Functionalized Metal-Free Carbon Nanosphere Catalyst for the Selective C-N Bond Formation under Open-Air Conditions

A versatile shape-controlled carbon nanomaterial that can efficiently catalyze the selective C-N coupling reactions under metal-free and open-air conditions was developed by applying N-doping and KOH activation strategies in candle soot (ANCS). The TEM and elemental mapping results showed the formation of sphere-shaped carbon particles as well as the uniform distribution of nitrogen species in the carbon framework. KOH activation enhanced the specific surface area of carbon, whereas N-doping enriched the electron-deficient nature by introducing functional N-based pyrrolic/graphitic structures in the carbon framework. The synergistic effect of N-doping and KOH activation significantly improved the catalytic efficiency of the carbon catalyst (ANCS), giving a 96% conversion of o-phenylenediamine (OPD) with a good selectivity to 2-phenylbenzimidazole (97%). In contrast, the pristine carbon exhibited very low activity (48% conversion of the OPD and 36% selectivity to 2-phenylbenzimidazole). Besides, the ANCS nanomaterial provided a facile catalytic approach for the homo- and cross-C-N condensation of various aromatic amines and diamines to produce diverse functional imines and benzimidazoles at mild conditions. This work provided promising insights into developing advanced, metal-free carbon-based catalysts for selective C-N coupling reactions to produce valuable drug motifs.

Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein-Ponndorf-Verley Reduction

The development of highly active acid-base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid-base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein-Ponndorf-Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)-OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).

Cerium Oxide Nanoparticles Confined in Doped Mesoporous Carbons: A Strategy to Produce Catalysts for Imine Synthesis

A group of (doped N or P) carbons were synthesized using soluble starch as a carbon precursor. Further, ceria nanoparticles (NPs) were confined into these (doped) carbon materials. The obtained solids were characterized by various techniques such as N2 physisorption, XRD, TEM, SEM, XPS, and XAS. These materials were used as catalysts for the oxidative coupling between benzyl alcohol and aniline as the model reaction. Ceria immobilized on mesoporous-doped carbon shows higher activity than the other materials, benchmark catalysts, and most of the previously reported catalysts. The control of the ceria NP size, the presence of Ce3+ cations, and an increment in the disorder in the ceria NP structure caused by a support-ceria interaction could increase the number of oxygen vacancies and improve its catalytic performance. CN-meso/CeO2 was also used as the catalyst for a rich scope of substrates, such as substituted aromatic alcohols, linear alcohols, and different types of amines. The influence of various reaction parameters (substrate content, reaction temperature, and catalyst content) on the activity of this catalyst was also checked.

Effect of surface acidity and basicity of supported Ni catalysts on the N-alkylation of isopropylamine with isopropanol

The effect of surface acidity and basicity on the adsorption for isopropanol, isopropylamine and diisopropylamine and their interactions on Ni catalyst was revealed, which was related to the N-alkylation of isopropylamine with isopropanol.

Regioselective Mercury(I)/Palladium(II)-Catalyzed Single-Step Approach for the Synthesis of Imines and 2-Substituted Indoles

An efficient synthesis of ketimines was achieved through a regioselective Hg(I)-catalyzed hydroamination of terminal acetylenes in the presence of anilines. The Pd(II)-catalyzed cyclization of these imines into the 2-substituted indoles was satisfactorily carried out by a C-H activation. In a single-step approach, a variety of 2-substituted indoles were also generated via a Hg(I)/Pd(II)-catalyzed, one-pot, two-step process, starting from anilines and terminal acetylenes. The arylacetylenes proved to be more effective than the alkyl derivatives.

Selective N-Alkylation of Amines with DMC over Biogenic Cu-Zr Bimetallic Nanoparticles

Herein, we report the green synthesis of copper-zirconium bimetallic nanoparticles (Cu-Zr BNPs) from aqueous solutions using Azadirachta indica leaf extract as a reducing and stabilizing agent. The CuO, ZrO2 NP, and Cu-Zr BNP samples were characterized by X-ray diffraction and Fourier transform infrared (FTIR) spectroscopy, and the morphologies of the samples were analyzed by high-resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction analysis (SAED). The synthesized Cu-Zr BNPs have been employed as efficient catalysts for the selective N-methylation of aromatic and aliphatic amines with dimethyl carbonate. The effect of process conditions on the percentage conversion of benzylamine with dimethyl carbonate as a model reaction has been investigated. The Cu-Zr bimetallic nanoparticle catalytic system in a 1:2 molar ratio was able to convert amines into the corresponding N-methylated amines with a selectivity up to 91% at 180 °C in 4 h. The analysis of catalytic reusability confirmed that the reported heterogeneous catalyst can be used for five consecutive cycles without much loss in activity. Thus, the current protocol can be considered as a simpler, reproducible, and environmentally benign approach for N-methylation of amines.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

High-surface-area mesoporous CeO₂ (hsmCeO₂ ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m²  g^-1 ), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵  spins g^-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol  g ceria - 1  h^-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Efficient imine synthesis via oxidative coupling of alcohols with amines in an air atmosphere using a mesoporous manganese–zirconium solid solution catalyst

Mesoporous Mn₁Zr_xO_y solid solution enables efficient imine formation from catalytic oxidative coupling of alcohols and amines at low temperature in an air atmosphere without base additives.

A magnetic CoRu-CoOX nanocomposite efficiently hydrogenates furfural to furfuryl alcohol at ambient H2 pressure in water

A one-pot synthesized CoRu-CoOX nanocomposite was reported as a magnetically recoverable catalyst for selective hydrogenation of furfural to furfuryl alcohol in water at ambient H2 pressure.

Tuning effect of amorphous Fe2O3 on Mn3O4 for efficient atom-economic synthesis of imines at low temperature: improving [O] transfer cycle of Mn3+/Mn2+ in Mn3O4

A novel Fe₂O₃ modified Mn₃O₄ catalyst (Fe₅Mn₅-100) has been prepared by adopting a simple co-precipitation method following low temperature baking. Fe₅Mn₅-100 showed exceptionally high catalytic activity for the production of imine.