Silica supported Brönsted acids as catalyst in organic transformations: A comprehensive review

N-Formamide as a carbonyl precursor in the catalytic synthesis of Passerini adducts under aqua and mechanochemical conditions

A new simple, efficient, and environmentally friendly protocol is presented for the catalytic synthesis of α-acyloxycarboxamides using N-formamides as a carbonyl precursor under aqua and mechanochemical conditions. Immobilized sulfuric acid on silica gel was employed for the synthesis of desired products, via the reaction of benzoic acid, 1-napthylisocyanide and various heterocyclic N-formamides. After a careful optimization of the reaction conditions, the desired Passerini products were obtained in high to excellent yields in short reaction times (10-30 min) at room temperature. The highly efficient and environmentally friendly method provides a facile access to a library of α-acyloxycarboxamides derivatives for future research on bioactivity screening.

SiO2-supported HClO4 catalyzed synthesis of (Z)-thiazolylhydrazonoindolin-2-ones and their electrochemical properties

In this paper, an environmentally benign silica-supported perchloric acid (HClO₄-SiO₂) catalyzed green FCDR strategy has been developed for the synthesis of (Z)-THIs (6) with high stereospecificity via an intramolecular hydrogen bond (IHB) directed approach, involving the reaction of methyl ketones (1), N-bromosuccinimide (NBS) (2), isatins (4) and thiosemicarbazide (5) in ethanol at reflux temperature for 45-60 min in one-pot. The reaction proceeds through the construction of C-Br (α-bromination), C-S & C-N (heterocyclization), and CN (condensation) bonds in one pot. The absolute structure of the compound (Z)-3-(2-(4-(4-bromophenyl)thiazol-2-yl)hydrazono)indolin-2-one (6e) has been confirmed by single-crystal XRD analysis. Further, the role of IHB on Z-configuration of the synthesized (Z)-THIs is proved by single-crystal XRD and ¹H NMR studies. Wide substrate scope, good functional group tolerance, scalability, improved safety since the method circumvents the use of highly lachrymatric α-bromoketones as starting materials, high product yields (up to 98%), short reaction times, reusable solid Brønsted acid catalyst (HClO₄-SiO₂), and products that do not require column chromatography purification are all attractive features of this FCDR strategy. Electrochemical properties of THIs (6) are examined by cyclic voltammetry. The HOMO and LUMO energy level of THIs, 6a, 6c, 6d, 6j, 6o-6v, 6y, and 6aa are comparable with the reported ambipolar materials, and the HOMO levels of other THIs, 6b, 6e-6i, 6n, 6w, 6x, 6z and 6 ab-6ae are similar with the most commonly used hole transporting materials (HTMs).

Catalytic Performance of Immobilized Sulfuric Acid on Silica Gel for N-Formylation of Amines with Triethyl Orthoformate

In the search for convenient, green, and practical catalytic methods for the current interest in organic synthesis, a simple, green, and highly efficient protocol for N-formylation of various amines was carried out in the presence of immobilized sulfuric acid on silica gel (H2SO4–SiO2). All reactions were performed in refluxing triethyl orthoformate (65 °C). The product formamides were obtained with high-to-excellent yields within 4 min to 2 h. The current approach is advantageous, due to its short reaction time and high yields. The catalyst is recyclable with no significant loss in catalytic efficiency.

Bis-indolylation of aldehydes and ketones using silica-supported FeCl3: molecular docking studies of bisindoles by targeting SARS-CoV-2 main protease binding sites

We report herein an operationally simple, efficient and versatile procedure for the synthesis of bis-indolylmethanes via the reaction of indoles with aldehydes or ketones in the presence of silica-supported ferric chloride under grindstone conditions. The prepared supported catalyst was characterized by SEM and EDX spectroscopy. The present protocol has several advantages such as shorter reaction time, high yield, avoidance of using harmful organic solvents during the reaction and tolerance of a wide range of functional groups. Molecular docking studies targeted toward the binding site of SARS-CoV-2 main protease (3CL^pro or M^pro) enzymes were investigated with the synthesized bis-indoles. Our study revealed that some of the synthesized compounds have potentiality to inhibit the SARS-CoV-2 M^pro enzyme by interacting with key amino acid residues of the active sites via hydrophobic as well as hydrogen bonding interactions.

Silica supported FeCl₃ catalyzed simple protocol for the synthesis of bis-indolylmethanes was explored via grindstone chemistry. Synthesized compounds were screened virtually as inhibitor by targeting the binding site of SARS-CoV-2 main protease enzyme.

The synthesis and characterization of Fe2O3@SiO2-SO3H nanofibers as a novel magnetic core-shell catalyst for formamidine and formamide synthesis

Over the past several decades, the fabrication of novel ceramic nanofibers applicable in different areas has been a frequent focus of scientists around the world. Aiming to introduce novel ceramic core-shell nanofibers as a magnetic solid acid catalyst, Fe2O3@SiO2-SO3H magnetic nanofibers were prepared in this study using a modification of Fe2O3@SiO2 core-shell nanofibers with chlorosulfonic acid to increase the acidic properties of these ceramic nanofibers. The products were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscope (EDS), vibrating sample magnetometer (VSM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The prepared nanofibers were used as catalysts in formamide and formamidine synthesis. The treatment of aqueous formic acid using diverse amines with a catalytic amount of Fe2O3@SiO2-SO3H nanofibers as a reusable, magnetic and heterogeneous catalyst produced high yields of corresponding formamides at room temperature. Likewise, the reaction of diverse amines with triethyl orthoformate led to the synthesis of formamidine derivatives in excellent yields using this novel catalyst. The catalytic system was able to be recovered and reused at least five times without any catalytic activity loss. Thus, novel core-shell nanofibers can act as efficient solid acid catalysts in different organic reactions capable of being reused several times due to their easy separation by applying magnet.

A Brief Overview of Recent Progress in Porous Silica as Catalyst Supports

Porous silica particles have shown applications in various technological fields including their use as catalyst supports in heterogeneous catalysis. The mesoporous silica particles have ordered porosity, high surface area, and good chemical stability. These interesting structural or textural properties make porous silica an attractive material for use as catalyst supports in various heterogeneous catalysis reactions. The colloidal nature of the porous silica particles is highly useful in catalytic applications as it guarantees better mass transfer properties and uniform distribution of the various metal or metal oxide nanocatalysts in solution. The catalysts show high activity, low degree of metal leaching, and ease in recycling when supported or immobilized on porous silica-based materials. In this overview, we have pointed out the importance of porous silica as catalyst supports. A variety of chemical reactions catalyzed by different catalysts loaded or embedded in porous silica supports are studied. The latest reports from the literature about the use of porous silica-based materials as catalyst supports are listed and analyzed. The new and continued trends are discussed with examples.

Porous Silica-Based Organic-Inorganic Hybrid Catalysts: A Review

Hybrid organic-inorganic catalysts have been extensively investigated by several research groups in the last decades, as they allow combining the structural robust-ness of inorganic solids with the versatility of organic chemistry. Within the field of hybrid catalysts, synthetic strategies based on silica are among the most exploitable, due to the convenience of sol-gel chemistry, to the array of silyl-derivative precursors that can be synthesized and to the number of post-synthetic functionalization strategies available, amongst others. This review proposes to highlight these advantages, firstly describing the most common synthetic tools and the chemistry behind sol-gel syntheses of hybrid catalysts, then presenting exemplificative studies involving mono- and multi-functional silica-based hybrid catalysts featuring different types of active sites (acid, base, redox). Materials obtained through different approaches are described and their properties, as well as their catalytic performances, are compared. The general scope of this review is to gather useful information for those approaching the synthesis of organic-inorganic hybrid materials, while providing an overview on the state-of-the art in the synthesis of such materials and highlighting their capacities.

A Low-Cost, Well-Designed Catalytic System Derived from Household Waste "Egg Shell": Applications in Organic Transformations

A waste feedstock-derived economical basic alternative catalyst is described in this review. Eggshell is one of the household wastes created in tons of weight daily. Therefore, in order to reduce the environmental pollution-related problems, its use in heterogeneous catalysis can be attributed as a great contribution for the chemical and material science society to carry out several known reactions and for the much-needed energy alternative biodiesel production as low-cost catalytic system. Keeping green chemistry in mind, industrial use of these catalysts may also reduce the use of other traditionally used high-cost chemical catalytic systems.

Heterogeneous acidic catalysts for the tetrahydropyranylation of alcohols and phenols in green ethereal solvents

The application of heterogeneous catalysis and green solvents to the set up of widely employed reactions is a challenge in contemporary organic chemistry. We applied such an approach to the synthesis and further conversion of tetrahydropyranyl ethers, an important class of compounds widely employed in multistep syntheses. Several alcohols and phenols were almost quantitatively converted into the corresponding tetrahydropyranyl ethers in cyclopentyl methyl ether or 2-methyltetrahydrofuran employing NH₄HSO₄ supported on SiO₂ as a recyclable acidic catalyst. Easy work up of the reaction mixtures and the versatility of the solvents allowed further conversion of the reaction products under one-pot reaction conditions.

Conversion of glucose into lactic acid using silica-supported zinc oxide as solid acid catalyst

Zinc oxide (ZnO) has been proven to be highly effective in converting biomass into fine chemicals. It possesses several limitations, such as leaching in hydrothermal reactions and difficulty with regard to its recovery. Supporting ZnO on silica improves its recovery, stability and recyclability. In this study, we produced silica-supported ZnO by incipient wetness impregnation (IWI) method for the conversion of glucose into lactic acid. The presence of the ZnO provided active sites for isomerization to occur. The highest yield of lactic acid was 39.2% at 180 °C for 60 min. Prolonged reaction time and higher reaction temperature promoted further degradation of lactic acid into acetic acid. The yield of lactic acid decreased after the first cycle and decreased slightly for the nine consecutive cycles.

Microwave-assisted expeditious approach towards benzimidazole acrylonitrile derivatives exploring a new silica supported SBPTS catalyst

An efficient and eco-friendly synthesis of benzimidazole-acrylonitrile derivatives.

H2SO4-Mediated Stereocontrolled Annulation of Oxygenated Naphthalenes and 4-Alkenols: One-Pot Synthesis of Tetanthrenes

H₂SO₄-mediated one-pot stereocontrolled (4 + 2) annulation of 4-alkenols and oxygenated naphthalenes provided tetanthrenes in CH₂Cl₂ at 25 °C for 10 h. The use of various Brønsted acids or Lewis acids was investigated for a facile and efficient transformation. A plausible mechanism has been proposed.

Brønsted Acid-Mediated Cycloaromatization of 1H-Indol-2-yl Propargyl Benzoates to 7H-Benzo[c]carbazoles

A synthetic method for the efficient assembly of benzo[c]carbazole derivatives that relies on silica gel-activated benzoic acid-mediated cycloaromatization of 1H-indol-2-yl propargyl benzoates under atmospheric conditions is described. Robust with a variety of substitution patterns tolerated, the reaction provides a one-step strategy to construct a member of the N-heterocycles family in good to excellent yields. A tentative mechanism is proposed in which the cycloaromatization process is thought to involve a Brønsted acid-mediated formal 1,3-acyloxy migration/6π-electrocyclization pathway.

Synthesis of tricyanomethanesulfonic acid as a novel nanostructured and recyclable solid acid: application at the synthesis of biological henna-based chromenes

A novel and unique nanostructured solid acid, namely tricyanomethanesulfonic acid (TCMS), was designed, synthesized, and attained as an ecofriendly, reusable catalyst for the one-pot three-component synthesis of 2-amino-3-cyano-4-aryl-5,10-dioxo-5,10-dihydro-4H-benzo[g]chromene derivatives from the reaction between aromatic aldehydes, 2-hydroxynaphthalene-1,4-dione (henna), and malononitrile under solvent-free conditions at room temperature with good to excellent yields and a short reaction time. The TCMS catalyst was prepared and fully characterized by FTIR, 1H NMR, 13C NMR, mass, X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric (TG), and derivative thermal gravimetric (DTG) analyses. The nanostructured solid acid catalyst was simply recycled at least eight times without any loss of catalytic activity. This process is improved as a safe and appropriate procedure for the synthesis of 2-amino-3-cyano-4-aryl-5,10-dioxo-5,10-dihydro-4H-benzo[g]chromenes using an environmentally friendly and green recyclable catalyst. Thus, TCMS is a good candidate for the synthesis of new molecular organic frameworks and gelators.

Fatty Acid Esterification with Polyols over Acidic Montmorillonite

The production of fatty acid esters from stearic, oleic, and palmitic acids and polyols (ethylene glycol and glycerol) was investigated in this work. A series of montmorillonite-based clays catalysts (KSF, KSF/0, KP10, and K10), having different physicochemical properties, were used as acidic catalysts. The influence of the specific surface area and the acidity of the catalysts on the esterification rate were explored. The best catalytic activities were obtained with KSF catalyst. The optimization of various factors on the reaction was also studied, including catalyst concentration, reaction temperature and molar ratio (polyol / fatty acid). The yield rate reached 94% under the optimum conditions and the recovery rate maintained more than 96% after 5 batches.

Nickel containing ionic liquid based ordered nanoporous organosilica: a powerful and recoverable catalyst for synthesis of polyhydroquinolines

The synthesis, characterization and catalytic application of a novel nickel containing ionic liquid based ordered mesoporous organosilica are demonstrated.

An efficient solvent-free synthesis of bis(indolyl)methane-based naked eye chemosensor for Cu2+ion from β-chloro-α,β-unsaturated aldehydes using PMA-Cellulose as a solid phase reusable catalyst

A green one pot synthesis of novel bis(indolyl)methanes as naked eye chemosensors towards Cu²⁺ion has been developed from β-chloro-α,β-unsaturated aldehydes using new reusable solid acid support/catalyst PMA-Cellulose.