Low Catalyst Loading Research Articles

Photocatalytic Cascade Trifluoromethylation/Cyclization : Selective Synthesis of Trifluoroethylated Benzopyran and Benzopyranone Derivatives

We have realized the photocatalytic radical cascade cyclization reaction, enabling the one‐step synthesis of rare trifluoroethyl benzopyran derivatives with a wide variety of structures and excellent yields. Furthermore, the approach exhibits low catalyst load, strong regional control and wide applicability under mild conditions. The selectivity of this transformation is influenced by the length of the carbon chain of the benzaldehyde, as demonstrated by the density functional theory calculations. Additionally, scaling up the photocatalytic process can be easily accomplished.

Iodine-Catalyzed Carbonyl-Alkyne Metathesis Reactions.

The reaction between aldehydes or ketones and alkynes-the carbonyl-alkyne metathesis-constitutes a very useful strategy for the synthesis of α,β-unsaturated carbonyls. We now demonstrate that iodine is a highly efficient catalyst for both the intra- and intermolecular metathesis reaction in very small concentrations (0.1-1 mol %). Our protocol outperforms other catalytic systems, is operationally very simple, cheap, metal-free, and tolerates a large variety of functional groups (e. g., -CN, -CO2Me, -Br, -OH) at very low catalyst loadings. We can furthermore show that iodine-catalyzed carbonyl-alkyne metatheses can be combined with other iodine-catalyzed reactions in one-pot procedures to afford larger and more complex molecular structures. Finally, our mechanistic studies indicate that the iodonium ion is the active catalyst under the reaction conditions.

ATRP with ppb Concentrations of Photocatalysts.

In atom transfer radical polymerization (ATRP), dormant alkyl halides are intermittently activated to form growing radicals in the presence of a CuI/L/X-CuII/L (activator/deactivator) catalytic system. Recently developed very active copper complexes could decrease the catalyst concentration to ppm level. However, unavoidable radical termination results in irreversible oxidation of the activator to the deactivator species, leading to limited monomer conversions. Therefore, successful ATRP at a low catalyst loading requires continuous regeneration of the activators. Such a regenerative ATRP can be performed with various reducing agents under milder reaction conditions and with catalyst concentrations diminished in comparison to conventional ATRP. Photoinduced ATRP (PhotoATRP) is one of the most efficient methods of activator regeneration. It initially employed UV irradiation to reduce the air-stable excited X-CuII/L deactivators to the activators in the presence of sacrificial electron donors. Photocatalysts (PCs) can be excited after absorbing light at longer wavelengths and, due to their favorable redox potentials, can reduce X-CuII/L to CuI/L. Herein, we present the application of three commercially available xanthene dyes as ATRP PCs: rose bengal (RB), rhodamine B (RD), and rhodamine 6G (RD-6G). Even at very low Cu catalyst concentrations (50 ppm), they successfully controlled PhotoATRP. Well-defined polymers with preserved livingness were prepared under green LED irradiation, with subppm concentrations ([PC] ≥ 10 ppb) of RB and RD-6G or 5 ppm of RD. Interestingly, these PCs efficiently controlled ATRP at wavelengths longer than their absorption maxima but required higher loadings. Polymerizations proceeded with high initiation efficiencies, yielding polymers with narrow molecular weight distributions and high chain-end fidelity. UV-vis, fluorescence, and laser flash photolysis studies helped to elucidate the mechanism of the processes involved in the dual-catalytic systems, comprising parts per million of Cu complexes and parts per billion of PCs.

Cooperative Diarylborinic Acid/Chloride-Catalyzed Formal SNi Reaction of cis-4-Hydroxymethyl-1,2-Cyclopentene Oxides.

A catalytic formal SNi reaction was designed to achieve stereoretentive products for cis-4-hydroxymethyl-1,2-cyclopentene oxides by using diarylborinic acid as a dual role catalyst and chloride as a catalytic transient nucleophile through a double-displacement mechanism. This reaction offers the advantages of a low catalyst loading of 0.1 mol % and wide substrate scope, even including N-substituents. The use of chiral boron acid as a catalyst for this reaction was also attempted.

Alkylaluminum Complexes Featuring Bridged Bis-Formylfluorenimide Ligands for Hydroboration of Aldehyde, Ketone, and Imines.

Three bis-formylfluorenimide ligands with different bridging groups were designed and synthesized, leading to the successful preparation of six novel alkylaluminum complexes through their reaction with alkylaluminum reagents (AlMe3 or AlEt3). Complexes 1 and 2 were obtained by the reaction of 1,2-propylene-bridged diamine (L1) with AlMe3 or AlEt3. By reacting 1,2-cyclohexylene-bridged diamine (L2) with AlMe3 or AlEt3 to obtain complexes 3 and 4. The above ligands formed a bidentate four-coordinate structure with alkylaluminum, which involved the elimination of one alkyl group as the ligand reacted with alkylaluminum. The complexes 5 and 6 were synthesized through the reaction of 1,2-phenylene-bridged diamine (L3) with AlEt3 in toluene or tetrahydrofuran. Notably, L3 exhibited unique reactivity compared with the other ligands, which formed a tridentate four-coordinated structure when reacting with alkylaluminum. The formation of the tridentate complex resulted from the introduction of a benzimidazole derivative or tetrahydrofuran (THF) molecule along with the elimination of two alkyl groups during its coordination with alkylaluminum. All complexes were characterized via 1H NMR, 13C NMR, and elemental analysis, with structural determination confirmed through X-ray. Furthermore, the catalytic activity in the hydroboration reaction of aldehyde, ketone, and imines was investigated with these complexes as catalysts. Among them, complex 1 demonstrated excellent catalytic performance (up to 99% yield) and broad substrate compatibility (more than 30 substrates) at low catalyst loading (1 mol %) under mild reaction conditions.

Expeditious and selective synthesis of 2,4,5-trialkyl-1H-imdazoles and benzimidazoles via multicomponent one-pot reaction catalyzed by Schiff base dioxomolybdenum(VI) complex

Imidazoles and benzimidazoles are privileged heterocyclic compounds that can be extensively found in medicinal and materials chemistry, however, the experimental procedures for their synthesis are limited to multicomponent condensation reactions and normally are strongly dependent upon the desired substitution pattern, consequently several procedures with different conditions and catalysts are available in literature. Accordingly, the development of synthetic methodologies to afford these compounds remains a very attractive research problem. In this regard, dioxomolybdenum based catalysts are emerging as versatile and non-expensive reagents for a myriad of chemically relevant transformations. Within this context, an expedient and efficient synthesis of 2,4,5-trialkyl-1H-imidazoles and benzimidazoles from a multicomponent one-pot condensation has been developed with a tridentate Schiff base dioxomolybdenum(VI) complex, resulting in an efficient, simple and readily available catalyst to promote the aforementioned reaction. This reaction offers advantages such as good to excellent yields, good selectivity of products, low catalyst loadings and simple reaction work-up.

Ag nanoparticles on arginine-cyanoguanidine functionalized magnetic g-C3N4: A catalyst for nitroaromatic hydrogenation and regioselective click reactions

This study describes the development of a novel hybrid nanocatalyst that was obtained by doping magnetic g-C3N4 with Ag nanoparticles and modifying it with arginine and cyanoguanidine (Ag@Fe3O4-g-C3N4-Arg-CG). Comprehensive characterization of the nanocatalyst using techniques such as FTIR, XRD, SEM, and TGA confirmed its structural and morphological properties. The catalytic efficiency of the synthesized nanostructure was evaluated in two key reactions: the reduction of nitroaromatic compounds and a click reaction for 1,2,3-triazole synthesis. The results demonstrate that Ag@Fe3O4-g-C3N4-Arg-CG effectively reduced various nitroaromatic compounds to substituted anilines at room temperature using NaBH4 as the reducing agent. Nitrobenzene reduction did not proceed in aprotic solvents such as acetonitrile, CH2Cl2, and dimethylformamide, whereas it exhibited a high reaction yield in protic solvents such as ethanol and water. The highest yield (100 %) was observed in water at 50 °C using H2O solvent. Additionally, the nanohybrid exhibited significant potential for “green” click chemistry by efficiently synthesizing 1,2,3-triazoles under mild conditions, with low catalyst loading. Its magnetic properties facilitated easy recovery, and the catalyst maintained high activity over six cycles, making it suitable for sustainable applications in organic transformations.

Ligand‐Controlled Regioselective Hydroacylation Reactions of Methylenecyclobutanes under Rhodium Catalysis

AbstractIn this study, we report a Rh(I)/dppf catalytic system that can tune the regioselectivity of methylenecyclobutanes. Under the optimal reaction conditions, a series of 1,3‐disubstituted cyclobutanes were prepared in high yields (up to 95%) with excellent regioselectivity (>20:1 r.r.). Scale‐up synthesis of the cyclobutane product at a low catalyst loading (2 mol%) demonstrates the robustness of this dppf‐controlled hydroacylation.

Self-supported FeCoNi(OHy)Ox oxy-hydroxide doped with Cr and Cu as robust low-loading catalyst for the alkaline oxygen evolution reaction

Self-supported FeCoNi(OHy)Ox oxy-hydroxides co-doped with Cr and Cu were obtained from a CrFeCoNiCu low ordered high entropy precursor synthesised by electrodeposition onto carbon cloth fibres (act-CrFeCoNiCu/CC). For comparison a dopant-free FeCoNi(OHy)Ox catalyst (act-FeCoNi/CC) was also synthesised following the same route. No heat treatment was performed to retain their low ordered structure. When assessed towards the oxygen evolution reaction (OER) in alkaline electrolyte, act-CrFeCoNiCu/CC and act-FeCoNi/CC catalyst reached a comparable current density of 10 mA cm−2 at ca. 1.6 VRHE (η10 of 380–390 mV). Nevertheless, Cr and Cu co-doping translated into improved stability. Evident degradation was observed during OER for act-FeCoNi/CC after 12 h operating at 100 mA cm−2, whereas no signs of degradation were detected for act-CrFeCoNiCu/CC under the same reaction conditions. The latter catalyst even delivered a constant potential for 160 h at 50 mA cm−2 aided to the stabilizing effect of Cr and Cu co-doping.

Magnetic BiFeO3 nanoparticles: a robust and efficient nanocatalyst for the green one-pot three-component synthesis of highly substituted 3,4-dihydropyrimidine-2(1H)-one/thione derivatives.

In this research, magnetic bismuth ferrite nanoparticles (BFO MNPs) were prepared through a convenient method and characterized. The structure and morphological characteristics of the prepared nanomaterial were confirmed through analyses using Fourier-transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), elemental mapping, powder X-ray diffraction (XRD), N2 adsorption-desorption isotherms and vibrating sample magnetometry (VSM) techniques. The obtained magnetic BFO nanomaterial was investigated, as a heterogeneous Lewis acid, in three component synthesis of 3,4-dihydropyrimidin-2 (1H)-ones/thiones (DHPMs/DHPMTs). It was found that the BFO MNPs exhibit remarkable efficacy in the synthesis of various DHPMs as well as their thione analogues. It is noteworthy that this research features low catalyst loading, good to excellent yields, environmentally friendly conditions, short reaction time, simple and straightforward work-up, and the reusability of the catalyst, distinguishing it from other recently reported protocols. Additionally, the structure of the DHPMs/DHPMTs was confirmed through 1H NMR, FTIR, and melting point analyses. This environmentally-benign methodology demonstrates the potential of the catalyst for more sustainable and efficient practices in green chemistry.

High-Shear Enhancement of Biginelli Reactions in Macromolecular Viscous Media.

Chemical reactions and transformations in non-traditional vessels have gained significant interest in recent years. Flow chemistry, with its advantages in mixing, mass transfer, scalability, and automation, is a driving force behind this paradigm shift. In particular, the Vortex Fluidic Device (VFD) has emerged as a versatile tool across various applications, from organic synthesis to materials science. In this study, the role of the VFD in performing the Biginelli reaction, a multicomponent reaction widely used in pharmaceutical and polymer science, for a post-polymerization modification is explored. By conducting the Biginelli reaction in the VFD, rapid product formation with low catalyst loading and without the need for high temperatures is achieved. However, the critical need to understand and know solution viscosity, especially within the context of modifying macromolecules is highlighted.

Peptide Catalyzed Conjugate Additions to β-Nitroacrylates - Steric Bulk Increases the Reaction Rate.

The organocatalytic conjugate addition of aldehydes to β-nitroacrylates provides direct access to β-ester-γ-nitroaldehydes and, thereby, common structural motifs of many bioactive compounds. However, the deactivation of amine-based catalysts by alkylation with the highly electrophilic nitroacrylates hampers this reaction. Here, we show that the peptide H-Mep-dPro-dGlu-NH2, which is reluctant to under alkylation,catalyzes this reaction at low catalyst loading (0.5-1 mol%) within short reaction times (15-60 min) to yield a broad range of β-ester-γ-nitroaldehydes with high stereoselectivity. Kinetic studies revealed that increased steric bulk on the β-nitroacrylate enhances the reaction rate by hindering catalyst alkylation.

Synthesis, Characterization, and Catalysis of Planar Chiral Ferrocene‐based N‐Heterocyclic Carbene Ligands and Its Metal Complexes

This study presents the development of a novel planar chiral ferrocene‐based NHC ligand and its corresponding metal complexes. The key unit of the imidazolium backbone, planar chiral ortho‐isopropyl ferrocenyl amine, was synthesized starting from (R)‐Ugi's amine. Imidazolium was obtained through the sequential condensation of glyoxal with ferrocenyl amine, followed by the ring‐closure of 1,3‐diferrocenyldiimine with chloromethylethyl ether. Chiral imidazolylidene NHC (IFcPr) metal complexes were prepared by treating imidazolium with silver oxide or base followed by transmetalation with appropriate metal salts. The steric and electronic properties of the IFcPr ligand were assessed using %Vbur and TEP, respectively. These assessments indicated that the ligand was as bulky as the adamantyl NHC ligand and exhibited strong donor ability similar to that of a triazolylidene NHC ligand. The catalytic performance of the copper and palladium complexes was evaluated in the asymmetric borylation of ethyl cinnamate and Suzuki–Miyaura coupling reactions, respectively. The IFcPr‐Cu complex catalyzed the borylation of ethyl cinnamate with bis(pinacolato) diboron, followed by oxidation to yield the hydroxy ester with 58% yield and 30% ee. In contrast, the IFcPr‐Pd‐PEPPSI complex provided the axial chiral binaphthyl compound with 59% yield and 51% ee at a low catalyst loading (0.1 mol%, TON = 590).

Porous transport layer influence on overpotentials in PEM water electrolysis at low anode catalyst loadings

Achieving widespread use of polymer electrolyte membrane water electrolysis (PEMWE) for clean hydrogen generation requires enhancing the efficiency and utilization of low iridium-loaded electrolyzers. Novel porous transport layer (PTL) design and engineering can play a critical role in addressing this challenge. Despite the fact that the PTL is an important component of PEM electrolyzers, it has received little attention, particularly in terms of the mechanistic understanding of the influence of PTL characteristics on overpotentials. Herein, we investigate the impact of the PTL porosity, thickness, and structure on the PEM water electrolyzer performance. We demonstrate that improving the contact between PTL and the catalyst layer by inserting a microporous layer facilitates electron transfer and oxygen removal leading to reduced cell voltage. We observe that this strategy proves particularly effective at low catalyst loadings.

Microwave‐Assisted Synthesis of Spiro Heterocyclic Scaffolds Using Graphite Oxide as a Heterogeneous Carbocatalyst and Study of their Anti‐Microbial Activities

AbstractEven though an astonishing variety of spirooxindole derivatives has been synthesized until now, the look for rapid, highly efficacious, environmental‐friendly and more skilful methods is yet an ongoing challenge. Here, we present a microwave‐assisted synthetic approach for the one‐pot synthesis of spirooxindole‐pyrimidine and spiro‐chromeno indoline‐trione derivatives in water using graphite oxide (GO) as a charismatic heterogeneous metal‐free carbocatalyst.Microwave‐assisted synthesis reduces the reaction times from hours (conventional heating) to minutes. The use of a heterogeneous catalyst allows for efficient recovery and reusability up to eight cycles without losing much of its effectiveness, thereby enhancing sustainability. This method offers high product yields of diverse substrates with low catalyst loading and simple work‐up and catalyst handling. In addition, the antimicrobial activities for some randomly taken compounds have also been studied against gram positive bacteria Staphylococcos aureus (MTCC96) and gram negative bacteria Escherichia coli(MTCC730).

Iridium‐Pyrazolato Complex Catalyzed Dehydrogenative Alkylation of Ketones and Arylacetonitriles with Primary Alcohols

Herein, we report the use of an air‐stable iridium pyrazolato complex as an effective catalyst for the α‐alkylation of ketones and arylacetonitriles with primary alcohols, employing borrowing hydrogen strategy. The alkylation reactions were efficiently conducted at 120 °C in toluene utilizing a very low catalyst loading (0.1 mol%). A broad range of ketones and arylacetonitriles, in combination with various aromatic and aliphatic primary alcohols, were successfully transformed into the corresponding α‐alkylated products. Additionally, quinoline derivatives were synthesized from 2‐aminobenzylalcohol and ketones using dehydrogenative coupling strategy. A mechanistic investigation unveiled that the key step involved alcohol activation via metal‐ligand cooperation.

Stereoselective Access to Spiro-isoindolinone Scaffolds via Catalytic Asymmetric aza-Piancatelli Rearrangement.

A chiral Brønsted acid catalyzed asymmetric aza-Piancatelli rearrangement of 3-furyl-3-hydroxyisoindolinones with anilines has been developed, enabling the divergent construction of chiral spiro-cyclopentenone-isoindolinones in high yields with excellent diastereo- and enantioselectivities even under a low catalyst loading of 0.5 mol%.

Gold Trifluoromethyl Complexes as Efficient Regioselective Catalysts in Alkyne Hydration.

Gold(III) complexes containing trifluoromethyl ligands are efficient catalysts in the hydration of alkynes, operating at low catalyst loadings, without additives, using environmentally friendly solvents and at mild conditions (60 °C). Hydration of terminal and internal alkynes provides the corresponding ketones in quantitative yields without special precautions as dry solvents or inert atmospheres. Remarkably, hydration of asymmetric internal alkynes proceeds with moderate to notable regioselectivities, providing mixtures of the two possible isomers with ratios up to 90 : 10.

Highly Functionalized SuFEx-able Hub Bearing All-Carbon Quaternary Center via Rapid Brønsted Superbase Catalysis.

A rapid Brønsted superbase catalysis for highly functionalized sulfur(VI)-fluoride exchange (SuFEx) hubs bearing all-carbon quaternary centers was developed. Remarkable functional group tolerance toward esters and nitriles was achieved by Michael addition of cyanoacetate derivatives to ethenesulfonyl fluoride with low catalyst loadings (∼0.5 mol %) within a short reaction time (0.5-30 min). Gram-scalability, water tolerance, and compatibility in polymeric catalysis showcase its unique practicability. SuFEx click conjugations were demonstrated using various amines, including DNA-tethered biomolecules.

B(C6F5)3-Catalyzed Multicomponent Reactions of 2,3-Diketoesters, Amines, Allenes, and Nucleophiles: Synthesis of 2α-Functionalized Pyrroles.

An efficient B(C6F5)3-catalyzed multicomponent reaction of 2,3-diketoesters, amines, allenes, and nucleophiles was reported, which afforded 2α-functionalized pyrroles in moderate to good yields. The reaction features low catalyst loading, usage of a small amount of solvent, high atom economy, tunable installation of diverse functional groups at 2α-position, and water being formed as the byproduct. This is the first multicomponent reaction that combines vicinal tricarbonyl compounds with allenes.