3,307 research outputs found
Pincer ligands as powerful tools for catalysis in organic synthesis
No full textPresent trends in homogeneous catalysis are moving toward the development of multi-step, one-pot processes where salt waste is reduced to a minimum and in the lowest possible volumes of solvent. Energy efficiency is also maximized to produce the most cost-effective end product(s). However, these one pot cascade, tandem, or domino catalyzed reaction sequences rely on catalyst, substrate and solvent compatibility, on catalyst stability and on the possibility to protect these catalysts from mutual deactivation. This can be facilitated, for example, by binding these to suitable (in)soluble supports or by using compatible, stable catalysts that can be used to mediate a variety of different reactions. Pincer organometallics are powerful tools that can be used to achieve many of these objectives. The synthesis and properties of a variety of “molecularly enlarged” pincer organometallics displaying both surprising stability and versatile catalytic properties are discussed. A fascinating, recent development is the site-selective inhibition of a lipase, i.e., Cutinase, with a single pincer-metal catalytic site (see X-ray). Similarly, partial inhibition of lipases immobilized on beads with a Ru-based racemization catalyst yields a bio-organometallic hybrid catalyst, which in combination with still uninhibited lipase on these beads, can operate as a heterogeneous bifunctional catalytic material for the kinetic resolution of racemic alcohols to afford enantiopure product(s). -------------------------------------------------------------------------------
Organocopper Compounds: From Elusive to Isolable Species, from Early Supramolecular Chemistry with RCuI Building Blocks to Mononuclear R2–nCuII and R3–mCuIII Compounds. A Personal View
No full textThe first reports on copper-mediated organic reactions and speculations about the role of presumed organocopper compounds as intermediates or transient species date back to the early 20th century. Since that time, copper salt mediated and much later, copper-catalyzed C–X bond forming reactions (X = C, O, N) have been developed. Phenyl- and methylcopper(I) (highly explosive as a dry solid) were the first organocopper materials synthesized by Reich (1923) and Gilman (1952), respectively. However, it was not until the late 1960s and early 1970s that the first pure organocopper(I) compounds, which are also stable at room temperature, were isolated and structurally characterized. Recently, by the application of clever design and new synthetic approaches even organocopper(II) and -(III) compounds have been isolated, species that during the greater part of the last century were considered elusive. Significantly, these species had for some time been surmised, via kinetic and computational studies, to play a key role in copper-mediated and -catalyzed C–X bond forming reactions. In this personal account the various stages along which organocopper chemistry developed, with a steady pace, will be highlighted
Sulfonate Functionalisation of Transition Metal Complexes: A Versatile Tool Towards Catalyst Recovery
No full textThis thesis describes the synthesis and application of sulfonate-functionalised ligands in organometallic chemistry and (aqueous) catalysis. Due to their ability to trigger specific solubility, different NHC ligand precursors bearing a butyl-sulfonate chain were synthesised. The formation of transition metal complexes containing gold and rhodium was enabled by a simple procedure involving the initial synthesis of a silver complex and a transmetallation with a suitable metal precursor and an organic salt. Metallodendrimers containing eight metal complexes immobilised via ionic interactions were synthesised following this straightforward one-pot protocol. This particular strategy was extended to the synthesis of homo-metallodendrimers (Au/Au) and was modified to allow the formation of hetero-metallodendrimers (Au/Pt). In addition, a new class of sulfonate-functionalised arene ruthenium complexes was developed. Due to their highly water-solubility these complexes act as catalysts in the asymmetric transfer hydrogenation of acetophenone in water. The catalyst was recycled and was still active after several runs, giving the alcohol products in excellent enantiopurity
'Dialyzable' Carbosilane Dendrimers as Soluble Supports in Organic Synthesis: Proof of principle, application and diafiltration performance
No full textSince the development of polystyrene as (insoluble) support for the synthesis of peptides by Merrifield in the 1960s, the application of supported synthesis in both industrial and academic research has increased. The introduction of dendrimers as supports solved many of the problems arising from the inhomogeneous nature of the solid supports. Dendrimers are highly soluble in various reaction media and standard spectroscopic analysis in solution phase is well applicable. Carbosilane dendrimers take a special position due to their chemical robustness and stability. Initially, these dendrimers were used as support for homogeneous catalysts, and could easily be removed from the reaction mixture by nano- or diafiltration. The application of carbosilane dendrimers as support in organic synthesis and optimization of the filtration steps in this procedure are subject of this thesis. Carbosilane dendrimers can be functionalized with various substrates, like 2-bromopyridines, which can be attached via lithiation chemistry. The complete attach-modify-release sequence was optimized using model reactions. It appeared that the conditions used for the model reactions could not always be directly translated to the dendrimer-supported syntheses. A new method for the loading of the dendrimers with bromopyridines was developed, which makes use of a hydrosilylation reaction. The supported bromopyridines were converted into a series ofcompounds and the products were released from the dendrimers. During the filtration experiments with membranes of various pore sizes, it appeared that the dendrimers are also retained by membranes that have a theoretical pore size larger than the dimensions of the dendrimer, and that the speed of filtration increases with increasing pore size. Since the applied dendrimers are flexible, especially under the ‘shear flow’ conditions of the filtration, it is expected that more rigid dendrimers will be retained better by membranes, and that even higher filtration speeds will be achieved. Two rigid core molecules and two dendritic wedges were synthesized and used for the formation of several new rigid dendrimers. The dendrimers were loaded with different dye molecules to detect them during the filtration experiments. The rigid dendrimers are retained somewhat better by membranes with larger pore sizes, but generally the difference with the ‘flexible’ dendrimers is not big. The results obtained from UV/Vis spectroscopy, GPC and modeling studies generally correspond to the results of the diafiltration experiments, showing a correlation between the dimensions and the molecular weights of the dendrimers. Some exceptions suggest that besides structural parameters of the dendrimers also other effects, like the presence of solvent molecules, play a role in determining the dimensions of the dendrimers in solution. In order to compare the results obtained with the dendrimers directly to the results of SPOS, one dendrimer is loaded with three typical SPOS linker groupings: benzylbromide-, DEAM- and REM-linkers. SPOS methodologies were used to investigate the application of these dendritic supports. For all three linkers the yields and purities of the products appeared not to be as high as for comparable SPOS results. This was partly due to the incompatibility of the used purification method (passive dialysis) with water-labile and water-soluble products
Cellulose at Work: Carbon-Supported Base Metal Nanoparticles, Catalytic Graphitisation and the Growth of Carbon Nanostructures
No full textWithin the field of chemistry both nanotechnology as well as green chemistry are considered to be of considerable scientific as well as societal relevance. Nanotechnology describes the study of materials with at least one dimension not exceeding the 100 nm size range. These nanosized materials possess mechanical, optical, electrical and/or magnetic properties that deviate from those observed for the bulk material. Green chemistry involves the development of environmentally benign chemical processes using renewable resources to access reactants, reagents, solvents and thus sustainable products. The research described in this thesis combines both nanotechnology and green chemistry for the development of functional base metal nanomaterials for which a range of applications are foreseen. A novel method is developed to obtain base metal nanoparticles of copper, nickel, cobalt or iron supported onto (graphitic) carbon bodies. Spherical microcrystalline cellulose spheres are used as the precursor for these carbon bodies. The method entails the loading of hydrophilic microcrystalline cellulose spheres by wet impregnation with aqueous solutions of base metal salts followed by drying. During pyrolysis of the dried loaded microcrystalline cellulose spheres under stagnant and inert conditions in the temperature range of 400-800 °C several processes occur. Whereas the cellulose is converted into an amorphous carbonaceous material, the dispersed metal salts are converted into their metal oxides. Finally, the highly dispersed supported nanosized metal oxide particles are quantitatively reduced yielding dispersed base metal nanoparticles without the need of an external H2(g) source, i.e. explosive H2(g) is not required, instead cellulose pyrolysis products provide the required reducing atmosphere. It is shown that base metal nanoparticles capable of forming metal carbides (nickel, cobalt and iron) convert the amorphous carbonaceous support into graphitic carbon nanostructures. This is of considerable interest for the development of efficient (ferromagnetic) absorbents. Under appropriate conditions in which the size of the base metal nanoparticles (nickel or iron) is carefully controlled, it is found that the metal nanoparticles act as efficient (heterogeneous) growth catalysts for either carbon nanofiber or multi-walled carbon nanotube growth in the presence of an external carbon source. This allows for a large scale synthesis of these materials. In the case of the iron nanoparticles, conditions could be found in which upon heat treatment in a stagnant and inert atmosphere both the formation of iron nanoparticles as well as a concomitant growth of multi-wall carbon nanotubes occurred consecutively in the absence of both an external H2(g) as well as an external carbon source. These experiments suggest viable pathways for the still debatable formation of (multi-wall) carbon nanotubes in nature. These results indicate that these base metal nanoparticles will also be active as heterogeneous catalyst in other processes. Finally preliminary experiments have been executed showing that the materials developed can also be applied in electrochemistry (novel electrodes) and magnetic resonance imaging
Bio-inspired iron and manganese complexes derived from mixed N,O ligands for the oxidation of olefins
No full textThis Thesis describes the synthesis and structural analysis of bio-inspired iron and manganese complexes used for the catalytic oxidation of olefin substrates. The development of catalytic systems for oxidation chemistry that are based on first row transition metals and that apply a green oxidant like hydrogen peroxide is an ongoing research topic in the fields of homogeneous catalysis, bioinorganic chemistry, and green chemistry. For the inspiration in the design of such systems, biological oxidation reactions that involve a metallo-enzyme are used as the starting point. The initial design of the ligand used in the catalytic system is based on the active site structure of one particular or of a family of metallo-enzymes. Further catalyst development and optimization is then carried out by ligand variation and reaction parameter optimization. This bio-inspired approach in catalyst design may on the one hand lead to discrete structural analogues of the active site of a metallo-enzyme (structural modeling) and may on the other hand lead to the development of efficient transition metal catalysts that are of use to organic synthesis (functional modeling). The first part of this thesis is dedicated to a study on manganese complexes derived from mixed N,O ligands, while the second part focuses on iron complexes based on related, yet different mixed N,O ligands. The interest in the use of mixed N,O ligands in the development of bio-inspired oxidation catalysts stems from the structure and activity of a class of mono-nuclear non-heme iron enzymes that feature a so-called 2-His-1-carboxylate facial triad in their active site. In this triad, the iron center is coordinated in a facial manner by only three endogenous donor ligands, i.e. two histidines and one glutamate or aspartate. This particular structural feature leads to a uniquely diverse reactivity of enzymes that contain this active site. Efforts to model the structural and reactivity aspects of the facial triad enzymes have mainly focused on the use of all-nitrogen ligands and have to a much lesser extent made use of mixed N,O ligand donor sets. During the research described in this thesis a library of bio-inspired iron and manganese complexes was synthesized and structurally analyzed. The metal complexes derived from the mixed N,O ligands are good structural and functional mimic of their biological counterparts. The complexes were all tested in catalytic oxidation reactions of olefin substrates, showing a diversity of activities. The information obtained by studying these synthesized ligands and their complexes can be used as a lead for the further development of bio-inspired iron and manganese complexes for the oxidation of olefins and other substrates
Incorporation of an n-Butylsulfonate Functionality To Induce Aqueous Solubility on Ruthenium(II) η6-Arene Complexes
No full textThe reaction of the new building block sodium n-butyl sulfonate cyclohexadiene 3 with RuCl3·xH2O leads to the formation of different dimeric halide-bridged ruthenium complexes, depending on the reaction conditions. Isolation of pure dimeric complexes from these mixtures was not successful. Changing the cyclohexadiene preligand in the reaction to its neutral isobutyl-protected analogue 2 did allow for the synthesis and isolation of neutral halide-bridged dimeric complexes 5 (chloride bridged) and 6 (iodide bridged) in good to excellent yields. Upon deprotection of the isobutyl sulfonate groups in 6 using sodium iodide, the anionic, water-soluble dimeric complex 7 was obtained in near-quantitative yield. Complexes 6 and 7 are starting materials for the synthesis of water-soluble heteroleptic Ru arene complexes, as was demonstrated by the reaction of 6 with (S,S)-TsDPEN, followed by a reaction with NaI, to yield the mononuclear water-soluble Ru arene TsDPEN complex 8. In preliminary experiments 8 was found to be an effective catalyst in the asymmetric transfer hydrogenation (ATH) of acetophenone to quantitatively provide (S)-phenylethanol in 94% e
Mechanistic Insights into the Rhenium-Catalyzed Alcohol-To-Olefin Dehydration Reaction
No full textRhenium-based complexes are powerful catalysts for the dehydration of various alcohols to the corresponding olefins. Here, we report on both experimental and theoretical (DFT) studies into the mechanism of the rhenium-catalyzed dehydration of alcohols to olefins in general, and the methyltrioxorhenium-catalyzed dehydration of 1-phenylethanol to styrene in particular. The experimental and theoretical studies are in good agreement, both showing the involvement of several proton transfers, and of a carbenium ion intermediate in the catalytic cycle
Assignment of phantom bands in the solid-state infrared and Raman spectra of coronene: the importance of a minute out-of-plane distortion
No full textThe molecular geometry and the normal modes properties of coronene are investigated by means of DFT B3LYP and restricted/Hartree–Fock calculations utilizing basis sets of triple zeta +polarization quality. The interpretation of the infrared and Raman spectra of coronene, especially in solid state, is critically revised. The phantom bands in the solid state, previously not understood, are readily assigned after considering a minute out-of-plane molecular distortion from D6h to C2h
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