Uncatalyzed Reaction Research Articles

Mechanism, Stereoselectivity, and Role of O2 in Aza-Diels-Alder Reactions Catalyzed by Dinuclear Molybdenum Complexes: A Theoretical Study.

The aza-Diels-Alder-type reaction between imines and functionalized alkenes is one of the most versatile approaches to obtain piperidine derivatives. When using the Lewis acid [Mo2(OAc)4] (CAT) as a catalyst, it was found that the activation of CAT by O2 was essential for an efficient reaction. In this paper, the mechanism and stereoselectivity of the aza-Diels-Alder reaction between aromatic acyl hydrozones 1 and Danishefsky diene 2 under uncatalyzed and catalyzed (CAT not activated by O2 and CAT activated by O2) conditions have been studied by density functional theory (DFT) calculation. The results show that the uncatalyzed reaction is difficult to proceed at room temperature due to the high energy barrier. The CAT not activated by molecular oxygen has catalytic activity but not too much. When CAT is activated by O2, CATO2 may be the correct catalytic species, which results in a dramatic increase of reaction activity. The reaction mechanisms with/without the catalyst are different. The uncatalyzed reaction is concerted for both the endo and exo pathways. For the CAT-catalyzed reaction, the endo pathway is concerted, but the exo pathway is nonconcerted and involves two steps. The endo product is the main product for the reaction catalyzed by CAT, while for reactions catalyzed by CATO1 and CATO2, the endo and exo products can be obtained. The reaction activity is directly correlated to the atomic charges of two coupling C atoms. Our work explains the experimental results, determines the structure of the O2-activated catalyst species, and provides predictions for the reaction activity and stereoselectivity controlling.

Shaping a soot combustion Ce0.5Pr0.5Ox catalyst

A Ce0.5Pr0.5Ox soot combustion catalyst has been shaped combining this material prepared in two different structures (nanoparticles and MS-macroporous) in a single active phase. Nanoparticles, prepared by reversed microemulsion method, provide high activity to the hybrid material, and the MS structure, obtained by a hard template synthesis method, provides high thermal stability and improved soot-catalyst contact. Ce0.5Pr0.5Ox nanoparticles present high surface area (148 m2/g; 5 nm) and improved redox properties with regard to other Ce0.5Pr0.5Ox structures, and the macropores size of MS Ce0.5Pr0.5Ox are large enough (around 100 nm) to properly disperse Ce0.5Pr0.5Ox nanoparticles inside and also to host soot particles, creating a high reactive catalyst environment. As a result, the as prepared Ce0.5Pr0.5Ox hybrid catalyst, mixed in loose-contact mode with soot, decreased soot combustion by ca. 100 °C with regard to the uncatalyzed reaction, and thermal aging at 800 °C only decreased this difference to ca. 75 °C. The as-prepared counterpart hybrid catalyst obtained dispersing pure ceria nanoparticles on pure ceria MS support was also active for soot combustion, but the activity was almost lost completely upon thermal aging due to ceria sintering.

Synthesis and Characterization of rGO@ZnO Nanocomposites for Esterification of Acetic Acid.

In the present work, the aim is to synthesize reduced graphene oxide (rGO) and zinc:reduced graphene oxide composite catalysts (ZnO:rGO) for esterification of acetic acid with n-heptanol. The physical and chemical characteristics of the rGO and rGO-metal oxide composite catalysts such as textural surface characteristics, surface morphology, thermal stability, functional groups, and elemental analysis were studied. The surface areas of rGO, ZnO(0.5 M), and ZnO(1 M) were recorded, respectively, at 31.72, 27.65, and 36.19 m2 g–1. Furthermore, esterification reaction parameters such as the reaction time, catalyst dosage, and reaction temperature for acetic acid were optimized to check the feasibility of rGO-metal oxide composites for a better conversion percentage of acetic acid. The optimized catalyst was selected for further optimization, and the optimum reaction parameters found were 0.1 wt % of catalyst, 160 min reaction duration, and 100 °C reaction temperature with a maximal yield of 100%. At 110 °C, the reaction conducted in the presence of 0.1 g of catalyst displayed more yield than the uncatalyzed reaction.

Hydroboration reactions using transition metal borane and borate complexes: an overview.

In recent years, the chemistry of transition metal borane and borate complexes has advanced fast with reasonable growth. Frequent utilisation of these complexes in hydrofunctionalisation reactions is one of the key driving forces for their development. As a result, the important role of borate complexes in the hydroboration/hydrosilylation/hydroamination of unsaturated organic species has been successfully demonstrated together with the isolation of many different boron-containing transition metal complexes such as borataallyl, vinylborane, silyl complexes featuring the known bonding modes of boron. Both the uncatalysed and catalysed hydroboration reactions using the transition metal borane/borate complexes are known, which show these complexes' huge potential. Careful investigation and fine-tuning of the electronic and steric properties of the borane/borate ligands has facilitated the synthesis of these transition metal complexes which are convenient for use in the hydroboration reactions. Furthermore, the systematic development of this field has established the connection between the structure and reactivity of these complexes and their utilisation in hydroboration reactions. This Frontier sheds light on the recent developments that have been made with hydroboration reactions using transition metal borane/borate complexes. Also, in this Frontier we have provided meaningful synthetic methods to make new boron-containing transition metal complexes together with mechanistic insights for some of these reactions.

Modeling and Designing Particle-Regulated Amyloid-like Assembly of Synthetic Polypeptides in Aqueous Solution.

In cells, actin and tubulin polymerization is regulated by nucleation factors, which promote the nucleation and subsequent growth of protein filaments in a controlled manner. Mimicking this natural mechanism to control the supramolecular polymerization of macromolecular monomers by artificially created nucleation factors remains a largely unmet challenge. Biological nucleation factors act as molecular scaffolds to boost the local concentrations of protein monomers and facilitate the required conformational changes to accelerate the nucleation and subsequent polymerization. An accelerated assembly of synthetic poly(l-glutamic acid) into amyloid fibrils catalyzed by cationic silica nanoparticle clusters (NPCs) as artificial nucleation factors is demonstrated here and modeled as supramolecular polymerization with a surface-induced heterogeneous nucleation pathway. Kinetic studies of fibril growth coupled with mechanistic analysis demonstrate that the artificial nucleators predictably accelerate the supramolecular polymerization process by orders of magnitude (e.g., shortening the assembly time by more than 10 times) when compared to the uncatalyzed reaction, under otherwise identical conditions. Amyloid-like fibrillation was supported by a variety of standard characterization methods. Nucleation followed a Michaelis-Menten-like scheme for the cationic silica NPCs, while the corresponding anionic or neutral nanoparticles had no effect on fibrillation. This approach shows the effectiveness of charge-charge interactions and surface functionalities in facilitating the conformational change of macromolecular monomers and controlling the rates of nucleation for fibril growth. Molecular design approaches like these inspire the development of novel materials via biomimetic supramolecular polymerizations.

Nature of C−I⋅⋅⋅π Halogen Bonding and its Role in Organocatalysis

AbstractThe nature of the C−I⋅⋅⋅π halogen bonding and its mode of activation in organocatalysis have been quantitatively explored in detail by means of Density Functional Theory (DFT) calculations. To this end, the uncatalyzed homo‐[4+2]‐cycloaddition reaction involving 2‐alkenylindoles is compared to the analogous process mediated by iodine‐substituted azolium salts. It is found that the catalysts establish relatively strong C−I⋅⋅⋅π noncovalent interactions with the indole reactant, which are characterized by a high degree of covalency. This interaction results in a significant acceleration of the cycloaddition by lowering the activation barrier up to 6 kcal/mol with respect to the uncatalyzed reaction. The calculations predict that this barrier can be further reduced by increasing the electrophilicity of the catalyst. Our quantitative analyses reveal that the origin of the catalysis is found mainly in a significant reduction of the steric (Pauli) repulsion between the diene and dienophile, therefore confirming the generality of the recently introduced Pauli‐repulsion lowering concept.

Three‐Component Reaction to 1,4,2‐Diazaborole‐Type Heteroarene Systems

AbstractThe borane FmesBH2 reacts in a three‐component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro‐1,4,2‐diazaborole derivatives. In a related way, annulated BN‐indolizine derivatives became conveniently available, as were dihydro‐1,4,2‐oxaza‐ or thiazaborole derivatives. The nucleophilic framework of a dihydro‐1,4,2‐diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3‐dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro‐1,4,2‐diazaborole derivative with N‐phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X‐ray diffraction.

Three-Component Reaction to 1,4,2-Diazaborole-Type Heteroarene Systems.

The borane FmesBH2 reacts in a three-component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro-1,4,2-diazaborole derivatives. In a related way, annulated BN-indolizine derivatives became conveniently available, as were dihydro-1,4,2-oxaza- or thiazaborole derivatives. The nucleophilic framework of a dihydro-1,4,2-diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3-dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro-1,4,2-diazaborole derivative with N-phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X-ray diffraction.

Decarbonylative Fluoroalkylation at Palladium(II): From Fundamental Organometallic Studies to Catalysis.

This Article describes the development of a decarbonylative Pd-catalyzed aryl-fluoroalkyl bond-forming reaction that couples fluoroalkylcarboxylic acid-derived electrophiles [RFC(O)X] with aryl organometallics (Ar-M'). This reaction was optimized by interrogating the individual steps of the catalytic cycle (oxidative addition, carbonyl de-insertion, transmetalation, and reductive elimination) to identify a compatible pair of coupling partners and an appropriate Pd catalyst. These stoichiometric organometallic studies revealed several critical elements for reaction design. First, uncatalyzed background reactions between RFC(O)X and Ar-M' can be avoided by using M' = boronate ester. Second, carbonyl de-insertion and Ar-RF reductive elimination are the two slowest steps of the catalytic cycle when RF = CF3. Both steps are dramatically accelerated upon changing to RF = CHF2. Computational studies reveal that a favorable F2C-H---X interaction contributes to accelerating carbonyl de-insertion in this system. Finally, transmetalation is slow with X = difluoroacetate but fast with X = F. Ultimately, these studies enabled the development of an (SPhos)Pd-catalyzed decarbonylative difluoromethylation of aryl neopentylglycol boronate esters with difluoroacetyl fluoride.

In Situ Generated Amorphous Molybdenum Sulfide on Reduced Graphene Oxide Nanocomposite Catalyst for Hydrogen Evolution in a Biphasic Liquid System

AbstractIn situ deposition of catalysts are drawing attention to the liquid/liquid interfaces by using raw materials for the energy conversion reactions such as hydrogen evolution and oxygen reduction. Herein, in situ generation of amorphous molybdenum sulfide on reduced graphene oxide (rGO/MoSx) is investigated in the hydrogen evolution reaction (HER) by decamethylferrocene electron donor at the liquid/liquid interfaces by using (NH4)2MoS4 and graphene oxide precursors in the aqueous phase. rGO/MoSx catalyst shows better catalytic activity than the uncatalyzed reaction and free‐MoSx, which increase the HER rate 57‐ and 1.7‐fold, respectively. The enhanced catalytic activity of rGO/MoSx catalyst is related to the increased surface area, active sites and conductivity of rGO. The catalytic activity of rGO/MoSx are examined by four‐electrode voltammetry and also two‐phase reactions. The obtained rGO/MoSx catalyst are characterized in detail by structural and morphological techniques.

A simple and green method for determination of ethanol in liquors by the conductivity measurement of the uncatalyzed esterification reaction

A simple and green procedure for quantifying the ethanol concentration based on the conductometric method and the uncatalyzed esterification reaction was demonstrated. The reaction progress between ethanol and acetic acid was followed by detecting the electrical conductivity, corresponding to the amount of hydrogen ion and acetate anion from acetic acid dissociation. By fixing the concentration of acetic acid, the decrease of conductivity with increasing the ethanol concentration was observed. The inversely linear relation between the conductivity and ethanol concentrations in the range of 0.0–99.9%v/v was successfully obtained. The ethanol content can be efficiently quantified, with good intra-day and inter-day precision (<3.45% RSD). The proposed method was successfully employed for determining the ethanol concentration in liquor samples, validated by the GC method. Therefore, this proposed strategy provides an alternative practical method through conductometry for the quantitative detection of non-electrolytic ethanol solution.

How Lewis Acids Catalyze Ene Reactions

AbstractThe catalytic effect of various Lewis acids (LAs) on the ene reaction between propene (ene) and but‐3‐en‐2‐one (enophile) was studied quantum chemically using density functional theory and with coupled‐cluster theory. The studied LAs efficiently accelerate the ene reaction by lowering the reaction barrier up to 12 kcal mol−1compared to the uncatalyzed reaction. Our detailed activation strain and Kohn‐Sham molecular orbital analyses reveal that coordination of a LA catalyst to the enophile decreases the reaction barrier of the ene reaction by inducing an asymmetry in the π‐electronic system, which increases the asynchronicity and hence relieves the otherwise highly destabilizing activation strain and Pauli repulsion between the closed‐shell filled π‐orbitals of the ene and enophile. In all, these findings further demonstrate the generality of thePauli‐lowering catalysisconcept.

The evolutionary origins of peroxynitrite signalling

Peroxynitrite is a reactive intermediate formed in vivo through uncatalysed reaction of superoxide and nitric oxide radicals. Despite significant interest in detecting peroxynitrite in vivo and understanding its production, little attention has been given to the evolutionary origins of peroxynitrite signalling. Herein we focus on two enzymes that are key to the biosynthesis of superoxide and nitric oxide, NADPH oxidase 5 (NOX5) and endothelial nitric oxide synthase (eNOS), respectively. Multiple sequence alignments of both enzymes including homologues from all domains of life, coupled with a phylogenetic analysis of NOX5, suggest eNOS and NOX5 are present in animals as the result of horizontal gene transfer from ancestral cyanobacteria to ancestral eukaryotes. Therefore, biochemical studies from other laboratories on a NOX5 homologue in Cylindrospermum stagnale and an eNOS homologue in Synechococcus sp. PCC 7335 are likely to be of relevance to human NOX5 and eNOS and to the production of superoxide, nitric oxide and peroxynitrite in humans.

Carbene-Catalyzed Activation of Formyl-phenylacetic Esters for Access to Chiral Dihydroisoquinolinones.

A carbene-catalyzed reaction to synthesize chiral dihydroisoquinolinones via an o-quinodimethane (o-QDM) intermediate is disclosed. o-QDM reacts with cyclic sulfonic imines via annulation to afford highly enantioenriched dihydroisoquinolinone products. ESI-HRMS studies suggest a stepwise Mannich addition and acylation reaction pathway, and the pathways of the catalytic and uncatalyzed background reactions are evaluated via DFT calculations.

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Concerted versus ionic mechanisms of the α and γ extensions in uncatalyzed Mukaiyama reaction between β,γ-unsaturated bis silyl ketene acetal and benzaldehyde: A DFT study

The uncatalyzed regioselective Mukaiyama aldol reaction between β,γ-unsaturated bis silyl ketene acetal and benzaldehyde has been studied theoretically using density functional theory with the M06-2X exchange–correlation functional. These DFT calculations mostly demonstrate that (i) the α and γ adducts in uncatalyzed Mukaiyama reaction can proceed through two mechanisms, ionic and concerted, (ii) the concerted mechanism is favored for the α adduct and it is disfavored for the γ adduct, (iii) under 12 and 17 kbar, the α adduct reaction path is below the γ adduct reaction path whereas if the pressure decreases to 5 kbar, the opposite is obtained.

Entrapment of the Fastest Known Carbonic Anhydrase with Biomimetic Silica and Its Application for CO2 Sequestration.

Capturing and storing CO2 is of prime importance. The rate of CO2 sequestration is often limited by the hydration of CO2, which can be greatly accelerated by using carbonic anhydrase (CA, EC 4.2.1.1) as a catalyst. In order to improve the stability and reusability of CA, a silica-condensing peptide (R5) was fused with the fastest known CA from Sulfurihydrogenibium azorense (SazCA) to form R5-SazCA; the fusion protein successfully performed in vitro silicification. The entrapment efficiency reached 100% and the silicified form (R5-SazCA-SP) showed a high activity recovery of 91%. The residual activity of R5-SazCA-SP was two-fold higher than that of the free form when stored at 25 °C for 35 days; R5-SazCA-SP still retained 86% of its activity after 10 cycles of reuse. Comparing with an uncatalyzed reaction, the time required for the onset of CaCO3 formation was shortened by 43% and 33% with the addition of R5-SazCA and R5-SazCA-SP, respectively. R5-SazCA-SP shows great potential as a robust and efficient biocatalyst for CO2 sequestration because of its high activity, high stability, and reusability.

Mechanisms and Dynamics of Synthetic and Biosynthetic Formation of Delitschiapyrones: Solvent Control of Ambimodal Periselectivity.

The mechanism and dynamics for the formation of the delitschiapyrone family of natural products are studied by density functional theory (DFT) calculations and quasiclassical molecular dynamics simulations with DFT and xTB. In the uncatalyzed reaction, delitschiapyrones A and B are formed by Diels-Alder reactions through a single transition state and a post-transition state bifurcation that favors formation of delitschiapyrone B. In water and most likely in the enzyme, the acidic hydroxyquinone ionizes, and the resulting conjugate base undergoes cycloaddition preferentially to delitschiapyrone A. We demonstrate a new type of biosynthetic transformation and variable selectivity from a (4 + 2)/(4 + 3) ambimodal transition state.

Ruthenium (III) catalysed and uncatalysed oxidation of torsemide by hexacyanoferrate (III) in aqueous alkaline medium: A kinetic comparative approach

The kinetics approach of oxidation of torsemide (TOR) by hexacyanoferrate (III) [HCF (III)] has been identified spectrophotometrically at 420 ​nm in the alkaline medium in the presence and absence of catalyst ruthenium (III) at 25 ​°C, by keeping ionic strength (1 ​× ​10−2 ​mol ​dm−3) constant. The reaction exhibits at the stoichiometry ratio 1:2 of TOR and HCF (III), for uncatalysed and catalysed reactions. In the absence and presence of the catalyst, the order of the reactions obtained for TOR and HCF (III) was unity. However, the rate of the reactions enhanced by the increase in the concentration of catalyst, as well as the rate increases with an increase in alkaline concentration. The activation parameters for the reaction at the slow step were identified, and the effect of temperature on the rate of the reaction was analysed. A suitable mechanism has been demonstrated by considering the obtained results. The derived rate laws are reliable with analysed experimental kinetics.

Catalytic pyrolysis of used tires on noble-metal-based catalysts to obtain high-value chemicals: Reaction pathways

A systematic study on the use of noble metals (Pd, Pt, Au) supported on titanate nanotubes (NT-Ti) for selectively producing BTX and p-cymene from waste tire pyrolysis is provided here. All the materials were characterized for chemical, textural and structural properties using a range of analytical techniques. The M/NT-Ti (M: Pd, Pt, or Au) catalysts exhibit low nanoparticle sizes (1.8 <NPs<2.2 nm), and a homogeneous pore size distribution. The catalysts demonstrated excellent activity for converting WT into BTX-enriched oil when tested in a micropyrolysis system coupled to chromatography/mass spectrometry (Py–GC/MS). The BTX production was enhanced by the presence of catalysts with a selectivity order as follows Pd > Pt ≈ Au > support > non-catalyst. The Py-GC/MS suggest that the catalysts participate in the secondary reactions of dealkylation, dehydrogenation, isomerization, aromatization, and cyclization leading to a higher formation of BTX than the uncatalyzed reaction. Finally, a comprehensive reaction pathway describing the catalytic pyrolysis of WT over Pd/NT-Ti was proposed by studying the catalytic pyrolysis of individual polymers constituting the waste tires, and D,L-Limonene.