Ring-opening Pathways Research Articles

High-pressure pyrolysis of isoprenoid hydrocarbon p-menthane in a tandem micro-reactor with online GC–MS/FID

Isoprenoids have been synthesized in large quantities through metabolic engineering method using some microorganisms as hosts and biomass as carbon resources. An isoprenoid cyclic hydrocarbon, p-menthane, is considered as a promising “drop-in” fuel to substitute traditional cycloalkane components, however, its pyrolysis data is lacking so far which is a significant part of the fuel combustion chemistry, especially under high pressures. In this work, pyrolysis experiments of p-menthane were carried out in a tandem micro-reactor under temperature of 450–800 °C and pressure of 2 MPa. 39 species were identified and quantified by online GC–MS/FID, based on which some initial pyrolysis pathways were determined. The early production of 4-methyl-1-cyclohexene and 1-isopropyl-1-cyclohexene indicates that methyl and isopropyl scission happen at the initial stage followed by H-transfer reaction and dehydrogenation. The activation energy Ea and pre-exponential factor A of the overall reaction were calculated as 225 kJ mol−1 and 1.17 × 1012 s−1, respectively. For secondary reactions, many conjugated dienes such as 1,3-butadiene and 2-methyl-1,3-butadiene and small olefins such as ethylene and propene are generated through the ring-opening pathways. Benzene and toluene are the most abundant products among aromatics detected which are formed through dehydrogenation and demethylation of some early species like 4-methyl-1-cyclohexene and side-chain scission of aromatics. A particular product 1,3-hexadien-5-yne was detected which is generated through ring-opening pathways followed by H-transfer and demethylation reactions and could isomerized to benzene. A lumped kinetic model was proposed to describe the pyrolysis of p-menthane which exhibited good agreement with the experimental data.

Ibuprofen biodegradation by hospital, municipal, and distillery activated sludges

ABSTRACTIbuprofen (IBU) has been considered as one of emergent pharmaceutical contaminants in environments due to its occurrences in natural water bodies. Some reports suggested that the IBU was biodegradable but details about biodegradation pathways and functional microbial community were still not fully clear. This study was aimed to assess IBU biodegradation using three different activated sludges (i.e. H: hospital, M: municipal, and D: distillery) with foci on surmising degradation pathway based on UPLC/Q-ToF/MS (ultra-performance liquid chromatography quadrupole time of flight mass spectrometry) analyses and concluding microbial community according to high-throughput sequencing for partial 16S rRNA gene. Results showed that some IBU (∼5 mg/L) was able to be degraded only by sludges H and M during 2–5 days incubation under aerobic conditions. From LC/MS analysis of degradation byproducts, two major ring-opening precursors were identified in extracted ion chromatogram data. Ring-opening degradation pathways leading to the formation of low-molecular-weight carboxylic acids were elucidated. Additionally, the principal coordinate analyses using Fast UniFrac method for the partial 16S rRNA gene showed the microbial communities in the three sludges were significantly different but typically with high proportion of sequences matched gene fragments from Proteobacteria phylum. Some sequences with first matches with previously reported IBU degraders (i.e. Nocardia sp., Sphingomonas sp., and Variovorax sp.) were only found in the sludges H and M which showed capability for IBU biodegradation. These results demonstrated some functional microbes in activated sludges from hospital and municipal WWTPs had potential to break down IBU into smaller molecules.

The ring-opening channel and the influence of Rydberg states on the excited state dynamics of furan and its derivatives.

One important relaxation pathway for photo-excited five-membered heterocyclic organic molecules is ring-opening via a dissociative πσ* state. In this study, we investigate the influence of this pathway in furan and several hydrogenated and methylated derivatives by combining time-resolved photoelectron spectroscopy with time-dependent density functional theory and coupled cluster calculations. We find strong experimental evidence that the ring-opening channel is the major relaxation channel in furan, 2,3-dihydrofuran, and 2-methylfuran (2-MF). In 2,5-dimethylfuran (25-DMF), however, we observe that the molecules relax either via a π3s Rydberg state or through a direct return to the ground state by undergoing ring-puckering motions. From the supporting calculations, for 2-MF and 25-DMF, we predict that there is strong mixing between the πσ* state and the π3s Rydberg state along the ring opening pathway. However, in 25-DMF, no crossing between the πσ*/π3s state and the initially excited ππ* state can be found along the ring opening coordinate, effectively blocking this channel.

Effect of the wavelength on the pathways of 2-MIB and geosmin photocatalytic oxidation in the presence of Fe-N co-doped TiO2

In this study, 2-methylisoborneol (2-MIB) and geosmin (GSM) were degraded through photocatalytic oxidation process. For 2-MIB degradation under visible light irradiation, the highest removal rate was obtained in the presence of the 0.001% Fe – 0.5% N co-doped TiO2 calcined at 500 °C. The removal rates of 2-MIB and GSM were significantly affected by the wavelength of the lamp used during the oxidation, with the effective order of 254 nm > 365 nm > 420 nm > 475 nm. Higher steady-state concentration of ·OH, which was obtained at shorter wavelength, led to higher T&O removal efficiency. The degradation intermediates of 2-MIB and GSM during the photocatalytic oxidation, including a series of alcohols, aldehydes and ketones, were associated with the dehydration and ring-opening pathways. In most cases, for 2-MIB photocatalytic degradation, intermediates with ring structure were yielded under visible light, while the chain compounds tended to be generated under UV irradiation. However, intermediate products of GSM were not varied from each other significantly when the wavelength changed. The abundances of the GSM degradation intermediates were larger than those of the 2-MIB degradation intermediates because GSM was easier to be oxidized than 2-MIB. The apparent degradation rates of 2-MIB and GSM in real water were lower than those in deionized water, and the humic substances in real water were decomposed into low-molecular weight substances.

Why Cyclopropanation is not Involved in Photoinduced α‐Alkylation of Ketones with Diazo Compounds

Diazo compounds are widely used reagents in organic synthesis. Direct photolysis leads to singlet carbenes whereas, in the presence of a photosensitizer, carbenes in the triplet state are generated. However, their reactivity under light irradiation remains poorly understood. Herein, we report photocatalytic, direct alkylation of enamines, generated from ketones, with diazo esters in the presence of free‐base porphyrins acting as photoredox catalysts. Based on the experimental and theoretical studies, we propose a plausible mechanism involving generation of an enamine radical cation that subsequently reacts with diazo compounds giving the radical thus excluding the controversial cyclopropanation–ring opening pathway.

Ti-Doped Pd-Au Catalysts for One-Pot Hydrogenation and Ring Opening of Furfural

Pd-Au bimetallic catalysts with different Pd/Au atomic ratios, supported on ordered structured silica (Hexagonal mesoporous silica—HMS, or Santa Barbara Amorphous-15—SBA-15) were evaluated for one-pot hydrogenation of furfural to 1,2-pentanediol. The surface and structural properties of the catalysts were deeply investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), N2 adsorption isotherms (BET), Infrared spectroscopy (IR), and acid capacity measurements. XPS studies revealed that Ti doped supports had higher dispersion of the active phase, particularly in the case of Pd-Au materials in which Ti played an important role in stabilizing the metallic species. Among the various process conditions studied, such as temperature (160 °C), catalyst amount (10% w/w), and reaction time (5 h), H2 pressure (500 psi) was found to improve the 1,2-pentanediol selectivity. The SBA silica bimetallic Ti-doped system showed the best performance in terms of stability and reusability, after multiple cycles. Under specific reaction conditions, the synergism between Pd-Au alloy and Ti doping of the support allowed the ring opening pathway towards the formation of 1,2-pentanediol in furfural hydrogenation.

Rapid transformation of sulfinate salts into sulfonates promoted by a hypervalent iodine(III) reagent.

An alternative method for forming sulfonates through hypervalent iodine(III) reagent-mediated oxidation of sodium sulfinates has been developed. This transformation involves trapping reactive sulfonium species using alcohols. With additional optimization of the reaction conditions, the method appears extendable to other nucleophiles such as electron-rich aromatic systems or cyclic ethers through a ring opening pathway.

A new approach to molecular dynamics with non-adiabatic and spin-orbit effects with applications to QM/MM simulations of thiophene and selenophene.

We present surface hopping dynamics on potential energy surfaces resulting from the spin-orbit splitting, i.e., surfaces corresponding to the eigenstates of the total electronic Hamiltonian including the spin-orbit coupling. In this approach, difficulties arise because of random phases of degenerate eigenvectors and possibility of crossings of the resulting mixed states. Our implementation solves these problems and allows propagation of the coefficients both in the representation of the spin free Hamiltonian and directly in the "diagonal representation" of the mixed states. We also provide a detailed discussion of the state crossing and point out several peculiarities that were not mentioned in the previous literature. We also incorporate the effect of the environment via the quantum mechanics/molecular mechanics approach. As a test case, we apply our methodology to deactivation of thiophene and selenophene in the gas phase, ethanol solution, and bulk liquid phase. First, 100 trajectories without spin-orbit coupling have been calculated for thiophene starting both in S1 and S2 states. A subset of 32 initial conditions starting in the S2 state was then used for gas phase simulations with spin-orbit coupling utilizing the 3-step integrator of SHARC, our implementation of the 3-step propagator in Newton-X and two new "one-step" approaches. Subsequently, we carried out simulations in ethanol solution and bulk liquid phase for both thiophene and selenophene. For both molecules, the deactivation of the S2 state proceeds via the ring opening pathway. The total population of triplet states reaches around 15% and 40% after 80 fs for thiophene and selenophene, respectively. However, it only begins growing after the ring opening is initiated; hence, the triplet states do not directly contribute to the deactivation mechanism. For thiophene, the resulting deactivation lifetime of the S2 state was 68 fs in the gas phase, 76 fs in ethanol solution, and 78 fs in the liquid phase, in a good agreement with the experimental value of 80 fs (liquid phase). For selenophene, the obtained S2 lifetime was 60 fs in the gas phase and 62 fs for both ethanol solution and liquid phase. The higher rate of intersystem crossing to the triplet states in selenophene is likely the reason for the lower fluorescence observed in selenium containing polymer compounds.

Reductive Decomposition Mechanism of Prop-1-ene-1,3-sultone in the Formation of a Solid–Electrolyte Interphase on the Anode of a Lithium-Ion Battery

A novel electrolyte additive, prop-1-ene-1,3-sultone (PES), has recently attracted great attention due to its formation of effective solid–electrolyte interphase (SEI) films and remarkable cell performance in lithium-ion batteries. Herein, the reductive decomposition of PES is investigated through density functional calculations combined with a self-consistent reaction field method, in which the bulk solvent effect is accounted for by the geometry optimization and transition-state search. We examine three ring-opening pathways, namely, O–C, S–C, and S–O bond-breaking processes. Our calculations reveal that the Li+ ion plays a pivotal role in the reductive decomposition of PES. While the most kinetically favored process—the S–O bond breaking—is effectively blocked via the formation of an intermediate structure, namely, the Li+-participated seven-membered ring, the other decomposition processes via O–C and S–C bond breaking lead to stable decomposition products. The constituents of SEI observed in previous ...

Supercritical water oxidation of quinazoline: Reaction kinetics and modeling

This paper presents a first quantitative kinetic model for supercritical water oxidation (SCWO) of quinazoline that describes the formation and interconversion of intermediates and final products at 673–873 K. The set of 11 reaction pathways for phenol, pyrimidine, naphthalene, NH3, etc, involved in the simplified reaction network proved sufficient for fitting the experimental results satisfactorily. We validated the model prediction ability on CO2 yields at initial quinazoline loading not used in the parameter estimation. Reaction rate analysis and sensitivity analysis indicate that nearly all reactions reach their thermodynamic equilibrium within 300 s. The pyrimidine yielding from quinazoline is the dominant ring-opening pathway and provides a significant contribution to CO2 formation. Low sensitivity of NH3 decomposition rate to concentration confirms its refractory nature in SCWO. Nitrogen content in liquid products decreases whereas that in gaseous phase increases as reaction time prolonged. The nitrogen predicted by the model in gaseous phase combined with the experimental nitrogen in liquid products gives an accurate nitrogen balance of conversion process.

Main-chain degradable single-chain cyclized polymers as gene delivery vectors

Single-chain technology (SCT) allows the manipulation of polymeric architectures at an individual polymer chain level, providing a new platform for the fabrication of nanoscale polymeric objects. However, it remains problematic to apply this newborn technology to the biological and medical fields, since synthesis of single-chain polymeric nanoparticles relies heavily on controlled/living radical polymerization of vinyl based monomers, yielding a persistent non-degradable carbon-carbon based backbone. Moreover, the ultrahigh dilution conditions often required for single-chain polymer nanoparticle synthesis limits large-scale applicability. A versatile approach to achieve backbone degradability in single-chain cyclized polymers was developed by combining ring-opening addition polymerization and intramolecular cyclization into a one-pot RAFT copolymerization of cyclic and mono/multi-vinyl monomers system under concentrated conditions. The in situ intramolecular cyclization of individual propagating chains was achieved by kinetic control and statistical manipulation of mono- and multi-vinyl monomer copolymerization. The cyclic allylsulfide monomer 3-methylidene-1,9-dioxa-5,12,13-trithiacyclopentadecane-2,8-dione (MDTD) was copolymerized via the ring-opening pathway to introduce disulfide groups into the vinyl-based backbone without compromising the single chain propagation nature. Backbone degradable single chain polymeric nanoparticles were obtained with molecular weights of 10kDa and MDTD incorporation ratios of 4.7%. Chemical degradation of the nanoparticles confirmed both their single chain nature, as well as backbone degradability. The single-chain cyclized polymeric nanoparticles were evaluated for their gene transfection capabilities. The backbone degradable nanoparticles displayed high transfection efficiencies and low cytotoxicities in both 3T3 and HeLa cells.

Electron-donating groups and high ring strain promoted ring opening of methylenecyclopropanes catalyzed by rhodium and iridium complexes

The possible ring-opening pathways of X-substituted MCPs (X = NO2, CF3, F, H, Ph, CH3, OH, NH2) promoted by MH(CO)(PY3)3 (M = Rh, Ir, Y = H, Ph) and Wilkinson's catalyst-[RhCl(PY3)3] (Y = H, Ph) were studied by density functional theory. The ring-opening reaction was initiated by the orbital interaction between the transition metal of catalyst and reactant MCP. The strong coordination ability of Ir with CC double bond and the inertness of complex intermediate caused the difficulty in further reaction steps of catalyst dissociation and hydride transfer, rationalizing that IrH(CO)(PPh3)3 catalyzed ring-opening reaction occurred at higher temperature (50 °C) than the RhH(CO)(PPh3)3 catalyzed one (room temperature). Without the medium hydride in RhCl(PPh3)3 catalyst, the CC bond of MCP cannot insert into the RhH bond and the hydride cannot be borrowed from RhCl(PPh3)3 like RhH(CO)(PPh3)3 catalyst, hence making the RhCl(PPh3)3-catalyzed reaction much more difficult to occur. The ring-opening step is predicted to be the rate-determining step for MH(CO)(PPh3)3 catalyst, which is different from the Cp2LnH (Ln = La, Lu) catalyzed reactions. Multifaceted factors of substituents including ring strain, electron-donating ability, and the steric effect of the substituents were revealed for MH(CO)(PPh3)3 catalyzed reactions when M = Rh and Ir. The hydroxyl substituted MCP (X = OH) bears not only higher ring strain but also the stronger electron-donating ability, leading to the lowest activation energy among the studied systems. Understanding of the multifaceted factors of substituents is useful for regulating the ring-opening reaction of substituted MCP promoted by transition metal complexes.

Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures

The pyrolysis of decalin was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at the pressures of 4, 20, and 101kPa and the temperatures from 920 to 1500K. Dozens of pyrolysis species were identified and their mole fraction profiles were measured as a function of temperature, particularly several free radicals and aromatic species. Besides, a detailed kinetic model consisting of 484 species and 1774 reactions was proposed and used to simulate the present experimental data. The rate of production analysis and sensitivity analysis was performed in order to provide deep insight into the pyrolysis kinetics of decalin. The decomposition of decalin is initiated by the C–C bond dissociation reactions of both decalin and its C10H18 isomers, while the H-atom abstraction reactions consume most of decalin. Monocyclic aromatic hydrocarbons (MAHs) are mainly produced in the fuel decomposition process instead of the mass growth process from small molecules, indicating that the two fused six-membered ring structure offers a solid infrastructure to form MAHs. Polycyclic aromatic hydrocarbons (PAHs) are mainly produced in the mass growth process from PAH precursors such as benzyl radical (A1CH2), phenyl radical (A1-), and 1,3-cyclopentadienyl radical (CYC5H5). The relatively high degree of saturation of decalin facilitates ring-opening pathways in its decomposition. This enhances the formation of MAHs but limits the formation of PAHs, leading to the lower sooting tendency of decalin than tetralin. Furthermore, the model was also validated on the shock tube pyrolysis data of decalin in the literature.

ChemInform Abstract: Controlling Catalytic Selectivity via Adsorbate Orientation on the Surface: From Furfural Deoxygenation to Reactions of Epoxides

AbstractReview: 72 refs.

Gas-Phase Anionic σ-Adduct (Trans)formations in Heteroaromatic Systems.

Anions of nitroderivatives of thiophene and furan were subjected to the reactions with selected C-H acids in the gas phase. Various structures and reaction pathways were proposed for the observed ionic products. In general, the reactions of heteroaromatic anions with C-H acids may be divided into three groups, depending on the proton affinity difference between C-H acid’s conjugate base and heteroaromatic anion (ΔPA). The proton transfer from C-H acid to heteroaromatic anion is a dominant process in the reactions for which ΔPA < 0 kcal mol−1, whereas the reactions with high ΔPA (ΔPA > 16 kcal mol−1) do not lead to any ionic products. The formation of σ-adducts and products of their further transformations according to the VNS, SNAr, cine, and tele substitution mechanisms have been proposed for reactions with moderate ΔPA. The other possible mechanisms as SN2 reaction, nucleophilic addition to the cyano group, ring-opening pathway, and halogenophilic reaction have also been discussed to contribute in the reactions between heteroaromatic anions and C-H acids. Graphical ᅟ Electronic supplementary materialThe online version of this article (doi:10.1007/s13361-015-1122-1) contains supplementary material, which is available to authorized users.

Controlling Catalytic Selectivity via Adsorbate Orientation on the Surface: From Furfural Deoxygenation to Reactions of Epoxides.

Specificity to desired reaction products is the key challenge in designing solid catalysts for reactions involving addition or removal of oxygen to/from organic reactants. This challenge is especially acute for reactions involving multifunctional compounds such as biomass-derived aromatic molecules (e.g., furfural) and functional epoxides (e.g., 1-epoxy-3-butene). Recent surface-level studies have shown that there is a relationship between adsorbate surface orientation and reaction selectivity in the hydrogenation pathways of aromatic oxygenates and the ring-opening or ring-closing pathways of epoxides. Control of the orientation of reaction intermediates on catalytic surfaces by modifying the surface or near-surface environment has been shown to be a promising method of affecting catalytic selectivity for reactions of multifunctional molecules. In this Perspective, we review recent model studies aimed at understanding the surface chemistry for these reactions and studies that utilize this insight to rationally design supported catalysts.

MIMICKING THE SECONDARY SPHERE OF HOMOPROTOCATECHUATE DIOXYGENASE (HPCD) ENZYME ACTIVE SITE; A THEORETICAL STUDY

Fe-HPCD is a non-heme extradiol enzyme that operates in the oxidative ring opening pathways of aromatic compounds.[1,2] Recent studies showed that the proton transfer to afford 3 or 4 from 0 (Figure 1) is assisted by an His200 residue that is lying in the secondary sphere of HPCD active site.[3,4] Thus, the main role of His200 is acting as a Bronsted base. Herein, we present a Density Functional Theory analysis of the energetics of a modified first coordination shell with the objective of mimicking the role of His200 by incorporation of various functional groups on one of the imidazole rings (Figure 1). The aim is to create a proton transfer agent that is adequate in terms of thermodynamic and kinetic parameters in comparison with the native protein environment. Figure 1 shows the reaction mechanism and possible functional groups capable to act as a proton shuttle.

Regarding initial ring opening of propylene oxide in its copolymerization with CO2 catalyzed by a cobalt(III) porphyrin complex.

Cobalt(III) tetraphenylporphyrin chloride (TPPCoCl) was experimentally proved to be an active catalyst for poly(propylene carbonate) production. It was chosen as a model catalyst in the present work to investigate the initiation step of propylene oxide (PO)/CO2 copolymerization, which is supposed to be the ring opening of the epoxide. Ring-opening intermediates (1-7) were detected by using (1) H NMR spectroscopy. A first-order reaction in TPPCoCl was determined. A combination of monometallic and bimetallic ring-opening pathways is proposed according to kinetics experiments. Addition of onium salts (e.g., bis(triphenylphosphine)iminium chloride, PPNCl) efficiently promoted the PO ring-opening rate. The existence of axial ligand exchange in the cobalt porphyrin complex in the presence of onium salts was suggested by analyzing collected (1) H NMR spectra.

Hydroxide-ion induced degradation pathway for dimethylimidazolium groups in anion exchange membranes

The alkaline stability of imidazolium-based anion exchange membranes was studied by FT-IR and 13C solid state NMR. Imidazolium groups are susceptible to attack by hydroxide ions and undergo degradation in alkaline conditions through a ring-opening mechanism. Density functional theory (DFT) studies were performed to better understand the degradation pathway and the relative stability of imidazolium cations on exposure to hydroxide. The ring-opening degradation pathway follows the following three steps: (a) a nucleophilic reaction, (b) a ring-opening reaction, and (c) a rearrangement reaction. The structural changes induced in the imidazole ring were studied using a model of the dimethylimidazolium cation ([DMIM]+). The energy changes along the degradation pathway were obtained from reaction path calculations. The effect of hydroxide ion concentration and dielectric constant of surrounding medium on reaction energy barriers were studied. The hydroxide-ion induced degradation of the dimethylimidazolium cation was found to be more facile in high-pH environments. Good solvation of the dimethylimidazolium cation and the hydroxide ion was helpful in minimizing the degradation and enhancing the chemical stability of anion exchange membranes based on these cations.

On the Way to Biofuels from Furan: Discriminating Diels–Alder and Ring-Opening Mechanisms

We performed kinetics experiments and quantum calculations to investigate the reaction of furan to benzofuran catalyzed by the acidic zeolite HZSM-5, which is a key step in the conversion of biomass to biofuels through catalytic fast pyrolysis. The reaction was studied experimentally by placing the zeolite in contact with solution-phase furan and detecting the benzofuran product over the temperature range 270–300 °C, yielding an apparent activation energy of 72 ± 3 kJ/mol. The reaction was modeled in gas and zeolite phases to determine the energetics of the following two competing pathways: a Diels–Alder mechanism often assumed in interpretations of experimental data and a ring-opening pathway predicted by the chemoinformatic software RING. Quantum calculations on the zeolite/guest system were performed using the ONIOM embedded cluster approach. We computed the energetics of reactants, products, and all intermediate steps. Locating relevant transition states fell beyond our computational resources because...