Catalytic Conditions Research Articles

Direct probing of single-molecule chemiluminescent reaction dynamics under catalytic conditions in solution

Chemical reaction kinetics can be evaluated by probing dynamic changes of chemical substrates or physical phenomena accompanied during the reaction process. Chemiluminescence, a light emitting exoenergetic process, involves random reaction positions and kinetics in solution that are typically characterized by ensemble measurements with nonnegligible average effects. Chemiluminescent reaction dynamics at the single-molecule level remains elusive. Here we report direct imaging of single-molecule chemiluminescent reactions in solution and probing of their reaction dynamics under catalytic conditions. Double-substrate Michaelis–Menten type of catalytic kinetics is found to govern the single-molecule reaction dynamics in solution, and a heterogeneity is found among different catalyst particles and different catalytic sites on a single particle. We further show that single-molecule chemiluminescence imaging can be used to evaluate the thermodynamics of the catalytic system, resolving activation energy at the single-particle level. Our work provides fundamental insights into chemiluminescent reactions and offers an efficient approach for evaluating catalysts.

Thermodynamic and Kinetic Insights into Hydrogen Adsorption and Dissociation on the Ru (0001) Surface under STM and Catalytic Conditions

Thermodynamic and Kinetic Insights into Hydrogen Adsorption and Dissociation on the Ru (0001) Surface under STM and Catalytic Conditions

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules.

Nickel excels at facilitating selective radical chemistry, playing a pivotal role in metalloenzyme catalysis and modern cross-coupling reactions. Radicals, being nonpolar and neutral, exhibit orthogonal reactivity to nucleophilic and basic functional groups commonly present in biomolecules. Harnessing this compatibility, we delve into the application of nickel-catalyzed radical pathways in the synthesis of noncanonical peptides and carbohydrates, critical for chemical biology studies and drug discovery.We previously characterized a sequential reduction mechanism that accounts for chemoselectivity in cross-electrophile coupling reactions. This catalytic cycle begins with nickel(I)-mediated radical generation from alkyl halides, followed by carbon radical capture by nickel(II) complexes, and concludes with reductive elimination. These steps resonate with mechanistic proposals in nickel-catalyzed cross-coupling, photoredox, and electrocatalytic reactions. Herein, we present our insights into each step involving radicals, including initiation, propagation, termination, and the nuances of kinetics, origins of selectivity, and ligand effects.Radical generation from C(sp3) electrophiles via one-electron oxidative addition with low-valent nickel radical intermediates provides the basis for stereoconvergent and cross-electrophile couplings. Our electroanalytical studies elucidate a concerted halogen atom abstraction mechanism, where electron transfer is coupled with halide dissociation. Using this pathway, we have developed a nickel-catalyzed stereoselective radical addition to dehydroalanine, facilitating the synthesis of noncanonical peptides. In this application, chiral ligands modulate the stereochemical outcome through the asymmetric protonation of a nickel-enolate intermediate.The capture of the alkyl radical by nickel(II) expands the scope of cross-coupling, promotes reductive elimination through the formation of high-valent nickel(III) species, and governs chemo- and stereoselectivity. We discovered that nickel(II)-aryl efficiently traps radicals with a barrier ranging from 7 to 9 kcal/mol, followed by fast reductive elimination. In contrast, nickel(II)-alkyl captures radicals to form a nickel(III) species, which was characterized by EPR spectroscopy. However, the subsequent slow reductive elimination resulted in minimal product formation. The observed high diastereoselectivity of radical capture inspired investigations into C-aryl and C-acyl glycosylation reactions. We developed a redox auxiliary that readily couples with natural carbohydrates and produces glycosyl radicals upon photoredox activation. Nickel-catalyzed cross-coupling of the glycosyl radical with bromoarenes and carboxylic acids leads to diverse non-natural glycosides that can facilitate drug discovery.Stoichiometric studies on well-defined d8-nickel complexes have showcased means to promote reductive elimination, including ligand association, oxidation, and oxidative addition.In the final section, we address the influence of auxiliary ligands on the electronic structure and redox activity of organonickel intermediates. Synthesis of a series of low-valent nickel radical complexes and characterization of their electronic structures led us to a postulate that ligand redox activity correlates with coordination geometry. Our data reveal that a change in ligand redox activity can shift the redox potentials of reaction intermediates, potentially altering the mechanism of catalytic reactions. Moreover, coordinating additives and solvents may stabilize nickel radicals during catalysis by adjusting ligand redox activity, which is consistent with known catalytic conditions.

Air-Stable Ni Catalysts Prepared by Liquid-Phase Reduction Using Hydrosilanes for Reactions with Hydrogen.

The liquid-phase reduction method for the preparation of metal nanoparticles (NPs) by the reduction of metal salts or metal complexes in a solvent with a reducing agent is widely used to prepare Ni NPs that exhibit high catalytic activity in various organic transformations. Intensive research has been conducted on control of the morphology and size of Ni NPs by the addition of polymers and long-chain compounds as protective agents; however, these agents typically cause a decrease in catalytic activity. Here, we report on the preparation of Ni NPs using hydrosilane (Ni-Si) as a reducing agent and a size-controlling agent. The substituents on silicon can control not only the size but also the crystal phase of the Ni NPs. The prepared Ni NPs exhibited high catalytic performance for the hydrogenation of unsaturated compounds, aromatics, and heteroaromatics to give the corresponding hydrogenated products in high yields. The unique feature of Ni catalysts prepared by the hydrosilane-assisted method is that the catalysts can be handled under air as opposed to conventional Ni catalysts such as Raney Ni. Characterization studies indicated that the surface hydroxide was reduced under the catalytic reaction conditions with H2 at around 100 °C and with the assistance of organosilicon compounds deposited on the catalyst surface. The hydrosilane-assisted method presented here could be applied to the preparation of supported Ni catalysts (Ni-Si/support). The interaction between the Ni NPs and a metal oxide support enabled the direct amination of alcohols with ammonia to afford the primary amine selectively.

Production of Biodiesel from Nonedible Parkia biglobosa Oil under Acidic Condition.

In this study, biodiesel was produced from Parkia biglobosa oil via optimization of transesterification reaction conditions, (methanol to oil ratio, catalyst concentration, reaction temperature, and reaction time) under sulphuric acid catalyst (H2SO4). The oil was first extracted from Parkia biglobosa seeds using the Soxhlet extraction method. The physicochemical properties of the biodiesel were analysed and then compared to international standards. Subsequently, the oil was then used to produce biodiesel at optimized transesterification reaction conditions. The free fatty acid (FFA) content of the oil was 1.61% w/w, while the saponification value (mgKOH/g) was 191.65. The maximum yield (percentage weight) of the biodiesel produced was 93.4% at the maximum transesterification conditions of methanol-to-oil molar ratio of 6 : 1, sulphuric acid catalyst amount of 3 wt%, reaction temperature of 65°C, and reaction time of 1.5 h. The biodiesel produced was within the limits of international standards as per the specification by ASTM D6751 (American standard), EN 14241 (European standard), and Ghana Standard Authority. It was therefore recommended that biodiesel from Parkia biglobosa seed oil under acidic catalytic condition is a potential new substitute for petroleum diesel for commercialization purposes.

The nature of extraframework aluminum species and Brønsted acid site interactions under catalytic operating conditions

A systematic investigation of hydrated extraframework aluminum (EFAl) species interacting with Brønsted acid sites (BAS) in H-ZSM-5 is presented to understand the cooperative active site structure under catalytic operating conditions. Static models of EFAl species confined in the H-ZSM-5 unit cell show that isolated BAS protonate neutral EFAl species to form cations. Ab-initio molecular dynamics (AIMD) simulations and enhanced sampling performed at the temperature for methanol-to-hydrocarbon conversion reveal two regimes of stable EFAl species, namely the [Al(OH)2]+ ion existing with two bonds to the zeolite scaffold or as a pore-guest in the form of [Al(OH)2(H2O)2]+. Our results indicate that hydrogen-bonding plays a significant role in BAS-EFAl active site structure, especially at higher BAS density and that EFAl species can function as both Bronsted and Lewis acidic components to alter proton transfer kinetics as well as shape selectivity within these microporous solids.

Solar light promoted CO2 hydrogenation to CH4 over photo-thermal responsive dispersed Co on defective CeO2 composite derived from MOFs

The conversion of the greenhouse gas CO2 represents a pragmatic and environmentally friendly approach. However, the substantial energy demands of the reaction hamper its widespread application. Here, we designed an efficient catalyst prepared by Ce-MOF and ZIF-67 precursor, namely Co/CeO2, for photothermal CO2 methanation under light and auxiliary heating. CeO2 synthesized by Ce-MOF has a good structure and abundant oxygen vacancies, meanwhile, Co has abundant active sites and strong adsorption capacity and is well-dispersed on CeO2. As a result, the catalyst exhibits high CH4 yield of 885.27 mmol⋅g−1⋅h−1 and CO yield of 24.5 mmol⋅g−1⋅h−1. Moreover, it has good selectivity and catalytic durability. Through the analysis of the reaction pathway under the conditions of photothermal catalysis, the hydrogenation reaction of formate can efficiently generate CH4 and H2O due to the promotion of hot carriers. This work demonstrated the advantages of photothermal co-catalysis and provides a potential strategy for efficient CO2 reduction.

Palladates of the [NHC⋅H][Pd(NH2)(CC)Cl2] Type as Effective Pre‐catalysts for the Formation of C−C and C−N Bonds

AbstractThe environmentally friendly, simple synthesis of a family complexes of the general formula [NHC⋅H][Pd(NH2)(CC)Cl2] (CC=biphenyl) is presented. The catalytic activity of these air‐ and moisture‐stable complexes was evaluated as a function of the imidazolium counterion. These pre‐catalysts can be converted, under catalytic conditions, to Pd‐NHC species enabling the Suzuki–Miyaura, Sonogashira, and Buchwald–Hartwig amination reactions under very mild operating conditions.

Chiral-Boron-Complex-Catalyzed Asymmetric [3 + 2] Cycloaddition of β-Trifluoromethyl α,β-Unsaturated Ketones with N,N'-Cyclic Azomethine Imines.

The (R)-3,3'-(3,5-(CF3)2-C6H3)2-BINOL-boron-complex-catalyzed asymmetric 1,3-dipolar cycloaddition of β-trifluoromethyl α,β-unsaturated ketone with N,N'-cyclic azomethine imines was developed to afford N,N'-bicyclic pyrazolidine derivatives bearing a stereogenic carbon center containing CF3 motifs in high yields with excellent diastereo- and enantioselectivities (up to >20:1 dr, and >99% ee). This catalytic system features mild reaction conditions, high efficiency, and a broad substrate scope.

Engineering the activity and stability of ZIF-8(Zn/Co)@g-C3N4 nanocomposites and their synergistic action in converting atmospheric CO2 into cyclic carbonates

The development of novel catalytic materials that integrate multifunctional sites has significant implications for expanding the utilization of CO2 resources. However, simultaneously achieving high activity and stability remains a formidable challenge. In this study, a series of ZIF-8(Zn/Co)@g-C3N4 nanocomposites were prepared by employing a thermo-physical compounding strategy that involved the combination of nitrogen-rich graphitic carbon nitride (g-C3N4) nanosheets with ZIF-8(ZnCo). The influences of different compositions of g-C3N4 and ZIF-8(Zn/Co) on the catalyst structure were systematically investigated. Subsequently, the catalytic activities of these nanocomposites towards the cycloaddition reaction between CO2 and epoxide were examined under different conditions. The presence of abundant Lewis base sites in g-C3N4 facilitates CO2 activation, while multiple Lewis acid sites in ZIF-8(Zn/Co) enable efficient epoxide activation. By working synergistically with a co-catalyst, tetrabutylammonium bromide (TBAB), CO2 and epoxides can be efficiently reacted to synthesize the corresponding cyclic carbonates under mild or even atmospheric pressure conditions. The catalytic reaction conditions were optimized, and both the catalyst's recycling performance and the scope of epoxides with various substituents were investigated. The integration of g-C3N4 and ZIF-8(Zn/Co) endows the catalytic material with exceptional structural stability and remarkable catalytic activity, thereby providing a new platform for highly efficient CO2 conversion.

Total synthesis of cassane‐type diterpenoid pikrosalvin

AbstractThis paper presents the first successful total synthesis of pikrosalvin, a compound naturally derived from Salvia officinalis, grounded on the structural framework delineated by Brieskorn and Fuchs. Our synthetic approach was underpinned by a biomimetic strategy inspired by the Stork–Eschenmoser hypothesis for the biosynthesis of terpenes via polyene cyclization. Starting with commercially available vanillic acid and geraniol, we strategically assembled pikrosalvin's structural core, consisting of a decalin A/B ring, an aromatic C ring, and a butyrolactone D ring. Challenges related to protective groups and specific catalytic conditions were effectively addressed, resulting in high product yields. We further demonstrated the efficacy and broad applicability of a combined‐acid‐catalyzed polyene cyclization methodology in the context of complex natural product synthesis. This study sets the stage for future biological and pharmacological investigations of pikrosalvin.

Ligand-based electron transfer enables molecular polypyridyl Zn(II) complexes for efficient photocatalytic CO2 reduction

Electron transfer process plays a critical role in photocatalytic CO2 reduction. Owing to low redox activity of Zn(II) cation under catalytic conditions, it remains a challenge to use Zn(II) complexes as efficient molecular catalysts for CO2 photoreduction. Herein, we overcome the obstacle and demonstrate two polypyridyl Zn(II) complexes can be utilized for selective photoreduction of CO2 to CO. Thanks to more efficient ligand-based electron transfer, the polypyridyl Zn(II) complex modified with electron-donating carbazole groups displays a higher TON of CO up to 116 than the other unsubstituted polypyridyl Zn(II) complex, which is among the most efficient Zn(II) complexes molecular catalysts in CO2 photoreduction. Photocatalytic mechanism has been studied by UV–vis absorption spectroscopy and computational calculations. These results show a promising way to develop Zn(II) complexes for efficient CO2 photoreduction through ligand modification and ligand-based electron transfer.

Alcohol‐Treated Nickel‐Aluminium Catalyst for One‐Step Highly‐Selective Butane‐1,4‐Diol Synthesis from 2‐Butyne‐1,4‐Diol

AbstractThe development of low‐cost nickel‐based catalysts for direct and selective hydrogenation of 2‐butyne‐1,4‐diol (BYD) to butane‐1,4‐diol (BAD) under mild conditions is an important and attractive target both in fundamental research and industrialization but remains a formidable challenge. The primary industrial production method for BAD synthesis is a two‐step reaction route, which suffers from complicated catalysis conditions and high equipment costs. Herein, we develop a high‐performance catalyst via a facile alcohol‐treated strategy for highly selective BAD synthesis at moderate operation conditions. The as‐synthesized NA‐80E catalyst exhibits outstanding BAD selectivity of 98.82 % and BYD conversion of 100 % at 60 °C and 4 MPa, outperforming most reported results for BAD formation in a one‐step process and even being comparable to those obtained by the two‐step hydrogenation reaction route under much high temperatures and pressures. Crucially, we found that after facile alcohol (ethanol) treatment, an intriguing phenomenon of suppression of adjacent acid‐assisted hydrogenolysis via extra acidic Al species at the NiO‐Al2O3 interface is observed, contributing to the precise enhancement of BAD selectivity by inhibiting the production of butanol (BOL). This facile alcohol‐treated method along with the revealed mechanism of blocked hydrogenolysis opens vast possibilities for designing high‐performance and highly‐selective hydrogenation catalysts.

Single-component systems of iron(II) half-sandwich complexes catalyze epichlorohydrin/CO2 condensation

Half-sandwich Fe(II) complexes bearing 2-(diphenylphosphino)ethylamine chelating ligand [(η5-C5H5)Fe(CO)(NH2(CH2)2PPh2)]X, where X = I (1), Br (2), Cl (3), OTf (4) were successfully prepared under mild reaction conditions. Their catalytic activity was evaluated systematically on the (chloromethyl)ethylene carbonate synthesis starting from CO2 and epichlorohydrin. The influence of the counterion has been established, where the single-component catalyst 1 showed low catalytic activity under mild conditions for CO2 and epoxide condensation. The exchange of iodide by bromide showed a higher catalytic activity and a higher conversion with compound 2 as a catalyst, while a decrease and no conversion were observed for 3 (X = Cl) and 4 (X = OTf), respectively. Optimization of the catalytic conditions was carried out using epichlorohydrin in dry chloroform, along with modification of the reaction conditions. Of the synthesized compounds (1–4), only 2 showed conversions to (chloromethyl)ethylene carbonate >99 % at 100 °C, 0.34 MPa of CO2, 14 h and 2 % mol of catalyst, or by modifying the temperature to 80 °C and the pressure to 0.69 MPa of CO2. Catalyst 2 proved to be very selective, although it has modest activity, reaching low values of TON (124) and TOF (8.17 h−1).

Synthesis of Inexpensive Ternary Metal Oxides by a Co-Precipitation Method for Catalytic Oxidation of Carbon Monoxide.

By using a simple co-precipitation method, new Fe2 O3 -based nanocatalysts (samples) were synthesized. The samples were composites of two or three transition metal oxides, MOx (M=Fe, Mn, Co, Ni, and Cu). The average size of CuO crystallites in the composites composed of two oxide components (CuO-Fe2 O3 ) was about 14.3 nm, while in those composed of three (CuO-MnOx -Fe2 O3 ), the composite's phase compositions were almost in the amorphous form when annealing the sample at 300 °C. The latter sample had a specific surface area higher than that of the former, 207.9 and 142.1 g/m2 , respectively, explaining its higher catalytic CO oxidation. The CO conversion over the CuO-MnOx -Fe2 O3 -300 catalyst (1 g of catalyst, 2600 ppm of CO concentration in air, and 1.0 L/min of gas flow rate) begins at about 40 °C; the temperature for 50 % CO conversion (t50 ) is near 82 °C; and CO removal is almost complete at t99 ≈110 °C. The activity of the optimal sample was tested in different catalytic conditions, thereby observing a high durability of 99-100 % CO conversion at 130 °C. The obtained results were derived from XRD, FTIR, BET, SEM, elemental analysis and mapping, as well as catalytic experiments.

Catalytic C-H Trifluoromethylation of Arenes and Heteroarenes via Visible Light Photoexcitation of a Co(III)-CF3 Complex.

A cobalt photocatalyst for direct trifluoromethylation of (hetero)arene C(sp2)-H bonds is described and shown to operate via visible light activation of a Co-CF3 intermediate, which functions as a combined chromophore and organometallic reaction center. Chemical oxidations of previously reported (OCO)Co complexes containing a redox-active [OCO] pincer ligand afford a Co-CF3 complex two oxidation states above Co(II). Computational and spectroscopic studies are consistent with formulation of the product as [(OCO•)CoIII(CF3)(THF)(OTf)] (II) containing an open-shell [OCO•]1- radical ligand bound to a S = 0 Co(III) center. II is thermodynamically stable, but exposure to blue (440 nm) light induces Co-CF3 bond homolysis and release of •CF3, which is trapped by radical acceptors including TEMPO•, (hetero)arenes, or the radical [OCO•] ligand in II. The latter comprises a competitive degradation pathway, which is overcome under catalytic conditions by using excess substrate. Accordingly, generation of II from the reaction of [(OCO)CoIIL] (III) (L = THF, MeCN) with Umemoto's dibenzothiophenium trifluoromethylating reagent (1) followed by photolytic Co-CF3 bond activation completes a photoredox catalytic cycle for C-H (hetero)arene trifluoromethylation utilizing visible light. Electronic structure and photophysical studies, including time-dependent density functional theory (TDDFT) calculations, suggest that Co-CF3 bond homolysis at II occurs via an ligand-to-metal charge-transfer (LMCT) (OCO0)CoII(CF3) state, revealing ligand redox activity as a critical design feature and establishing design principles for the use of base metal chromophores for selectivity in photoredox bond activations occurring via free radical intermediates.

Immobilization of horseradish peroxidase on metal-organic framework to improve enzyme activity for enhanced chemodynamic therapy

Bio-enzymes have the advantages of strong substrate specificity, high catalytic efficiency, and minimal toxic side effects, making them promising drugs in cancer therapy. However, the poor stability and cellular penetrability of uncoated protein in the physiological environment severely restricts the direct application of Bio-enzyme. To address it, we report a metal-organic framework (MOF), Hf-DBA (H2DBA, biphenyl carboxylic acid ligands). The morphology of the Hf-DBA was revealed by TEM and the diameter was in the range of 200 to 350 nm. Hf-DBA acted a carrier for intracellular delivery and protection of horseradish peroxidase (HRP). The prepared HRP@Hf-DBA can catalyze the excess H2O2 in the tumor cells to generation of •OH for chemodynamic therapy (CDT). Compared with free HRP, the catalytic activity of HRP@Hf-DBA is significantly improved, and the optimal catalytic conditions are explored. The catalytic stability of HRP@Hf-DBA remained above 70% after 12 cycles of catalysis. After treatment with HRP@Hf-DBA, the apoptosis rates of A549 and Hela cells was 71.64%, and 76.86%. The results in vitro show that HRP@Hf-DBA can effectively inhibit the growth of tumor cells through enhanced CDT.

Group I Alkoxides and Amylates as Highly Efficient Silicon-Nitrogen Heterodehydrocoupling Precatalysts for the Synthesis of Aminosilanes.

Group I alkoxides are highly active precatalysts in the heterodehydrocoupling of silanes and amines to afford aminosilane products. The broadly soluble and commercially available KOt Amyl was utilized as the benchmark precatalyst for this transformation. Challenging substrates such as anilines were found to readily couple primary, secondary, and tertiary silanes in high conversions (>90 %) after only 2 h at 40 °C. Traditionally challenging silanes such as Ph3 SiH were also easily coupled to simple primary and secondary amines under mild conditions, with reactivity that rivals many rare earth and transition-metal catalysts for this transformation. Preliminary evidence suggests the formation of hypercoordinated intermediates, but radicals were detected under catalytic conditions, indicating a mechanism that is rare for Si-N bond formation.

Assessing the changes in thermal and catalytic pyrolysis of lignocellulose when shifting from batch to continuous biomass feeding modes

Lignocellulosic biomass pyrolysis has been earlier studied in a large variety of reaction systems. However, there is a lack of information on the effects of passing from batch to continuous feeding mode (BM vs CM) despite being a necessary step in the scaling up of the process. In this sense, the main aim of this work is to conduct a comparative study of lignocellulosic biomass pyrolysis using the same reactor but varying the biomass feeding methods under thermal and catalytic conditions and different thermal zone temperatures (350 and 500 °C). Under thermal conditions, significant differences have been observed between both feeding systems. When operating in CM, the production of char decreases, particularly at 500 °C, from 27.3 to 18.9 wt % while the generation of gases increases. This can be attributed to the differences in the feeding mode since the CM allows a higher biomass heating rate, resulting in a more efficient lignocellulose decomposition. On the other hand, the balance between the higher residence time and the lower concentration of the vapours reduces the occurrence of secondary cracking reactions. Consequently, the share of water in the liquid fraction diminishes in CM, although only a notable increase of bio-oil* (bio-oil in water-free basis) is observed at 500 °C (39.9 vs 45.0 wt %). The bio-oil* molecular composition is also modified by this shifting, leading to higher sugars and oxygenated-aromatics in the CM reactor at 500 °C. The different operation modes also affect the interaction between the vapours and the catalyst when the n-ZSM-5 is introduced into the reactor. The vapours are more diluted in CM in comparison with the batch one, allowing a more progressive and efficient catalytic conversion. As a result, less coke is deposited over the zeolite catalyst leading to an enhanced production of aromatic hydrocarbons, achieving yields of ca. 7.6 wt %.

Organocatalytic C−H Functionalization of Simple Alkanes

AbstractThe direct functionalization of inert C(sp3)‐H bonds to form carbon‐carbon and carbon‐heteroatom bonds offers vast potential for chemical synthesis and therefore receives increasing attention. At present, most successes come from strategies using metal catalysts/reagents or photo/electrochemical processes. The use of organocatalysis for this purpose remains scarce, especially when dealing with challenging C−H bonds such as those from simple alkanes. Here we disclose the first organocatalytic direct functionalization/acylation of inert C(sp3)‐H bonds of completely unfunctionalized alkanes. Our approach involves N‐heterocyclic carbene catalyst‐mediated carbonyl radical intermediate generation and coupling with simple alkanes (through the corresponding alkyl radical intermediates generated via a hydrogen atom transfer process). Unreactive C−H bonds are widely present in fossil fuel feedstocks, commercially important organic polymers, and complex molecules such as natural products. Our present study shall inspire a new avenue for quick functionalization of these molecules under the light‐ and metal‐free catalytic conditions.