Enolate Species Research Articles

Nontraditional Aldol Condensation Performance of Highly Efficient and Reusable Cs+ Single Sites in β-Zeolite Channels.

Aldol reactions (self- and cross-aldol condensations) for conjugated enone synthesis were efficiently performed on large-sized Cs+ single sites (1 wt %) confined in β-zeolite channels in toluene, which showed the highest level of catalytic aldol condensation activity among reported zeolite catalysts. In general, aldol condensation reactions for C-C bond synthesis can proceed by acids (e.g., H+), bases (e.g., OH-), enolate species, and acidic or basic solid catalysts. However, the Cs+ single site/β sample without significant acid-base property showed unprecedented, efficient, and reusable catalysis for self-aldol and cross-aldol condensations. Intrinsically inactive Cs+ single sites due to the noble-gas electronic structure were transformed to active Cs+ single sites in β-zeolite channels. Cs+/β has many advantages such as broad substrate scope, eco-friendliness, high product selectivity and yield, and simple work-up procedure. Thus, the Cs+ single site/β provides an attractive and useful methodology for practical C-C bond synthesis. On the basis of the Cs+/β characterization by X-ray photoelectron spectroscopy (XPS), in situ X-ray absorption fine structure (XAFS) (X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)), and temperature-programmed desorption (TPD), density functional theory (DFT) calculations of the self- and cross-aldol condensation reaction pathways involving the transition states on the Cs+ single site in β-zeolite channel revealed nontraditional concerted interligand bond rearrangement mechanisms.

High-Temperature Catalytic Oxidation of 1,2-Dichlorethane: An Alternative Applicable Method for Safe Elimination of CVOCs

The application of catalytic removal of chlorinated volatile organic compounds (CVOCs) is restricted by the problems of catalyst deactivation and toxic byproducts. In order to alleviate these problems, the effects of high reaction temperature on the catalytic oxidation of CVOCs are explored by employing thermostable catalysts. Transition metal (Mn, Cu, and Fe)-substituted thermostable hexaaluminate catalysts are prepared and used in the catalytic oxidation of 1,2-dichloroethane (DCE). Mn-substitution and Ru-loading could induce rich active oxygen species and promote reducibility, oxygen mobility, and surface acidity, leading to the improved reaction performance consequently. The inhibition effect on generating chlorinated byproducts by elevating the reaction temperature and the influence of rising temperature on the reaction process are investigated. Catalytic oxidation of DCE over LaMn3 or Ru/LaMn3 at the high-temperature region (over 500 °C) could convert the pollutant into small inorganic molecules (CO2, H2O, HCl, etc.) with scarce chlorinated byproducts. Meanwhile, Ru/LaMn3 presents an excellent structural stability and steady reaction activity at 650 °C. Additionally, DCE oxidation occurs via the pathway of enolic species, vinyl chloride–acetaldehyde–acetate species over LaMn3 and Ru/LaMn3. These results suggest that the high-temperature catalytic oxidation of CVOCs might be a practicable method for safe elimination of CVOCs.

Dopamine Adsorption on Rutile TiO2(110): Geometry, Thermodynamics, and Core-Level Shifts from First Principles.

The modification of the rutile TiO2(110) surface with dopamine represents the best example of the functionalization of TiO2-based nanoparticles with catecholamines, which is of great interest for sunlight harvesting and drug delivery. However, there is little information on the dopamine–TiO2(110) adsorption complex in terms of thermodynamic properties and structural parameters such as bond coordination and orientation of the terminal ethyl–amino group. Here, we report a density functional theory (DFT) investigation of dopamine adsorption on the TiO2(110) surface using the optB86b-vdW functional with a Hubbard-type correction to the Ti 3d orbitals, where Ueff = 3 eV. Guided by available X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) data, our simulations identify enolate species with bidentate coordination at a submonolayer coverage, which are bonded to two neighboring 5-fold-coordinated Ti atoms at the TiO2(110) surface through both deprotonated oxygen atoms of the dopamine, i.e., in a bridging fashion. The process is highly exothermic, involving an adsorption energy of −2.90 eV. Calculated structural parameters suggest that the molecule sits approximately upright on the surface with the amino group interacting with the π-like orbitals of the aromatic ring, leading to a gauche-like configuration. The resulting NH···π hydrogen bond in this configuration can be broken by overcoming an energy barrier of 0.22 eV; in this way, the amino group rotation leads to an anti-like conformation, making this terminal group able to bind to other biomolecules. This mechanism is endothermic by 0.07 eV. Comparison of existing spectroscopic data with DFT modeling shows that our computational setup can reproduce most experimentally determined parameters such as tilt angles from NEXAFS and chemical shifts in XPS, which allows us to identify the preferred mode of adsorption of dopamine on the TiO2(110) surface.

A π–Cu(II)−π Complex as an Extremely Active Catalyst for Enantioselective α-Halogenation of N-Acyl-3,5-dimethylpyrazoles

Novel chiral π–copper(II)−π complex catalyzed enantioselective α-chlorination and -bromination of N-acyl-3,5-dimethylpyrazoles are described. The π–copper(II)−π complexation of Cu(OTf)2 with 3-(2-naphthyl)-l-alanine-derived amides greatly increases the Lewis acidity and triggers the in situ generation of enolate species without an external base, which has a suppressing effect for α-chlorination and -bromination due to undesired halogen bonding. This strategy provides facile access to α-halogenated compounds in high yield with excellent enantioselectivity. X-ray crystallographic and ESR analyses of the catalyst complexes suggest that the release of two counteranions (2TfO–) from the copper(II) center might be crucial for the efficient activation of N-acyl-3,5-dimethylpyrazoles.

New aspects on a low-medium temperature mechanism of H2-assisted C3H6-SCR over xAg-CeZr catalyst

The spherical xAg-CeZr catalysts were prepared by a continuous solvothermal-impregnation method and applied to H2-assisted C3H6-SCR. Adding H2 significantly promoted NO conversion between 150 and 500 °C. The outstanding NO conversion and N2 selectivity over 2%Ag-CeZr at 200 °C achieved 71.7% and 45.3%, and its C3H6 conversion rate was significantly improved. Multi-technology characterizations results showed that active Ag+, Ce3+ and Oα species were important factors for the excellent activity in H2-C3H6-SCR. The in situ FT-IR analysis indicated that NO adsorption capacity, the reactivity of adsorbed NO species (monodentate nitrate, bidentate nitrate and monodentate nitrite) and CxHyOz species (enolic species, formate, carboxylate), and the formation of crucial intermediates were remarkably improved on account of adding H2, further promoting the SCR catalytic performance. Different sequences of H2 admission did affect its promotion mechanism to the reaction system. We proposed a main pathway for H2-C3H6-SCR over Ag-CeZr: NO + C3H6 + H2 + O2 → NO2–/NO3– + CxHyOz → ONCs → –NCO + –CN → N2 + CO2 + H2O. The present work confirmed that H2 would enhance the C3H6-SCR performance through different pathways over spherical Ag-CeZr catalysts.

Reactive interaction of isopropanol with Co3O4(1 1 1) and Pt/Co3O4(1 1 1) model catalysts

The structure and chemical composition of the catalyst may change under reaction conditions resulting in changes of the active sites and, thereby, loss of selectivity. In this work, we investigated the reactive interaction of isopropanol with well-defined Co3O4(111)/Ir(100) and Pt/Co3O4(111)/Ir(100) model catalysts by means of synchrotron radiation photoelectron spectroscopy (SRPES), near ambient pressure X-ray photoelectron spectroscopy (NAP−XPS), scanning tunneling microscopy (STM), and temperature programmed desorption (TPD). We found that adsorption at 150 K yields molecularly adsorbed isopropanol on both model catalysts accompanied by small fractions of isopropoxide and enolate species on Co3O4(111) and supported Pt nanoparticles, respectively. The reactive interaction of isopropanol-derived species with the model catalysts upon annealing in UHV and in 1×10-7 mbar and 1 mbar of isopropanol results in the reduction of Co3O4(111) followed by its conversion to CoO(111) and, finally, to metallic Co and Pt–Co alloy. The mechanism of isopropanol decomposition reveals remarkable sensitivity to the oxidation state and morphology of the model catalysts. On as-prepared Pt/Co3O4(111) catalyst, Pt particles densely cover the Co3O4(111) substrate steering isopropanol decomposition to acetone and hydrogen. Selective channels towards acetone and propene, both accompanied by water, open after temperature-driven sintering of the supported Pt particles. The selectivity of these channels is controlled by the degree of reduction of the Co3O4(111) substrate and the chemical composition of the supported nanoparticles. At high degrees of reduction of Co3O4(111), the formation of both propene and acetone through selective channels decline due to the strong preference for C–C bond scission. Under NAP conditions, the formation of acetone resumes after complete reduction of model catalysts to metallic Co and Pt–Co alloy.

Gold-Catalyzed Oxidative Aminocyclizations of Propargyl Alcohols and Propargyl Amines to Form Two Distinct Azacyclic Products: Carbene Formation versus a 3,3-Sigmatropic Shift of an Initial Intermediate.

Gold-catalyzed oxidations of propargyl alcohols with nitrones by using a P(tBu)2 (o-biphenyl)Au+ catalyst, afforded bicyclic annulation products from the Mannich reactions of gold enolates. The same reactions of propargyl amines with nitrones by using the same gold catalyst gave distinct oxoarylation products. Our DFT calculations indicate that oxidation of propargyl alcohols with nitrones by using electron-rich gold catalysts lead only to gold carbenes, which can generate gold enolates or oxoarylation intermediates with enolate species having a barrier smaller than that of oxoarylation species.

Investigation of Water and Sulfur Tolerance of Precipitable Silver Compound Ag/Al2O3 Catalysts in H2-Assisted C3H6-SCR of NOx.

Ag/Al2O3 catalysts containing different precipitable silver compounds (AgCl, Ag2SO4, and Ag3PO4) were synthesized and investigated for NOx reduction in H2-assisted C3H6-selective catalytic reduction (SCR). The samples were systematically characterized by N2 adsorption, X-ray diffraction (XRD), UV–Vis, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM). N2 adsorption revealed that the introduction of anions (Cl–, SO42–, and PO43–) did not significantly affect the surface and structural properties of the Al2O3 support. However, XRD patterns and HR-TEM images indicated that the addition of Cl– anions caused the agglomeration of silver species to form large AgCl particles on the AgCl/Al2O3 catalysts. In contrast, the silver species dispersed well on Ag2SO4/Al2O3 and Ag3PO4/Al2O3 catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that partial oxidation of C3H6 on Ag2SO4/Al2O3 produced large amounts of reactive enolic species, while it tended to yield inert formate on AgCl/Al2O3. As a result, Ag2SO4/Al2O3 catalysts, especially 3% Ag2SO4/Al2O3, exhibited superior water and sulfur tolerance in H2-assisted C3H6-SCR.

Blue Light-Promoted Formal [4+1]-Annulation of Diazoacetates with o-Aminoacetophenones: Synthesis of Polysubstituted Indolines and Computational Study.

A blue light-promoted formal [4+1]-annulation of diazoacetates with o-aminoacetophenones has been reported, which provides an environmentally friendly method for the synthesis of polysubstituted indoline derivatives in moderate to good yields with excellent diastereoselectivities. Detailed mechanistic studies through density functional theory calculations reveal that the (E)-enol species is the key intermediate in this transformation, and the excellent diastereoselectivity is enabled via H-bonding in the intramolecular Aldol-type addition.

Catalytic combustion of CVOCs over CrxTi1-x oxide catalysts

CrxTi1-x oxide catalysts prepared by a sol–gel process were investigated in catalytic combustion of 1,2-dichloroethane, vinyl chloride, chlorobenzene, 1,2-dichlorobenzene, and trichloroethylene. FT-IR spectra on Cr0.1Ti0.9 showed that 1,2-dichloroethane, vinyl chloride, and chlorobenzene adsorbed through the hydrogen bond between TiOH and Cl atoms and then interacted with surface Cr6+=O in Cr2O72− species to form a reactive enolic species, which was further oxidized with surface oxygen. The activity was greatly dependent on Cr6+ as both a strong Lewis acid and reducible sites. TOFCr6+ at 200 °C was 0.59–1.12 × 10−3 s−1. Chlorination products were not observed with the conversion approaching completion below 300 °C. The activity on a stream feed containing 1000 ppm reactants at 245 and 285 °C was maintained for at least 100 h. The formation of Cr-O-Ti effectively inhibited the escape of Cr as Cl2CrO2. It was confirmed by physically closed contact of Cr2O3 and TiO2 that a synergism between Cr6+ and Ti-OH was responsible for the high activity and stability of CrxTi1-x catalysts.

Effect of initial γ-irradiation on infrared laser ablation of poly(vinyl alcohol) studied by infrared spectroscopy

The effect of initial γ-irradiation of poly(vinyl alcohol) (PVA) on the kinetics of polymer ablation under CO2 laser irradiation was studied using infra-red spectroscopy. The rate of laser ablation rapidly decreases with increasing time of laser ablation up to 10 s. Initial γ-irradiation of a dose up to 100 kGy reduces the rate of the laser ablation, just like the effect of laser exposure. The number of OH and CH groups decrease in PVA both with laser irradiation and with γ-irradiation. The number of OH groups decreases more under laser irradiation. Unsaturated bonds are formed in γ–irradiated PVA on the surface of the crater formed by laser ablation of the polymer that has undergone radiolysis. Increasing the dose of initial γ-irradiation from 100 to 2300 kGy leads to an increase in the intensity of the IR bands of the unsaturated bonds. The number of IR bands in the spectra of the laser ablation crater of PVA does not change with the dose of initial γ-irradiation. A dose of radiolysis of 3500 kGy significantly changes the IR spectra of both the ablation crater and the coating formed from the ablation products. Many bands characteristic for PVA are no longer present and the intensities of bands corresponding to unsaturated bonds are increased. A computational model for loss of water and ketone formation was developed for PVA and PVA including a diol using G3(MP2). The highest activation energy barrier step in the decomposition reaction was formation of the enol species and water was shown to catalyze all decomposition steps.

Atom-Condensed Fukui Function in Condensed Phases and Biological Systems and Its Application to Enzymatic Fixation of Carbon Dioxide.

Local reactivity descriptors such as atom-condensed Fukui functions are promising computational tools to study chemical reactivity at specific sites within a molecule. Their applications have been mainly focused on isolated molecules in their most stable conformation without considering the effects of the surroundings. Here we propose to combine quantum mechanics/molecular mechanics Born-Oppenheimer molecular dynamics simulations to obtain the microstates (configurations) of a molecular system using different representations of the molecular environment and calculate Boltzmann-weighted atom-condensed local reactivity descriptors based on conceptual density functional theory. Our approach takes the conformational fluctuations of the molecular system and the polarization of its electron density by the environment into account, allowing us to analyze the effect of the molecular environment on reactivity. In this contribution, we apply the method mentioned above to the catalytic fixation of carbon dioxide by crotonyl-CoA carboxylase/reductase and study if the enzyme alters the reactivity of its substrate compared with an aqueous solution. Our main result is that the protein environment activates the substrate by the elimination of solute-solvent hydrogen bonds from aqueous solution in the two elementary steps of the reaction mechanism: the nucleophilic attack of a hydride anion from NADPH on the α,β-unsaturated thioester and the electrophilic attack of carbon dioxide on the formed enolate species.

Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure-Reactivity Relationship.

Benz(o)hydrazide (BH) is the basic aryl hydrazide; aryl hydrazides have been pursued in the course of drug discovery. Oxidations of BH and phenylacetic hydrazide (PAH) by hexachloroiridate(IV) ([IrCl6]2−) were investigated by use of stopped-flow spectral, rapid spectral scan, RP-HPLC and NMR spectroscopic techniques. The oxidation reactions followed well-defined second-order kinetics and the observed second-order rate constant k′ versus pH profiles were established over a wide pH range. Product analysis revealed that BH and PAH were cleanly oxidized to benzoic acid and phenylacetic acid, respectively. A reaction mechanism was proposed, resembling those suggested previously for the oxidations of isoniazid (INH) and nicotinic hydrazide (NH) by [IrCl6]2−. Rate constants of the rate-determining steps were evaluated, confirming a huge reactivity span of the protolysis species observed previously. The enolate species of BH is extremely reactive towards reduction of [IrCl6]2−. The determined middle-ranged negative values of activation entropies together with rapid scan spectra manifest that an outer-sphere electron transfer is probably taking place in the rate-determining steps. The reactivity of neutral species of hydrazides is clearly not correlated to the corresponding pKa values of the hydrazides. On the other hand, a linear correlation, logkenolate = (0.16 ± 0.07)pKenol + (6.1 ± 0.8), is found for the aryl hydrazides studied so far. The big intercept and the small slope of this correlation may pave a way for a rational design of new antioxidants based on aryl hydrazides. The present work also provides the pKa values for BH and PAH at 25.0 °C and 1.0 M ionic strength which were not reported before.

Hetero diacylation of 1,1-diborylalkanes: Practical synthesis of 1,3-diketones

An efficient protocol for the synthesis of asymmetric 1,3-diketones was reported through diacylation of 1,1-diborylalkanes using two different acyl sources. In this transformation, an enolate boron species was initially formed by introducing an acyl group, then it was trapped by another acyl group to form 1,3-diketone. This method not only provided the gateway to obtain a series of 1,3-diketones, but also afforded an operationally simple and efficient access to pyrazoles and isoxazoles.

Selective catalytic reduction of NOx with ethanol and other C1–4 oxygenates over Ag/Al2O3 catalysts: A review

Research results regarding selective catalytic reduction (SCR) of NOx with ethanol and other C1-4 oxygenates as reductants over silver-alumina catalysts are summarized. The aspects of the process mechanism, nature of active sites, role of alumina and silver (especially in the formation of bifunctional active sites), effects of reductants and reaction conditions are discussed. It has been determined that key stages of the process include formation of reactive enolic species, their interaction with NOx and formation of nitroorganic compounds which transform to NCOads species and further to N2. The results obtained over various silver-alumina catalysts demonstrate the perspectives of their application for reducing the level of nitrogen oxides in engine emissions, including in the presence of water vapor and sulfur oxides. Ways to improve the catalysts for the SCR of NOx with C1-4 oxygenates are outlined.

A Brønsted Acid-Catalyzed Multicomponent Reaction for the Synthesis of Highly Functionalized γ-Lactam Derivatives.

Brønsted acids catalyze a multicomponent reaction of benzaldehyde with amines and diethyl acetylenedicarboxylate to afford highly functionalized γ-lactam derivatives. The reaction consists of a Mannich reaction of an enamine to an imine, both generated in situ, promoted by a phosphoric acid catalyst and a subsequent intramolecular cyclization. The hydrolysis of the cyclic enamine substrate can provide enol derivatives and, moreover, a second attack of the amine on the carboxylate can afford amide derivatives. An optimization of the reaction conditions is presented in order to obtain selectively cyclic enamines that can afford the enol species after selective hydrolysis.

Ultrafast Excited-State Intermolecular Proton Transfer in Indigo Oligomer.

It has been a long-lasting debate whether indigo undergoes excited-state proton transfer and how this contributes to its photostability. A prevailing point of view is that a sub-picosecond excited-state intramolecular single proton transfer occurs; however, it has been studied mostly under dilute solution conditions. In this work, excited-state structural dynamics of indigo oligomers formed at millimolar concentration in dimethyl sulfoxide is investigated using femtosecond visible pump spectroscopy, infrared and visible probe spectroscopies, and steady-state infrared and fluorescence spectroscopies. Experimental evidence indicates the presence of transient intermolecular electronic excited-state proton transfer, which is supported by quantum-chemistry computations. The formed enol species disappears with a time constant of 200-300 fs, followed by a relatively slow nonradiative relaxation to the electronic ground state. Our results reveal new photochemistry of indigo particularly in its oligomeric state.

Catalytic Enhancement of Aldol Condensation by Oxygen Vacancy on CeO2 Catalysts

AbstractAldol condensation is a very useful reaction for biomass upgrading by coupling small molecule platform compounds into high value‐added products. Here we choose acetaldehyde (AcH) condensation, the rate‐determining step in bioethanol transformations, as a targeted reaction, and prepared four CeO2 catalysts with different concentrations of surface oxygen vacancies to investigate the role of oxygen vacancies in this reaction. To the best of our knowledge, it is the first time to demonstrate that there is a linear correlation between the activity of aldol condensation and the concentration of oxygen vacancies. Based on in‐situ FTIR studies, we conclude that oxygen vacancies are the Lewis acid sites for activation and stabilization of AcH, while lattice oxygen act as base sites for the formation of enolate species which are the key intermediates for AcH coupling reactions. Moreover, the proposed reaction mechanism indicates that Ce cations which are weak Lewis acid sites are not involved in the AcH condensation.

Enantioselective Oxidative Cyclization/Mannich Addition Enabled by Gold(I)/Chiral Phosphoric Acid Cooperative Catalysis

An enantioselective Mannich-type reaction of 3-butynol and nitrones is described, which affords dihydrofuran-3-ones in good yields and with excellent enantioselectivities. The reaction is initiated by gold-catalyzed alkyne oxidation and modification of the resulting gold carbene species with a tethered hydroxy group to form enolate species; the reaction terminates with an enantioselective Mannich-type addition with the assistance of chiral phosphoric acid (CPA) and hydrogen bonding. This novel pattern of alkyne transformation involving chemical bond cleavage, and a fragment modification and reassembly process, provides an atom- and step-economic method, and is the first example of cooperative asymmetric catalysis in gold-catalyzed alkyne oxidations via an α-oxo gold carbene route.

Mechanistic insights into the Knoevenagel condensation reaction over ZnO catalysts: Direct observation of surface intermediates using in situ FTIR

Liquid phase Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was studied over ZnO under solvent-free conditions. An induction period of approximately 1 h was observed, the duration of which is dependent on the pre-treatment method adopted for the catalyst and whether the catalyst was exposed to either ethyl cyanoacetate or benzaldehyde prior to reaction. In-situ infra-red experiments show that the adsorption of ethyl cyanoacetate on ZnO results in the formation of an enol intermediate, while the adsorption of benzaldehyde on ZnO results in the formation of an acetal-type intermediate. A proposed reaction mechanism comprises the formation of ethyl trans-α-cyanocinnamate as a reaction intermediate. The key step in the reaction involves interaction between the acetal and the enol species. The induction period arises as a consequence of the formation and relative stability of the surface OH groups from the reactants when the reagents adsorb on the ZnO surface.