FB particles [3]. Fig. 1 Double-logarithmic plots of the reaction rate (R) and light intensity (I) observed in the FB or ML-photocatalyzed acetic-acid decomposition under high-intense 365-nm UV-LED photoirradiation (open circles). Data obtained by the diffraction grating type illuminator were also plotted; closed triangles, circles and squares correspond to irradiation wavelength of 320, 350 and 380 nm.

Photo‐Initiated Reduction of CO2 by H2 on Silica Surface

AbstractInvited for this month's cover is the group of Prof. Kimberly Gray at Northwestern University. The Cover picture depicts a strategy to convert CO2 to CO selectively by vacuum UV photolysis via a chain radical mechanism. The Full Paper itself is available at 10.1002/cssc.201702341.

A Visible Light-Driven Minisci-Type Reaction with N-Hydroxyphthalimide Esters.

A visible light-promoted protocol for the redox-neutral coupling of N-hydroxyphthalimide esters with different N-heterocyclic compounds is described. The reaction proceeds through an alkyl radical intermediate generated by reductive decarboxylation of N-hydroxyphthalimide esters. In contrast to the original Minisci protocol, polyalkylation can largely be avoided. Mechanistic investigations revealed a radical chain mechanism which in some cases can proceed even if no photocatalyst is added. This valuable and functional group-tolerant reaction produces substituted heterocycles in moderate to excellent yield. The use of inexpensive starting materials and LEDs as the light source are key features of this C–C bond formation.

Tris(trimethylsilyl)silane-mediated Reductive Decyanation and Cyano Transfer Reactions of Malononitriles

Reductive decyanation reactions of malononitriles were achieved with tris(trimethylsilyl)silane as a radical mediator. The reaction proceeds via a radical chain mechanism involving a silyl radical addition to the malononitrile to form an imidoyl radical followed by α-cleavage to give a silyl isocyanide and an α-cyano radical. The reaction of a 3-butenyl-substituted malononitrile afforded a decyano/cyanosilylation product in good yield through 1,4-cyano transfer.

Kinetic analysis supporting multielectron reduction of oxygen in bismuth tungstate-photocatalyzed oxidation of organic compounds

Light-intensity dependence of the rate of carbon-dioxide liberation in the photocatalytic decomposition of acetic acid by bismuth tungstate particles suspended in an aqueous solution under aerobic conditions was measured by monochromatic photoirradiation using a monochromator (lower intensity <20 mW) and high-intensity UV-LED (higher intensity <300 mW). The light-intensity dependence of both flake ball-shaped micrometer-sized particles (FB-BWO) and wet ball-milled nanometer-sized particles (ML-BWO) seemed to be bimodal, i.e., first and 0.5th orders in the lower and higher intensity ranges, respectively. Approximately 1.5th and second-order light-intensity dependences were also observed for ML-BWO at the lowest intensity range and for FB at the highest intensity range, respectively. The light-intensity dependences could be reproduced by a kinetic model that was derived on the basis of the assumption of oxygen reduction via two-electron (and possibly four-electron at the highest intensity region) transfer and a radical chain mechanism with peroxy radicals as chain carriers. The calculated threshold intensity between the first and 0.5th-order light-intensity dependences for FB-BWO was appreciably lower than that of ML-BWO, suggesting that the higher FB-BWO activity is attributable to the larger effective particle size for accumulation of two (or four) electrons.

α-Arylation of alkylamines with sulfonylarenes through a radical chain mechanism.

In the presence of a substoichiometric amount of a tert-butoxy radical precursor, the reaction of alkylamines with sulfonylarenes was found to give α-arylated alkylamines through homolytic aromatic substitution, where a radical chain is operative.