Mechanism and kinetics of the atmospheric degradation of perfluoropolymethylisopropyl ether by OH radical: a theoretical study

In the present work, the mechanism and kinetics of the reaction of perfluoropolymethylisopropyl ether (PFPMIE) with OH radical are studied. The reaction between PFPMIE and OH radical is initiated through breaking of C–C or C–O bond of PFPMIE. These reactions lead to the formation of COF₂ molecules and alkyl radical. The pathways corresponding to the reaction between PFPMIE and OH radical have been modelled using density functional theory methods M06-2X and MPW1K with 6-31G(d,p) basis set. It is found that the C–C bond breaking reaction is most favourable than the C–O bond breaking reaction. The subsequent reactions of the alkyl radicals, formed from the C–C bond breaking reactions, are studied in detail. The rate constant for the initial oxidation reactions is calculated using canonical variational transition state theory with small curvature tunnelling corrections over the temperature range of 278–350 K. From the calculated reaction, potential energy surface and rate constant, the lifetime and global warming potential of PFPMIE are studied.     

Degradation mechanism of sulfa drugs based on theoretical prediction and experimental examination in TiO2 suspension

Photodegradations of Sulfamethazine (SMT) and Sulfametoxydiazine (SMD) in TiO₂ suspension were investigated experimentally and theoretically under UV irradiation. The degradations of SMT and SMD by TiO₂ photocatalysis increased with prolonging the reaction time, and three main intermediates for SMT and four main intermediates for SMD were indentified by LC/MS. The N–S bond, the C–C bond (benzene ring) in the SMT, and the N–C bond, the S–N bond and the C–O bond in the SMD, had higher frontier electron densities. Initial photodegradation of SMT proceeded mainly via the break of the N–S bond and the hydroxylation of SMT, and initial photodegradation of SMD was similar to SMT. There existed a plausible consistency between the experimental results and the theoretical calculations on the basis of results obtained.     

Ab initio molecular dynamics study for C–Br dissociation within BrH<Subscript>2</Subscript>C–C≡C<Subscript>(ads)</Subscript> adsorbed on an Ag(111) surface: a short-time Fourier transform approach

The reaction dynamics for C–Br dissociation within BrH₂C–C≡CH_(ads) adsorbed on an Ag(111) surface has been investigated by combining density functional theory-based molecular dynamics simulations with short-time Fourier transform (STFT) analysis of the dipole moment autocorrelation function. Two possible reaction pathways for C–Br scission within BrH₂C–C≡CH_(ads) have been proposed on the basis of different initial structural models. Firstly, the initial perpendicular orientation of adsorbed BrH₂C–C≡CH_(ads) with a stronger C–Br bond will undergo dynamic rotation leading to the final parallel orientation of BrH₂C–C≡CH_(ads) to cause the C–Br scission, namely, an indirect dissociation pathway. Secondly, the initial parallel orientation of adsorbed BrH₂C–C≡C_(ads) with a weaker C–Br bond will directly cause the C–Br scission within BrH₂C–C≡CH_(ads), namely, a direct dissociation pathway. To further investigate the evolution of different vibrational modes of BrH₂C–C≡CH_(ads) along these two reaction pathways, the STFT analysis is performed to illustrate that the infrared (IR) active peaks of BrH₂C–C≡CH_(ads) such as vCH₂ [2956 cm^?1(s) and 3020 cm^?1(as)], v≡CH (3320 cm^?1) and vC≡C (2150 cm^?1) gradually vanish as the rupture of C–Br bond occurs and then the resulting IR active peaks such as C=C=C (1812 cm^?1), ω-CH₂ (780 cm^?1) and δ-CH (894 cm^?1) appear due to the formation of H₂C=C=CH_(ads) which are in a good agreement with experimental reflection adsorption infrared spectrum (RAIRS) at temperatures of 110 and 200 K, respectively. Finally, the total energy profiles indicate that the reaction barriers for the scission of C–Br within BrH₂C–C≡CH_(ads) along both direct and indirect dissociation pathways are very close due to a similar rupture of C–Br bond leading to a similar transition state.     

Sequential Pd2(dba)3 participated C–C bond cleavage of O-bromophenyl cyclobutanones/Michael addition en route to benzospirones

A tandem Pd₂(dba)₃ participated C–C bond cleavage of O-bromophenyl cyclobutanone derivatives/Michael addition reaction sequence was realized. We disclosed the first intramolecular C–Br bond triggered ring opening reaction of arylcyclobutanones, distinct from related reports in which the reactions were initiated by arylboron, silane or unsaturated chemical motifs, among others. The in situ generated palladium species underwent ring expansion process leading to methyleneindanones, which further reacted with dba to provide benzospirones in one step.     

Palladium-Catalyzed Decarboxylative Alkynylation of α-Acyloxyketones by C(sp3)−O Bond Cleavage

Palladium-catalyzed decarboxylative alkynylation of α-acyloxyketones triggered by C(sp³)−O bond cleavage is disclosed. The decarboxylation strategy featuring a neutral reaction condition enabled an unprecedent catalytic alkynylation of a ketone enolate. The reaction was applied to a variety of substrates, giving desired products in good yields. We successfully obtained X-ray crystallography of a new palladium–enolate intermediate that was synthesized by a reaction of [Pd(cod)(CH₂TMS)₂] with XPhos and α-acyloxyketone at room temperature, indicating facile C(sp³)−O bond disconnection.     

Effect of bond orbital interactions on the conformational stabilities of 1,2-difluoroethane and 2,3-difluorobutanes

Several hypotheses have been proposed to explain the origin of the conformational stabilities of 1,2-difluoroalkanes, for which bond orbital interactions are an important factor. However, there is a limit to the effectiveness of the traditional approach focusing on only the antiperiplanar interactions between bonding and antibonding orbitals such as σ_C–H/σ_C–F*, σ_C–C/σ_C–F*, and σ_C–F/σ_C–F*, which cannot actually explain the conformational stabilities of 2,3-difuluorobutanes. In this study, to elucidate the effect of bond orbital interactions on the conformational stabilities of 1,2-difluoroethane, erythro-2,3-difluorobutane, and threo-2,3-difluorobutane, we extended the range of interactions considered to beyond these conventional interactions. The results showed that for 1,2-difluoroethane, the conformational stability can be understood by considering all antiperiplanar bond orbital interactions around the C1–C2 bond, and for 2,3-difluorobutanes, by considering all antiperiplanar bond orbital interactions around the C2–C3 bond in addition to bond orbital interactions between the methyl groups.     

Computational SN2‐Type Mechanism for the Difluoromethylation of Lithium Enolate with Fluoroform through Bimetallic C−F Bond Dual Activation

The reaction mechanism for difluoromethylation of lithium enolates with fluoroform was analyzed computationally (DFT calculations with the artificial force induced reaction (AFIR) method and solvation model based on density (SMD) solvation model (THF)), showing an S_N2‐type carbon–carbon bond formation; the “bimetallic” lithium enolate and lithium trifluoromethyl carbenoid exert the C?F bond “dual” activation, in contrast to the monometallic butterfly‐shaped carbenoid in the Simmons–Smith reaction. Lithium enolates, generated by the reaction of 2 equiv. of lithium hexamethyldisilazide (rather than 1 or 3 equiv.) with the cheap difluoromethylating species fluoroform, are the most useful alkali metal intermediates for the synthesis of pharmaceutically important α‐difluoromethylated carbonyl products.     

Synthesis of Tofisopam by Way of Photoinduced CO2 Fixation

Herein reported is a unique synthetic route of Tofisopam, an anxiolytic drug containing a 2,3‐benzodiazepine core structure. 3,4‐Dimethoxypropylbenzene and 3,4‐dimethoxybenzoic acid, which are both of plant origin, and CO₂ constitute its carbon skeleton. These three renewable substances are united by two C?C bond forming reactions, i.e., a Friedel–Crafts acylation reaction and a photoinduced carboxylation reaction to construct the major carbon framework. Finally, a methyl group is introduced by a Kumada‐type cross‐coupling reaction to furnish Tofisopam. Various analogs of Tofisopam are readily synthesized by introducing other substituents than a methyl group at the last C?C bond forming step.     

Studies on Cu-catalyzed asymmetric alkynylation of tetrahydroisoquinoline derivatives

Enantioselective C–C bond formations between the sp³ C–H bond of prochiral CH₂ and terminal alkynes via the cross-dehydrogenative coupling (CDC) reaction were studied. Efficient asymmetric syntheses of alkynyl tetrahydroisoquinoline derivatives were achieved by using a catalytic amount of CuOTf together with PyBox chiral ligand. When dihydroisoquinolinium salts were used as electrophiles, the combination of CuBr/QUINAP provided the best results for asymmetric syntheses of alkynyl tetrahydroisoquinoline derivatives. The factors influencing the enantioselectivity were studied.     

Metal‐Free Cross‐Dehydrogenative Coupling (CDC): Molecular Iodine as a Versatile Catalyst/Reagent for CDC Reactions

The development of ecofriendly methods for carbon–carbon (C?C) and carbon–heteroatom (C?Het) bond formation is of great significance in modern‐day research. Metal‐free cross‐dehydrogenative coupling (CDC) has emerged as an important tool for organic and medicinal chemists as a means to form C?C and C?Het bonds, as it is atom economical and more efficient and greener than transition‐metal catalyzed CDC reactions. Molecular iodine (I₂) is recognized as an inexpensive, environmentally benign, and easy‐to‐handle catalyst or reagent to pursue CDCs under mild reaction conditions, with good regioselectivities and broad substrate compatibility. This review presents the recent developments of I₂‐catalyzed C?C, C?N, C?O, and C?S/C?Se bond‐forming reactions for the synthesis of various important organic molecules by cross‐dehydrogenative coupling.     

Thiozonation and thiozonolysis of triatomic sulfur (S<Subscript>3</Subscript>) on the C<Subscript>70</Subscript> fullerene: a DFT study

Density functional theory calculations have been carried out to investigate the [2?+?x] x?=?1, 2, and 3 cycloaddition reactions (paths A, B, and C) of triatomic sulfur (S₃) with the C₇₀ fullerene in terms of geometry, energies, and electronic structures. The thiozonation (S₃) on the hexagon–hexagon and hexagon–pentagon bonds of the C₇₀ fullerene through 1,3-dipolar reaction, i.e., [2?+?3] cycloaddition, is generally exothermic, while through the chelotrope additions, i.e., [2?+?1] cycloaddition, are endothermic. The results indicate that the 1,3-dipolar cycloaddition is the most preferable path. Having more negative values of reaction energies E_r together with the lower barrier heights, thiozonation of the hexagon–hexagon bonds is thermodynamically and kinetically more favorable than hexagon–pentagon ones. Moreover, the addition of thiozone to the hexagon–hexagon bonds near the pole area of the C₇₀ leads to more negative reaction energies. Therefore, it is established that the arrangement and position of C=C bonds play an important role in the thiozonation of C₇₀ fullerene. Thiozonolysis of triatomic sulfur (S₃) indicates that S–S bond cleavage has not occurred, instead a sulfur bridge over a C–C bond or a four-membered ring of 1,2-dithietane-1-sulfide is preferred to be formed.     

Co-oligomerization of ethylene and higher linear alpha olefins. I. Co-oligomerization with the sulfonated nickel ylide-based catalytic system

Co-oligomers of ethylene and a series of linear α-olefins (propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 1-decene) were synthesized with a homogeneous catalyst consisting of sulfonated nickel ylide and diethylaluminum ethoxide at 90°C. GC analysis of the co-oligomerization products allowed complete structural identification of all reaction products, α-olefins with linear and branched chains, vinylidene olefins, and linear olefins with internal double bonds. The article describes the reaction scheme of ethylene–olefin co-oligomerization. The scheme includes chain initiation reactions (insertion of ethylene or an olefin into the Ni? H bond), chain propagation reactions, and chain termination reactions via β-hydride elimination. Primary and secondary inertions of α-olefins into the Ni? H bond in the initiation stage proceed with nearly equal probabilities. Higher olefins participate in the chain growth reactions (insertion into the Ni? C bond) also both in primary and secondary insertion modes. The primary insertion of an α-olefin molecule into the Ni? C bond produces the β-branched Ni? CH₂? CR₁R₂ group. This group is susceptible to β-hydride elimination with the formation of vinylidene olefins. However, the Ni? CH₂? CR₁R₂ groups can participate in further ethylene insertion reactions and thus form vinyl oligomerization products with branched alkyl groups. On the other hand, the secondary insertion of an α-olefin molecule into the Ni? C bond produces the α-branched Ni? CR₁R₂ bond which does not participate in further chain growth reactions and undergoes the β-hydride elimination reaction with the formation of linear reaction products with internal double bonds. Most co-oligomer molecules contain only one α-olefin fragment. However, the analysis of ethylene-propylene and ethylene-1-heptene co-oligomers allowed identification of products with two olefinic fragments which are also formed in the copolymerization reactions with small yields.     

Lewis Acid Promoted Single C–F Bond Activation of the CF3 Group: SN1′-Type 3,3-Difluoroallylation of Arenes with 2-Trifluoromethyl-1-alkenes

Activation of the sp³ C−F bond in 2-trifluoromethyl-1-alkenes was accomplished through treatment with a Lewis acid. In the presence of an equimolar amount of EtAlCl₂, the (trifluoromethyl)alkenes readily underwent an S_N1′-type reaction with arenes through a Friedel–Crafts-type mechanism via elimination of a fluoride ion to afford 3,3-difluoroallylated arenes in good yields. This selective activation of one C−F bond of the CF₃ group provides a synthetic method for accessing biologically and synthetically important 1,1-difluoro-1-alkenes.     

A Facile One‐Step Synthesis of 2‐Arylindazoles via Reductive Cyclization of N‐(2‐nitroarylidene)amines

A mild and efficient synthesis of 2‐arylindazole derivatives via the reductive cyclization of nitro‐aryl substrates mediated by a low‐valent titanium reagent (TiCl₄/Sm/Et₃N) has been developed. The attractive features of the current method include an N–N bond formation and the selective reduction of the C = N bond and nitro group, both of which were easily achieved in one‐pot by controlling the pH of the reaction mixture.     

Product analysis and mechanism of toluene degradation by low temperature plasma with single dielectric barrier discharge

A single dielectric barrier discharge (DBD) low-temperature plasma reactor was set up, and toluene was selected as the representative substance for volatile organic compounds (VOCs), to study the reaction products and degradation mechanism of VOCs degradation by low-temperature plasma. Different parameters effect on the concentration of O₃ and NO_x during the degradation of toluene were studied. The exhaust in the process of toluene degradation was continuously detected and analyzed, and the degradation mechanism of toluene was explored. The results showed that the concentration of O₃ increased with the increase of the power density and discharge voltage of the plasma device. However, as the initial concentration of toluene increased, the concentration of O₃ basically keep steady. The concentration of NO_x in the by-products increased with the discharge voltage, power density, and initial concentration of toluene in the plasma device, and the concentration of NO₂ was much higher than the concentration of NO. The degradation process of toluene was detected and analyzed. The results showed that the degradation mechanism of toluene by plasma includes high energy electron bombardment reaction, active radical reaction and ion molecule reaction. Among them, the effect of high-energy electrons on toluene degradation is the largest, followed by the effect of free radicals, in which oxygen radicals participated in the reaction mainly through the formation of C–O bond, CO bond, (CO)–O– bond and –OH radical, while nitrogen radicals participate in the reaction mainly through the formation of C–NH₂, (CNH)- bond, CN bond and C–NO₂ bond. The results can provide some data supports for the study of low-temperature plasma degradation of VOCs.     

Chemical and thermal stability of N‐heterocyclic ionic liquids in catalytic C–H activation reactions

¹H‐NMR spectrum analyses are applied to study the chemical and thermal stability of selected N‐heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C–H bond of methane and convert it into the C–O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt‐based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH₄⁺). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Theoretical study of the gas-phase ethane C–H and C–C bonds activation by bare niobium cation

The potential energy surfaces for the reaction of bare niobium cation with ethane, as a prototype of the C–H and C–C bonds activation in alkanes by transition metal cations, have been investigated employing the Density Functional Theory in its B3LYP formulation. All the minima and key transition states have been examined along both high- and low-spin surfaces. For both the C–H and C–C activation pathways the rate determining step is that corresponding to the insertion of the Nb cation into C–H and C–C bond, respectively. However, along the C–H activation reaction coordinate the barrier that is necessary to overcome is 0.13 eV below the energy of the ground state reactants asymptote, while in the C–C activation branch the corresponding barrier is about 0.58 eV above the energy of reactants in their ground state. The overall calculated reaction exothermicities are comparable. Since the spin of the ground state reactants is different from that of both H–Nb⁺–C₂H₅ and CH₃–Nb⁺–CH₃ insertion intermediates and products, spin multiplicity has to change along the reaction paths. All the obtained results, including Nb⁺–R binding energies for R fragments relevant to the examined PESs, have been compared with existing experimental and theoretical data.     

Electron spin resonance studies of the anaerobic and aerobic photolysis of alkylcobaloximes

Aerobic and anaerobic photolysis of methyl(pyridine)cobaloxime, benzyl(pyridine)cobaloxime and analogous compounds in CHCl₃ results only in an electron transfer reaction from an equatorial ligand producing photo-reduction of Co^III to Co^II, the complex retaining its axial ligands.If after the anaerobic photolysis of benzyl(pyridine)cobaloxime the oxygen is introduced without any further photolysis we obtain an ESR spectrum of nitroxide, arising from the attack of a benzyl radical on the dimethylglyoxime equatorial ligand.For the other complexes, homolytic cleavage of the CoC bond occurs and in the presence of oxygen gives rise to the superoxide cobalt complex adduct Py(Co^IIIO₂^?.During photolysis of methyl(pyridine)cobaloxime in isopropanol homolytic cleavage of the CoC bond occurs in preference to electron transfer reaction from the equatorial ligands.The anaerobic photolysis of benzyl(pyridine)cobaloxime in isopropanol or in water at 113–133 K results in an electron transfer reaction. However, at 170 K we observe the formation of the Co^II complex arising from CoC bond cleavage.A mechanism for photo-induced insertion of oxygen in the CoC bond is proposed.     

Theoretical study of mechanism of extraction reaction between silylene carbene and its derivatives and ethylene oxide

The mechanism of the oxide extraction reaction between singlet silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide has been investigated with density functional theory, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6‐311G(d,p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists two steps, the first step is the two reactants firstly form an intermediate (INT) through a barrier‐free exothermic reaction; the second step is the INT then generates a product via a transition state (TS). This kind reaction has similar mechanism, when the silylene carbene and its derivatives [X₂Si = C: (X = H, F, Cl, CH₃)] and ethylene oxide close to each other, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X₂Si = C: gives a p → p donor–acceptor bond, thereby leading to the formation of INT. As the p → p donor–acceptor bond continues to strengthen (that is, the C? O bond continues to shorten), the INT generates product (P + C₂H₄) via TS. It is the substituent electronegativity, which mainly affects the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Fe or FeNO Catalysis? A Quantum Chemical Investigation of the [Fe(CO)3(NO)]−‐Catalyzed Cloke–Wilson Rearrangement

A quantum chemical investigation of the Bu₄N[Fe(CO)₃(NO)]‐catalyzed Cloke–Wilson rearrangement of vinyl cyclopropanes is reported. It was found that allylic C?C bond activation can proceed through a S_N2′ or S_N2‐type mechanism. The application of the recently reported intrinsic bond orbital (IBO) method for all structures indicated that one Fe?N π bond is directly involved. Further analysis showed that during the reaction oxidation occurs at the NO ligand exclusively.