Olefinic Carbon Research Articles

A Computational Study of Heteroatom Analogues of Selenoxide and Selenone syn Eliminations.

Selenoxide syn elimination is a widely used method for the synthesis of alkenes because it proceeds under exceptionally mild conditions, typically with excellent regio- and stereoselectivity. Surprisingly, hetero-selenoxide eliminations, where one or both olefinic carbon atoms are replaced with heteroatoms, have been little investigated, and their selenonyl counterparts even less so. A variety of such reactions, where the heteroatoms included combinations of O, N and S, as well as C, were investigated computationally. Selenoxides typically have lower activation energies and are slightly endothermic, while the corresponding selenones display higher activation energies and are exothermic in the gas state. The results are consistent with concerted, five-centre processes, leading to the formation of dioxygen, aldehydes, diazenes and imines from seleninyl or selenonyl peroxides, esters, hydrazines and amines, respectively. The more acidic selenenyl hydrodisulfide analogue undergoes proton transfer to the basic selenoxide oxygen atom instead of concerted elimination, resulting in the formation of a zwitterion. However, the formation of the corresponding selenonyl zwitterion is disfavoured compared to concerted syn elimination. The effects of solvents were also computed along with changes in enthalpy, entropy and free energy. Solvent effects were variable, while free energy calculations indicated overall ΔG values ranging between 3.60 and -32.12 kcal mol-1 for the syn eliminations of methyl methanethioseleninate and methaneperoxyselenonic acid, respectively. These computations suggest that the olefin-forming selenoxide syn elimination may be more general than currently understood and that replacement of the two carbon atoms with heteroatoms can lead to viable processes.

Hydrogenation of hexene catalyzed by a ruthenium (II) complex with N‐heterocyclic carbene ligands

AbstractIn this study, we investigated the mechanism of the inactivated hexene hydrogenation reaction catalyzed by a ruthenium (II) complex containing “N‐heterocyclic carbene” (NHC) ligands, specifically SIMes and CBA, using DFT calculations. Our focus was on RuH(OSO2CF3)(CO)(SIMes)(CBA), which exhibits excellent catalytic behavior. We tested the B3LYP‐D3, cam‐B3LYP, and TPSSh functionals. The hydrogenation reaction is initiated by the release of SIMes rather than CBA due to the lower associated dissociation energy. Our findings indicate a reaction mechanism consisting of two consecutive steps, each involving one hydrogen atom migration. The first step, considered as the kinetically limiting transition state, exhibits a Gibbs free activation barrier of 12.9 kcal mol−1. This step involves two asynchronous processes. The first one describes the migration of the ruthenium hydride to the internal carbon of the olefine function, transitioning from π to σ coordination mode, which promotes the formation of a bond between ruthenium and the terminal olefinic carbon. The second process involves the oxidation of ruthenium from Ru(II) to Ru(IV). This oxidation is crucial as it enables the decomposition of the H2 molecule into two hydrogen atoms bonded to the ruthenium atom. The geometrical structures of the Hidden Reaction Intermediate Ru(II) complex and the quasi‐transition state of the second process have been determined by means of the RIRC technique. The second step entails the migration of one of the newly formed hydrides of the Ru(IV) complex to the terminal olefinic carbon, resulting in the release of hexane with a weak activation Gibbs free energy of .8 kcal mol−1. Lastly, we explored the use of dichloromethane as a solvent, considering the PCM model. The presence of the solvent significantly decreases the energy dissociation of SIMes from 17.9 to 9.0 kcal mol−1, providing notable benefits.

Oxidative Nitrogen Insertion into Silyl Enol Ether C═C Bonds.

Here, we demonstrate a fundamentally new reactivity of the silyl enol ether functionality utilizing an in situ-generated iodonitrene-like species. The present transformation inserts a nitrogen atom between the silyl enol ether olefinic carbons with the concomitant cleavage of the C═C bond. Overall, this facile transformation converts a C-nucleophilic silyl enol ether to the corresponding C-electrophilic N-acyl-N,O-acetal. This unprecedented access to α-amido alkylating agents enables modular derivatization with carbon and heteroatom nucleophiles and the unique late-stage editing of carbon frameworks. The reaction efficiency of this transformation is well correlated with enol ether nucleophilicity as described by the Mayr N scale. Applications presented herein include late-stage nitrogen insertion into carbon skeletons of natural products with previously unattainable regioselectivity as well as modified conditions for 15N labeling of amides and lactams.

Robust Ru/Ce@Co Catalyst with an Optimized Support Structure for Propane Oxidation.

Short carbon chain alkanes, as typical volatile organic compounds (VOCs), have molecular structural stability and low molecular polarity, leading to an enormous challenge in the catalytic oxidation of propane. Although Ru-based catalysts exhibit a surprisingly high activity for the catalytic oxidation of propane to CO2 and H2O, active RuOx species are partially oxidized and sintered during the oxidation reaction, leading to a decrease in catalytic activity and significantly inhibiting their application in industrial processes. Herein, the Ru/Ce@Co catalyst is synthesized with a specific structure, in which cerium dioxide is dispersed in a thin layer on the surface of Co3O4, and Ru nanoparticles fall preferentially on cerium oxide with high dispersity. Compared with the Ru/CeO2 and Ru/Co3O4 catalysts, the Ru/Ce@Co catalyst demonstrates excellent catalytic activity and stability for the oxidation of propane, even under severe operating conditions, such as recycling reaction, high space velocity, a certain degree of moisture, and high temperature. Benefiting from this particular structure, the Ru/Ce@Co (5:95) catalyst with more Ce3+ species leads to the Ru species being anchored more firmly on the CeO2 surface with a low-valent state and has a strong potential for adsorption and activation of propane and oxygen, which is beneficial for RuOx species with high activity and stability. This work provides a novel strategy for designing high-efficiency Ru-based catalysts for the catalytic combustion of short carbon alkanes.

Fluorine‐Effect‐Enabled Photocatalytic 4‐Exo‐Trig Cyclization Cascade to Access Fluoroalkylated Cyclobutanes

AbstractCyclobutanes are popular structural units in bioactive compounds and versatile intermediates in synthetic chemistry, but their synthesis is challenging owing to high ring strain. In this study, a novel method for highly regio‐ and diastereoselective synthesis of fluoroalkylcyclobutanes bearing vicinal quaternary and tertiary stereocenters is realized by a photocatalytic 4‐exo‐trig cyclization cascade of thioalkynes or trifluoromethylalkenes. Density functional theory calculations reveal that a unique fluorine effect, arising from hyperconjugative π→σ*C‐F interactions, accounts for the regio‐reversed radical addition at the sterically hindered alkene carbon, which facilitates an unprecedented 4‐exo‐trig ring closure. This chemistry enables the direct and controllable construction of medicinally valuable quaternary‐carbon‐containing cyclobutanes from readily available raw materials, nicely complementing the existing methods.

Fluorine-Effect-Enabled Photocatalytic 4-Exo-Trig Cyclization Cascade to Access Fluoroalkylated Cyclobutanes.

Cyclobutanes are popular structural units in bioactive compounds and versatile intermediates in synthetic chemistry, but their synthesis is challenging owing to high ring strain. In this study, a novel method for highly regio- and diastereoselective synthesis of fluoroalkylcyclobutanes bearing vicinal quaternary and tertiary stereocenters is realized by a photocatalytic 4-exo-trig cyclization cascade of thioalkynes or trifluoromethylalkenes. Density functional theory calculations reveal that a unique fluorine effect, arising from hyperconjugative π→σ*C-F interactions, accounts for the regio-reversed radical addition at the sterically hindered alkene carbon, which facilitates an unprecedented 4-exo-trig ring closure. This chemistry enables the direct and controllable construction of medicinally valuable quaternary-carbon-containing cyclobutanes from readily available raw materials, nicely complementing the existing methods.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

Inspection of microwave self‐healing efficiency in carbon nanotube reinforced polymer composites for aerospace applications

AbstractThe aerospace industry is evolving very rapidly every day, and due to the low operational and maintenance costs, unmanned aerial vehicles (UAVs) are utilized for many duties, including imaging, patrol, surveillance, and delivery. Flying platforms prioritize effective load‐carrying capacity and light weight. To achieve this, lightweight materials with sufficient strength are utilized, and design optimizations are implemented. This study investigates material development for a UAV propeller, taking into account the possible consequences of a bird strike or hard landing such as micro damage occurrence. In this study, a twin‐screw extruder was used to produce hybrid composites by blending a thermoplastic, polyamide‐6 (PA6) with olefin block copolymers (OBC) and carbon nanotubes (CNT). After manufacturing test specimens by injection molding, tensile and Charpy impact tests were performed. OBC increased the elongation capacity and impact resistance of the PA6 through maleic anhydride (MAH) grafting while reducing the tensile strength. CNT incorporation compensated for this drop, but it rendered the composites more brittle. More importantly, due to the CNT's microwave (MW) absorption capacity, the hybrid composites have gained self‐healing properties. Extended MW exposure time and high MW powers may cause localized burning of the material, resulting in a decrease in its self‐healing efficiency. Following the failure of the examined composites, SEM microscopy revealed various toughening mechanisms in the composites. The use of a modified Halpin‐Tsai model to estimate the elastic characteristics of CNT‐reinforced composites revealed promising results, with minimal discrepancies when compared to experimental data.Highlights CNTs were found efficient for the self‐healing behavior which is critical for improving the lifetime and planning maintenance for UAV propellers. CNT content, MW power & exposure time all impact the self‐healing efficiency. Extended MW exposure time and high MW powers can cause localized burning of the material, resulting in a decrease in its self‐healing efficiency. CNTs created bridge effects, ultimately leading to an enhancement in the strength of the composites. The use of a modified Halpin‐Tsai model yielded good accuracy with experimental data.

Hydrogen transfer reaction in butene catalytic cracking over ZSM-5

Hydrogen transfer reaction (HTR) is the pivotal side reaction in the catalytic cracking process of low carbon olefins. The intricate reaction pathways and product diversity of HTR directly impact the selective formation of ethylene and propylene. Therefore, elucidating the key HTR in various reaction pathways and defining the hydrogen transfer index (HTI) as a criterion lay a scientific foundation for precisely regulating HTR during olefin catalytic cracking process. Herein, the influence of the acid strength of ZSM-5 zeolites on the HTR degree was analyzed in butene catalytic cracking. Results showed that isobutane was the predominant component of HTR products, mainly derived from HTR during the dimerization-cracking of pentene (butene primary cracking product). Subsequent pentene catalytic cracking experiments validated this conclusion. Thus, the HTI in butene or pentene cracking process was defined as follows: for butene cracking process, HTI = Si–C4H10/SC5H10; for pentene cracking process, HTI = Si–C4H10/SC4H8. The HTI accurately reflected the extent of HTR with respect to the acid properties of the catalysts. Moreover, the reaction network of butene catalytic cracking process was optimized, providing a comprehensive explanation for the intriguing phenomenon of decreasing butene conversion with increasing reaction temperature when the acid strength of ZSM-5 was weak. Finally, a high-performance butene catalytic cracking catalyst, De-TS-1-0.25%P, was developed, exhibiting high olefin selectivity (92.33 %) and outstanding stability (307 h) in the conversion of butene.

Effect of ligand structure and metal center on ethylene oligomerization with N, N′ Schiff base late transition complexes: experiments and calculations

A series of N, N’ bis(pyridine-imine) Schiff base late transition metal (Fe, Co, and Ni) complexes with different ligand structures and catalytic active sites were synthesized. Ethylene oligomerization results showed that bis(pyridine-imine) iron complex had the highest catalytic activity (10.8 × 105 g/mol Fe h), and the catalytic activity of cobalt complex was the lowest (2.5 × 105 g/mol Co h); nickel complex had the highest selectivity toward high carbon olefins (28.6%), while the iron complex had the lowest (0.2%). The structure-activity relationship between the catalyst structure and catalytic properties for ethylene oligomerization was further studied by density functional theory (DFT) calculations. The bis(pyridine-imine) iron complex had the highest energy gap value and the most stable catalyst structure, which was more conducive to oligomerization; and the N–M–N bond of the nickel complex had the smallest bond angle, which was beneficial for chain growth. Based on the Cossee mechanism, a mechanism for ethylene oligomerization catalyzed by bis(pyridine-imine) metal complexes was proposed. To further study the micro reaction process of ethylene oligomerization, a molecular model of pyridine imine nickel complex and its catalytic ethylene oligomerization reaction path were proposed. The structure and energy of the reaction intermediate and transition state were calculated using DFT. The molecular model of the nickel complex had a lower ethylene insertion barrier and β-H elimination of energy barriers, indicating that the molecular model of the pyridine imine nickel complex had ethylene oligomerization active centers.

Congested C(sp3)-rich architectures enabled by iron-catalysed conjunctive alkylation.

Catalytic cross-coupling by transition metals has revolutionized the formation of C-C bonds in organic synthesis. However, the challenge of forming multiple alkyl-alkyl bonds in crowded environments remains largely unresolved. Here, we report the regioselective functionalization of olefins with sp3-hybridized organohalides and organozinc reagents using a simple (terpyridine)iron catalyst. Aliphatic groups of various sizes are successfully installed on either olefinic carbon, furnishing a diverse array of products with congested cores featuring C- or heteroatom-substituted stereocenters. The method enables access to valuable but synthetically challenging C(sp3)-rich molecules, including alicyclic compounds bearing multiple contiguous stereocenters through annulation cascades. Mechanistic and theoretical studies suggest a stepwise iron-mediated radical carbometallation pathway followed by outer-sphere C-C bond formation, which potentially opens the door to a broader scope of transformations and new chemical space.

Adsorption and activation of small molecules on boron nitride catalysts.

Hexagonal boron nitride possesses a unique layered structure, high specific surface area and similar electronic properties as graphene, which makes it not only a promising catalyst support, but also a highly effective metal-free catalyst in the booming field of green chemistry. Reactions involving small molecules (e.g., oxygen, low carbon alkanes, nitrogen and carbon dioxide) have always been a hot topic in catalytic research, especially associated with the adsorption and activation regime of different forms of small molecules on catalysts. In this review, we have investigated the adsorption of different small molecules and the relevant activation mechanisms of four typical chemical bonds (OO, C-H, NN, CO) on hexagonal boron nitride. Recent progress on approaches adopted to enhance the activation capacity such as doping, defect engineering and heterostructuring are summarized, highlighting the potential applications of nonmetallic hexagonal boron nitride catalysts in various reactions. This comprehensive investigation offers a reference point for the enhanced mechanistic understanding and future design of effective and sustainable catalytic systems based on boron nitride.

Ionothermal synthesis of ZSM-5/AlPO4-5 composite zeolites and their catalytic performances in the methanol-to-olefin reaction

Composite zeolites, specifically ZSM-5/AlPO4-5 ([E]-Z5/Al5, [B]-Z5/Al5 and [H]-Z5/Al5), were prepared using tri-substituted imidazolium bromide ionic liquids (ILs). The ILs used were 1-ethyl-2,3-dimethylimidazolium bromide ([EMMIM]Br), 1-butyl-2,3-dimethylimidazolium bromide ([BMMIM]Br) and 1-hexyl-2,3-dimethylimidazolium bromide ([HMMIM]Br). The structure, morphology and acidity of the composite zeolites were analyzed using various techniques, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FT-IR), N2 isothermal adsorption-desorption, thermogravimetric analysis (TGA), temperature-programmed desorption of ammonia (NH3-TPD) and nuclear magnetic resonance spectroscopy (NMR). The catalysts obtained demonstrate outstanding performance in the methanol-to-olefin (MTO) reaction, with a catalytic lifetime extended by approximately tenfold and an approximately 10 % increase in the yield of light carbon olefins compared to the traditional hydrothermal Z5/Al5 catalyst. There is a strong relationship between the selectivity and stability of the MTO reaction with the optimized grain size and catalyst acidity.

Decomposition Kinetics of Perfluoro‐(4‐methyl‐2‐pentene) (C6F12) and its Extinguishing Performance Study

AbstractThe urgent desire for Halon substitution has initiated the development of potential environment friendly alternatives since the Montreal Protocol and subsequent amendments were introduced. Atmospheric degradable agents such as halogenated alkenes, fluorinated ketones and cyclic compounds have been extensively studied. Among them, perfluoro‐(4‐methyl‐2‐pentene) (C6F12) has an analogous structure to C6F12O (Novec 1230) and has been regarded as one kind of potential substitute. In this paper, the main decomposition kinetics of C6F12 are studied numerically. In addition, the interactions between C6F12 and flames are studied experimentally. The results show that the suppression effectiveness of C6F12 is comparable to C6F12O, and the temperature rise is not obvious. The low chemical reactivity is believed to be caused by the perfluorinated methyl and isopropyl groups adjacent to olefinic carbon decreasing the reactivity of C6F12 toward OH radicals. Chemical kinetics simulation indicates that H‐induced reactions dominate the initial decomposition of C6F12, and a lot of isomers will be formed via a series of isomerization reactions and subsequent decomposition reactions at high temperature. These findings suggest the promising applicability of C6F12 and worth further study.

Diastereoselective Hydroacylation of Cyclopropenes by Visible-Light Photocatalysis.

An efficient and general strategy for the hydroacylation of cyclopropene is disclosed for synthesizing various 2-acylcyclopropane derivatives under mild reaction conditions. High functional group tolerance of this protocol features a novel route to access a divergent synthesis of acylated cyclopropane in a diastereoselective manner by photoinduced decarboxylation of α-ketoacid followed by acyl radical addition to cyclopropene. Additionally, the regioselective addition of acyl radical at the least substituted olefinic carbon center with trans-selective fashion makes this protocol more appealing toward natural product development.

Origin of Enantio- and Chemoselectivity in the Synthesis of Spirocycles via Palladium/Xu-Phos-Catalyzed Cascade Heck/Remote C(sp2)-H Alkylation: A Computational Mechanistic Study.

Density functional theory (DFT) calculations were performed to study the mechanism and factors affecting the enantio-, regio-, and chemoselectivities in the palladium/Xu-Phos-catalyzed cascade Heck/remote C(sp2)-H alkylation reaction. The active catalyst is found to be able to sustain coordination with P and S atoms and can adapt its coordination mode to accommodate the significant steric hindrance between the ligand and substrate, unlike previous findings that showed coordination with P and O atoms. The reaction is established to occur in sequence through the oxidative addition of the aryl iodide to Pd(0), intramolecular alkene insertion, C(sp2)-H bond activation, and C(sp2)-C(sp3) bond reductive elimination. The C(sp2)-C(sp3) bond reductive elimination is identified as the rate-determining step, and the intramolecular alkene insertion as the enantioselectivity-determining step. The high enantioselectivity originates from the stronger electronic interaction between the catalyst and substrate; the exclusive 5-exo-regioselectivity is due to the stronger nucleophilicity of the terminal alkene carbon atom, and the chemoselectivity of C-H activation over carboiodination is driven by thermodynamics.

Bioassay of Phytochemicals Isolated from Chloroform Extracts of Azadirachta indica Leaves

The present study investigated the chemical constituents isolated from Azadirachta indica chloroform extract. The extraction of leaves was done using Soxhlet extraction apparatus. To isolate and identify the antibacterial fraction from Azadirachta indica chloroform extract, TLC-bioautography was carried out. Phenol 3,5-bis (1,1-dimethyl ethyl), Phthalic acid bis (7-methyloclyl), Dido decyl phthalates, Oxalic acid, allyl hexadecyl ester, and 2-Piperidinone, N-(4-bromo-n-butyl) were the five primary antibacterial chemicals identified by the GC-MS study. The findings of the FTIR study revealed the presence of functional groups C-H str, C=O str, and C=C str as well as alcohol and the carboxylate ion. While the 13C NMR data demonstrated the existence of carbonyl, aromatic carbon, quaternary carbon, olefinic carbon, and methyl group, and the 1H NMR results revealed the presence of aliphatic OH, methyl, aromatic OH, and olefinic proton. The phytoconstituents detected using GC-MS analysis showed a wide range of pharmacological properties, including antioxidant, antibacterial, antifungal, anti-inflammatory, and antimalarial effects. Thus, Azadirachta indica plant has a high concentration of medicinal phytoconstituents that can be used to make antimicrobial medications to fight against plant pathogenic bacteria.

Industrial Regenerator Model for SMTO Technology.

In order to alleviate the current domestic oil shortage, China has studied the technology of using coal as the source to produce low carbon olefins, among which methanol to olefin (MTO) is an important process. Since the coke deposition of MTO catalyst is inevitable, the deactivated MTO catalysts need to be regenerated by continuous coke combustion to recover the activity. This paper used the actual industrial data to study the gas-solid two-phase fluidized bed of a SMTO regenerator and the coke combustion kinetics of the deactivated SAPO-34 catalyst, and established the mathematical model of the SMTO industrial regenerator. The kinetic parameters in the model were obtained and validated with different data, which showed that the model is reliable and can accurately predict the industrial reaction results and provide guidance for the SMTO production operation.

A substrate descriptor based approach for the prediction and understanding of the regioselectivity in caged catalyzed hydroformylation.

The use of data driven tools to predict the selectivity of homogeneous catalysts has received considerable attention in the past years. In these studies often the catalyst structure is varied, but the use of substrate descriptors to rationalize the catalytic outcome is relatively unexplored. To study whether this may be an effective tool, we investigated both an encapsulated and a non-encapsulated rhodium based catalyst in the hydroformylation reaction of 41 terminal alkenes. For the non-encapsulated catalyst, CAT2, the regioselectivity of the acquired substrate scope could be predicted with high accuracy using the Δ13C NMR shift of the alkene carbon atoms as a descriptor (R2 = 0.74) and when combined with a computed intensity of the CC stretch vibration (ICC stretch) the accuracy increased further (R2 = 0.86). In contrast, a substrate descriptor approach with an encapsulated catalyst, CAT1, appeared more challenging indicating a confined space effect. We investigated Sterimol parameters of the substrates as well as computer-aided drug design descriptors of the substrates, but these parameters did not result in a predictive formula. The most accurate substrate descriptor based prediction was made with the Δ13C NMR shift and ICC stretch (R2 = 0.52), suggestive of the involvement of CH-π interactions. To further understand the confined space effect of CAT1, we focused on the subset of 21 allylbenzene derivatives to investigate predictive parameters unique for this subset. These results showed the inclusion of a charge parameter of the aryl ring improved the regioselectivity predictions, which is in agreement with our assessment that noncovalent interactions between the phenyl ring of the cage and the aryl ring of the substrate are relevant for the regioselectivity outcome. However, the correlation is still weak (R2 = 0.36) and as such we are investigating novel parameters that should improve the overall regioselectivity outcome.

Critical role of sodium migration in iron-based FT- zeolite tandem catalyst system for syngas hydrogenation to gasoline

Exploration the role of promotor in the Fischer-Tropsch zeolite (FTO-ZEO) tandem catalyst systems is of paramount importance in improving the selectivity of target products. Herein, by varying the contents of Na promotor in xNa-FeOx, the evolution of Na in FTO-ZEO system during reaction and the resultant catalytic performance for FT synthesis have been investigated. When the content of Na falls in the range of 0.5–5 %, the syngas conversion performance on bare FeOx catalysts reaches a performance-platform period, while that on tandem catalysts show a volcano-like trend. With a set of control experiments, it is speculated that there is a migration process of Na from xNa-FeOx to the H-ZSM-5 moiety. The changed acidity of H-ZSM-5 causes the transformation of products toward higher carbon alkanes and aromatics in the gasoline range with a selectivity of 71.5 %. This work makes an important progress by showing the phenomenon of Na migration during reaction, which puts forward the influence of promoters on FTO-ZEO system and gives a new insight of zeolites modification.