Norbornene polymerization using multinuclear nickel catalysts based on a polypropyleneimine dendrimer scaffold

Four multinuclear nickel complexes derived from generation 1 (G1) and generation 2 (G2) dendrimeric salicylaldimine ligands based poly(propyleneimine) dendrimer scaffolds of the type, DAB-(NH₂)_n (n = 4 or 8, DAB = diaminobutane) were evaluated as catalysts precursors in the polymerization of norbornene, using methylaluminoxane as co-catalyst. All four catalyst evaluated were found to be active for norbornene polymerization giving polymers with moderate to high molecular weights and low polydispersity indices. The polymerization results indicate that there is some sort of dendritic effect, in that the catalyst activity appears to be influenced by the dendrimer generation.     

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

Chloromethylsilane functionalised dendrimers: synthesis and reactivity

Tetraallylsilane was functionalised using (chloromethyl)dimethylsilane to give the first generation chloromethyl terminated dendrimer 1. The resulting dendrimer was successfully reacted with K[CpM(CO)₂] (Cp=η⁵-C₅H₅; M=Fe, Ru) to give Si[(CH₂)₃SiMe₂CH₂MCp(CO)₂]₄ functionalised dendrimers in satisfactory yield. Reaction of dendrimer 1 with NaI in acetone gave the -SiMe₂CH₂I functionalised dendrimer, while reactions of 1 with K[CpM(CO)₃] (M=Mo, W, Re), Li[C₅Me₄H], Na[C₅Me₄H], the cobaloxime nucleophile or tert-BuLi were not successful.     

Viologen-based benzylic dendrimers: selective synthesis of 3,5-bis(hydroxymethyl)benzylbromide and conformational analysis of the corresponding viologen dendrimer subunit

Convergent and divergent strategies for the synthesis of viologen dendrimers with 1,3,5-tri-methylene-branching units are discussed. The title compound is easily transformed into 1-[3,5-bis(hydroxymethyl)benzyl]-4-(pyridin-4-yl)pyridinium hexafluorophosphate, which is used in sequential growth and activation steps as a CB₂ compound in the cascade-type dendrimer synthesis (B = -OH, activation = -OH → Br). Analysis of the dendrimer structure reveals that three torsional angles, that is, τ₁ between the two pyridinium units, τ₂ between the methylene and pyridinium and τ₃ between the methylene and phenyl, determine the conformational space of the dendrimers. We report here the crystal structure of 1-[3,5-bis(hydroxymethyl)benzyl]-4-(pyridin-4-yl)pyridinium as PF₆⁻ salt which represents the smallest subunit of the dendrimer that shows the same three torsional angles. The crystal structure together with the results from PM3 calculations opens an avenue to judge the structure of benzylic viologen-based dendrimers.     

An iminophosphine dendrimeric ligand and its evaluation in the Heck reaction

DAB-dendr-(NH₂)₃₂ has been modified to produce two novel dendrimeric P,N-ligands, DAB-dendr-[1,2-(NCHC₆H₄PPh₂)]₃₂ and DAB-dendr-[1,2-(NHCH₂C₆H₄PPh₂)]₃₂. These were both found to activate electron rich aryl bromides in the Heck reaction. The reactions seem to proceed better in a novel solvent system consisting of an equimolar mixture of a tertiary amine with acetic acid. A marked dependence of the conversion on the [equivalents of dendrimeric ligand]/[Pd] ratio has been observed with conversion decreasing as the ratio of dendrimer equivalents to Pd increases. This phenomenon is attributed to interactions of the arms bearing the metal with neighboring ones within the dendrimer.     

Thermodynamic parameters for the dissolution of n-alkanes and n-alkanols in a liquid-crystalline polypropylenimine dendrimer: A gas-chromatographic study

Thermodynamic parameters for the dissolution of n-alkanes (C₇-C₁₁) and n-alcohols (C₅-C₉) under infinite dilution in the columnar and isotropic phases of a polypropylenimine dendrimer have been determined by inverse gas chromatography. Thermodynamic functions have been investigated as a function of the phase state of a mesogen, the length of alkyl chains of sorbates, and temperature. For the dissolution of low-molecular-mass organic compounds, a higher endothermic effect corresponds to a larger entropy factor with its positive deviation from ideality. The dominant effect of the entropy constituent of the Gibbs energy on the dissolution of sorbates in a high-molecular-mass LC solvent has been revealed. The Rohrschneider constants have been estimated for the stationary phase based on the dendrimer. The experimental data indicate the low polarity of the sorbent that is close to those of phenyl-containing siloxane phases.     

Theory of orientational relaxation of individual specified units in a dendrimer

The theory of orientational relaxation properties of certain individual segments in a dendrimer macromolecule depending on the generation number and the position of a given segment in the dendrimer was developed. The time dependence for the dipole moment after switching an electric field off and the frequency dependence of the permittivity for this segment were calculated, which are determined by the autocorrelation function P ₁ of the average projection of the single element. The dielectric properties of the dendrimer at the random distribution of dipole moments (e.g., as a result of sorption of solvent polar groups on the macromolecule) are considered. The time and frequency dependences of the autocorrelation function P ₂ for the mean squared projection of the single element that are detectable by means of some experimental techniques (NMR, luminescence, birefringence, etc.) were studied. The theory qualitatively agrees with both the computer simulation results on the autocorrelation function P ₁ for the dendrimer macromolecules and the available experimental data on the dependence of orientational mobility for the terminal segments of the number of generations.     

A third generation chiral phosphorus-containing dendrimer as ligand in Pd-catalyzed asymmetric allylic alkylation

The synthesis of a third generation phosphorus-containing dendrimer possessing 24 chiral iminophosphine end groups derived from (2S)-2-amino-1-(diphenylphosphinyl)-3-methylbutane is described. In situ complexation of this dendrimer by [Pd(η³-C₃H₅)Cl]₂ affords a catalyst, which is used in asymmetric allylic alkylations of rac-(E)-diphenyl-2-propenyl acetate and pivalate. The percentage of conversion, the yield of isolated 2-(1,3-diphenylallyl)-malonic acid dimethyl ester, and its enantiomeric excess have been measured in each case, and were found to be good to very good (ee from 90% to 95%). Furthermore, the dendritic catalyst can be recovered and reused at least two times, with almost the same efficiency.     

Synthesis,Redox Activity of Rigid Ferrocenyl Dendrimers,and Isolation of Robust Ferricinium and Class‐II Mixed‐Valence Dendrimers

The coupling reactions of ethynylferrocene with trihalogenoarenes do not lead to ethynylferrocenyl arenes that are soluble enough to form the basis of a suitable construction of stiff ferrocenylethynyl arene‐cored dendrimers, which explains the previous lack of reports on stiff ferrocenyl dendrimers. However, rigid ferrocenyl‐terminated dendrimers have been synthesized from 1,3,5‐tribromo‐ and triiodobenzene through Sonogashira and Negishi reactions with 1,2,3,4,5‐pentamethyl‐1′‐ethylnylferrocene ( 1 a ), according to 1→2 connectivity. With compound 1 a , the construction of a soluble dendrimer ( 10 a ) that contained 12 ethynylpentamethylferrocenyl termini was achieved. Stiff dendrimer 10 a shows a single, reversible cyclic voltammetry (CV) wave (with adsorption), which disfavors the hopping heterogeneous electron‐transfer mechanism that is postulated for redox‐terminated dendrimers that contain flexible tethers. The selectivity of these Sonogashira reactions allows the synthesis of an arene‐cored dendron ( 2 c ) that contains both ethynylferrocenyl and 1,2,3,4,5‐pentamethyl‐ferrocenylethynyl redox groups, thus leading to the construction of a dendrimer ( 7 c ) that contains both types of differently substituted ferrocenyl groups with two well‐separated reversible CV waves. Upon selective oxidation, this mixed dendrimer ( 7 c ) leads to a class‐II mixed‐valence dendrimer, 7 c [PF₆]₃, as shown by Mössbauer spectroscopy, whereas oxidation of the related fully pentamethylferrocenylated dendrimer ( 7 a ) leads to the all‐ferricinium dendrimer, 7 a [PF₆]₆.     

Decomposition of poly(amidoamine) (PAMAM) dendrimers under O2 plasma conditions

Results of FTIR and XPS measurements of bulk and γ-Al₂O₃-supported fourth generation hydroxyl-terminated (G4OH) poly(amidoamine) (PAMAM) dendrimers during O₂ plasma treatment at room temperature indicate that such treatment can effectively decompose the structure of the dendrimer. No new functionalities appear in the dendrimer structure during such treatment, but instead fragmentation of this structure is taking place. The dendrimer backbone likely disintegrates into small molecules (i.e., NO_x, CO, CO₂) which in the case of the supported dendrimers interact with the γ-Al₂O₃ support and form surface nitrate and carboxy species. Subsequent treatment in H₂ at 300 °C leads to the complete removal of the nitrogen-containing species from the surface of γ-Al₂O₃. However, carbon-containing species still remain on the surface in relatively small amounts. Finally, XRD measurements further show that the crystallinity of the support is not affected substantially by the O₂ plasma treatment.     

A new colorimetric and fluorimetric sensor for metal cations based on poly(propilene amine) dendrimer modified with 1,8-naphthalimide

A new fluorescent first generation poly(propylene amine) dendrimer (PPI), peripherally modified with 4(butylamino-substituted-1,8-naphthalimide), has been synthesized and characterized. Its photophysical characteristics in organic solvents of different polarities were studied, and the influence of sodium hydroxide on its spectral characteristics in N,N-dimethylformamide is discussed. The complexes formed between the dendrimer and metal cations in solution have been studied with regard to the potential applications of the new dendrimer as a colorimetric and fluorescent sensor for metal ions. The fluorescence intensity of the dendrimer in the presence of metal cations (Zn²⁺, Co²⁺, Pb²⁺, Mn²⁺, Cu²⁺ and Fe³⁺) increases substantially revealing its sensor potential.     

Dendrimer polyaluminosilicates as a matrix for filled coatings

Sol-gel synthesis of filled polyaluminosilicates with macromolecules having a dendrimer morphology (M _calc ≈ 480 000) was developed. The software package Gaussian B3LYP/6-31G(d) was used to assess the possible mechanism by which nuclei of dendrimer molecules are created, with the subsequent generation of crowns of branched ensembles capable of accommodating up to 70 wt % nano-Al₂O₃ as a filler. The data furnished by transmission electron microscopy of the samples are in agreement with the suggested mechanism of generation and growth of dendrimers. The main results were obtained on filled aluminosilicate samples that contain 70 wt % nano-Al₂O₃ and exhibit a high wear resistance in tribological tests.     

Thermodynamics of the sorption of organic vapors in a carbosilane dendrimer and a hyperbranched polymer

Fluorinated derivatives of the carbosilane G6 dendrimer and the hyperbranched polymer of the same chemical structure are synthesized and characterized via the method of inverse gas chromatography. For various organic compounds (n-alkanes, aromatic hydrocarbons, partially fluorinated and perfluorinated organic compounds), the following thermodynamic parameters of their interaction with the above polymers are defined: solubility coefficient S, the sorption enthalpy, the partial molar enthalpy of mixing, the Flory-Huggins parameter, and the solubility parameter. The thermodynamic behavior of vapors in the dendrimer is similar to that in the hyperbranched polymer. For a dendrimer and a hyperbranched polymer, a linear correlation between the logarithm of S and T _cr ² is established, where T _cr is the critical temperature of a probe. For both polymers, the partial enthalpy of mixing of n-alkanes is shown to be independent of the dimensions of the probe molecules. This behavior is typical of the vapor sorption in linear polymers at temperatures above the glass transition temperature.     

New super- and supramolecular receptor systems — cages, chains, squares, and dendrimers incorporating macrocycles as structural elements

The selective use of tert-butoxycarbonyl protecting group chemistry has enabled efficient syntheses of multilinked macrocyclic derivatives incorporating tetraaza and aza-thia donor rings. Mixed ring species have also been prepared as well as a dendrimer incoporating nine N₂S₂-donor macrocyclic sites. The interaction of these species with selected metal ions is discussed. A series of cage molecules with varying cavity sizes has been prepared and their interaction with both (selected) metal ions and small molecules described. The formation of new large molecular squares and catenanes involving self-assembly is also presented.     

PAMAM (poly-amido amine) dendrimer supported copper nanoparticles for chemoselective nitro reduction

Present paper reports the synthesis of biocompatible poly (amidoamine) (PAMAM) dendrimer by Michael addition (up to Generation 2) which has been used as a matrix for preparing copper nanoparticles using green solvent. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy – dispersive X-ray spectroscopy (EDX) were used to determine the surface morphology and elemental composition of the nanoparticle. The results confirmed the FCC structure of the prepared dendrimer encapsulated Cu nanoparticles with an average crystallite size of 4 ​nm. Purely aqueous-phase chemoselective reduction of nitro group in various aromatic nitro substrates such as p-nitrophenol, p-nitrobenzaldehyde and 2, 4 dinitrophenol have been employed in the presence of eco-friendly dendrimer encapsulated Cu nanoparticles using hydrazine hydrate as reducing agent. The nanocatalyst showed high reusability up to seven runs exhibiting no significant loss in its catalytic activity. As the synthesised material is found to be biocompatible, this catalytic strategy can be used to catalyse similar biochemical reactions.     

Rapid Bacterial Recognition over a Wide pH Range by Boronic Acid-Based Ditopic Dendrimer Probes for Gram-Positive Bacteria

We have developed a convenient and selective method for the detection of Gram-positive bacteria using a ditopic poly(amidoamine) (PAMAM) dendrimer probe. The dendrimer that was modified with dipicolylamine (dpa) and phenylboronic acid groups showed selectivity toward Staphylococcus aureus. The ditopic dendrimer system had higher sensitivity and better pH tolerance than the monotopic PAMAM dendrimer probe. We also investigated the mechanisms of various ditopic PAMAM dendrimer probes and found that the selectivity toward Gram-positive bacteria was dependent on a variety of interactions. Supramolecular interactions, such as electrostatic interaction and hydrophobic interaction, per se, did not contribute to the bacterial recognition ability, nor did they improve the selectivity of the ditopic dendrimer system. In contrast, the ditopic PAMAM dendrimer probe that had a phosphate-sensing dpa group and formed a chelate with metal ions showed improved selectivity toward S. aureus. The results suggested that the targeted ditopic PAMAM dendrimer probe showed selectivity toward Gram-positive bacteria. This study is expected to contribute to the elucidation of the interaction between synthetic molecules and bacterial surface. Moreover, our novel method showed potential for the rapid and species-specific recognition of various bacteria.     

Curcumin Loaded Dendrimers Specifically Reduce Viability of Glioblastoma Cell Lines

Glioblastoma (GB) is a deadly and aggressive cancer of the CNS. Even with extensive resection and chemoradiotherapy, patient survival is still only 15 months. To maintain growth and proliferation, cancer cells require a high oxidative state. Curcumin, a well-known anti-inflammatory antioxidant, is a potential candidate for treatment of GB. To facilitate efficient delivery of therapeutic doses of curcumin into cells, we encapsulated the drug in surface-modified polyamidoamine (PAMAM) dendrimers. We studied the in vitro effectiveness of a traditional PAMAM dendrimer (100% amine surface, G4 NH₂), surface-modified dendrimer (10% amine and 90% hydroxyl-G4 90/10-Cys), and curcumin (Cur)-encapsulated dendrimer (G4 90/10-Cys-Cur) on three species of glioblastoma cell lines: mouse-GL261, rat-F98, and human-U87. Using an MTT assay for cell viability, we found that G4 90/10-Cys-Cur reduced viability of all three glioblastoma cell lines compared to non-cancerous control cells. Under similar conditions, unencapsulated curcumin was not effective, while the non-modified dendrimer (G4 NH₂) caused significant death of both cancerous and normal cells. By harnessing and optimizing the components of PAMAM dendrimers, we are providing a promising new route for delivering cancer therapeutics. Our results with curcumin suggest that antioxidants are good candidates for treating glioblastoma.     

Organoiron activation combined with electron- and proton transfer: implications in biology, organic synthesis, catalysis and nanosciences

This Account summarizes the results obtained in our research group on the intra- and intermolecular organoiron activation of substrates by combining the coordination of arenes by CpFe^+/0 and electron and/or proton transfer. The concepts involved are those of electron and proton reservoirs, activation of O₂ by single electron transfer in solution, mimic and inhibition of the reactivity of superoxide radical anion, materials synthesis (for instance fullerene anions), electronic communication between two metals connected by a hydrocarbon bridge, activation of arene ligands for multiple functionalization, giant dendrimer synthesis and electron transfer in catalysis (redox and electron-transfer-chain).     

Identification of diamine linkers with differing reactivity and their application in the synthesis of melamine dendrimers

Diamine linkers for the synthesis of dendrimers based on melamine were identified using competition reactions. The relative reactivity of the surveyed cyclic monoamines varies by 40 times, expanding the previously identified series to an overall relative reactivity range of 320 times. Azetidine is 40 times more reactive than the cyclic, nine-membered ring (C₈H₁₇N), and 320 times more reactive than benzylamine. Reactivity differences are attributed to pK_a values and sterics. Diamines incorporating these groups are useful linkers that can be employed in dendrimer synthesis. Specifically, the nucleophilicity of the individual amine groups comprising 3-aminoazetidine, 3-aminopyrrolidine, and 4-aminopiperidine varies by 100 times, 70 times, and 20 times, respectively. These linkers are incorporated into a generation three dendrimer.     

Switching the selectivity of a polyglycerol dendrimer monomolecularly imprinted with d-(−)-fructose

A polyglycerol dendrimer monomolecularly imprinted with d-(−)-fructose (Fru) was synthesized. The dendrimer formed adducts with several monosaccharides, Fru, d-(+)-galactose, d-(+)-glucose, d-(+)-mannose, and methyl-α-d-mannopyranoside (MMan), by removal of four water molecules. The dendrimer preferred Fru in the absence of N,N,N′,N′-tetramethyldiaminomethane (TMDAM), whereas it preferred MMan in the presence of TMDAM.