Pd-Loaded Cellulose NanoSponge as a Heterogeneous Catalyst for Suzuki-Miyaura Coupling Reactions

The (eco)design and synthesis of durable heterogeneous catalysts starting from renewable sources derived from biomass waste represents an important step for reducing environmental impacts of organic transformations. Herein, we report the efficient loading of Pd(II) ions on an eco-safe cellulose-based organic support (CNS), obtained by thermal cross-linking between TEMPO-oxidized cellulose nanofibers and branched polyethyleneimine in the presence of citric acid. A 22.7% w/w Pd-loading on CNS was determined by the ICP-OES technique, while the metal distribution on the xerogel was evidenced by SEM-EDS analysis. XPS analysis confirmed the direct chelation of Pd(II) ions by means of the high number of amino groups present in the network, so that further functionalization of the support with specific ligands was not necessary. The new composite turned to be an efficient heterogeneous pre-catalyst for promoting Suzuki-Miyaura coupling reactions between aryl halides and phenyl boronic acid in water, obtaining yields higher than 90% in 30 min, by operating in a microwave reactor at 100 °C and with just 2% w/w of CNS-Pd catalyst with respect to aryl halides (4.5‱ for Pd). At the end of first reaction cycle, Pd(II) ions on the support resulted in being reduced to Pd(0) while maintaining the same catalytic efficiency. In fact, no leaching was observed at the end of reactions, and five cycles of recycling and reusing of CNS-Pd catalyst provided excellent results in terms of yields and selectivity in the desired products.

Pd@Py2PZ@MSN as a Novel and Efficient Catalyst for C–C Bond Formation Reactions

In this study, a novel catalyst is introduced based on the immobilization of palladium onto dipyrido (3,2-a:2′,3′-c) phenazine–modified mesoporous silica nanoparticles. The dipyrido (3,2-a:2′,3′-c) phenazine (Py₂PZ) ligand is synthesized in a simple method from the reaction of 1,10-phenanthroline-5,6-dione and 3,4-diaminobenzoic acid as starting materials. The ligand is used to functionalize mesoporous silica nanoparticles (MSNs) and modify their surface chemistry for the immobilization of palladium. The palladium-immobilized dipyrido (3,2-a:2′,3′-c) phenazine–modified mesoporous silica nanoparticles (Pd@Py₂PZ@MSNs) are synthesized and characterized by several characterization techniques, including TEM, SEM, FT-IR, TGA, ICP, XRD, and EDS analyses. After the careful characterization of Pd@Py₂PZ@MSNs, the activity and efficiency of this catalyst is examined in carbon–carbon bond formation reactions. The results are advantageous in water and the products are obtained in high isolated yields. In addition, the catalyst showed very good reusability and did not show significant loss in activity after 10 sequential runs.

Dendritic structured palladium complexes: magnetically retrievable, highly efficient heterogeneous nanocatalyst for Suzuki and Heck cross-coupling reactions

The recyclable nanomagnetic Pd-complex PAMAM G₀-Pd@γ-Fe₂O₃ is reported for catalytic C–C cross-coupling reactions of challenging substrates. Mainly, a great variety of aryl chlorides can be used as substrates for Suzuki–Miyaura and Mizoroki–Heck reactions under mild reaction conditions (60–90 °C) and low catalyst loading (<1 mol% Pd) in aqueous media. The presence of numerous polar groups in the polymer matrix increases the solubility of the catalyst in water, thus facilitating its operation in aqueous environments. The immobilization of the catalyst on the surface of a magnetic platform allows its effective recovery and reuse without significant loss of catalytic activity for at least six cycles with total leaching of <1% palladium metal, meeting the requirements for acceptable metal residues in the pharmaceutical industry.

The recyclable nanomagnetic Pd-complex PAMAM G₀-Pd@γ-Fe₂O₃ is reported for catalytic C–C cross-coupling reactions of challenging substrates.

Magnetically Directed Co-nanoinitiators for Cross-Linking Adhesives and Enhancing Mechanical Properties

Magnetically directed localized polymerization is of immense interest for its extensive impacts and applications in numerous fields. The use of means untethered from an external magnetic field to localize initiation of polymerization to develop a curing system is a novel concept, with a sustainable, efficient, and eco-friendly approach and a wide range of potential in both science and engineering. However, the conventional means for the initiation of polymerization cannot define the desirable location of polymerization, which is often exacerbated by the poor temporal control in the curing system. Herein, the copper-immobilized dendrimer-based magnetic iron oxide silica (MNPs-G2@Cu²⁺) co-nanoinitiators are rationally designed as initiators for redox radical polymerization. The nanoinitiators are magnetically responsive and therefore enable localized polymerization using an external magnetic field. In this work, anaerobic polymerization of an adhesive composed of triethylene glycol dimethacrylate, tert-butyl peroxybenzoate, and MNPs-G2@Cu²⁺ as the magnetic co-nanoinitiators has been investigated. The use of a magnet locates and promotes redox free radical polymerization through the synergistic functions between peroxide and MNPs-G2@Cu²⁺ co-nanoinitiators. The mechanical properties of the resulting polymer are considerably reinforced because the MNPs-G2@Cu²⁺ co-nanoinitiators concurrently play another crucial role as nanofillers. This strategy provides a novel approach for magnetically tunable localized polymerization, which allows new opportunities to govern the formulation of advanced adhesives through polymerization under hazard-free conditions for various promising applications.

Prospects and Applications of Palladium Nanoparticles in the Cross-coupling of (hetero)aryl Halides and Related Analogues

Discovering efficient methods for the formation of carbon-carbon bonds is a central ongoing theme in organic synthesis. Cross-coupling reactions catalysed by metal nanoparticles are attractive alternatives to the traditional use of metal counterparts due to the catalytic tunability, selectivity, recyclability and reusability of the nanoparticles. The ongoing search for sustainable processes demands that reusable and environmentally benign catalysts are used. While the advantages of nanoparticles catalysts over bulk catalysts cannot be overemphasised, the problem of sintering, agglomeration and leaching are drawbacks to their full industrial applications. Hence, efforts are being made towards advancing the efficiency of the catalytic nanoparticle systems over the years. This review presents the progress, the challenges and the prospects of palladium nanoparticle with focus on Heck, Suzuki, Hiyama and Sonogashira cross-coupling reactions involving (hetero) aryl halides and the analogues.

A decennary update on applications of metal nanoparticles (MNPs) in the synthesis of nitrogen- and oxygen-containing heterocyclic scaffolds

Heterocycles have been found to be of much importance as several nitrogen- and oxygen-containing heterocycle compounds exist amongst the various USFDA-approved drugs. Because of the advancement of nanotechnology, nanocatalysis has found abundant applications in the synthesis of heterocyclic compounds. Numerous nanoparticles (NPs) have been utilized for several organic transformations, which led us to make dedicated efforts for the complete coverage of applications of metal nanoparticles (MNPs) in the synthesis of heterocyclic scaffolds reported from 2010 to 2019. Our emphasize during the coverage of catalyzed reactions of the various MNPs such as Ag, Au, Co, Cu, Fe, Ni, Pd, Pt, Rh, Ru, Si, Ti, and Zn has not only been on nanoparticles catalyzed synthetic transformations for the synthesis of heterocyclic scaffolds, but also provide an inherent framework for the reader to select a suitable catalytic system of interest for the synthesis of desired heterocyclic scaffold.

Carbonylative Suzuki-Miyaura cross-coupling by immobilized Ni@Pd NPs supported on carbon nanotubes

In this study, a novel carbon nanotube (CNT) based nanocatalyst (Ni@Pd/CNT) was synthesized by modifying CNTs using Ni@Pd core–shell nanoparticles (NPs). Ni@Pd/CNT was used in catalytic carbonylative cross-coupling between 4-iodoanisole and phenylboronic acid. The Ni@Pd NPs possessed a magnetic nickel (Ni) core with a palladium (Pd) structural composite shell. Thus, the use of Ni had led to a reduced consumption of Pd without sacrificing the overall catalytic performance, simultaneously making it reusable as it could be conveniently recovered from the reaction mixture by using an external magnetic field. Immobilization of the Ni@Pd NPs on carbon nanotubes not only prevented their aggregation, but also significantly enhanced the accessibility of the catalytically active sites. The abovementioned approach based on carbon nanotubes and Ni@Pd NPs provided a useful platform for the fabrication of noble-metal-based nanocatalysts with easy accessibility and low cost, which may allow for an efficient green alternative for various catalytic reductions.

In this study, a novel carbon nanotube (CNT) based nanocatalyst (Ni@Pd/CNT) was synthesized by modifying CNTs using Ni@Pd core–shell nanoparticles (NPs).

Atom-efficient synthesis of 2,4,6-trisubstituted 1,3,5-triazines via Fe-catalyzed cyclization of aldehydes with NH4I as the sole nitrogen source

An atom-efficient, straightforward method for the synthesis of 2,4,6-triaryl-1,3,5-triazines via iron-catalyzed cyclization of aldehydes with NH₄I as the sole nitrogen source is demonstrated. This strategy works smoothly under air atmosphere, and affords symmetrical 2,4,6-trisubstituted and unsymmetrical 1,3,5-triazines with yields from 18% to 72%. Compared to other methods, the present protocol provides a straightforward and atom-efficient approach to 2,4,6-trisubstituted 1,3,5-triazines using an inexpensive, easily available ammonium salt as the sole nitrogen source. Research into the preliminary mechanism indicates that N-benzylidenebenzimidamides are involved in this cyclization reaction.

An atom-efficient, straightforward method for the synthesis of 2,4,6-triaryl-1,3,5-triazines via iron-catalyzed cyclization of aldehydes with NH₄I as the sole nitrogen source is demonstrated.

Palladium nanoparticles immobilized on a nano-silica triazine dendritic polymer: a recyclable and sustainable nanoreactor for C–S cross-coupling

Dendrimers are of great interest due to their special structural topology and chemical versatility. Owing to their properties, dendrimers have found practical applications in catalytic processes as efficient nanoreactors. Therefore, we herein report an environmentally attractive strategy and highly efficient route for the synthesis of a wide variety of diaryl sulfides using palladium nanoparticles immobilized on a nano-silica triazine dendritic polymer (Pd_np-nSTDP) as a nanoreactor. In this manner, different diaryl or aryl heteroaryl sulfides and bis(aryl/heteroarylthio)benzene/anthracene/pyridine derivatives were prepared via C–S cross-coupling reactions of aryl halides with diaryl/diheteroaryl disulfides under thermal conditions and microwave irradiation. The catalyst could be easily recovered and reused several times without any significant loss of its activity.

Efficient synthesis of diaryl sulfides via C–S cross-coupling of aryl halides with disulfides catalyzed by Pd_np-nSTDP is reported.

Transition metal-free assembly of 1,3,5-triazines using ethyl bromodifluoroacetate as C1 source

Transition-metal and oxidant-free annulation of amidine to access 2,4-disubstituted-1,3,5-triazines using ethyl bromodifluoroacetate as C1 source via quadruple cleavage.

Facile synthesis of palladium nanoparticles on hierarchical hollow silica spheres and its catalytic properties in Suzuki-reaction

A novel well-dispersed Pd@HHSS catalyst was synthesized by reduction of Pd(OAc)2 immobilized on HHSS we reported. When the ratios of Pd/SiO2 were 5 : 100 and 10 : 100, the Pd nanoparticles size was about 5-10 nm. The Pd@HHSS catalyst (Pd/SiO2 = 10 : 100) showed high catalytic activity in Suzuki-reaction with yields of 91-99% and the catalyst is reusable after four successive cycles without obvious loss of catalytic activity.

Coimmobilization of β-Agarase and α-Neoagarobiose Hydrolase for Enhancing the Production of 3,6-Anhydro-l-galactose

Here we report a simple and efficient method to produce 3,6-anhydro-l-galactose (l-AHG) and agarotriose (AO3) in one step by a multienzyme system with the coimmobilized β-agarase AgWH50B and α-neoagarobiose hydrolase K134D. K134D was obtained by AgaWH117 mutagenesis and showed improved thermal stability when immobilized via covalent bonds on functionalized magnetic nanoparticles. The obtained multienzyme biocatalyst was characterized by Fourier transform infrared spectroscopy (FTIR). Compared with free agarases, the coimmobilized agarases exhibited a relatively higher agarose-to-l-AHG conversion efficiency. The yield of l-AHG obtained with the coimmobilized agarases was 40.6%, which was 6.5% higher than that obtained with free agarases. After eight cycles, the multienzyme biocatalyst still preserved 46.4% of the initial activity. To the best of our knowledge, this is the first report where two different agarases were coimmobilized. These results demonstrated the feasibility of the new method to fabricate a new multienzyme system onto magnetic nanoparticles via covalent bonds to produce l-AHG.

Novel Multicomponent Synthesis of Pyridine-Pyrimidines and Their Bis-Derivatives Catalyzed by Triazine Diphosphonium Hydrogen Sulfate Ionic Liquid Supported on Functionalized Nanosilica

In this Research Article, we report an efficient synthesis of 1,3-dimethyl-5-aryl-7-(pyridine-3(2)(4)-yl)pyrimidine-2,4(1H,3H)-diones via a three-component reaction of aryl aldehydes, 1,3-dimethyl-6-aminouracil and carbonitriles in the presences of triazine diphosphonium hydrogen sulfate ionic liquid supported on functionalized nanosilica (APTADPHS-nSiO₂) as a reusable catalyst under microwave irradiation and solvent-free conditions. The bis-derivatives of pyridine-pyrimidines were also efficiently prepared from dialdehydes and dinitriles. In addition, 3-methyl-1H-pyrazole-5-amine was used successfully instead of 1,3-dimethyl-6-aminouracil under the same conditions to afford the corresponding products in high yields. The catalyst can be reused at least five times without any significant loss of its activity. The easy recovery, reusability and excellent activity of the catalyst as well as easy workup are other noteworthy advantages of this method.

Pd nanoparticles immobilized on carbon nanotubes with a polyaniline coaxial coating for the Heck reaction: coating thickness as the key factor influencing the efficiency and stability of the catalyst

Pd/PANI@CNTs were synthesized using a low-cost and simple method. The thickness of the PANI layer is the key in determining the stability of the catalyst in the Heck reaction.

A dendritic TiO2–Co(ii) nanocomposite based on the melamine catalyzed one-pot aerobic photocatalytic synthesis of benzimidazoles

The nanocrystalline TiO₂ surface modified with cobalt melamine dendrimer amine complex under ultrasonic agitation produced a novel low band-gap photocatalyst (Co(ii)–TD@TiO₂). This catalyst system involves photooxidation of alcohols to aldehydes followed by cyclocondensation with 1,2-phenylenediamine to afford benzimidazoles in one-pot operation with good to excellent yield.

Copper-catalyzed ambient-temperature decarboxylative annulation of isatins with amidine hydrochlorides: a facile access to 2-(1,3,5-triazin-2-yl)aniline derivatives

A copper-catalyzed domino protocol for the synthesis of 2-(1,3,5-triazin-2-yl)anilines has been developed from isatins and amidine hydrochlorides at ambient temperature.

iEDDA Reaction of the Molecular Iodine-Catalyzed Synthesis of 1,3,5-Triazines via Functionalization of the sp3 C-H Bond of Acetophenones with Amidines: An Experimental Investigation and DFT Study

The present work reports an inverse electron demand Diels-Alder (iEDDA)-type reaction to synthesize 1,3,5-trizines from acetophenones and amidines. The use of molecular iodine in a catalytic amount facilitates the functionalization of the sp³ C-H bond of acetophenones. This is a simple and efficient methodology for the synthesis of 1,3,5-triazines in good to excellent yields under transition-metal-free and peroxide-free conditions. The reaction is believed to take place via an in situ iodination-based oxidative elimination of formaldehyde. DFT calculations at the M062X/6-31+G(d,p) level were employed to investigate the reaction mechanism. Reaction barriers for the cycloaddition as well as a formaldehyde expulsion steps were computed, and a multistep mechanism starting with the nucleophilic attack by benzamidine on an in situ generated imine intermediate has been proposed. Both local and global reactivity descriptors were used to study the regioselectivity of the addition steps.