Interactions between PAMAM-NH2 and 6-Mercaptopurine: Qualitative and Quantitative NMR studies

Despite being used as an anti-leukemic drug, the poor solubility of 6-mercaptopurine (6-MP) limits its use in topical and parenteral applications. Dendrimers are commonly used as drug carriers to improve their solubility in aqueous solution. In this work, the interactions between 6-MP and the amine-terminated poly(amidoamine) dendrimers (PAMAM-NH₂ ) were investigated by various NMR technology. The chemical shift titrations disclosed that the 6-MP interacted with the surface of PAMAM-NH₂ mainly through electrostatics. The determination of diffusion coefficient and relaxation measurements further confirmed the presence of interactions in 6-MP/PAMAM-NH₂ complexes. In addition, the encapsulation of 6-MP within the cavity of PAMAM-NH₂ was revealed through nuclear Overhauser effect spectroscopy and Saturation Transfer Double Difference analysis. Finally, the binding strength (H-8 is 100% and H-2 is 70%) of 6-MP to PAMAM-NH₂ was quantitatively expressed using epitope maps. This study provides a systematic methodology for qualitative and quantitative studies of the interactions between dendrimers and drug molecules in general.

Synthesis and characterization of supported stabilized palladium nanoparticles for selective hydrogenation in water at low temperature

Zirconia supported vacant phosphotungstate stabilized Pd nanoparticles (Pd-PW11/ZrO2) were prepared using a simple impregnation and post reduction method, characterized and their efficiency for selective C[double bond, length as m-dash]C hydrogenation of unsaturated compounds explored. The establishment of a hydrogenation strategy at low temperature using water as solvent under mild conditions makes the present system environmentally benign and green. The catalyst shows outstanding activity (96% conversion) with just a small amount of Pd(0) (0.0034 mol%) with high substrate/catalyst ratio (29 177/1), TON (28 010) and TOF (14 005 h-1) for cyclohexene (as a model substrate) hydrogenation. The catalyst was recovered by simple centrifugation and reused for up to five catalytic cycles without alteration in its activity. The present catalyst was found to be viable towards different substrates with excellent activity and TON (18 000 to 28 800). A study on the effect of addenda atom shows that the efficiency of the catalyst can be enhanced greatly by increasing the number of counter protons. This challenging strategy would greatly benefit sustainable development in chemistry as it diminishes the use of organic solvents and offers economic and environmental benefits as water is cheap and non-toxic.

An overview on synthetic strategies for the construction of star-shaped molecules

Strategies for the synthesis of star-shaped molecules have been in high demand in the last decades due to the importance of those compounds in various fields. The distinctly different properties of these compounds compared to their linear analogues make them versatile building blocks for the formation of mesophases of interesting mesomorphic and photophysical properties. Moreover, the applications of star-shaped molecules as building units for dendrimers as well as in supramolecular host-guest chemistry have also been recently studied. The star-shaped molecules mentioned in this review are classified according to the central core as well as the type of side arms. The properties and applications of these compounds are described in the appropriate contexts. This report summarizes the recent advances in this area.

Dendrimer-Encapsulated Pd Nanoparticles, Immobilized in Silica Pores, as Catalysts for Selective Hydrogenation of Unsaturated Compounds

Heterogeneous Pd-containing nanocatalysts, based on poly (propylene imine) dendrimers immobilized in silica pores and networks, obtained by co-hydrolysis in situ, have been synthesized and examined in the hydrogenation of various unsaturated compounds. The catalyst activity and selectivity were found to strongly depend on the carrier structure as well as on the substrate electron and geometric features. Thus, mesoporous catalyst, synthesized in presence of both polymeric template and tetraethoxysilane, revealed the maximum activity in the hydrogenation of various styrenes, including bulky and rigid stilbene and its isomers, reaching TOF values of about 230000 h-1. Other mesoporous catalyst, synthesized in the presence of polymeric template, but without addition of Si(OEt)4, provided the trans-cyclooctene formation with the selectivity of 90-95 %, appearing as similar to homogeneous dendrimer-based catalysts. Microporous catalyst, obtained only on the presence of Si(OEt)4, while dendrimer molecules acting as both anchored ligands and template, demonstrated the maximum activity in the hydrogenation of terminal linear alkynes and conjugated dienes, reaching TOF values up to 400000 h-1. Herein the total selectivity on alkene in the case of terminal alkynes and conjugated dienes reached 95-99 % even at hydrogen pressure of 30 atm. The catalysts synthesized can be easily isolated from reaction products and recycled without significant loss of activity.

Palladium functionalized phosphinite polyethyleneimine grafted magnetic silica nanoparticles as an efficient catalyst for the synthesis of isoquinolino[1,2-b]quinazolin-8-ones

Novel 5-methyl-8H-isoquinolino[1,2-b]quinazolin-8-one derivatives are synthesized using a reusable immobilized palladium catalyzed intramolecular carbon–carbon bond formation.

Physico-chemical properties of β-diketone phosphorus-containing dendrimers

Protolytic, absorbance and fluorescence properties of β-diketone phosphorus-containing dendrimers based on cyclotriphosphazene core were studied. Dendrimer solutions in acetone are characterized by intense absorbance band at ≈ 340 nm (ε340≈ 8.5·104L/mol·сm) and fluorescence band with maximum at 440 nm. Position of these maxima does not change in various solvents, unlike the bands of monomer β-diketone. It was found that dendrimer aggregation is accompanied by appearance of a second absorbance band ε400≈ 4.5·103L/mol·сm, by red shift of emission spectra ∆λ ≈ 10 nm and also by decrease in surface tension of acetone solution. Ability of dendrimer aggregates to solubilize organic substrates was observed with the fluorescent indicator acridine.

Palladium(0) Nanoparticle Formation, Stabilization, and Mechanistic Studies: Pd(acac)₂ as a Preferred Precursor, [Bu₄N]₂HPO₄ Stabilizer, plus the Stoichiometry, Kinetics, and Minimal, Four-Step Mechanism of the Palladium Nanoparticle Formation and Subsequent Agglomeration Reactions

Palladium(0) nanoparticles continue to be important in the field of catalysis. However, and despite the many prior reports of Pd(0)n nanoparticles, missing is a study that reports the kinetically controlled formation of Pd(0)n nanoparticles with the simple stabilizer [Bu4N]2HPO4 in an established, balanced formation reaction where the kinetics and mechanism of the nanoparticle-formation reaction are also provided. It is just such studies that are the focus of the present work. Specifically, the present studies reveal that Pd(acac)2, in the presence of 1 equiv of [Bu4N]2HPO4 as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 ± 1 psig initial H2 pressure at 22.0 ± 0.1 °C to yield 7 ± 2 nm palladium(0) nanoparticles. Studies of the balanced stoichiometry of the Pd(0)n nanoparticle-formation reaction shows that 1.0 Pd(acac)2 consumes 1.0 equiv of H2 and produces 1.0 equiv of Pd(0)n while also releasing 2.0 ± 0.2 equiv of acetylacetone. The inexpensive, readily available HPO4(2-) also proved to be as effective a Pd(0)n nanoparticle stabilizer as the more anionic, sterically larger, "Gold Standard" stabilizer P2W15Nb3O62(9-). The kinetics and associated minimal mechanism of formation of the [Bu4N]2HPO4-stabilized Pd(0)n nanoparticles are also provided, arguably the most novel part of the present studies, specifically the four-step mechanism of nucleation (A → B, rate constant k1), autocatalytic surface growth (A + B → 2B, rate constant k2), bimolecular agglomeration (B + B → C, rate constant k3), and secondary autocatalytic surface growth (A + C → 1.5C, rate constant k4), where A is Pd(acac)2, B represents the growing, smaller Pd(0)n nanoparticles, and C represents the larger, most catalytically active Pd(0)n nanoparticles. Additional details on the mechanism and catalytic properties of the resultant Pd(0)n·HPO4(2-) nanoparticles are provided in this work.

Nanocasting synthesis of an ordered mesoporous CeO2-supported Pt nanocatalyst with enhanced catalytic performance for the reduction of 4-nitrophenol

Ordered mesoporous ceria was used to support Pt nanoparticles stabilized by dendrimers and enhance catalytic reduction of 4-nitrophenol.

Pd(0) encapsulated nanocatalysts as superior catalytic systems for Pd-catalyzed organic transformations

In the last decade, Pd(0) nanoparticles have attracted increasing attention due to their outstanding utility as nanocatalysts in a wide variety of key chemical reactions.

Magnetic and dendritic catalysts

The recovery and reuse of catalysts is a major challenge in the development of sustainable chemical processes. Two methods at the frontier between homogeneous and heterogeneous catalysis have recently emerged for addressing this problem: loading the catalyst onto a dendrimer or onto a magnetic nanoparticle. In this Account, we describe representative examples of these two methods, primarily from our research group, and compare them. We then describe new chemistry that combines the benefits of these two methods of catalysis. Classic dendritic catalysis has involved either attaching the catalyst covalently at the branch termini or within the dendrimer core. We have used chelating pyridyltriazole ligands to insolubilize catalysts at the termini of dendrimers, providing an efficient, recyclable heterogeneous catalysts. With the addition of dendritic unimolecular micelles olefin metathesis reactions catalyzed by commercial Grubbs-type ruthenium-benzylidene complexes in water required unusually low amounts of catalyst. When such dendritic micelles include intradendritic ligands, both the micellar effect and ligand acceleration promote faster catalysis in water. With these types of catalysts, we could carry out azide alkyne cycloaddition ("click") chemistry with only ppm amounts of CuSO4·5H2O and sodium ascorbate under ambient conditions. Alternatively we can attach catalysts to the surface of superparamagnetic iron oxide nanoparticles (SPIONs), essentially magnetite (Fe3O4) or maghemite (γ-Fe2O3), offering the opportunity to recover the catalysts using magnets. Taking advantage of the merits of both of these strategies, we and others have developed a new generation of recyclable catalysts: dendritic magnetically recoverable catalysts. In particular, some of our catalysts with a γ-Fe2O3@SiO2 core and 1,2,3-triazole tethers and loaded with Pd nanoparticles generate strong positive dendritic effects with respect to ligand loading, catalyst loading, catalytic activity and recyclability. In other words, the dendritic catalysts were more efficient and more stable than their nondendritic γ-Fe2O3@SiO2 analogues. The bulk at the dendritic periphery helps to localize the metal nanoparticles at the SPION core surface, which confers these advantages. We could also use sonification as a remarkably simple and efficient method to impregnate the SPIONs with dendrimer-encapsulated PdNPs. Catalysis within the hydrophobic dendrimer pockets that include ligands leads to rapid turnover with or without a γ-Fe2O3@SiO2 core. In addition, catalytically active metal nanoparticles are more robust when they are loaded onto the surface of a γ-Fe2O3@SiO2 dendritic core. Herein, we illustrate this chemistry with examples including olefin metathesis, click chemistry, cross carbon-carbon bond forming reactions, and selective alcohol oxidation.

Linkage of the PAMAM type dendrimer with the gel type resin based on glycidyl methacrylate terpolymer as a method of preparation of the polymer support for the recyclable palladium catalyst for Suzuki–Miyaura cross-coupling reactions

Poly(amidoamine) PAMAM type hyperbranched systems were immobilized on an epoxy functionalized polymer based on glycidyl methacrylate (the GMA resin).

Reliable palladium nanoparticle syntheses in aqueous solution: the importance of understanding precursor chemistry and growth mechanism

The consideration of precursor chemistry and growth mechanism enables the reliable synthesis of palladium nanoparticles even in the absence of stabilizing agents.

Synthesis of dendrimer-templated Pt nanoparticles immobilized on mesoporous alumina for p-nitrophenol reduction

Dendrimer-templated mesoporous alumina-supported Pt nanocatalysts were prepared and used to catalyze reduction reaction after calcination at different temperatures in nitrogen.

Pd nanoparticles in dendrimers immobilized on silica-polyamine composites as catalysts for selective hydrogenation

New heterogeneous hydrogenation catalysts, based on Pd nanoparticles and polypropyleneimine (PPI) dendrimers of the third generation that have been covalently grafted to a silica surface modified with polyallylamine (PAA) have been synthesized. The final products were characterized by TEM, XPS, and solid-state NMR spectroscopy. The synthesized materials are effective catalysts for selective hydrogenation of dienes to monoenes and phenyl acetylene to styrene at very high substrate/Pd ratios with turnover rates higher than related Pd nanoparticle catalysts. The synthesized catalysts can be reused without any loss of activity in the case of styrene and isoprene.