Anodic initiation of the hydrogen sulfide reactions with the hydrocarbons of C n H2n?2 series

We examined the interaction of alkynes and alkadienes with hydrogen sulfide under the conditions of electrochemical initiation of the reaction. One-electron oxidation of hydrogen sulfide leads to the generation of reactive particles (thiyl radical and proton), which react with unsaturated hydrocarbons by two different routes. Hydrogen sulfide acts as a bifunctional reagent forming new C-S bonds, but also exhibits the properties of hydrogenating agent. Varying the experiment duration, reactants ratio, and solvent, it is possible to influence the composition and amount of the formed organic sulfur compounds.     

Probing the Regioselectivity with Encapsulated H2: Diels–Alder Reaction of an Open-Cage C60 Derivative with Anthracene

The regioselectivity in the Diels–Alder reaction of an unsymmetrical open-cage C₆₀ derivative with anthracene was studied. By using an encapsulated H₂ molecule as a magnetic probe, the product population was successfully evaluated in detail, indicating the formation of approximately ten compounds as major components. The nucleus-independent chemical shift (NICS) calculations showed a close resemblance to the observed ¹H NMR spectrum, which allowed for a facile characterization of the products. Theoretical studies on the formation of all 29 possible anthracene adducts were also performed. The results indicated that the regioselectivity is strongly governed by steric factors, values of the frontier orbital coefficients, and thermodynamic stabilities. Single-crystal X-ray analysis of the dominant compound revealed the supramolecular architecture between the anthracene moiety and the π-sphere of a neighboring molecule.     

Can quasiclassical trajectory calculations reproduce the extreme kinetic isotope effect observed in the muonic isotopologues of the H + H2 reaction?

Rate coefficients for the mass extreme isotopologues of the H + H(2) reaction, namely, Mu + H(2), where Mu is muonium, and Heμ + H(2), where Heμ is a He atom in which one of the electrons has been replaced by a negative muon, have been calculated in the 200-1000 K temperature range by means of accurate quantum mechanical (QM) and quasiclassical trajectory (QCT) calculations and compared with the experimental and theoretical results recently reported by Fleming et al. [Science 331, 448 (2011)]. The QCT calculations can reproduce the experimental and QM rate coefficients and kinetic isotope effect (KIE), k(Mu)(T)/k(Heμ)(T), if the Gaussian binning procedure (QCT-GB)--weighting the trajectories according to their proximity to the right quantal vibrational action--is applied. The analysis of the results shows that the large zero point energy of the MuH product is the key factor for the large KIE observed.     

RhIII-Catalyzed C6-Selective Oxidative C−H/C−H Crosscoupling of 2-Pyridones with Thiophenes

A rhodium(III)-catalyzed C6-selective dehydrogenative cross-coupling of 2-pyridones with thiophenes was developed for the synthesis of 6-thiophenyl pyridin-2(1H)-one derivatives. In this reaction, the excellent site selectivity was controlled by the 2-pyridyl directing group on the nitrogen of the pyridone ring. Control experiments indicated that the N-pyridyl was essential for the transformation. To the best of our knowledge, this procedure is the first successful example of the direct C6 heteroarylation of 2-pyridones with electron-rich thiophene derivatives. 4-Pyridone was also used as substrate to generate the corresponding C2 heteroarylated product. Moreover, this pyridyl directing group was readily removable to generate the biheteroaryl structures with a free N−H group.     

Recent progress in catalytic oxygenation of aromatic C–H groups with the environmentally benign oxidants H2O2 and O2

Developing efficient approaches for the selective oxygenation of aromatic C–H groups has been a challenging goal, interesting from both fundamental and practical perspectives. Designing catalyst systems for the direct production of phenol from benzene holds significant practical promise; on the contrary, novel strategies for the late-stage selective introduction of oxygen into aromatic rings of complex organic molecules could be useful for the synthesis of fine chemicals, including biologically active compounds. Another aspect of the overall picture is toughening environmental constraints, resulting in a steady increase in the number of catalyst systems relying on environmentally benign oxidants such as H₂O₂ and O₂. Overall, today, the realm of selective catalytic aromatic C–H oxidations is developing rapidly. This review summarizes the progress made in homo- and heterogeneous catalyst systems of aromatic C–H oxygenations with hydrogen peroxide and molecular oxygen, achieved in the last decade.     

Theoretical studies of the radical reaction H2CSiH2 + H

Ab initio molecular orbital calculations on the transition states and barrier heights for the addition of atomic hydrogen to silaethylene are carried out. The activation energy for the addition to the silicon site is lower than that to the carbon site, while the exothermicity is smaller.     

Can an OH radical form a strong hydrogen bond? A theoretical comparison with H2O

In this study, we apply UCCSD/6-31++G** to investigate the ability of an OH radical acting as a hydrogen bond acceptor with HF, HCl, and H(2)O (HO...HX; X=F, Cl, OH) or as a hydrogen bond donor with H(2)O and H(2)S (OH...XH(2); X=O and S). We also replace OH with H(2)O and make a fair comparison between them. Additionally, the counterpoise method (CP) has been used to examine the effect of basis set superposition error (BSSE). Our results reveal that OH is a stronger hydrogen bond donor but a weaker hydrogen bond acceptor than H(2)O. This conclusion is independent of the correction for BSSE and can be rationalized by the NBO analysis, the results of which indicate that OH radical has a lower n(O) and sigma*(O-H) in energy than that of H(2)O.     

Study on the reaction of C60 with DL-valine and cyclohexanone

Reaction of C60 with DL-valine and cyclohexanone in refluxing chlorobenzene under N2 yields three [60]fulleropyrrolidine derivatives (mono-, bis- and triadduct) which are characterized on the basis of spectroscopies and cyclic voltammetry. It is found that the ratio of the three products greatly depends on the reaction conditions.     

Rate constant for the reaction C2H5 + HBr → C2H6 + Br

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ?mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ～4 kJ?mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.     

Can 1,3-dimethylcyclobutadiene and carbon dioxide co-exist inside a supramolecular cavity?

The calculated free energy barrier at 175 K between 1,3-dimethylcyclobutadiene and carbon dioxide inside a calixarene host (ωB97XD/6-311G(d,p)+polarizable continuum solvent model) has the low value of ~8-10.5 kcal mol(-1). This value casts doubt on the recently claimed isolation and X-ray structure determination at 175 K of 1,3-dimethylcyclobutadiene and carbon dioxide as separate species inside such a cavity.     

Cocrystal Formation Precedes the Mechanochemically Acetate-Assisted C−H Activation with [Cp*RhCl2]2

This work reports the experimentally studied mechanochemical formation of rhodacycles by ball milling pyridine- and quinoline-derived substrates and [Cp*RhCl₂]₂ in the presence of NaOAc. Ex-situ analysis of the mechanochemical reactions using powder X-ray diffraction (PXRD), solid-state UV-vis spectroscopy and ATR-FTIR spectroscopy revealed the formation of unexpected cocrystals between the substrates and the rhodium dimer prior to the C−H activation step. This sequence of events differs from the generally accepted steps in solution in which cleavage of [Cp*RhCl₂]₂ is initiated by acetate ions. Additionally, the mechanochemical approach enabled the synthesis of the six-membered rhodacycle [Cp*Rh(2-benzilpyridine)Cl], a metal complex repeatedly reported as inaccessible in solution. Altogether, the results of this investigation clarify some of the fundamental aspects of mechanochemical cyclometallations.     

Plasma-catalytic Selective Reduction of NO with C2H4 in the Presence of Excess Oxygen

This paper reports observations of significant synergistic effects between dielectric barrier discharge (DBD) plasmas and Cu-ZSM-5 catalysts for C2H4 selective reduction of NOx at 250℃ in the presence of excess oxygen by using a one-stage plasma-over-catalyst (POC) reactor. With the reactant gas mixture of 530 ppm NO, 650ppm C2H4, 5.8% 02 in N2and GHSV = 12000h^-1, the pure catalytic, pure plasma-induced (discharges over fused silica pellets) and plasma-catalytic (in the POC reactor) NOx conversion are 39%, 1.5% and 79%, respectively. The in-situ optical emission spectra of the reactive systems imply some short-lived active species formed from plasma-induced and plasma-catalytic processes may be responsible to the observed synergistic effects in this one-stage POC system.     

C2H5C60H的加成产物的结构和光谱性质的量子化学研究

用INDO系列方法对C₂H₅C₆₀H的1,2-加成和1,4-加成两种产物异构体的结构进行了理论研究,结果表明1,2-C₂H₅C₆₀H具有Cs对称性,1,4-C₂H₅C₆₀H没有任何对称性,1,2-C₂H₅C₆₀H的总能量比1,4-C₂H₅C₆₀H的低。以此优化构型为基础,计算了两种产物异构体的电子吸收光谱,讨论了其光谱红移的原因,同时对产物的NMR谱进行了探讨。     

Site-Selective Electrochemical C−H Carboxylation of Arenes with CO2

Herein, a direct, metal-free, and site-selective electrochemical C−H carboxylation of arenes by reductive activation using CO₂ as the economic and abundant carboxylic source was reported. The electrocarboxylation was carried out in an operationally simple manner with high chemo- and regioselectivity, setting the stage for the challenging site-selective C−H carboxylation of unactivated (hetero)arenes. The robust nature of the electrochemical strategy was reflected by a broad scope of substrates with excellent atom economy and unique selectivity. Notably, the direct and selective C−H carboxylation of various challenging arenes worked well in this approach, including electron-deficient naphthalenes, pyridines, simple phenyl derivatives, and substituted quinolines. The method benefits from being externally catalyst-free, metal-free and base-free, which makes it extremely attractive for potential applications.     

Termolecular ion-molecule reactions in Titan’s atmosphere. II: The structure of the association adducts of HCNH+ with C2H2 and C2H4

The ion-molecule reactivity of the products formed in the association reactions of HCNH+ with C2H2 (C3H4N+) and C2H4 (C3H6N+) has been investigated to provide information on the structures of the adducts thus formed. The C3H4N+ and C3H6N+ adducts were formed in the reaction flow tube of a flowing afterglow sourced-selected ion flow tube (FA-SIFT) and their reactivity with a neutral molecular "probe" examined. The reactivity of possible known structural isomers for the C3H4N+ and C3H6N+ ions was investigated in both the FA-SIFT and an ion cyclotron resonance spectrometer (ICR). Ab initio investigations of the potential energy surfaces for both structures at the G2(MP2) level have also been performed and structures corresponding to local minima on both surfaces have been identified and evaluated. The results of these experimental and theoretical studies show that at room temperature, the C3H4N+ adduct ion contains two isomers; a less reactive one that is likely to be a four-membered cyclic covalent isomer (approximately 70%) and a faster reacting component that is probably an electrostatic complex (approximately 30%). The C3H6N+ adduct ion formed from HCNH+ + C2H4 at room temperature is a single isomer that is likely to be the four-membered covalently bound cyclic CH2CH2CHNH+ species.     

Infrared spectra of Rh atom reaction products with C2H2: the HRhCCH, RhCCH, RhCCH2, and Rh-η2-C2H2 molecules

Laser-ablated Rh atoms react with C(2)H(2) upon co-condensation in excess argon and neon to form the insertion product HRhCCH, the alkyne RhCCH, the vinylidene RhCCH(2), and the metallacycle complex Rh-η(2)-(C(2)H(2)). These species are identified through (13)C(2)H(2), C(2)D(2), and mixed C(2)HD isotopic substitutions and density functional theory isotopic frequency calculations. The HRhCCH molecule is characterized by the CH stretching mode at 3306.2 cm(-1) (Ar) and 3310.9 cm(-1) (Ne), the Rh-H stretching mode at 2090.8 cm(-1) (Ar) and 2111.0 cm(-1) (Ne), and two CCH deformation modes at 584.3 and 573.3 cm(-1) (Ar) and 587.1 and 580.3 cm(-1) (Ne). The absorptions for the vinylidene RhCCH(2) complex are observed at 3150.9 (Ar), 3147.2 (Ne) (CH stretching), 1690.1 (Ar), 1694.3 (Ne) (CC stretching), and 804.9 (Ar), 810.5 cm(-1) (Ne) (CCH deformation). The metallacycle Rh-η(2)-(C(2)H(2)) complex is also identified through CC stretching and CCH deformation modes. The insertion reaction of ground Rh atom to the C-H bond is spontaneous on the basis of the growth of HRhCCH absorptions upon annealing in both solid neon and argon. Here, we show that atomic Rh can convert acetylene to the simple Rh vinylidene complex, analogous to that found for ligand-supported Rh complexes.     

Quantum instanton calculation of rate constants for the C2H6 + H → C2H5 + H2 reaction: anharmonicity and kinetic isotope effects

Thermal rate constants and kinetic isotope effects for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The obtained results are in good agreement with experimental measurements at high temperatures. The detailed investigation reveals that the anharmonicity of the hindered internal rotation motion does not influence the rate too much compared to its harmonic oscillator approximation. However, the motion of the nonreactive methyl group in C(2)H(6) significantly enhances the rates compared to its rigid case, which makes conventional reduced-dimensionality calculations a challenge. In addition, the temperature dependence of kinetic isotope effects is also revealed.     

Activation of H2O2 by methyltrioxorhenium(VII) inside β-cyclodextrin

The effect of β-cyclodextrin on the catalytic stability and reactivity of methylrhenium trioxide (MTO), CH₃ReO₃, which has been used for activation of hydrogen peroxide toward oxidation and epoxidation reactions, was studied using UV–Vis spectrophotometery. The stability and reactivity of the new catalytic system (MTO/β-CD) to activate H₂O₂ toward oxidation of indigo blue dye were investigated in basic media. Furthermore, effects of inclusion stoichiometry, temperature and concentrations of hydrogen peroxide on the stability and reactivity of the MTO/β-CD system were investigated. The formation of the inclusion complex between MTO and β-CD was confirmed experimentally using the changes in the UV–Vis absorption spectra. The results of this study demonstrate that the complexation process was better guaranteed when the amount of β-CD is higher than that of MTO, using a 1:2 molar ratio of MTO:β-CD enhances both the activity and stability of the catalyst. The results showed that the stability of the catalytic system was enhanced in presence of β-CD with maintaining good reactivity of the MTO even in the presence of high concentration of NaOH. The changes of thermodynamic activation parameters (ΔH^≠ and ΔS^≠) for the oxidation reaction of indigo with H₂O₂ catalyzed by MTO/β-CD system were determined on the basis of the experimental data.