Mechanism Of Oxidative Coupling Research Articles

Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations sho...

Enantioselective Oxidative Homocoupling and Cross-Coupling of 2-Naphthols Catalyzed by Chiral Iron Phosphate Complexes.

Novel chiral iron phosphate complexes were prepared as catalysts for asymmetric oxidative coupling reactions. These catalysts were applied for the synthesis of enantio-enriched C1- and C2-symmetric BINOLs, in which the 3 and 3' positions are available for chemical modifications. It was proposed that the reaction takes place via an oxidative radical-anion coupling mechanism. A destructive BINOL racemization that competes with the enantioselective oxidative coupling of 2-naphthols was revealed, thereby offering new insights into this highly important reaction.

Catalytic consequences of cation and anion substitutions on rate and mechanism of oxidative coupling of methane over hydroxyapatite catalysts

The identity and rate constants of elementary steps in primary reactions of oxidative coupling of methane (OCM) over Pb2+ and/or F− substituted hydroxyapatite (HAP, Pb-HAP, HAP-F, and Pb-HAP-F) catalysts have been studied. The rigorous kinetics analysis suggests that HAP and HAP-F initiated the reaction between adsorbed methane and O2 following Langmuir-Hinshelwood behavior. The Pb-HAP and Pb-HAP-F, however, enabled the reaction between gaseous methane and adsorbed O2 in the Eley-Rideal mechanism. The F− substitution of OH− weakened both O2 adsorption and CH bond activation, leading to low methane conversion and slightly higher C2H6 selectivity. The substitution of Ca2+ by Pb2+ weakened both methane and oxygen adsorptions, but maintained CH bond activation and raised C2H6 selectivity. The present analysis explored for the first time the effects of cation and/or anion in HAP on OCM reactions in which the analysis has been detailed and quantified in the nature of the mechanism-based kinetic models.

Theoretical study of coupling p-aminothiophenol to hydroazo- and azo-adducts on Au(111).

Aminothiophenol/Au(111) has been adopted as an exemplary model in plasmonics research, including surface-enhanced Raman spectroscopy, due to its high plasmonic-induced spectral-signal enhancement. The present work was aimed at clarifying whether aminothiophenol on Au(111) is chemically stable in the absence of any photo- and plasmonic-induced effects. Briefly, first-principles calculations were employed to track the detailed mechanism of oxidative coupling of p-aminothiophenol (PATP) to its azo-adduct with an N = N bond, i.e., p,p'-dimercaptoazobenzene (DMAB). Our results show the following: first, in the presence of adsorbed O2, PATP fractures its N-H bond and transfers the hydrogen to a nearby oxygen. This pathway is more favorable than the transfer of H to Au, but the activation barrier of 0.9eV is still too high for the reaction to occur in the absence of thermal-, photo-, or plasmonic-activation. If this bar can be lifted, two such dehydrogenated PATP can couple themselves to form an adduct with a N-N bond, i.e., p,p'-dimercaptohydroazobenzene (DMHAB), and this reaction is exoergic with an energy barrier of 0.57eV. Again, this step is slow in the absence of moderate thermal activation or photo-/plasmonic-activation. Finally, dehydrogenation of DMHAB gives the azo-adduct of DMAB, and this reaction is spontaneous, with no energy barrier.

Electronic Origins of the Variable Efficiency of Room-Temperature Methane Activation by Homo- and Heteronuclear Cluster Oxide Cations [XYO2](+) (X, Y = Al, Si, Mg): Competition between Proton-Coupled Electron Transfer and Hydrogen-Atom Transfer.

The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](•+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](•+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.

ChemInform Abstract: Oxidative Coupling Reactions at Ruthenium(0) and Their Applications to Catalytic Homo‐ and Cross‐Dimerizations

AbstractReview: 164 refs.

ChemInform Abstract: TBAI‐Catalyzed Synthesis of α‐Ketoamides via sp3 C—H Radical/Radical Cross‐Coupling and Domino Aerobic Oxidation.

Abstract A TBAI-catalyzed one-pot synthesis of α-ketoamides via sp 3 C–H radical/radical cross-coupling and domino aerobic oxidation was developed. This synthesis is suitable for abroad range of substrates. The control experiments suggested a possible oxidative coupling mechanism.

Mechanistic insights into catalytic linear cross-dimerization between conjugated dienes and styrenes by a ruthenium(0) complex

The mechanistic studies for linear cross-dimerization between 2,3-dimethylbuta-1,3-diene and styrene by a Ru(0) complex, Ru(η6-naphthalene)(η4-1,5-COD) (1), are performed both by kinetic and computational studies. This reaction is basically zero-order to both of the diene and styrene concentrations and first-order to the catalyst concentration. The Hammett plot using p-substituted styrenes gives a linear relationship with a positive slope (ρ = +0.482). The deuterium isotope experiment clearly shows the present reaction being a formal 1,4-addition of a C–H bond in styrene to cisoid-1,3-diene. These kinetic studies show the reaction proceeding via oxidative coupling mechanism that is also supported by the DFT calculations.

TBAI-catalyzed synthesis of α-ketoamides via sp3 C–H radical/radical cross-coupling and domino aerobic oxidation

A TBAI-catalyzed one-pot synthesis of α-ketoamides via sp3 C–H radical/radical cross-coupling and domino aerobic oxidation was developed. This synthesis is suitable for abroad range of substrates. The control experiments suggested a possible oxidative coupling mechanism.

Insights into the reaction mechanism of CO oxidative coupling to dimethyl oxalate over palladium: a combined DFT and IR study.

Oxidative coupling of toxic pollutant CO to form the platform raw chemical material dimethyl oxalate (DMO) has been industrialized however the catalytic mechanism has been unknown so far. The reaction mechanism of CO oxidative coupling to form DMO on a Pd(111) surface has been investigated using density functional theory (DFT) and in situ diffuse reflectance infrared (DRIR) spectroscopy. DFT calculations and in situ DRIRS measurements indicate that two co-adsorbed intermediates COOMe and OCCO, initiate the reaction. C-C coupling occurs earlier due to a low coupling barrier and small steric hindrance. The results also suggest that Pd(111) is selective towards DMO over DMC, and that CO pre-adsorption and CO in excess effectively enhance the yield of DMO. The microscopic elucidation of this important reaction suggests improvements in coal-to-EG (CTEG) production which can be applied in practice to effectively enhance the yield and reduce the cost. The results may help with further fine-tuning and designing of high-efficient noble metal catalysts.

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO2

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H2C═CHAr using CO2 to form α-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO2 and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the Ni–H bond followed by reductive elimination produces the α-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative cou...

Electronic descriptors for analytical use of the benzidine‐based compounds and the mechanism of oxidative coupling of anilines

A number of hybrid exchange‐correlation functionals, namely, B3LYP, BMK, M06‐2X, PBE0, CAM‐B3LYP, BH&HLYP, and HSE06, have been employed to calculate the electronic spectra of a series of the N,N′‐derivatives of benzidine, their radical cations, and dications; the results are compared with available experimental data. In most cases studied, the time‐dependent density functional theory results based on the CAM‐B3LYP and BMK functionals indicate a better agreement with the experimental absorption bands in the UV–Vis spectra. The other five functionals provide statistically comparable but lower accuracy. On the basis of the time‐dependent density functional theory calculations with both the CAM‐B3LYP and BMK functionals and the 6‐311 + G(d,p) basis set including effect of polar medium simulation, the electronic spectrum of the benzidine dication has been completely assigned. Because the latter dication is formed during oxidative coupling of two aniline molecules, as the reductive reagents for analytical determination of strong oxidants, the mechanism of this reaction has been clarified on the ground of density functional theory calculations including effect of dispersion for all possible reaction paths and intermediates. By the analysis of spectral properties and ionization energies of several aniline derivatives, a few simple electronic descriptors that help to justify a reagent molecule as a potentially favorable for colorimetric use have been proposed. Copyright © 2014 John Wiley & Sons, Ltd.

Controllable synthesis of bis(1,2,3-triazole)s and 5-alkynyl-triazoles via temperature effect on copper-catalyzed Huisgen cycloaddition

A facile synthetic protocol has been developed for the controllable preparation of bis(1,2,3-triazole)s and 5-alkynyl-1,2,3-triazoles from alkyne and azide under different temperatures. Various azides and alkynes were used as substrates for the reactions and the successful applications in nucleoside analogues manifested the values of this method in syntheses of bioactive molecules. Besides, a possible temperature-guided triazolyl–copper intermediate aerobic oxidative coupling mechanism was proposed for this controllable reaction procedure.

Asymmetric Cross-Dimerization between Methyl Methacrylate and Substituted Alkene by Ru(0)–Bicyclononadiene Complex

New Ru(0)-naphthalene complexes containing a bicyclononadiene ligand catalyze the linear cross-dimerization between methyl methacrylate and substituted alkenes by an oxidative coupling mechanism. The chiral (S,S)-2-methylbicyclo[3.3.1]nona-2,6-diene complex (S,S)-1b catalyzes asymmetric linear cross-dimerization between methyl methacrylate and 2,5-dihydrofuran to give the cross-dimer in 74% yield in 80% ee.

Stoichiometric and Catalytic Cross Dimerization between Conjugated Dienes and Conjugated Carbonyls by a Ruthenium(0) Complex: Straightforward Access to Unsaturated Carbonyl Compounds by an Oxidative Coupling Mechanism

A series of stoichiometric and catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are studied. The reaction of Ru(η4-cisoid-1,3-butadiene)(η4-1,5-COD)(NCMe) (2a) with methyl acrylate gives a Ru(0) complex, Ru[methyl η4-cisoid-(2E,4E)-hepta-2,4-dienoate](η4-1,5-COD)(NCMe) (3aa) in 97% yield. Similar treatments of 2a with a series of tert-butyl acrylate, methyl crotonate, 3-buten-2-one, and N,N-dimethylacrylamide produce similar analogues of 3ac. When (E)-1,3-pentadiene complex 2d is employed in the reaction with methyl acrylate, the branched coupling product Ru[methyl η4-cisoid-(2E,4E)-4-methylhepta-2,4-dienoate](η4-1,5-COD)(NCMe) (3da-b) is dominantly obtained in 65% yield along with the linear product in 19% yield. In the case of the (E)-2,5-dimethylhexa-1,3-diene complex 2e, the corresponding branch product is exclusively obtained in 86% yield. The catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are established by 2. The origin of the presen...

Assessment of the abiotic transformation of 17β-estradiol in the presence of vegetable matter – II: The role of molecular oxygen

This study characterizes the effect of oxygen in the abiotic transformation of estrogens when they are contacted with a surrogate of the vegetable wastes found in sewage. 17β-Estradiol (E2) and 17β-14C4-estradiol (14C-E2) were utilized as model compounds. Batch experiments were run under both oxic and anoxic conditions. In order to accomplish an accurate mass balance of the target estrogen, two analyses were performed simultaneously: first, radioactivity counting, and second, quantitation of E2 and 14C-E2, as well as their transformation product estrone and 14C4-estrone, by Liquid Chromatography tandem Mass Spectrometry. Under oxic conditions, the total concentration of 14C-E2 was found to decrease by 78% in 72h (15% and 7% remained in the liquid and solid phases, respectively). Conversely, when the estrogens were contacted with the synthetic influent under anoxic conditions, E2 was quantitatively recovered after 72h (70% and 22% in aqueous and solid matrices, correspondingly). These results suggest that when the concentration of dissolved oxygen is null or limited, catalysis through an oxidative coupling mechanism is halted. Moreover, it was confirmed that the catalytic reaction occurred solely in the presence of the solid phase of the model vegetable matter.

0価ルテニウム錯体による置換アルケンおよびジエン類の触媒的鎖状カップリング反応：酸化的カップリング機構による化学，位置およびジアステレオ選択的炭素-炭素結合形成反応

0価ルテニウム錯体による置換アルケンおよびジエン類の触媒的鎖状カップリング反応：酸化的カップリング機構による化学，位置およびジアステレオ選択的炭素-炭素結合形成反応

Oxidative coupling of alcohols on gold: Insights from experiments and theory

Molecular level understanding of the mechanism of oxidative coupling of alcohols on metallic Au(111) activated by oxygen is achieved through a combination of experiments and theoretical calculations. The facility of the beta-H elimination of the alkoxys, which increases with the length of the alkyl chain, is identified to be critical in determining the product distributions. Dioxymethylene serves as a formaldehyde reservoir in the cross-coupling reaction between methanol and formaldehyde through its reversible formation and decomposition, contributing to the high selectivity for the coupling products.

Isolation of trans-2,5-Bis(methoxycarbonyl)ruthenacyclopentane by Oxidative Coupling of Methyl Acrylate on Ruthenium(0) as an Active Intermediate for Tail-to-Tail Selective Catalytic Dimerization

The trans-bis(methoxycarbonyl)ruthenacyclopentane complex [trans-Ru{C2H{C(O1)OMe}C3H2C4H2C5H{C(O2)OMe}-κ1C2,κ1O1,κ1C5,μ-κ1O2}(η4-1,5-COD)]2 (2) and the acetonitrile adduct Ru[C2H(CO2Me)CH2CH2C5H(CO2Me)-κ1C2,κ1C5](η4-1,5-COD)(NCMe)2 (3) have been obtained from the reaction of Ru(η6-naphthalene)(η4-1,5-COD) (1) with methyl acrylate and are active for the Ru(0)-catalyzed dimerization of methyl acrylate. Thus, complex 3 catalyzes the tail-to-tail dimerization of methyl acrylate in 52% yield at 140 °C in THF, suggesting that this dimerization proceeds by the oxidative coupling mechanism.

In muro feruloylation and oxidative coupling in monocots

Recently we have suggested that (glucurono)arabinoxylans [(G)AX] feruloylation and oxidative coupling occur both intra-protoplasmically and, extra-protoplasmically, in the plant cell wall. In this work we illustrate a model of two possible mechanisms of polysaccharide feruloylation and oxidative coupling in plants. Moreover, we take into consideration the possible role of in muro feruloylation as a rapid defense mechanism against potential plant pathogen and parasite infections.